Ideal Mic Preamp Input Impedance For Ribbon Mics

[Sean B]

I've been putting together some transformer-less discrete op-amp based mic pres, and I just noticed that 1500 ohms is definitely not the ideal input impedance for a Coles 4038. I was considering selling my 4038s because they were sounding so dull, and just weren't pleasing. Then I plug one into this pre that has a 100k ohm input Z, and the mics sounded COMPLETELY different. More and better bass. Way more top, smoother, less "cutting" top (after a high freq EQ boost). Now they only need a few dB of treble lift, instead of 10 or 12 dB, to make them sound spectrally balanced. And they seem to respond to transients a lot better, as if a compressor were turned off or something.

So is there a preamp input impedance figure that might be optimum for ribbon mics?

Thanks,

Sean B

[PStamler]

I think you may have found it, at least for the Coles. The folks at Benchmark make a preamp with an input impedance of about 7k, and in my experience it sounds a lot better on ribbon mics than standard 1.5k preamps.

In the old days RCA made preamps for ribbon mics with a transformer input; the secondary was hooked to the tube's grid with no terminating resistor, so the transformer was operating into a very high impedance, loaded only by the tube's input capacitance. In the preamps where the tube was a pentode, the capacitance was minimal, so the impedance facing the secondary was pretty close to infinite. All those preamps had very high input impedances, and RCA's ribbon mics were designed to give best results into them.

Peace,
Paul

[Scott Dorsey]

Sean B > wrote:
>
>So is there a preamp input impedance figure that might be optimum for ribbo=
>n mics?

Wes Dooley says to use the highest possible input Z, maybe 10K or more.
This runs counter to what I thought was true for many years, but I do agree
that it sounds better.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

On Thursday, August 22, 2013 8:34:18 AM UTC-4, Scott Dorsey wrote:
> Sean B > wrote: > >So is there a preamp input impedance figure that might be optimum for ribbo= >n mics? Wes Dooley says to use the highest possible input Z, maybe 10K or more. This runs counter to what I thought was true for many years, but I do agree that it sounds better. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis."

does the ribbon mic in question have a transformer in the mic or is the ribbon connected directly to the output?

Mark

[Scott Dorsey]

In article >,
> wrote:
>On Thursday, August 22, 2013 8:34:18 AM UTC-4, Scott Dorsey wrote:
>> Sean B > wrote: > >So is there a preamp inpu=
>t impedance figure that might be optimum for ribbo=3D >n mics? Wes Dooley s=
>ays to use the highest possible input Z, maybe 10K or more. This runs count=
>er to what I thought was true for many years, but I do agree that it sounds=
> better.
>
>does the ribbon mic in question have a transformer in the mic or is the rib=
>bon connected directly to the output?

They all have a transformer in them. The effective output Z of the ribbon is
less than an ohm, so it needs a massive step-up.

I want to say the 77DX has an an impedance on the order of 0.01 ohm and
a 1:50 step-up but don't quote me on this. Yes, this would mean that it
would want a fairly low-Z load. And that's what I thought for many years
but the response is flatter with a high-Z load.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

On Thursday, August 22, 2013 9:28:00 AM UTC-4, Scott Dorsey wrote:
> In article >, > wrote: >On Thursday, August 22, 2013 8:34:18 AM UTC-4, Scott Dorsey wrote: >> Sean B > wrote: > >So is there a preamp inpu= >t impedance figure that might be optimum for ribbo=3D >n mics? Wes Dooley s= >ays to use the highest possible input Z, maybe 10K or more. This runs count= >er to what I thought was true for many years, but I do agree that it sounds= > better. > >does the ribbon mic in question have a transformer in the mic or is the rib= >bon connected directly to the output? They all have a transformer in them. The effective output Z of the ribbon is less than an ohm, so it needs a massive step-up. I want to say the 77DX has an an impedance on the order of 0.01 ohm and a 1:50 step-up but don't quote me on this. Yes, this would mean that it would want a fairly low-Z load. And that's what I thought for many years but the response is flatter with a high-Z load. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis."

then a lot of it is the interaction of the inductanace of the xformer and the cable cap...similar to the situation with mag phono cart.

with Hi Z load, probably creates a pleasant sounding peak up in the treble.

