Hi-Z mics? where OH where have they gone!

[[email protected]]

I'm looking a mic that will work on a hi-z input (12Kohm) without a
preamp or impedance transformer. Two questions:

1) How will a 600ohm mic sound through this input? Will I get a decent
signal or do I need a mic with much high impedance?

2) Can anyone recommend a hi-z mic, not more than $100, that is NOT a
dinky computer microphone? And where I could get one? I would have
thought that some mics I consider to be "cheap", such as a PG58 or
even a "Sony dynamic mic" (radioshack style) for $40 would be hi-z,
but they are in fact 600ohm.

I admit this isn't really "pro" level equipment I'm asking about but
it's to accompany the Tascam Pocketstudio 5 that I got for my
girlfriend :D so come on, help a brother out, eh!

Dex

[Scott Dorsey]

> wrote:
>I'm looking a mic that will work on a hi-z input (12Kohm) without a
>preamp or impedance transformer. Two questions:

You can find one at a truckstop or a place that sells ham radio supplies.
Unbalanced high-Z mikes are still standard for communications applications
where high frequency response isn't needed.

>1) How will a 600ohm mic sound through this input? Will I get a decent
>signal or do I need a mic with much high impedance?

It won't be very loud unless you use a matching transformer.

>2) Can anyone recommend a hi-z mic, not more than $100, that is NOT a
>dinky computer microphone? And where I could get one? I would have
>thought that some mics I consider to be "cheap", such as a PG58 or
>even a "Sony dynamic mic" (radioshack style) for $40 would be hi-z,
>but they are in fact 600ohm.

The thing is, if you want a lot of windings around a coil, you are going
to have a lot of mass on the coil. So either you get no high end, or
you get a transformer with an integral step-up transformer if you want
a dynamic high-Z mike.

>I admit this isn't really "pro" level equipment I'm asking about but
>it's to accompany the Tascam Pocketstudio 5 that I got for my
>girlfriend :D so come on, help a brother out, eh!

Pocketstudio? Is this a thing with a 1/8" input that provides plug-in power?
If so, you will need a microphone that uses plug-in power and if you use
a dynamic mike you will need to add a blocking cap.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

On Dec 5, 7:33 am, wrote:
> I'm looking a mic that will work on a hi-z input (12Kohm) without a
> preamp or impedance transformer. Two questions:
>
> 1) How will a 600ohm mic sound through this input? Will I get a decent
> signal or do I need a mic with much high impedance?
>
> 2) Can anyone recommend a hi-z mic, not more than $100, that is NOT a
> dinky computer microphone? And where I could get one? I would have
> thought that some mics I consider to be "cheap", such as a PG58 or
> even a "Sony dynamic mic" (radioshack style) for $40 would be hi-z,
> but they are in fact 600ohm.
>
> I admit this isn't really "pro" level equipment I'm asking about but
> it's to accompany the Tascam Pocketstudio 5 that I got for my
> girlfriend :D so come on, help a brother out, eh!
>
> Dex

Go get an Audio-Technica CP8201 Lo-Z to Hi-Z transformer. It will cost
around $12. Use this on the Line B input to the PS5. Now use whatever
dynamic mic you prefer.

bobs

Bob Smith
BS Studios
we organize chaos
http://www.bsstudios.com

[Eeyore]

wrote:

> I'm looking a mic that will work on a hi-z input (12Kohm) without a
> preamp or impedance transformer. Two questions:

Firstly, 12 k ohms ISN'T High impedance. To be a genuine high impedance
input it would be at least 100 k ohms or so. Yours is more of a medium
inpedance input.


> 1) How will a 600ohm mic sound through this input?

Absolutely fine. The recommended load for microphones according to modern
thinking (for 35 years at least) is at least TEN times the impedance of
the microphone itself. This improves voltage transfer compared to
impedance matching and as a result improves signal to noise ratio.

Most studio mics are actually 200 ohms or so and the input impedance on
the majority of modern professioanl min inputs is typically ~ 2 k ohms.

There are plenty of 600 ohms mics though. Typically the cheaper Asian ones
that mimic well known brands like Shure are 600 ohms.


> Will I get a decent signal

You may do. It depends on the input sensitivity rather than the impedance
here. What exactly is the equipment ?


> or do I need a mic with much high impedance?

No.

Graham

[Eeyore]

Scott Dorsey wrote:

> > wrote:
> >I'm looking a mic that will work on a hi-z input (12Kohm) without a
> >preamp or impedance transformer. Two questions:
>
> You can find one at a truckstop or a place that sells ham radio supplies.
> Unbalanced high-Z mikes are still standard for communications applications
> where high frequency response isn't needed.
>
> >1) How will a 600ohm mic sound through this input? Will I get a decent
> >signal or do I need a mic with much high impedance?
>
> It won't be very loud unless you use a matching transformer.

How can you say that without knowing the sensitivity ? The impedance per se isn't
the issue.

Graham

[Eeyore]

wrote:

> I'm looking a mic that will work on a hi-z input (12Kohm) without a
> preamp or impedance transformer. Two questions:
>
> 1) How will a 600ohm mic sound through this input? Will I get a decent
> signal or do I need a mic with much high impedance?

Read the flipping manual !
http://www.tascam.com/i-864-232-128-0-7DF475C0.pdf

The 'B' input appears on both a 3.5mm jack designed for 'computer mics'
and also a 1/4 inch jack. Use the 1/4" jack.

That impedance is perfectly suitable for direct connection of a 200-600
ohm mic but you'll need to have it wired to an unbalanced jack.

The sensitivity of -40dBV will be adequate for many applications.

Graham

[Randy Yates]

Eeyore > writes:

> [...] the input impedance on the majority of modern professioanl min
> inputs is typically ~ 2 k ohms.

That figure sounds low. I would think these types of inputs are at least
10k ohms.
--
% Randy Yates % "She's sweet on Wagner-I think she'd die for Beethoven.
%% Fuquay-Varina, NC % She love the way Puccini lays down a tune, and
%%% 919-577-9882 % Verdi's always creepin' from her room."
%%%% > % "Rockaria", *A New World Record*, ELO
http://www.digitalsignallabs.com

[[email protected]]

On Dec 5, 10:56 pm, Randy Yates > wrote:
> Eeyore > writes:
> > [...] the input impedance on the majority of modern professioanl min
> > inputs is typically ~ 2 k ohms.
>
> That figure sounds low. I would think these types of inputs are at least
> 10k ohms.

The Tascam unit has 2 mic inputs. a 1/4" of 12Kohm and an 1/8th inch
of 2.2Kohm.

Anyway this is great advice, I'll get one of the cheaper 600ohm mics.
I'd get her a more expensive one but in combination with the other
stuff I'm buying, I don't wanna freak her out. Over-eager xmas
presents = "I'm a serial killer" !! :D

Dex

[Eeyore]

Randy Yates wrote:

> Eeyore > writes:
>
> > [...] the input impedance on the majority of modern professioanl min
> > inputs is typically ~ 2 k ohms.
>
> That figure sounds low. I would think these types of inputs are at least
> 10k ohms.

