A fixed gain amplifier. (Dead Thread)

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Very cool.

As I'm sure most of us know, most mic amps were fixed gain until fairly recently. Whole systems would be built from a number of 40dB gain blocks with attenuators inserted along the way as needed :wink: It may not have been the best from a S/N standpoint, but it surely worked more than well enough for decades. And you DO change the sonic signature of an amp to some extent as you vary its gain, so a fixed gain amp offers some predictability in that regard.
 
the amps I use in my studio predominantly are fixed gain. I would say I use those amps probably 20 times more than pre's with an adjustable gain structure. I dont know if thats chance or what, but it says something...

John- you are feeding this circuit 66v?

whats going on there in the top left of the drawing?

dave
 
What kind of output transformer?

What is the resistor value feeding the gates?

What kind of drafting program?

Looks cool!

What did you listen for when tweaking the feedback, or did you use the scope?

How long did it take you to design this thing?

Why not box it up and retire instead of giving it to us druids?
:green:
 
[quote author="NewYorkDave"]Very cool.

As I'm sure most of us know, most mic amps were fixed gain until fairly recently. Whole systems would be built from a number of 40dB gain blocks with attenuators inserted along the way as needed :wink: It may not have been the best from a S/N standpoint, but it surely worked more than well enough for decades. [/quote]

Yes, especially when they are 20dB gain blocks :razz:

I'll give a serious response soon. I already like the heroic use of multiple parts :green:
 
Another good one!
They are coming faster than I can build them.
A very good problem to have.
The monolithic op-amps in my studio are in trouble.
They need to run and hide because I will soon make them extinct.



Thanks Winston!
 
Winston, have you measured the input Z of the preamp? According to my calculations it ought to be about 501 ohms. After the "bizarre input Z behavior" thread, I'm curious.

Peace,
Paul
 
> input Z of the preamp? According to my calculations it ought to be about 501 ohms.

You (or I) must be misreading where the 15K feedback goes. As I read it, the 15K hits the 802 on the low side of the transformer winding. The high side is loaded with infinite ohms, some pFd, 8K of dead resistance, and 1300pFd||619 Zobel. Over most of the audio band, 8K. Reflected back to the primary at 1:2, about 2K.
 
[quote author="PRR"]> input Z of the preamp? According to my calculations it ought to be about 501 ohms.

You (or I) must be misreading where the 15K feedback goes. As I read it, the 15K hits the 802 on the low side of the transformer winding. The high side is loaded with infinite ohms, some pFd, 8K of dead resistance, and 1300pFd||619 Zobel. Over most of the audio band, 8K. Reflected back to the primary at 1:2, about 2K.[/quote]

I think I'm the one who's doing the misreading. Sorry, I saw the feedback as going to the hot side, as in the RCA preamp New York Dave diagrammed a few weeks ago. :oops:

Peace,
Paul
 

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