Mark

[Les Cargill[_4_]]

Scott Dorsey wrote:
> Sean B > wrote:
>>
>> So is there a preamp input impedance figure that might be optimum for ribbo=
>> n mics?
>
> Wes Dooley says to use the highest possible input Z, maybe 10K or more.

So it's one-tenth of a guitar pickup, then?

> This runs counter to what I thought was true for many years, but I do agree
> that it sounds better.

Interesting - a lot of consumer MI line inputs are 10k.

> --scott
>

--
Les Cargill

[Sean B]

On Thursday, August 22, 2013 11:56:06 AM UTC-5, wrote:

> then a lot of it is the interaction of the inductanace of the xformer and the cable cap...similar to the situation with mag phono cart.
>
> with Hi Z load, probably creates a pleasant sounding peak up in the treble.
> Mark

Bill Whitlock said the same thing in an email when I asked him about supplying mic input trannies with higher input Z, but I think there is more to it.. I think the transient behavior and the distortion behavior of the mic is greatly improved under light loading. Also, the mics are treble deficient to begin with, so a "peak" from the mic's output trannie might actually bring the overall response of the mic closer to flat (if the peak is not too narrow). And the boost you talk about is actually caused by the mic output trannie's leakage inductance resonating with the mic cable's capacitance; the Coles 4038s have a toroidal transformer in them which exhibit among the lowest leakage L of any trannie type. I will experiment with a 1 foot mic cable and see if I hear a significant difference (at the recording test I used a 25 foot cable).

Sean B

[Mike Rivers[_2_]]

On 8/22/2013 1:20 AM, Sean B wrote:
> I've been putting together some transformer-less discrete op-amp
> based mic pres, and I just noticed that 1500 ohms is definitely not
> the ideal input impedance for a Coles 4038.

For many years, the netlore was that ribbon mics were happiest when
looking into a low input impedance, but that didn't jive with what mic
inputs were built like in the days when the RCA ribbon mics were very
popular. The AEA ribbon mic preamp has an input impedance of about 18 k
ohms because that's what they think sound best with their mics (which
are patterned after the RCA designs).

I don't have a lot of ribbon mic experience except for Beyer M160 and
M260 mics which are a very different design from the RCA. I did have a
CAD Trion ribbon mic in here for a review several years back and found
that it sounded kind of flabby into a "standard" mic preamp, but when
playing with the input impedance switch on my Mackie 800R preamp (it
goes from 300 to 2400 ohms) that mic cleaned up considerably at the 300
ohm setting. I suspect, in that case, it had more to do with damping the
ribbon than loading the transformer.


--
For a good time, call http://mikeriversaudio.wordpress.com

[Neil Gould]

Les Cargill wrote:
> Scott Dorsey wrote:
>> Sean B > wrote:
>>>
>>> So is there a preamp input impedance figure that might be optimum
>>> for ribbo= n mics?
>>
>> Wes Dooley says to use the highest possible input Z, maybe 10K or
>> more.
>
> So it's one-tenth of a guitar pickup, then?
>
Could you have meant a guitar amp input impedance, or are you talking about
Piezo pickups? I have one guitar that has a pickup that is around 10kOhms,
and it's unusual. The rest are much lower than that.

>> This runs counter to what I thought was true for many years, but I
>> do agree that it sounds better.
>
> Interesting - a lot of consumer MI line inputs are 10k.
>
Modern instrument amp inputs are often 100k or higher. '60s gear sometimes
had 10k inputs.
--
best regards,

Neil

[Les Cargill[_4_]]

Neil Gould wrote:
> Les Cargill wrote:
>> Scott Dorsey wrote:
>>> Sean B > wrote:
>>>>
>>>> So is there a preamp input impedance figure that might be optimum
>>>> for ribbo= n mics?
>>>
>>> Wes Dooley says to use the highest possible input Z, maybe 10K or
>>> more.
>>
>> So it's one-tenth of a guitar pickup, then?
>>
> Could you have meant a guitar amp input impedance, or are you talking about
> Piezo pickups?

Both are roughly 1 megohm, and I should have said "one one hundredth."

