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Default Scott Hampton's JFET mic pre - component selection

Hi All. I recently came across an old Tape Op article by Scott Hampton describing a DIY JFET mic pre (and other equipment):
http://tapeop.com/tutorials/37/diy-jfet-mic-pre/

The transistors specified are not available to me anymore. Also, the input transformer is specified as the turns ration (1:10), but I'm not sure if the input/output impedances matter?

For the input transformer, I thought I could make this work, although the datasheet says not to pass DC through the windings, so I suppose that precludes phantom power?
http://docs-asia.electrocomponents.c...6b815288ea.pdf

In the JFP gain module, I came up with the following substitutions for the transistors:
2N5457 to J109 (http://au.element14.com/on-semicondu...-92/dp/1467947)

MPSA14 to BC517G (http://au.element14.com/on-semicondu...-to/dp/1705773)

ZTX653 to BC337 (http://au.element14.com/fairchild-se...-92/dp/1228215)

If someone could help me understand the input transformer and cast an eye over the transistor substitutions, I'd appreciate it.
Thanks.
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geoff geoff is offline
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Default Scott Hampton's JFET mic pre - component selection

On 31/08/2017 9:32 AM, wrote:
Hi All. I recently came across an old Tape Op article by Scott
Hampton describing a DIY JFET mic pre (and other equipment):
http://tapeop.com/tutorials/37/diy-jfet-mic-pre/


For the input transformer, I thought I could make this work, although
the datasheet says not to pass DC through the windings, so I suppose
that precludes phantom power?


No, there is no significant current through the primary, assuming the
6k8 resistors are 1% tolerance (or better).

If someone could help me understand the input transformer and cast
an eye over the transistor substitutions, I'd appreciate it. Thanks.


He does mention 'generic' wrt the transistors quite often, so nothing
too critical. I'm sure Scott will come up with some favorite candidates.

But the overall question remains "Why ?" .

geoff
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Scott Dorsey Scott Dorsey is offline
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Default Scott Hampton's JFET mic pre - component selection

wrote:
Hi All. I recently came across an old Tape Op article by Scott Hampton describing a DIY JFET mic pre (and other equipment):
http://tapeop.com/tutorials/37/diy-jfet-mic-pre/


This is a competent design using cheap commodity parts. The 2N5457 is kind of
doubtful but you might be able to select to find quiet ones. I'd bypass that
470uF cap too.

The transistors specified are not available to me anymore.


Which ones? All of those are basically the cheapest possible commodity parts.

Also, the input transformer is specified as the turns ration (1:10), but I'm not sure if the input/output impedances matter?


The thing about microphone input transformers is that you are matching a thing
that is very low (likely lower than the design impedance of the transformer)
to something that is mindbogglingly high (way higher than any possible design
impedance of any transformer). So any time you see impedances listed on a
microphone input transformer, they are kind of doubtful numbers anyway.

For the input transformer, I thought I could make this work, although the datasheet says not to pass DC through the windings, so I suppose that precludes phantom power?
http://docs-asia.electrocomponents.c...6b815288ea.pdf


This has no magnetic shielding, which could be a problem, but the output
impedance is very low. This looks like it's designed as an interstage line
level transformer rather than a microphone input transformer. But you
likely could make this work although I suspect that the low end will be a
bit slack.

I would strongly recommend using the Jensen JE-115KE which is really pretty
amazing and has lower distortion than anything else comparable.

In the bargain basement you might try using the Edcor MXCS series. They
are $30 and they still don't have any magnetic shielding and aren't all
that well wound, but hell, they are only $30.

Basically, most of the coloration and all of the problems in a preamp like
this all come from the transformer.


In the JFP gain module, I came up with the following substitutions for the transistors:
2N5457 to J109 (http://au.element14.com/on-semicondu...-92/dp/1467947)

MPSA14 to BC517G (http://au.element14.com/on-semicondu...-to/dp/1705773)

ZTX653 to BC337 (http://au.element14.com/fairchild-se...-92/dp/1228215)


2N5457, MPSA14, and ZTX653 are all currently manufactured parts. Those
subs are probably fine (and God knows the J109 will be quieter than the
2N5457).