NO.

Graham

[Don Pearce]

On Wed, 05 Dec 2007 22:56:52 -0500, Randy Yates >
wrote:

>Eeyore > writes:
>
>> [...] the input impedance on the majority of modern professioanl min
>> inputs is typically ~ 2 k ohms.
>
>That figure sounds low. I would think these types of inputs are at least
>10k ohms.

The input impedance of the typical mic preamp is of the order of 100k
or so. This is set by the op amp or discrete transistor stage used for
the input amplifier. As long as this figure is high, it won't load a
microphone adversely. What determines the optimum source impedance
(the actual impedance of the mic) is the way the noise generators in
that input amp balance each other. There are separate mechanisms for
generating current and voltage noise.

The input impedance of the typical mic preamp *system* is generally
around 12k, and this is because it has a requirement to be able to
supply phantom power to a condenser mic. It does this with a 48 volt
supply fed through a pair of 6.2k resistors, each to one arm of the
balanced pair. Turning off phantom power for microphones that don't
need it generally involves disconnecting the 48V from the ends of the
resistors, but leaving them connected to each other, leaving 12.4k
across the microphone input. This is usually what defines the input
impedance of a mic preamp.

The typical preamp is nominally ok from a fairly wide range of
microphone impedances, but you need to know the impedance of the mic
because that will determine its output voltage level. You need that to
know what range of gains the preamp must offer. A high impedance mic
will have a greater voltage output, so you need a lower gain figure
from the preamp. This doesn't mean that a high impedance, high output
mic will offer a better noise performance, because signal to noise
ratio is set by the ratio of powers, not voltages. That all gets a bit
complex, and determines the nature of the preamp's input circuitry.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Don Pearce wrote:

> The input impedance of the typical mic preamp is of the order of 100k
> or so.

NO.


> The input impedance of the typical mic preamp *system* is generally
> around 12k,

Complete nonsense.

Graham

[Eeyore]

Don Pearce wrote:

> The input impedance of the typical mic preamp *system* is generally
> around 12k, and this is because it has a requirement to be able to
> supply phantom power to a condenser mic. It does this with a 48 volt
> supply fed through a pair of 6.2k resistors, each to one arm of the
> balanced pair. Turning off phantom power for microphones that don't
> need it generally involves disconnecting the 48V from the ends of the
> resistors, but leaving them connected to each other, leaving 12.4k
> across the microphone input. This is usually what defines the input
> impedance of a mic preamp.

Utter rubbish.

You really shouldn't pontificate about stuff you're very clearly totally
unfamiliar with.

Just a casual glance at any of the many schematics available online for mic
input stages gives the lie to your claim.

Graham

[Eeyore]

Don Pearce wrote:

> What determines the optimum source impedance
> (the actual impedance of the mic) is the way the noise generators in
> that input amp balance each other.

NO.

Not 'balance'. The word you're looking for is ADD ! You can't have 'negative
noise' with one stage balancing out another.

Did you just have a stroke or something ? You're not normally this daft.

Graham

[Don Pearce]

On Thu, 06 Dec 2007 10:30:04 +0000, Eeyore
> wrote:

>
>
>Don Pearce wrote:
>
>> The input impedance of the typical mic preamp is of the order of 100k
>> or so.
>
>NO.
>

Go on then. You are going to have to do better than that. Looking at a
few circuits I find that I could revise that upwards in many cases to
probably over 1M. I can't find any that are lower.

>
>> The input impedance of the typical mic preamp *system* is generally
>> around 12k,
>
>Complete nonsense.
>

Again - more is needed.

Looking round at a few I find

Rane MS1B 10k
Neve 11k
Behringer 12k

d

--
Pearce Consulting
http://www.pearce.uk.com

[Don Pearce]

On Thu, 06 Dec 2007 10:35:19 +0000, Eeyore
> wrote:

>
>
>Don Pearce wrote:
>
>> What determines the optimum source impedance
>> (the actual impedance of the mic) is the way the noise generators in
>> that input amp balance each other.
>
>NO.
>

Yes.

>Not 'balance'. The word you're looking for is ADD ! You can't have 'negative
>noise' with one stage balancing out another.
>
>Did you just have a stroke or something ? You're not normally this daft.
>
>Graham

I mean exactly what I say.

When I speak of balance, what I mean is that for high source
impedances current noise dominates, and for low source impedances
voltage noise dominates. You need to find a balance between these two
to optimize the noise performance. You do this by choosing appropriate
input devices and setting their operating currents appropriately.

Why have you decided to claim that I am saying that the noise from one
stage is balancing that of another? That would be stupid.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >
> >> The input impedance of the typical mic preamp *system* is generally
> >> around 12k,
> >
> >Complete nonsense.
>
>
> Again - more is needed.
>
> Looking round at a few I find
>
> Rane MS1B 10k

I'm not familiar with that one.


> Neve 11k

Certainly not when I was there. What product are you looking at ?


> Behringer 12k

So why does the manual say 2.6 k ?

http://www.behringer.com/1622FX/index.cfm?lang=eng (model chosen entirely
at random).

Graham

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >
> >> What determines the optimum source impedance
> >> (the actual impedance of the mic) is the way the noise generators in
> >> that input amp balance each other.
> >
> >NO.
>
> Yes.

********.


> >Not 'balance'. The word you're looking for is ADD ! You can't have 'negative
> >noise' with one stage balancing out another.
> >
> >Did you just have a stroke or something ? You're not normally this daft.
>
>
> I mean exactly what I say.
>
> When I speak of balance, what I mean is that for high source
> impedances current noise dominates, and for low source impedances
> voltage noise dominates.

Well you're not using it with a high source impedance you ****ing clown ! Since
when was 200 ohms a high impedance.

Jesus H Christ !

STOP talking drivel about stuff you're not familiar with.

Graham

[Don Pearce]

On Thu, 06 Dec 2007 11:09:58 +0000, Eeyore
> wrote:

>
>
>Don Pearce wrote:
>
>> Eeyore wrote:
>> >Don Pearce wrote:
>> >
>> >> The input impedance of the typical mic preamp *system* is generally
>> >> around 12k,
>> >
>> >Complete nonsense.
>>
>>
>> Again - more is needed.
>>
>> Looking round at a few I find
>>
>> Rane MS1B 10k
>
>I'm not familiar with that one.
>
>
>> Neve 11k
>
>Certainly not when I was there. What product are you looking at ?
>
>
>> Behringer 12k
>
>So why does the manual say 2.6 k ?
>
>http://www.behringer.com/1622FX/index.cfm?lang=eng (model chosen entirely
>at random).
>
That is a input maximum level figure with the gain backed off. At high
gain where the noise level matters the input impedance is comprised
almost solely of the phantom power resistors. This is true of most
preamps.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Don Pearce]

On Thu, 06 Dec 2007 11:12:35 +0000, Eeyore
> wrote:

>
>
>Don Pearce wrote:
>
>> Eeyore wrote:
>> >Don Pearce wrote:
>> >
>> >> What determines the optimum source impedance
>> >> (the actual impedance of the mic) is the way the noise generators in
>> >> that input amp balance each other.
>> >
>> >NO.
>>
>> Yes.
>
>********.
>
>
>> >Not 'balance'. The word you're looking for is ADD ! You can't have 'negative
>> >noise' with one stage balancing out another.
>> >
>> >Did you just have a stroke or something ? You're not normally this daft.
>>
>>
>> I mean exactly what I say.
>>
>> When I speak of balance, what I mean is that for high source
>> impedances current noise dominates, and for low source impedances
>> voltage noise dominates.
>
>Well you're not using it with a high source impedance you ****ing clown ! Since
>when was 200 ohms a high impedance.
>

Read the title of the thread, will you? And do try to stop swearing -
it really doesn't bolster your position any.

