[fripholm]

for that i see, only the ne5534 or the THAT 1646 could the "noise genrators "

The problem is that in a FET compressor the signal level has to be extremely low (<<100mV) at the gain reduction FET for it to remain in its linear region. Otherwise it would distort heavily.

Your real "noise generator" in this case is the thermal noise added by resistor R10 (10k) and the input pot which in the worst case adds another 2.5k at its -6dB position. Both of these resistances account for about -112dBu of noise which is amplified by ~25dB by the next stage. This stage is a modified mic pre design by our fellow Samuel Groner which is extremely low noise but you still end up with a noise level that's at least 20 times higher. Being swamped by this amount of hiss, the contribution of the following stages to the output noise is virtually negligible.

However, you could try to replace the input opamps U2 and U3 by opamps that have even less noise than the good old NE5532. An LM4562 would be a good candidate - they have gotten quite affordable lately - but I doubt that you would see an improvement of more than a dB or two...

A real 1176 has a 27k resistor in the place of R10 which by itself produces almost 4dB more Johnson noise. Be aware that the sidechain of my OP76 is tuned around the value of R10 to achieve the same compression curves and knees of a real one which means that reducing its value any further is not possible without altering the compression characteristics that you've grown to love

[pstcho]

thanks for your reply, unfortunately the LM4562 are not available easily in my country for a fast test, but i had some OPA 2134 and the difference is un significative, same with TL072

i started to check a bit more carefully what i feel

when i activate the compressor i just hear a little bit of high frequency noise around 8K around 4dB so i could that just OCD, i just feels weird at the moment

of course the takes are great and noiseless, now with a good amount of gain and big compression i can go to -90dB of noise, that means absolutely nothing in a mix ....

just feels weird at the moment i press the bypass

i must really give big credit to this build, cause i have different 1176 build at different period and different components inside more or less well calibrated and this specific one ( yours ) is really the most transparent compression that i have heard so far, i mean i can slam vocals as f**k to look like a square and not hear the compression, its magic

so yeah, dont pay attention to my " request " cause its really not significant in the context of music !!  thanks a lot for this mindblowing build

[fripholm]

Thanks a lot for the kind words. Much appreciated

I have a bunch of LM4562 here that I could pop into one of my units and see if it makes a difference. Maybe over the weekend if I find the time...

But the fact that it doesn't get worse when using a TL072 with its much higher noise density may suggest that there's not much to hope for when aiming for the other direction. We'll see...

[pstcho]

yeah its rare in a well designed circuit to hear a big difference between op amps but yeah i enjoy a lot this compressor, didn't try on anything else than vocals but does magic

[fripholm]

Finally found some time to try out two LM4562s for U2 and U3.

At the same settings I could measure a slightly lower noise floor by about 0.2 to 0.3 dB compared to the NE5532. It's an improvement... but I wouldn't call it 'huge'. 

[pstcho]

same things with the OPA, doesn't really worth the investment, and doesn't need after all

[mgb54]

I've just finished building these and I want to say a big thanks to Thomas for his very detailed and generous help with troubleshooting an issue I had with my build (which was entirely my fault!). They sound fantastic and the small enclosure is going to be ideal for travelling with, once we are all travelling again...

DC offset and such VS. FET input opamps..

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