The 2N5457 is the best jfet that you can buy for less than four cents each.
The J109 is a much more expensive part at eleven cents, and correspondingly
it is a lot more consistent and likely to be less noisy.

You may have to fiddle with the bias resistor values. Then again, you will
likely have to fiddle with them even with the 2N5457. No two of these parts
will have the same mu or Gm.

If someone could help me understand the input transformer and cast an eye over the transistor substitutions, I'd appreciate it.


The important part of the whole thing is the input transformer. The rest
of the thing is just matching.

Be aware that no two of those JFETs will have the same gain, so I would
socket them and buy a couple dozen JFETs and select them for gain in the
circuit. This circuit has very little feedback so it will exaggerate
differences between JFETs and especially at this price point the differences
between JFETs will be significant. So select-on-test.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Scott Dorsey Scott Dorsey is offline
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Default Scott Hampton's JFET mic pre - component selection

Geoff wrote:
On 31/08/2017 9:32 AM, wrote:
Hi All. I recently came across an old Tape Op article by Scott
Hampton describing a DIY JFET mic pre (and other equipment):
http://tapeop.com/tutorials/37/diy-jfet-mic-pre/


For the input transformer, I thought I could make this work, although
the datasheet says not to pass DC through the windings, so I suppose
that precludes phantom power?


No, there is no significant current through the primary, assuming the
6k8 resistors are 1% tolerance (or better).


Agreed.

If someone could help me understand the input transformer and cast
an eye over the transistor substitutions, I'd appreciate it. Thanks.


He does mention 'generic' wrt the transistors quite often, so nothing
too critical. I'm sure Scott will come up with some favorite candidates.


The ones in there are fine and all readily available. I have become a huge
fan of the 2N4401/2N4403 for low-noise-low-impedance-low-cost circuits
but I'd see no reason not to use the original recommendations.

Personally I don't buy Zetex parts because my rep ****ed me off, but don't
consider that a reason for you not to buy them.

But the overall question remains "Why ?" .


Because it is fun to build things. Everybody should do something like this.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Scott Hampton's JFET mic pre - component selection



If someone could help me understand the input transformer and cast
an eye over the transistor substitutions, I'd appreciate it. Thanks.



a couple of things about that design

The dc voltage on the drain is slightly above the supply, and directly biases the MPSA14 Darlington transistor.


ABOVE should be changed to BELOW

The output impedance of the voltage follower is a function of the 47 kilo-ohm resistor on the base of the ZTX653. If this resistor is too big, the output will not have enough drive and will be clipped on one 1/2 of the waveform. On the opposite end of the spectrum, if you make it too small, more power will be dissipated than is needed in the output devices.


The "active load" makes little sense. You would be better off leaving out the ZTX653 anf the 47K and connect the load resistor directly to the emitter of the darlington. Increase the load from 100 Ohms to something like 1000 Ohms. It makes no sense to use a transistor as a resistor.

I agree with the other comments about the input transformer.

have fun

mark




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geoff geoff is offline
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Default Scott Hampton's JFET mic pre - component selection

On 1/09/2017 1:12 AM, Scott Dorsey wrote:
Geoff wrote:
p ****ed me off, but don't
consider that a reason for you not to buy them.

But the overall question remains "Why ?" .


Because it is fun to build things. Everybody should do something like this.
--scott


So why not build something that's going to be relatively (or
exceptionally) good ?!

geoff
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Default Scott Hampton's JFET mic pre - component selection

On 8/31/2017 6:09 PM, Geoff wrote:
So why not build something that's going to be relatively (or
exceptionally) good ?!


That's probably the reason why Scott suggested using the Jensen input
transformer.

--

For a good time, call http://mikeriversaudio.wordpress.com
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Default Scott Hampton's JFET mic pre - component selection

In article , Mike Rivers wrote:
On 8/31/2017 6:09 PM, Geoff wrote:
So why not build something that's going to be relatively (or
exceptionally) good ?!


That's probably the reason why Scott suggested using the Jensen input
transformer.


Right. In spite of this being kind of a cheap design, the bottleneck is
still the transformer. You put a good input transformer on it, and it will
probably sound pretty good.

