Urei 175b side chain mods

[beatnik]

I am working on a Urei 175b project and I am thinking to build a second unit to make a stereo compressor for the main bus, so I would really like to implement the option to link the side chains

I tried searching around but there is nothing specific about this circuit, there was a link to a thread in the meta but it's dead

But found out that other stereo vari mu just tie together the two sidechain signals, some adding diodes as well

Would this work fine for the 175 too ? I attached the schematic for quick reference

I also would like to add a stepped control with two-three positions high pass filter, i guess it would be a simple mod but i don't really know how to do it properly, nor I have found any other circuit schematics with the same feature to copy it. The Retro 176 which is based on the same urei circuit has the same features I'm looking to add but obviously no schematics around for that one

Any help is highly appreciated

 

[PRR]

Strap both C5s together.

A HPF is non-trivial.

 

[beatnik]

perfect ! thanks PRR . but why you think the hpf is meaningless ? sometimes during mixing and even more often on the "mastering" phase i try not to compress the low end of the mix too much and by using a sidechain filter i can compress the mix a bit harder without introducing too many artifacts. still sometimes is useful to have the mix bouncing with the kick drum and the bass but i would really like to have both options as this compressor will be used mainly on the mix buss but i also found that the hpf is useful for recording, i use it on vocals sometimes.

 

[Walrus]

PRR didn't say it was meaningless, he said non-trivial. That is, it is not very easy.

 

[beatnik]

ops it seems google translate didn't work out well this time

well i wouldn't mind if it's not super easy to implement. i'd really like to have this function

i guess some experimentation would be the best anyway but i don't know where to start

one thing i am sure it is possible to do it because Retro is using the same feature on their urei inspired compressor

 

[beatnik]

I have found some information relative to the Altec 436 circuit which has some similarities to the Urei 175 and someone was suggesting to alter the coupling caps going to the cathodes of the 6AL5

I will try to experiment with those but I would be very glad to hear more suggestions. The fact that PRR said is not very easy mod to do makes me think just switching those caps is not the best method

 

[Walrus]

Changing the coupling caps feeding the cathodes to a much lower value would certainly create a HPF, but simply using a switch to change values will probably cause some horrendous pops and bangs on the output! 

 

[PRR]

> Changing the coupling caps

Smaller caps to the diodes will slow the attack more than reduce the bass through the sidechain. It may cause "zipper noise" (*) on strong low frequency.

The 175 is *cheap*. It does exactly what it does, but does not make it easy to add features.

(*) Take that, Google Translate!

 

[beatnik]

thanks for the explanation PRR

i will do some experiments and post the results

 

[emrr]

Been down this road recently.  All easy.  Different and more caps in side chain for HPF works fine.

http://groupdiy.com/index.php?topic=60520.0

I've put smaller SC caps in a bunch of tube limiters to take bass out of the picture, works, but realize you are removing the largest energy portion, so unit will not compress as much, moving you closer to max output / distortion before compression happens.  There is a limit.  Ears are your guide.