GHR Audio

Heikki. could your transformer less input circuit work in the 670 Fairchild comp.

cheers

Dennis

Until Heikki arrives, I've attached a translated version of the document that is at the bottom of the vari mu home page.

https://ghr.fi/varimu.html


It's translated from Finnish

Towards the end it talks about the theory of transformerless.

skal1 wrote: ↑Tue Jul 23, 2024 1:19 pm Heikki. could your transformer less input circuit work in the 670 Fairchild comp.

Short answer is no. Slightly longer answer is maybe. Assuming you would keep rest of the 670 without any changes you could use something like in the picture below. Except Fairchild has very large control voltage which can be more than 60V and the signal peaks might be even larger, so anything with +-15V supply isn't enough. There are high voltage opamps and differential amplifiers available which you could use.

cheers for that, i thought it would kind of work, where would the center tap go pin 8 and 14 ? and i take it pin 9& 13 are o/p to the control grid ? whats that u3a op amp for.

best

denis

U3 pin 3 is the "center tap" where the control voltage goes. Since U3 is a buffer you wouldn't need power amplifier for the sidechain. Yes U2 pin 13 & 9 go to the grids. Fairchild input transformer has large step up ratio, U1 could be replaced with differential amplifier with adjustable gain.

Cheers Heikki thanks for the knowledge , not going to get around doing it any time soon to busy @ work


i think the first step will be figuring out the voltage gain of the i/p tx and then try to recreate that with an op amp

cheers

Denis

Here's a general replacement for input transformer in vari mu compressors. This won't work for Fairchild 660 because power supply max is +/- 45V. For Fairchild you would need opamp with higher power supply voltage.

Here are the KiCAD design files

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Picture of the board