The input stage is noisy and inconsistent.... but with the high transformer
ratio the noise won't be a big issue, and pretty much all of the front end
distortion at normal levels is going to be third and fifth harmonic that won't
sound offensive. The output stage is a follower and so the local feedback
will keep it clean in spite of that kind-of-noxious darlington.

It'll be a lot more colored than a "ring of three" style circuit with global
feedback, but I bet it will sound fine.

And.... if the noise gets to be annoying you can replace the fet with a
2SK170 and fiddle with that source resistor to get it biased right, then
add another gain stage and put some feedback around it. It's a good circuit
to take as a starting point for designing something better because it's an
easy circuit to understand and comparatively immune to component variations.
(Aside from the fact that no two will ever have the same gain because the
JFET mu varies so much).

So, I'd put this in the "you can have fun with this and make music with it
and be happy with it and it won't cost much" category. Don't go in expecting
a Millennia.
--scott


--

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



--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Scott Hampton's JFET mic pre - component selection

On Thursday, August 31, 2017 at 11:12:51 PM UTC+10, Scott Dorsey wrote:
Geoff wrote:
On 31/08/2017 9:32 AM, dän wrote:
Hi All. I recently came across an old Tape Op article by Scott
Hampton describing a DIY JFET mic pre (and other equipment):
http://tapeop.com/tutorials/37/diy-jfet-mic-pre/



For the input transformer, I thought I could make this work, although
the datasheet says not to pass DC through the windings, so I suppose
that precludes phantom power?


No, there is no significant current through the primary, assuming the
6k8 resistors are 1% tolerance (or better).


Agreed.

If someone could help me understand the input transformer and cast
an eye over the transistor substitutions, I'd appreciate it. Thanks.


He does mention 'generic' wrt the transistors quite often, so nothing
too critical. I'm sure Scott will come up with some favorite candidates.


The ones in there are fine and all readily available. I have become a huge
fan of the 2N4401/2N4403 for low-noise-low-impedance-low-cost circuits
but I'd see no reason not to use the original recommendations.

Personally I don't buy Zetex parts because my rep ****ed me off, but don't
consider that a reason for you not to buy them.

But the overall question remains "Why ?" .


Because it is fun to build things. Everybody should do something like this.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


I have become a huge fan of the 2N4401/2N4403 for low-noise-low-impedance-low-cost circuits


Have you tried 2SC3329/2SA1316 for low-noise? I've heard that they have one of the lowest base spreading resistances.
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Default Scott Hampton's JFET mic pre - component selection

On Thursday, August 31, 2017 at 11:09:47 PM UTC+10, Scott Dorsey wrote:
dän wrote:
Hi All. I recently came across an old Tape Op article by Scott Hampton describing a DIY JFET mic pre (and other equipment):
http://tapeop.com/tutorials/37/diy-jfet-mic-pre/


This is a competent design using cheap commodity parts. The 2N5457 is kind of
doubtful but you might be able to select to find quiet ones. I'd bypass that
470uF cap too.

The transistors specified are not available to me anymore.


Which ones? All of those are basically the cheapest possible commodity parts.

Also, the input transformer is specified as the turns ration (1:10), but I'm not sure if the input/output impedances matter?


The thing about microphone input transformers is that you are matching a thing
that is very low (likely lower than the design impedance of the transformer)
to something that is mindbogglingly high (way higher than any possible design
impedance of any transformer). So any time you see impedances listed on a
microphone input transformer, they are kind of doubtful numbers anyway.

For the input transformer, I thought I could make this work, although the datasheet says not to pass DC through the windings, so I suppose that precludes phantom power?
http://docs-asia.electrocomponents.c...6b815288ea.pdf


This has no magnetic shielding, which could be a problem, but the output
impedance is very low. This looks like it's designed as an interstage line
level transformer rather than a microphone input transformer. But you
likely could make this work although I suspect that the low end will be a
bit slack.

I would strongly recommend using the Jensen JE-115KE which is really pretty
amazing and has lower distortion than anything else comparable.

In the bargain basement you might try using the Edcor MXCS series. They
are $30 and they still don't have any magnetic shielding and aren't all
that well wound, but hell, they are only $30.