>Jesus H Christ !
>
>STOP talking drivel about stuff you're not familiar with.
>

I am right now looking at a ribbon mic with an impedance of 50k. To
make a low noise pre for that I had to use an op amp which you would
have slung out as being totally unacceptable for a 200 ohm mic. As I
say, you have to look at the source resistance and choose the right
balance of voltage and current noise to pin the minimum generated
noise power at that impedance.

It is very clear that while you may be familiar with this stuff in the
sense of having been around it, you haven't actually absorbed the
theory. Can you, for instance, tell me the equation you would use to
calculate noise figure given specified current noise, voltage noise
and source impedance?

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >> Eeyore wrote:
> >> >Don Pearce wrote:
> >> >
> >> >> The input impedance of the typical mic preamp *system* is generally
> >> >> around 12k,
> >> >
> >> >Complete nonsense.
> >>
> >> Again - more is needed.
> >>
> >> Looking round at a few I find
> >>
> >> Rane MS1B 10k
> >
> >I'm not familiar with that one.
> >
> >
> >> Neve 11k
> >
> >Certainly not when I was there. What product are you looking at ?
> >
> >
> >> Behringer 12k
> >
> >So why does the manual say 2.6 k ?
> >
> >http://www.behringer.com/1622FX/index.cfm?lang=eng (model chosen entirely
> >at random).
>
> That is a input maximum level figure with the gain backed off.

Where does it say that ? You're just making it up as you go alomg.


> At high gain where the noise level matters the input impedance is comprised

Very slightly.


> almost solely of the phantom power resistors.

Rubbish.


> This is true of most preamps.

Complete nonsense. The input Z is usually at the worst only very slightly
reduced at high gain.

The fact of the matter is that you're talking out of your backside.

Here's a mic amp schematic I found quickly with Google from an older Souncraft
console. The very first thing you can see is that input Z is dominated by R1
which has a value of 2k2.


Graham

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >> Eeyore wrote:
> >> >Don Pearce wrote:
> >> >
> >> >> What determines the optimum source impedance
> >> >> (the actual impedance of the mic) is the way the noise generators in
> >> >> that input amp balance each other.
> >> >
> >> >NO.
> >>
> >> Yes.
> >
> >********.
> >
> >
> >> >Not 'balance'. The word you're looking for is ADD ! You can't have 'negative
> >> >noise' with one stage balancing out another.
> >> >
> >> >Did you just have a stroke or something ? You're not normally this daft.
> >>
> >>
> >> I mean exactly what I say.
> >>
> >> When I speak of balance, what I mean is that for high source
> >> impedances current noise dominates, and for low source impedances
> >> voltage noise dominates.
> >
> >Well you're not using it with a high source impedance you ****ing clown ! Since
> >when was 200 ohms a high impedance.
>
> Read the title of the thread, will you? And do try to stop swearing -
> it really doesn't bolster your position any.

When dealing with halfwit posts like yours I find that very difficult.


> >Jesus H Christ !
> >
> >STOP talking drivel about stuff you're not familiar with.
> >
>
> I am right now looking at a ribbon mic with an impedance of 50k.

No it hasn't. No ribbon mic has an impedance of 50k. The transformer may however
need a load of 50k.


> To make a low noise pre for that I had to use an op amp which you would
> have slung out as being totally unacceptable for a 200 ohm mic.

That's a completely different subject. And not really 'pro'. Nor anything to do with
the OP's psot.


> As I say, you have to look at the source resistance and choose the right
> balance of voltage and current noise to pin the minimum generated
> noise power at that impedance.
>
> It is very clear that while you may be familiar with this stuff in the
> sense of having been around it, you haven't actually absorbed the
> theory. Can you, for instance, tell me the equation you would use to
> calculate noise figure given specified current noise, voltage noise
> and source impedance?

In ONE equation. Don't be daft. You'd want to split it up into 2 at least.

I know perfectly well how to add noise sources btw and I'm not rising to your silly
goads.

Graham

[Eeyore]

Don Pearce wrote:

> Read the title of the thread, will you?

The OP hasn't got a Hi-Z mic input nor does he need a Hi-Z mic.

Graham

[Randy Yates]

Eeyore > writes:

> Don Pearce wrote:
>
>> The input impedance of the typical mic preamp *system* is generally
>> around 12k, and this is because it has a requirement to be able to
>> supply phantom power to a condenser mic. It does this with a 48 volt
>> supply fed through a pair of 6.2k resistors, each to one arm of the
>> balanced pair. Turning off phantom power for microphones that don't
>> need it generally involves disconnecting the 48V from the ends of the
>> resistors, but leaving them connected to each other, leaving 12.4k
>> across the microphone input. This is usually what defines the input
>> impedance of a mic preamp.
>
> Utter rubbish.
>
> You really shouldn't pontificate about stuff you're very clearly totally
> unfamiliar with.
>
> Just a casual glance at any of the many schematics available online for mic
> input stages gives the lie to your claim.

Got an example?
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% > % 'Waterfall', *Face The Music*, ELO
http://www.digitalsignallabs.com

[Randy Yates]

(Don Pearce) writes:
> [nice explanation deleted]

Makes sense. I thought it might have something to do with phantom
powering. Thanks.
--
% Randy Yates % "Remember the good old 1980's, when
%% Fuquay-Varina, NC % things were so uncomplicated?"
%%% 919-577-9882 % 'Ticket To The Moon'
%%%% > % *Time*, Electric Light Orchestra
http://www.digitalsignallabs.com

[Don Pearce]

On Thu, 06 Dec 2007 13:49:24 +0000, Eeyore
> wrote:

>
>
>Don Pearce wrote:
>
>> Eeyore wrote:
>> >Don Pearce wrote:
>> >> Eeyore wrote:
>> >> >Don Pearce wrote:
>> >> >
>> >> >> What determines the optimum source impedance
>> >> >> (the actual impedance of the mic) is the way the noise generators in
>> >> >> that input amp balance each other.
>> >> >
>> >> >NO.
>> >>
>> >> Yes.
>> >
>> >********.
>> >
>> >
>> >> >Not 'balance'. The word you're looking for is ADD ! You can't have 'negative
>> >> >noise' with one stage balancing out another.
>> >> >
>> >> >Did you just have a stroke or something ? You're not normally this daft.
>> >>
>> >>
>> >> I mean exactly what I say.
>> >>
>> >> When I speak of balance, what I mean is that for high source
>> >> impedances current noise dominates, and for low source impedances
>> >> voltage noise dominates.
>> >
>> >Well you're not using it with a high source impedance you ****ing clown ! Since
>> >when was 200 ohms a high impedance.
>>
>> Read the title of the thread, will you? And do try to stop swearing -
>> it really doesn't bolster your position any.
>
>When dealing with halfwit posts like yours I find that very difficult.
>
>
>> >Jesus H Christ !
>> >
>> >STOP talking drivel about stuff you're not familiar with.
>> >
>>
>> I am right now looking at a ribbon mic with an impedance of 50k.
>
>No it hasn't. No ribbon mic has an impedance of 50k. The transformer may however
>need a load of 50k.
>
As it happens, the impedance of this ribbon mic (not the ribbon - you
seem to have major problems with boundary points) has a source
impedance of 50k. There is a transformer in the mic, and it needs a
load resistance of hundreds of k for proper operation.

>
>> To make a low noise pre for that I had to use an op amp which you would
>> have slung out as being totally unacceptable for a 200 ohm mic.
>
>That's a completely different subject. And not really 'pro'. Nor anything to do with
>the OP's psot.
>

What has the impedance to do with being pro? It was built at a time
when valve amps with input impedances in the megohms were the norm,
and it is a highly professional mic.

>
>> As I say, you have to look at the source resistance and choose the right
>> balance of voltage and current noise to pin the minimum generated
>> noise power at that impedance.
>>
>> It is very clear that while you may be familiar with this stuff in the
>> sense of having been around it, you haven't actually absorbed the
>> theory. Can you, for instance, tell me the equation you would use to
>> calculate noise figure given specified current noise, voltage noise
>> and source impedance?
>
>In ONE equation. Don't be daft. You'd want to split it up into 2 at least.
>
>I know perfectly well how to add noise sources btw and I'm not rising to your silly
>goads.
>

Well, that last bit tells us that you don't. Have a look here for my
calculation that identified the OP207 as the optimum op amp for my
ribbon mic - and also why it would be a poor choice for any more
modern, low impedance offering.

http://81.174.169.10/odds/opanoise

And there you will see the one equation that puts current noise and
voltage noise together.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Randy Yates wrote:

> Eeyore writes:
> > Don Pearce wrote:
> >
> >> The input impedance of the typical mic preamp *system* is generally
> >> around 12k, and this is because it has a requirement to be able to
> >> supply phantom power to a condenser mic. It does this with a 48 volt
> >> supply fed through a pair of 6.2k resistors, each to one arm of the
> >> balanced pair. Turning off phantom power for microphones that don't
> >> need it generally involves disconnecting the 48V from the ends of the
> >> resistors, but leaving them connected to each other, leaving 12.4k
> >> across the microphone input. This is usually what defines the input
> >> impedance of a mic preamp.
> >
> > Utter rubbish.
> >
> > You really shouldn't pontificate about stuff you're very clearly totally
> > unfamiliar with.
> >
> > Just a casual glance at any of the many schematics available online for mic
> > input stages gives the lie to your claim.
>
> Got an example?

I posted a link.

Don has totally lost the plot on this. He likes to sound authoritative but his
background isn't actually pro-audio so he makes a fool of himself like this from
time to time. Silly becasue his technical knowledge is otherwise pretty sound. I
suspect audio's mainly just a hobby with him.

Graham

[Eeyore]

Randy Yates wrote:

> (Don Pearce) writes:
> > [nice explanation deleted]

Shame it was so wrong.


> Makes sense. I thought it might have something to do with phantom
> powering. Thanks.

The phantom power resistors have to be included in the overal input
impedance calculation for sure, but in all other respects Don's
COMPLETELY wrong about typical input impedances.

Graham

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >>
> >> It is very clear that while you may be familiar with this stuff in the
> >> sense of having been around it, you haven't actually absorbed the
> >> theory.

I'm VERY strong on BOTH theory AND practice thank you.


> >>Can you, for instance, tell me the equation you would use to
> >> calculate noise figure given specified current noise, voltage noise
> >> and source impedance?
> >
> >In ONE equation. Don't be daft. You'd want to split it up into 2 at least.
> >
> >I know perfectly well how to add noise sources btw and I'm not rising to your silly
> >goads.
>
>
> Well, that last bit tells us that you don't.

**** off. Of course I bloody well do. There's a fairly good treartment of the subject of
adding voltage and current noise sources in either (or both) the TI Bifet Manual or
Nationals' Audio Hanbook, but in any event any competent designer can do it from first
principles.

Graham

[Don Pearce]

On Thu, 06 Dec 2007 14:27:49 +0000, Eeyore
> wrote:

>
>
>Don Pearce wrote:
>
>> Eeyore wrote:
>> >Don Pearce wrote:
>> >>
>> >> It is very clear that while you may be familiar with this stuff in the
>> >> sense of having been around it, you haven't actually absorbed the
>> >> theory.
>
>I'm VERY strong on BOTH theory AND practice thank you.
>
>
>> >>Can you, for instance, tell me the equation you would use to
>> >> calculate noise figure given specified current noise, voltage noise
>> >> and source impedance?
>> >
>> >In ONE equation. Don't be daft. You'd want to split it up into 2 at least.
>> >
>> >I know perfectly well how to add noise sources btw and I'm not rising to your silly
>> >goads.
>>
>>
>> Well, that last bit tells us that you don't.
>
>**** off. Of course I bloody well do. There's a fairly good treartment of the subject of
>adding voltage and current noise sources in either (or both) the TI Bifet Manual or
>Nationals' Audio Hanbook, but in any event any competent designer can do it from first
>principles.
>

But you evidently couldn't. That tells us all we need to know. We
appear to be fundamentally different types of person. You scream and
swear and bully, but produce nothing. I will explain rationally and
produce the goods. Before you post, you might want to ask yourself
"What would Christopher Robin say?".

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >
> >> It is very clear that while you may be familiar with this stuff in the
> >> sense of having been around it, you haven't actually absorbed the
> >> theory. Can you, for instance, tell me the equation you would use to
> >> calculate noise figure given specified current noise, voltage noise
> >> and source impedance?
> >
> >In ONE equation. Don't be daft. You'd want to split it up into 2 at least.
> >
> >I know perfectly well how to add noise sources btw and I'm not rising to your silly
> >goads.
> >
>
> Well, that last bit tells us that you don't.

And how many mic amps have you designed to make it into large scale manufacture (at least
1000 units) ?

In my own case it most cover around a dozen or more model series.

Graham

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
> >Don Pearce wrote:
> >> Eeyore wrote:
> >>
> >> >I know perfectly well how to add noise sources btw and I'm not rising to your silly
> >> >goads.
> >>
> >>
> >> Well, that last bit tells us that you don't.
> >
> >**** off. Of course I bloody well do. There's a fairly good treartment of the subject of
> >adding voltage and current noise sources in either (or both) the TI Bifet Manual or
> >Nationals' Audio Hanbook, but in any event any competent designer can do it from first
> >principles.
> >
>
> But you evidently couldn't. That tells us all we need to know.

**** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
undertand a detailed noise calculation and they already know you're talking crap. You're
obsessing over a calculation that'll be way over the heads of most people here and it's just
a subterfuge of yours to cover up your lack of knowledge of real world practice.


> We appear to be fundamentally different types of person.

Yes.

I know what I'm talking about and you don't. Here's some more real world schematics for you.

http://www.gyraf.dk/schematics/Mackie%20CR1604%20VLZ.pdf

Input impedance ~ 2k5

http://www.gyraf.dk/schematics/Behringer_ada8000_analouge.PDF

Input impedance ~ 2k2


Graham

[Don Pearce]

On Thu, 06 Dec 2007 15:07:32 +0000, Eeyore
> wrote:

>**** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
>undertand a detailed noise calculation and they already know you're talking crap. You're
>obsessing over a calculation that'll be way over the heads of most people here and it's just
>a subterfuge of yours to cover up your lack of knowledge of real world practice.

Are you one of those engineers? I thought they died out about 1970. I
mean the type that thinks theory is somehow different to practice and
should be ignored. I remember the chief engineer at my first job told
me that if theory and practice don't agree, you don't have the theory
right. Is that how your engineering career worked out? Sorry, but mine
was the reverse; the more I understood, the better theory matched
practice.

d

--
Pearce Consulting
http://www.pearce.uk.com

[John Hardy]

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>> Don Pearce wrote:
>>>> Eeyore wrote:
>>>>
>>>>> I know perfectly well how to add noise sources btw and I'm not rising to your silly
>>>>> goads.
>>>>
>>>> Well, that last bit tells us that you don't.
>>> **** off. Of course I bloody well do. There's a fairly good treartment of the subject of
>>> adding voltage and current noise sources in either (or both) the TI Bifet Manual or
>>> Nationals' Audio Hanbook, but in any event any competent designer can do it from first
>>> principles.
>>>
>> But you evidently couldn't. That tells us all we need to know.
>
> **** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
> undertand a detailed noise calculation and they already know you're talking crap. You're
> obsessing over a calculation that'll be way over the heads of most people here and it's just
> a subterfuge of yours to cover up your lack of knowledge of real world practice.
>
>
>> We appear to be fundamentally different types of person.
>
> Yes.
>
> I know what I'm talking about and you don't. Here's some more real world schematics for you.
>
> http://www.gyraf.dk/schematics/Mackie%20CR1604%20VLZ.pdf
>
> Input impedance ~ 2k5
>
> http://www.gyraf.dk/schematics/Behringer_ada8000_analouge.PDF
>
> Input impedance ~ 2k2
>
>
> Graham
>
>
>
Here is a simple schematic for the MPC-3000 card that I manufacture (see
page 2):

http://www.johnhardyco.com/pdf/MPC3000.pdf

This is essentially the same as the M-1, but a simpler approach and
easier to look at. The busier M-1 schematic is here (page 7):

http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf

Both preamps use the Jensen JT-16-B input transformer which has the
lowest impedance ratio of all the Jensen input transformers, 1:2. See
the specs for the JT-16-B here:

http://www.jensentransformers.com/datashts/16b.pdf

The typical input impedance is 1.61k, not including anything to do with
phantom supply resistors.

The more common Jensen JT-115K-E input transformer has the highest
impedance ratio of all the Jensen input transformers, 1:10. It has a
typical input of impedance of 1.40k, not including anything to do with
phantom supply resistors:

http://www.jensentransformers.com/datashts/115ke.pdf

The Jensen JT-13K7-A is their mid impedance ratio model (1:5). The
typical input impedance is 1.54k, not including anything to do with
phantom supply resistors.

http://www.jensentransformers.com/datashts/13k7a.pdf

The impedance ratio is typically chosen to work best with the noise
characteristics of the op-amp or vacuum tube that follows the
transformer. The JT-16-B is an ideal match to the 990C op-amp that is
used in all of my preamps.

All of these mic input transformers provide an input impedance or load
impedance that is approximately 10X the source impedance or output
impedance of the typical 150 ohm mic, which is considered by many people
to be an optimum ratio. There are other mic preamps that have much
higher input impedances, probably transformerless circuits, and your
mileage may vary.

Thank you.

John Hardy
The John Hardy Co.
www.johnhardyco.com

[Eeyore]

Don Pearce wrote:

> Eeyore wrote:
>
> >**** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
> >undertand a detailed noise calculation and they already know you're talking crap. You're
> >obsessing over a calculation that'll be way over the heads of most people here and it's just
> >a subterfuge of yours to cover up your lack of knowledge of real world practice.
>
> Are you one of those engineers? I thought they died out about 1970. I
> mean the type that thinks theory is somehow different to practice and
> should be ignored

Don't be ridiculous.

If theory and practice don't agree then you've misunderstood one of them.

In your case you've mostly misunderstood the practice.

The simple FACT of the matter is that you're COMPLETELY WRONG about typical mic input impedances.
FYI Paul Stamler is well known for example for promoting input Z's as low as 600 ohms to get the
best perceived performance from certain mics like the SM57 IIRC.

Rane would seem to be barking up the wrong tree with the MS1b

Graham

[Eeyore]

John Hardy wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> >> Eeyore wrote:
> >>> Don Pearce wrote:
> >>>> Eeyore wrote:
> >>>>
> >>>>> I know perfectly well how to add noise sources btw and I'm not rising to your silly
> >>>>> goads.
> >>>>
> >>>> Well, that last bit tells us that you don't.
> >>> **** off. Of course I bloody well do. There's a fairly good treartment of the subject of
> >>> adding voltage and current noise sources in either (or both) the TI Bifet Manual or
> >>> Nationals' Audio Hanbook, but in any event any competent designer can do it from first
> >>> principles.
> >>>
> >> But you evidently couldn't. That tells us all we need to know.
> >
> > **** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
> > undertand a detailed noise calculation and they already know you're talking crap. You're
> > obsessing over a calculation that'll be way over the heads of most people here and it's just
> > a subterfuge of yours to cover up your lack of knowledge of real world practice.
> >
> >
> >> We appear to be fundamentally different types of person.
> >
> > Yes.
> >
> > I know what I'm talking about and you don't. Here's some more real world schematics for you.
> >
> > http://www.gyraf.dk/schematics/Mackie%20CR1604%20VLZ.pdf
> >
> > Input impedance ~ 2k5
> >
> > http://www.gyraf.dk/schematics/Behringer_ada8000_analouge.PDF
> >
> > Input impedance ~ 2k2
>
>
>
> Here is a simple schematic for the MPC-3000 card that I manufacture (see
> page 2):
>
> http://www.johnhardyco.com/pdf/MPC3000.pdf
>
> This is essentially the same as the M-1, but a simpler approach and
> easier to look at. The busier M-1 schematic is here (page 7):
>
> http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
>
> Both preamps use the Jensen JT-16-B input transformer which has the
> lowest impedance ratio of all the Jensen input transformers, 1:2. See
> the specs for the JT-16-B here:
>
> http://www.jensentransformers.com/datashts/16b.pdf
>
> The typical input impedance is 1.61k, not including anything to do with
> phantom supply resistors.
>
> The more common Jensen JT-115K-E input transformer has the highest
> impedance ratio of all the Jensen input transformers, 1:10. It has a
> typical input of impedance of 1.40k, not including anything to do with
> phantom supply resistors:
>
> http://www.jensentransformers.com/datashts/115ke.pdf
>
> The Jensen JT-13K7-A is their mid impedance ratio model (1:5). The
> typical input impedance is 1.54k, not including anything to do with
> phantom supply resistors.
>
> http://www.jensentransformers.com/datashts/13k7a.pdf
>
> The impedance ratio is typically chosen to work best with the noise
> characteristics of the op-amp or vacuum tube that follows the
> transformer. The JT-16-B is an ideal match to the 990C op-amp that is
> used in all of my preamps.
>
> All of these mic input transformers provide an input impedance or load
> impedance that is approximately 10X the source impedance or output
> impedance of the typical 150 ohm mic, which is considered by many people
> to be an optimum ratio.

