Transformerless Vari Mu Compressor build thread

[Heikki]

That's right ! Which trimmer?
on SC board? An only trim the board that gives higher output?

Yes on the channel which gives the +7dBu output turn RV1 on that channels sidechain slightly counter clockwise intil the output drops to +6.5dBu.

 

[manulaudic]

Sounds good I am measuring these with AC meter so I will try to get voltages same point

 

[manulaudic]

Am I right if assume that channel which originally measured 3.89V gives the +7dBu output?

You can trim or try different tubes. I would probably just adjust the trimmer.

Sorry actually I measured too fast :
The channel that gave originally 3.89vdc measures 1,624v which is 6,43dbu and the channel that measured 4.15vdc shows 1,718v which is 6,92dbu

 

[scott2000]

Are your input levels being sent from the dac or wherever calibrated?

 

[manulaudic]

Are your input levels being sent from the dac or wherever calibrated?

I'm sending the sine wave through the test oscillator of logic with UAD interface and measuring the voltage sent on each channel to be 0.245V

 

[Heikki]

Increase the bias voltage with RV1 for the channel which has higher output.

 

[manulaudic]

Sweet so I adjusted RV1 on the side chain board that was giving the loudest output voltage to match the other one. Now it's great I can just turn the threshold on both channels so identical place and the volume output is more or less matched.
Though now the 1db LED is lit on the board I had to adjust, I saw your post about adjusting the resistor for it... but I mostly work by ear so that's fine,

I'm not sure if it's compressing more though on one channel Than the other, it's such a smooth compressor that I can't really tell... but sound is lush ! super

Manu

 

[Heikki]

Though now the 1db LED is lit on the board I had to adjust, I saw your post about adjusting the resistor for it... but I mostly work by ear so that's fine

Another option is to add 220k resistor parallel to 13k R6 on the LED board. That would make the resistance approximately 12k.

 

[fazeka]

Another option is to add 220k resistor parallel to 13k R6 on the LED board. That would make the resistance approximately 12k.

I thought R6 was supposed to be changed to 10k?

 

[Heikki]

I thought R6 was supposed to be changed to 10k?

There are two versions of the LED board out there. The original LED board did not have a trimmer and the new revision has a trimmer for the 1 dB led.

R6 is 10k in the LED board version which has a trimmer for the 1dB led.

The original LED boards don’t have a trimmer and in some cases the R6 needs to be changed from 13k to 12k.

 

[manulaudic]

Fantastic ! That's next on my to do list.

 

[zzzzz]

Very cool project... following

 

[fazeka]

There are two versions of the LED board out there. The original LED board did not have a trimmer and the new revision has a trimmer for the 1 dB led.

R6 is 10k in the LED board version which has a trimmer for the 1dB led.

The original LED boards don’t have a trimmer and in some cases the R6 needs to be changed from 13k to 12k.

Got it! Thank you!

 

[manulaudic]

Here are some pics from my build. The case is from Frank at frontpanel super good stuff and really really nice customer service.
I added the 220k resistor in parallel and that did the trick, now only the 0db LED is on.

The compressor is so good, love it, really can't turn the knobs in a position that would sound bad which is always what I want with gear I use.

Thanks Heikki for this great project and the help you all provided here !

I will most likely build another one cause its also really good on tracking with vocals etc...

I'd love to make one with needle VU In there

Last thing I need to do is add heatsink to U2 and U1

 

[Heikki]

Here are some pics from my build.

Great job, very neatly built. Did you use 6SS7 tubes?

 

[manulaudic]

Great job, very neatly built. Did you use 6SS7 tubes?

Yes like you mentioned in one email, sourced them from helsinki, they were a cheaper option than 6sk7 which they also had
I did one mistake though is I did not pay attention to the orientation of the tube sockets... I followed the steps and was careful to place them with standoffs before soldering but still made that one mistake...

Oh well, I realised when inserting the Tubes in place

 

[Heikki]

Hint for anyone else building this too. 6SS7 can be used instead of 6SK7.

 

[manulaudic]

Any other possible candidates? Would be nice to try different ones

 

[scott2000]

Hint for anyone else building this too. 6SS7 can be used instead of 6SK7.

Would this mean a smaller current ac/dc brick could be used in this case?

 

[Heikki]

Any other possible candidates? Would be nice to try different ones

6SG7 is pin compatible and should work. Pretty much any pin compatible remote cut-off tube will work but performance might not be as good.

Would this mean a smaller current ac/dc brick could be used in this case?

6SS7 does have only 150mA heater current where 6SK7 has 300mA. But I wouldn't want to have a wall wart which has only enough power to supply 6SS7 in case I would want to use 6SK7 later.