Basically, most of the coloration and all of the problems in a preamp like
this all come from the transformer.


In the JFP gain module, I came up with the following substitutions for the transistors:
2N5457 to J109 (http://au.element14.com/on-semicondu...-92/dp/1467947)

MPSA14 to BC517G (http://au.element14.com/on-semicondu...-to/dp/1705773)

ZTX653 to BC337 (http://au.element14.com/fairchild-se...-92/dp/1228215)


2N5457, MPSA14, and ZTX653 are all currently manufactured parts. Those
subs are probably fine (and God knows the J109 will be quieter than the
2N5457).

The 2N5457 is the best jfet that you can buy for less than four cents each.
The J109 is a much more expensive part at eleven cents, and correspondingly
it is a lot more consistent and likely to be less noisy.

You may have to fiddle with the bias resistor values. Then again, you will
likely have to fiddle with them even with the 2N5457. No two of these parts
will have the same mu or Gm.

If someone could help me understand the input transformer and cast an eye over the transistor substitutions, I'd appreciate it.


The important part of the whole thing is the input transformer. The rest
of the thing is just matching.

Be aware that no two of those JFETs will have the same gain, so I would
socket them and buy a couple dozen JFETs and select them for gain in the
circuit. This circuit has very little feedback so it will exaggerate
differences between JFETs and especially at this price point the differences
between JFETs will be significant. So select-on-test.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Thanks Scott! The original parts are not available from my preferred suppliers in Australia (RS and Element14). I checked Digikey after I read your post, and they were not much better, although Mouser seems to have them, and they now ship to Aus, but they have a $60 minimum order to get free shipping, and I couldn't find a suitable transformer there either.
Thanks for the transformer suggestions. If I can get those easily, they look like great options. I also found http://au.element14.com/oep-oxford-e...150/dp/1172344 from Element14, which is available with a mu-metal can. This one has a bit higher turns ratio than specified. Also the Edcor is available in 1:8 or 1:12. Would you opt for a higher or lower ratio, if the 1:10 specified transformer wasn't available?
Thanks again. BTW, I recently built your reamp box (http://www.recordingmag..com/resourc...tail/314.html). Works great for me!
Cheers,
Dan.


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Default Scott Hampton's JFET mic pre - component selection

On Friday, September 1, 2017 at 12:34:05 AM UTC+10, wrote:

If someone could help me understand the input transformer and cast
an eye over the transistor substitutions, I'd appreciate it. Thanks.



a couple of things about that design

The dc voltage on the drain is slightly above the supply, and directly biases the MPSA14 Darlington transistor.


ABOVE should be changed to BELOW

The output impedance of the voltage follower is a function of the 47 kilo-ohm resistor on the base of the ZTX653. If this resistor is too big, the output will not have enough drive and will be clipped on one 1/2 of the waveform. On the opposite end of the spectrum, if you make it too small, more power will be dissipated than is needed in the output devices.


The "active load" makes little sense. You would be better off leaving out the ZTX653 anf the 47K and connect the load resistor directly to the emitter of the darlington. Increase the load from 100 Ohms to something like 1000 Ohms. It makes no sense to use a transistor as a resistor.

I agree with the other comments about the input transformer.

have fun

mark


Thanks for the suggestions Mark. I might try your modification and compare it to the original.
-Dan.
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Default Scott Hampton's JFET mic pre - component selection

On 2/09/2017 10:19 AM, dän wrote:
.."

Thanks Scott! The original parts are not available from my preferred suppliers in Australia (RS and Element14). I checked Digikey after I read your post, and they were not much better, although Mouser seems to have them, and they now ship to Aus, but they have a $60 minimum order to get free shipping, and I couldn't find a suitable transformer there either.


Free shipping over $60 - that's good. Wonder what paid shipping would
cost from Mouser.

Last time I wanted just a few transistors from Digikey to NZ the
shipping alone was around that price ( = about 10 x the real cost).
Needless to say, I didn't get them from Digikey.....

geoff
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Default Scott Hampton's JFET mic pre - component selection

Free shipping over $60 - that's good. Wonder what paid shipping would
cost from Mouser.


It would be at least AUD$24. RS Components is free with no minimum order, at least to Aus.
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