Agreed.


> There are other mic preamps that have much
> higher input impedances, probably transformerless circuits, and your
> mileage may vary.

It's always intrigued me that US mics tend to be closer to 150 ohms and European ones 200 ohms.
That would explain your slightly lower input Z than I would design for.

Graham

[Don Pearce]

On Thu, 06 Dec 2007 09:42:03 -0600, John Hardy
> wrote:

>Eeyore wrote:
>>
>> Don Pearce wrote:
>>
>>> Eeyore wrote:
>>>> Don Pearce wrote:
>>>>> Eeyore wrote:
>>>>>
>>>>>> I know perfectly well how to add noise sources btw and I'm not rising to your silly
>>>>>> goads.
>>>>>
>>>>> Well, that last bit tells us that you don't.
>>>> **** off. Of course I bloody well do. There's a fairly good treartment of the subject of
>>>> adding voltage and current noise sources in either (or both) the TI Bifet Manual or
>>>> Nationals' Audio Hanbook, but in any event any competent designer can do it from first
>>>> principles.
>>>>
>>> But you evidently couldn't. That tells us all we need to know.
>>
>> **** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
>> undertand a detailed noise calculation and they already know you're talking crap. You're
>> obsessing over a calculation that'll be way over the heads of most people here and it's just
>> a subterfuge of yours to cover up your lack of knowledge of real world practice.
>>
>>
>>> We appear to be fundamentally different types of person.
>>
>> Yes.
>>
>> I know what I'm talking about and you don't. Here's some more real world schematics for you.
>>
>> http://www.gyraf.dk/schematics/Mackie%20CR1604%20VLZ.pdf
>>
>> Input impedance ~ 2k5
>>
>> http://www.gyraf.dk/schematics/Behringer_ada8000_analouge.PDF
>>
>> Input impedance ~ 2k2
>>
>>
>> Graham
>>
>>
>>
>Here is a simple schematic for the MPC-3000 card that I manufacture (see
>page 2):
>
>http://www.johnhardyco.com/pdf/MPC3000.pdf
>
>This is essentially the same as the M-1, but a simpler approach and
>easier to look at. The busier M-1 schematic is here (page 7):
>
>http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
>
>Both preamps use the Jensen JT-16-B input transformer which has the
>lowest impedance ratio of all the Jensen input transformers, 1:2. See
>the specs for the JT-16-B here:
>
>http://www.jensentransformers.com/datashts/16b.pdf
>
>The typical input impedance is 1.61k, not including anything to do with
>phantom supply resistors.
>
>The more common Jensen JT-115K-E input transformer has the highest
>impedance ratio of all the Jensen input transformers, 1:10. It has a
>typical input of impedance of 1.40k, not including anything to do with
>phantom supply resistors:
>
>http://www.jensentransformers.com/datashts/115ke.pdf
>
>The Jensen JT-13K7-A is their mid impedance ratio model (1:5). The
>typical input impedance is 1.54k, not including anything to do with
>phantom supply resistors.
>
>http://www.jensentransformers.com/datashts/13k7a.pdf
>
>The impedance ratio is typically chosen to work best with the noise
>characteristics of the op-amp or vacuum tube that follows the
>transformer. The JT-16-B is an ideal match to the 990C op-amp that is
>used in all of my preamps.
>
>All of these mic input transformers provide an input impedance or load
>impedance that is approximately 10X the source impedance or output
>impedance of the typical 150 ohm mic, which is considered by many people
>to be an optimum ratio. There are other mic preamps that have much
>higher input impedances, probably transformerless circuits, and your
>mileage may vary.
>
>Thank you.
>
>John Hardy
>The John Hardy Co.
>www.johnhardyco.com

Yes. Those transformers are specified for use with op amps like the
5534 (the example used by Jensen). This has an optimum source
resistance of about 7000 ohms, so it needs a step up transformer to
load it optimally from a microphone. The 5:1 ratio of the first
example brings a 150 ohm source up to 750 ohms, which is still not
good enough, but leaves a noise figure of about 3.6dB.

Unfortunately, the resulting 1.5k load on the mic kills the signal by
a further 0.8dB, which can't be recovered as SNR - it is gone for
good. Finally add in the 1dB of the transformer's insertion loss, and
you have a noise figure of about 5.4dB.

That 1.5k input impedance is of course nothing to do with the preamp
itself. It is caused by the 9.76k and 39k resistors that have to be
hung across the output of the transformer, presumably to keep it from
ringing.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Don Pearce]

On Thu, 06 Dec 2007 16:25:34 GMT, (Don Pearce)
wrote:

>On Thu, 06 Dec 2007 09:42:03 -0600, John Hardy
> wrote:
>
>>Eeyore wrote:
>>>
>>> Don Pearce wrote:
>>>
>>>> Eeyore wrote:
>>>>> Don Pearce wrote:
>>>>>> Eeyore wrote:
>>>>>>
>>>>>>> I know perfectly well how to add noise sources btw and I'm not rising to your silly
>>>>>>> goads.
>>>>>>
>>>>>> Well, that last bit tells us that you don't.
>>>>> **** off. Of course I bloody well do. There's a fairly good treartment of the subject of
>>>>> adding voltage and current noise sources in either (or both) the TI Bifet Manual or
>>>>> Nationals' Audio Hanbook, but in any event any competent designer can do it from first
>>>>> principles.
>>>>>
>>>> But you evidently couldn't. That tells us all we need to know.
>>>
>>> **** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
>>> undertand a detailed noise calculation and they already know you're talking crap. You're
>>> obsessing over a calculation that'll be way over the heads of most people here and it's just
>>> a subterfuge of yours to cover up your lack of knowledge of real world practice.
>>>
>>>
>>>> We appear to be fundamentally different types of person.
>>>
>>> Yes.
>>>
>>> I know what I'm talking about and you don't. Here's some more real world schematics for you.
>>>
>>> http://www.gyraf.dk/schematics/Mackie%20CR1604%20VLZ.pdf
>>>
>>> Input impedance ~ 2k5
>>>
>>> http://www.gyraf.dk/schematics/Behringer_ada8000_analouge.PDF
>>>
>>> Input impedance ~ 2k2
>>>
>>>
>>> Graham
>>>
>>>
>>>
>>Here is a simple schematic for the MPC-3000 card that I manufacture (see
>>page 2):
>>
>>http://www.johnhardyco.com/pdf/MPC3000.pdf
>>
>>This is essentially the same as the M-1, but a simpler approach and
>>easier to look at. The busier M-1 schematic is here (page 7):
>>
>>http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
>>
>>Both preamps use the Jensen JT-16-B input transformer which has the
>>lowest impedance ratio of all the Jensen input transformers, 1:2. See
>>the specs for the JT-16-B here:
>>
>>http://www.jensentransformers.com/datashts/16b.pdf
>>
>>The typical input impedance is 1.61k, not including anything to do with
>>phantom supply resistors.
>>
>>The more common Jensen JT-115K-E input transformer has the highest
>>impedance ratio of all the Jensen input transformers, 1:10. It has a
>>typical input of impedance of 1.40k, not including anything to do with
>>phantom supply resistors:
>>
>>http://www.jensentransformers.com/datashts/115ke.pdf
>>
>>The Jensen JT-13K7-A is their mid impedance ratio model (1:5). The
>>typical input impedance is 1.54k, not including anything to do with
>>phantom supply resistors.
>>
>>http://www.jensentransformers.com/datashts/13k7a.pdf
>>
>>The impedance ratio is typically chosen to work best with the noise
>>characteristics of the op-amp or vacuum tube that follows the
>>transformer. The JT-16-B is an ideal match to the 990C op-amp that is
>>used in all of my preamps.
>>
>>All of these mic input transformers provide an input impedance or load
>>impedance that is approximately 10X the source impedance or output
>>impedance of the typical 150 ohm mic, which is considered by many people
>>to be an optimum ratio. There are other mic preamps that have much
>>higher input impedances, probably transformerless circuits, and your
>>mileage may vary.
>>
>>Thank you.
>>
>>John Hardy
>>The John Hardy Co.
>>www.johnhardyco.com
>
>Yes. Those transformers are specified for use with op amps like the
>5534 (the example used by Jensen). This has an optimum source
>resistance of about 7000 ohms, so it needs a step up transformer to
>load it optimally from a microphone. The 5:1 ratio of the first
>example brings a 150 ohm source up to 750 ohms, which is still not
>good enough, but leaves a noise figure of about 3.6dB.
>
>Unfortunately, the resulting 1.5k load on the mic kills the signal by
>a further 0.8dB, which can't be recovered as SNR - it is gone for
>good. Finally add in the 1dB of the transformer's insertion loss, and
>you have a noise figure of about 5.4dB.
>
>That 1.5k input impedance is of course nothing to do with the preamp
>itself. It is caused by the 9.76k and 39k resistors that have to be
>hung across the output of the transformer, presumably to keep it from
>ringing.
>
>d

Oops! Misread the data sheet. I saw 1:5 and impedance on the same line
and assumed wrongly. So it is a 1:25 impedance step up which brings
150 ohms up to 3.75k, which knocks the op amp's noise figure back
somewhat, though it still falls short of the optimum operating point.

The input impedance is set by the 39k divided by 25, with top-end
fiddling provided by the 9k76.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Don Pearce wrote:

> On Thu, 06 Dec 2007 09:42:03 -0600, John Hardy
> > wrote:
>
> >Eeyore wrote:
> >>
> >> Don Pearce wrote:
> >>
> >>> Eeyore wrote:
> >>>> Don Pearce wrote:
> >>>>> Eeyore wrote:
> >>>>>
> >>>>>> I know perfectly well how to add noise sources btw and I'm not rising to your silly
> >>>>>> goads.
> >>>>>
> >>>>> Well, that last bit tells us that you don't.
> >>>> **** off. Of course I bloody well do. There's a fairly good treartment of the subject of
> >>>> adding voltage and current noise sources in either (or both) the TI Bifet Manual or
> >>>> Nationals' Audio Hanbook, but in any event any competent designer can do it from first
> >>>> principles.
> >>>>
> >>> But you evidently couldn't. That tells us all we need to know.
> >>
> >> **** OFF. You're a gesturing idiot. Very few people here (I can think of only about 2) would
> >> undertand a detailed noise calculation and they already know you're talking crap. You're
> >> obsessing over a calculation that'll be way over the heads of most people here and it's just
> >> a subterfuge of yours to cover up your lack of knowledge of real world practice.
> >>
> >>
> >>> We appear to be fundamentally different types of person.
> >>
> >> Yes.
> >>
> >> I know what I'm talking about and you don't. Here's some more real world schematics for you.
> >>
> >> http://www.gyraf.dk/schematics/Mackie%20CR1604%20VLZ.pdf
> >>
> >> Input impedance ~ 2k5
> >>
> >> http://www.gyraf.dk/schematics/Behringer_ada8000_analouge.PDF
> >>
> >> Input impedance ~ 2k2
> >>
> >>
> >> Graham
> >>
> >>
> >>
> >Here is a simple schematic for the MPC-3000 card that I manufacture (see
> >page 2):
> >
> >http://www.johnhardyco.com/pdf/MPC3000.pdf
> >
> >This is essentially the same as the M-1, but a simpler approach and
> >easier to look at. The busier M-1 schematic is here (page 7):
> >
> >http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
> >
> >Both preamps use the Jensen JT-16-B input transformer which has the
> >lowest impedance ratio of all the Jensen input transformers, 1:2. See
> >the specs for the JT-16-B here:
> >
> >http://www.jensentransformers.com/datashts/16b.pdf
> >
> >The typical input impedance is 1.61k, not including anything to do with
> >phantom supply resistors.
> >
> >The more common Jensen JT-115K-E input transformer has the highest
> >impedance ratio of all the Jensen input transformers, 1:10. It has a
> >typical input of impedance of 1.40k, not including anything to do with
> >phantom supply resistors:
> >
> >http://www.jensentransformers.com/datashts/115ke.pdf
> >
> >The Jensen JT-13K7-A is their mid impedance ratio model (1:5). The
> >typical input impedance is 1.54k, not including anything to do with
> >phantom supply resistors.
> >
> >http://www.jensentransformers.com/datashts/13k7a.pdf
> >
> >The impedance ratio is typically chosen to work best with the noise
> >characteristics of the op-amp or vacuum tube that follows the
> >transformer. The JT-16-B is an ideal match to the 990C op-amp that is
> >used in all of my preamps.
> >
> >All of these mic input transformers provide an input impedance or load
> >impedance that is approximately 10X the source impedance or output
> >impedance of the typical 150 ohm mic, which is considered by many people
> >to be an optimum ratio. There are other mic preamps that have much
> >higher input impedances, probably transformerless circuits, and your
> >mileage may vary.
> >
> >Thank you.
> >
> >John Hardy
> >The John Hardy Co.
> >www.johnhardyco.com
>
> Yes. Those transformers are specified for use with op amps like the
> 5534 (the example used by Jensen). This has an optimum source
> resistance of about 7000 ohms, so it needs a step up transformer to
> load it optimally from a microphone. The 5:1 ratio of the first
> example brings a 150 ohm source up to 750 ohms, which is still not
> good enough, but leaves a noise figure of about 3.6dB.
>
> Unfortunately, the resulting 1.5k load on the mic kills the signal by
> a further 0.8dB, which can't be recovered as SNR - it is gone for
> good. Finally add in the 1dB of the transformer's insertion loss, and
> you have a noise figure of about 5.4dB.
>
> That 1.5k input impedance is of course nothing to do with the preamp
> itself. It is caused by the 9.76k and 39k resistors that have to be
> hung across the output of the transformer, presumably to keep it from
> ringing.
>
> d
>
> --
> Pearce Consulting
> http://www.pearce.uk.com