>I have one guitar that has a pickup that is around 10kOhms,
> and it's unusual. The rest are much lower than that.
>

The effective impedance of a guitar *input* is about 1 megohm,
rule of thumb.

>>> This runs counter to what I thought was true for many years, but I
>>> do agree that it sounds better.
>>
>> Interesting - a lot of consumer MI line inputs are 10k.
>>
> Modern instrument amp inputs are often 100k or higher.

Yes. Usually higher. But *line* inputs - the little 1/4" inputs
that are not instrument nor mic/XLR - are frequently 10 or 20 k.

You would need a lot of gain to hear it, but if it's going t
a 24 bit digital recorder, you could
hyptothetically make up the gain after the fact - assuming
it's not then totally swamped in noise.


> '60s gear sometimes
> had 10k inputs.
>
--
Les Cargill

[PStamler]

Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:

http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html

Peace,
Paul

[hank alrich]

PStamler > wrote:

> Talking about electric guitar impedances...Some folks at the University of
> Illinois at Urbana-Champaign did extensive research on the output
> impedances of electric guitars. They found that while the pickups
> themselves had impedances that were quite high, the guitars as a whole had
> lower output impedances due to the controls, switching, etc. Look here:
>
> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_res
> ults.html
>
> Peace,
> Paul

Thanks for that link, Paul.

--
shut up and play your guitar * HankAlrich.Com
HankandShaidriMusic.Com
YouTube.Com/WalkinayMusic

[hank alrich]

Les Cargill > wrote:

> Scott Dorsey wrote:
> > Sean B > wrote:
> >>
> >> So is there a preamp input impedance figure that might be optimum for
> >> ribbo= n mics?
> >
> > Wes Dooley says to use the highest possible input Z, maybe 10K or more.
>
> So it's one-tenth of a guitar pickup, then?
>
> > This runs counter to what I thought was true for many years, but I do
> > agree that it sounds better.
>
> Interesting - a lot of consumer MI line inputs are 10k.

Then there's the Gordon preamp.

http://gordonaudio.com/specs.htm

--
shut up and play your guitar * HankAlrich.Com
HankandShaidriMusic.Com
YouTube.Com/WalkinayMusic

[Don Pearce[_3_]]

On Thu, 22 Aug 2013 23:17:08 -0700 (PDT), PStamler
> wrote:

>Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>
>http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>
>Peace,
>Paul

Not accessible from the UK apparently. Shame

d

[Paul Babiak]

On 08/23/2013 10:09 AM, Don Pearce wrote:
> On Thu, 22 Aug 2013 23:17:08 -0700 (PDT), PStamler
> > wrote:
>
>> Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>>
>> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>>
>> Peace,
>> Paul
>
> Not accessible from the UK apparently. Shame
>
> d
>
Try here:

http://web.archive.org/web/20110525060505/http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html

[Les Cargill[_4_]]

Don Pearce wrote:
> On Thu, 22 Aug 2013 23:17:08 -0700 (PDT), PStamler
> > wrote:
>
>> Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>>
>> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>>
>> Peace,
>> Paul
>
> Not accessible from the UK apparently. Shame
>
> d
>

We're apparently not quite over that Beatles thing yet :)

--
Les Cargill

[Don Pearce[_3_]]

On Fri, 23 Aug 2013 12:52:55 -0400, Paul Babiak >
wrote:

>On 08/23/2013 10:09 AM, Don Pearce wrote:
>> On Thu, 22 Aug 2013 23:17:08 -0700 (PDT), PStamler
>> > wrote:
>>
>>> Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>>>
>>> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>>>
>>> Peace,
>>> Paul
>>
>> Not accessible from the UK apparently. Shame
>>
>> d
>>
>Try here:
>
>http://web.archive.org/web/20110525060505/http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html

That got it, thanks.

d

[Don Pearce[_3_]]

On Fri, 23 Aug 2013 12:12:33 -0500, Les Cargill
> wrote:

>Don Pearce wrote:
>> On Thu, 22 Aug 2013 23:17:08 -0700 (PDT), PStamler
>> > wrote:
>>
>>> Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>>>
>>> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>>>
>>> Peace,
>>> Paul
>>
>> Not accessible from the UK apparently. Shame
>>
>> d
>>
>
>We're apparently not quite over that Beatles thing yet :)

Will we ever be? ;-)

d

[Les Cargill[_4_]]

PStamler wrote:
> Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>
> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>
> Peace,
> Paul
>


Interesting! Although it's pretty safe to say that for
quack stick or mag pickups, you get better transfer from
the higher Z inputs. Still, some amps don't have that
hi z of an input.

