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Old 28th August 2012, 01:20 AM   #7911
Salas is offline Salas  Greece
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Quote:
Originally Posted by lohk View Post
Microphones are very different in out level and impedance, but generally have about 100Ohm source impedance upwards, usually around 200 Ohms. Because level and sound pressure changes the gain needed varies from 50-60db to -say- 20-30db. Real life dynamics exceed the dynamics we know from music productions by far, so the headroom should be very high or the distortion characteristics smooth.

Usually mic preamps have either differential amps as inputs or transformers, also because they are usually balanced.
The simplest I could think of is a Jfet dif amp input handing over to a Jfet input opamp. It can swing 26Vpp this one. Its 60dB gain can be padded down by switching resistors parallel to R3 or by shunting a pot. G=(R3/R4)+1. Balanced Mic transformer coupling offers the best CMRR and sound anyway so its preferable if in hand to keep the electronics SE and minimal. Jfet cascoding, other worthwhile Jfet types, easily applicable. Housekeeping like opamp decoupling, sizing C1 to likely impedance it will meet, PSU etc. goes without saying.
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Old 28th August 2012, 07:12 AM   #7912
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The inputs to U1 are ~ 10V above power/signal ground.
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Old 28th August 2012, 07:35 AM   #7913
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@ 7.5V
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Old 28th August 2012, 09:17 AM   #7914
AndrewT is offline AndrewT  Scotland
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Are you/we OK with that?
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Old 28th August 2012, 09:37 AM   #7915
lohk is offline lohk  Europe
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Thank you , Salas.
But topologies like this have been done many times, sometimes with cascoding, and other variations.

What I meant - and maybe it was a bit naive - was, if it may be possible to use your circuit as a micpre. I like the simplicity and the possible "sound".
Of course with a unbalanced input cables must be quite short and only unbalanced or non-active microphones can be used. But there are ribbon microphones, which usually have a step-up transformer built in, and some other very quiet dynamic microphones, which could possibly benefit. And for balancing I can use a transformer anyway.

I use another simple mic-pre in an experimental state right now, and was wondering if your circuit may not be better.

It uses two of these stages in series and a transformer in front.
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Old 28th August 2012, 09:40 AM   #7916
Salas is offline Salas  Greece
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Originally Posted by AndrewT View Post
Are you/we OK with that?
They nil out since they are common DC to inverting and non inverting inputs. The offset and the signal is what gets amplified. C1 is for the inevitable offset especially without single die dual Jfets like 2SK389 which are commonly unavailable these days.
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Old 28th August 2012, 09:49 AM   #7917
Salas is offline Salas  Greece
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Quote:
Originally Posted by lohk View Post
Thank you , Salas.
But topologies like this have been done many times, sometimes with cascoding, and other variations.

What I meant - and maybe it was a bit naive - was, if it may be possible to use your circuit as a micpre. I like the simplicity and the possible "sound".
Of course with a unbalanced input cables must be quite short and only unbalanced or non-active microphones can be used. But there are ribbon microphones, which usually have a step-up transformer built in, and some other very quiet dynamic microphones, which could possibly benefit. And for balancing I can use a transformer anyway.

I use another simple mic-pre in an experimental state right now, and was wondering if your circuit may not be better.

It uses two of these stages in series and a transformer in front.
I see what you are after, but if you will choose this non feedback gain controlled approach you should settle for about 40dB middle of the way gain because the bias points would need some determined swing headroom, and it will not be that easy to change parameters on the fly. Maybe a 1:10 mic trafo and a 30dB output voltage divider switchable pad can make it more versatile. Will check out and give you a flat gain one based on the simplistic. Any simplistic is +20dB on nominal without the passive RIAA loss so it takes some recalculation.
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Old 28th August 2012, 05:00 PM   #7918
RCruz is offline RCruz  Switzerland
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After posting elsewere about the qualities of another riaa preamp I could not resist to leave my other opinion here.

Some say the Simplistic has special staging qualities. I did find it to be true and never heard anything aproaching the Simplistic in this area (not even the best jfet EB implementations that I had the oportunity to evaluate). Off course this can only be obtained with a very carefull layout implementation and carefull choice of components. IMO the type of shunt used is determinant.

Apart from staging, the Simplistic can have the sweetest mid/trebble causing a magical transe state on the listener....

Off course it might be difficult to get all this at first start (it took me a while to get here) but diy difficulties should never stop those who are really determined.

This post serves to remove all doubts about my subjective evaluation of this superlative design by Salas.
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Old 28th August 2012, 05:08 PM   #7919
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RC, Did you build the Simplistic with a power supply per channel or a single power supply?
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Old 28th August 2012, 05:12 PM   #7920
RCruz is offline RCruz  Switzerland
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It responds heavily to PSU info. My best build has true double mono with Salas V12R shunts.
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