[Paul Stamler]

"Don Pearce" > wrote in message
...
> >> [...] the input impedance on the majority of modern professioanl min
> >> inputs is typically ~ 2 k ohms.
> >
> >That figure sounds low. I would think these types of inputs are at least
> >10k ohms.
>
> The input impedance of the typical mic preamp is of the order of 100k
> or so. This is set by the op amp or discrete transistor stage used for
> the input amplifier. As long as this figure is high, it won't load a
> microphone adversely. What determines the optimum source impedance
> (the actual impedance of the mic) is the way the noise generators in
> that input amp balance each other. There are separate mechanisms for
> generating current and voltage noise.
>
> The input impedance of the typical mic preamp *system* is generally
> around 12k, and this is because it has a requirement to be able to
> supply phantom power to a condenser mic. It does this with a 48 volt
> supply fed through a pair of 6.2k resistors, each to one arm of the
> balanced pair. Turning off phantom power for microphones that don't
> need it generally involves disconnecting the 48V from the ends of the
> resistors, but leaving them connected to each other, leaving 12.4k
> across the microphone input. This is usually what defines the input
> impedance of a mic preamp.

Excuse me, but Don has just exposed a remarkable lack of experience in the
world of professional audio -- or, at the very least, a lack of reading spec
sheets for professional mic preamps.

Virtually every mic preamp in the business has a nominal input impedance of
1.0-2.5k. Go look up the specs. If there's a transformer on the input, its
secondary is typically loaded with a resistor of 10x the secondary impedance
when fed from a 150 ohm source. A 1:10 transformer has a nominal 15k
secondary impedance with a 150 ohm source, and is loaded with 150k (plus,
perhaps, a Zobel network). Reflected back through the transformer, that's an
input impedance of 1.5k. Add a pair of 6.81k resistors (the phantom power
standard specifies 6.8k, *not* 6.2k, another error), and the parallel
resistance of 13.62k with the transformer's input impedance works out to
about 1.35k input Z. Transformerless preamps do the loading with a simple
resistor, plus the phantom resistors, and as mentioned before the total
typically works out between 1.0k and 2.5k.

There are a very few exceptions. Some preamps have begun adding circuits to
lower the input impedance, in order to terminate certain microphones
(notably the Shure SM57) with a load that adds more damping. That's usually
switchable, as in the Universal Audio circuit (which switches taps on the
input transformer) or the ADL 600 (which switches in parallel resistors).
I've run across a few prosumer devices with input Z of about 600 ohms (the
Groove Tubes Brick is the one that springs to mind).

The only preamps I am aware of on the commercial market with input
impedances as high as Don suggests are specialty items intended for classic
ribbon microphones, which were designed to work best into a higher
impedance. The most notable of these is from AEA, and I believe its input
impedance (set by a load resistor) is about 12k. There's no phantom, since
it's intended to be used only with ribbon mics.

Don, before you fire back your usual counterblast, go read about a dozen
spec sheets for typical mic preamps.

Peace,
Paul

[Mike Rivers]

On Dec 6, 12:18 pm, "Paul Stamler" > wrote:

> The only preamps I am aware of on the commercial market with input
> impedances as high as Don suggests are specialty items intended for classic
> ribbon microphones, which were designed to work best into a higher
> impedance. The most notable of these is from AEA, and I believe its input
> impedance (set by a load resistor) is about 12k. There's no phantom, since
> it's intended to be used only with ribbon mics.

I believe the AEA TRP input impedance is 18k. The Gordon has a very
high input impedance, 2 megohms if I recall, which contributes to its
version of "clean." I recall seeing a German (some Steinberg relation,
I believe) home studio preamp half a dozen or so years ago that had an
input impedance of about 3k, and the current Mackie Onyx crop is 2.4k
(the 800R has a couple of inputs with switchable resistors).

[Paul Stamler]

Meanwhile, when all the shouting dies down, and the distracting chaff about
low-noise-figure design has drifted to the ground, the input impedance of
*almost every* professional preamp sold in the world, bar the odd device
intended for ribbons, is 2.6k or lower. Period.

As for how many equations you need to compute the noise figure, yes, you can
do it with a single monster equation, but if you're writing a spreadsheet
it's easier to split it into two or three. Makes the proofreading easier.
One to compute the voltage and current noise, one to add that to the
source's noise (obtaining the equivalent input noise), and one to compare
the result with the source noise to obtain the noise figure. The advantage,
aside from being less unwieldy, is that you pop out both ways of expressing
the performance -- noise figure and EIN. Occasionally useful from a
human-engineering point of view.

Peace,
Paul

[[email protected]]

On Dec 6, 5:50 am, Eeyore >
wrote:
> Don Pearce wrote:
> > Read the title of the thread, will you?
>
> The OP hasn't got a Hi-Z mic input nor does he need a Hi-Z mic.
>
> Graham

The OP's PS5 only has approximately 40 dB of gain on the Line B 1/4 in
jack input. If the OP wants to use a Shure SM58 with -54.5 dBV/Pa he
might find the approximately 20 dB voltage gain from an 1:10 to 1:14
impedance transformer useful with his PS5. I believe the AT CP8201 is
a 1:14.

bobs

Bob Smith
BS Studios
we organize chaos
http://www.bsstudios.com

[Eeyore]

wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> > > Read the title of the thread, will you?
> >
> > The OP hasn't got a Hi-Z mic input nor does he need a Hi-Z mic.
>
> The OP's PS5 only has approximately 40 dB of gain on the Line B 1/4 in
> jack input. If the OP wants to use a Shure SM58 with -54.5 dBV/Pa he
> might find the approximately 20 dB voltage gain from an 1:10 to 1:14
> impedance transformer useful with his PS5. I believe the AT CP8201 is
> a 1:14.

Loading that with 12k would hardly be very clever.

I never can really make out why this consumer crap is designed this way.
It would hardly have been difficult to organise a more suitable input
impedance or sensitivity, although I suppose the -40dBV was designed to
avoid easy overloading.

Graham