I am still disconnecting my wah pedal when I don't
need it. The buffer doesn't sound that good; there
are true bypass mods for it because of this so it's
not just me.

--
Les Cargill

[Mike Rivers[_2_]]

On 8/23/2013 2:17 AM, PStamler wrote:

> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html

This hasn't worked all day. uiuc.edu is there but
online.physics.uiuc.edu doesn't load. Got another shot at it? I'm
arguing with a couple of fussy guitar players about replacement pots
(which started out being about pots in things other than guitars). Of
course the guitar players think the sound of anything but a genuine
vintage replacement pot sucks.

I told one of them to take up the trombone.


--
For a good time, call http://mikeriversaudio.wordpress.com

[Mike Rivers[_2_]]

On 8/23/2013 10:09 AM, Don Pearce wrote:

> Not accessible from the UK apparently. Shame

Nor from Falls Church VA either, at least not so far today.



--
For a good time, call http://mikeriversaudio.wordpress.com

[Mike Rivers[_2_]]

On 8/23/2013 12:52 PM, Paul Babiak wrote:

> Try here:

http://tinyurl.com/leu466n

Thanks. That worked. Guess it slipped out of the active course work and
into the archive. Interesting reading. I wonder if that test setup would
show the difference between a carbon composition pot (probably the
original equipment), an all carbon pot (does anyone make those any
more?) and a deposited carbon pot.

The argument is about replacing pots in "vintage" gear, having gone off
track with guitar pots.


--
For a good time, call http://mikeriversaudio.wordpress.com

[Neil Gould]

Mike Rivers wrote:
> On 8/23/2013 12:52 PM, Paul Babiak wrote:
>
>> Try here:
>
> http://tinyurl.com/leu466n
>
> Thanks. That worked. Guess it slipped out of the active course work
> and into the archive. Interesting reading. I wonder if that test
> setup would show the difference between a carbon composition pot
> (probably the original equipment), an all carbon pot (does anyone
> make those any more?) and a deposited carbon pot.
>
> The argument is about replacing pots in "vintage" gear, having gone
> off track with guitar pots.
>
Why argue? Guitar pots are a part of a musical instrument, and some players
"play" them, too. ;-) There are some aspects of guitar pots that make it
desirable to use "genuine" replacements or avoid "genuine" replacements,
depending on the taste of the player. For example, their size, position of
the shaft, and other physical qualities may keep them from fitting well into
the guitar. Then, there's their "feel"... there are perceptable differences
in their handling. Fortunately, there are lots of places to get guitar pots,
"genuine" or otherwise, so an argument should be avoidable.
--
best regards,

Neil

[Don Pearce[_3_]]

On Fri, 23 Aug 2013 12:52:55 -0400, Paul Babiak >
wrote:

>On 08/23/2013 10:09 AM, Don Pearce wrote:
>> On Thu, 22 Aug 2013 23:17:08 -0700 (PDT), PStamler
>> > wrote:
>>
>>> Talking about electric guitar impedances...Some folks at the University of Illinois at Urbana-Champaign did extensive research on the output impedances of electric guitars. They found that while the pickups themselves had impedances that were quite high, the guitars as a whole had lower output impedances due to the controls, switching, etc. Look here:
>>>
>>> http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>>>
>>> Peace,
>>> Paul
>>
>> Not accessible from the UK apparently. Shame
>>
>> d
>>
>Try here:
>
>http://web.archive.org/web/20110525060505/http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html

Ok, interesting but is that really from a university? Looks more like
a high school science club project. Basic circuit analysis.

d

[Scott Dorsey]

Don Pearce > wrote:
>>
>>http://web.archive.org/web/20110525060505/http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html
>
>Ok, interesting but is that really from a university? Looks more like
>a high school science club project. Basic circuit analysis.

Looks like a senior level undergraduate project to me, and a respectable
one for that level.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."