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Transistor JFET/MOSFET Mic Preamp with transformer schematic / layout?
Transistor JFET/MOSFET Mic Preamp with transformer schematic / layout?
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Old 4th January 2021, 01:31 PM   #1
22uf is offline 22uf  United Kingdom
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Default Transistor JFET/MOSFET Mic Preamp with transformer schematic / layout?

I have a sm57 which I use to record with. However I don’t really have a proper pre. I would like to build my own without IC chips. Ideally something like a MOSFET. However, I would also like it to have a transformer.

Are there any simple schematics or layouts for something like this? It doesn’t need phantom power. Just a simple simple single mic pre for a dynamic mic, no IC, and a transformer.

I have tried to look for something, but no luck at all...

In fact, it is hard to find ANY layouts/schematics for mic pre’s, especially compared to how easy it is to find guitar pedal circuits!

Last edited by 22uf; 4th January 2021 at 01:33 PM.
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Old 4th January 2021, 02:00 PM   #2
JMFahey is offline JMFahey  Argentina
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Straight from the mouth ofthe horse:

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forget inconsistent spread-all-over-the-place Fets , even less MosFets, Shure used reliable consistent Bipolar transistors.
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Old 4th January 2021, 02:26 PM   #3
duncan2 is offline duncan2  United Kingdom
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Very impressive and I agree most of the good designs use BJT,s .
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Old 5th January 2021, 06:19 AM   #4
22uf is offline 22uf  United Kingdom
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Quote:
Originally Posted by JMFahey View Post
Straight from the mouth ofthe horse:







forget inconsistent spread-all-over-the-place Fets , even less MosFets, Shure used reliable consistent Bipolar transistors.

Is there a simpler version of this, with only one channel?
Also, why on the schematic does it say ‘external battery’?
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Old 5th January 2021, 06:26 AM   #5
22uf is offline 22uf  United Kingdom
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I am only using one microphone in mono, going to a cassette deck via aux - so a rather simple pre would suit my situation better. I just didn’t want the pre to sound sterile. I am after an older design without IC’s.
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Old 5th January 2021, 08:22 AM   #6
JMFahey is offline JMFahey  Argentina
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You got it here.

No need to build 4 channels, you may very well simplify it, no big deal.

Define your desired configuration, what you will feed from it , and we may help you chop it down.

As in: a single channel will NOT need a 4-to-1 mixer, you might not need a VU Meter, etc.

Guess you will not need a balanced out either, chop that off too.

This preamp uses transformer input.

Do not avoid it, it provides 90% of that smooth vintage sound you are craving.

You *might* chop Mixer down to as little as "Channel 2" only, using Q2-Q6 , and feeding tape recorder Aux In straight from R16 wiper, go figure.

If gain is not enough you might add the Mixer stage (here "mixing" just 1 channel) using Q9-Q10

You do not need the powerful line out stage using Q12-13-14 , it´s a mini Power Amp designed to send powerful signal up to 200 yards away.
This was a PROFESSIONAL Mic Mixer/Preamp, often used to send Audio to typical Radio/TV transmitting van up to 2 blocks away in the middle of a hurricane or whatever.

It has its own supply but since it was often used live in complex situations, there was also a portable battery pack to feed it in the field.

I would protoboard or perfboard build just Channel 2, feed it 15V from any supply or a couple 9V batteries in series, plug your microphone in and send output to tape recorder.

If enough, it´s enough.

If not, add an extra stage, following original circuit.
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Last edited by JMFahey; 5th January 2021 at 08:24 AM.
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Old 13th January 2021, 01:12 PM   #7
22uf is offline 22uf  United Kingdom
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Thanks JMF,
After looking at the prices of all the components, I see that the transformers are where the expense is at, so I guess I might as well go for a tube pre!

I will make another thread.
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Old 13th January 2021, 05:34 PM   #8
campsquire is offline campsquire  United States
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I have that Sure microphone preamp. It's pretty good. I understand four of these were used tandem at the Woodstock music festival.
But....input transformers are kind of expensive.
I'd suggest this Elliot Sound Products project:
Low Noise Microphone Preamp
I've built one - for its' simplicity I don't think you can do better. It simulates well and performs will in reality with very low distortion and noise, and good headroom.
This and a power supply consisting of a 20V Antek toroidal transformer A0120, a 3" x 3" perf-board, a low power diode bridge, a 2200uf electrolytic capacitor, power cord, fuse holder and fuse, and connectors, you got yourself a mic preamp for under $20.00.
The circuit itself is so cheap I'd build at least a pair. It's about 1/4 that total cost.
An enclosure might be the big ticket item.
In fact if that Sure mic preamp weren't so iconic I'd gut the original circuit and replace with four of these.
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