Transformerless Vari Mu Compressor build thread

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Heikki

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There was a lot more interest in this project than I expected, so here is a help thread.

Schematics, BOM and instructions can be found here. In case I don’t pay server hosting fees, backup is here.

Scott2000 has already built his and he should have some extra boards to sell. He might also have kit/partial kit to sell. He is located in the US so anyone in that part of the world still interested getting boards should contact him. He will probably chime in here how best to contact him.

Black Market thread for boards

Big thanks to Scott for helping with the instructions. Now the instructions should make at least some sense.

Here is a RightMark Audio Analyzer test report made with using Focusrite Scarlett 2i2. (Using the line input where 0dBFS = +22dBu)

Sound samples, same song with different amounts of compression.

Mouser cart which should have almost all of the components needed. Missing are tubes, tube sockets and the 36V wall wart. Double check everything before ordering! Does NOT have all the components needed for the rotary switch sidechain version!

Here is a Mouser cart for the rotary switch version. Best double check everything is there! Lorlin CK1054 rotary switches can be probably gotten cheaper somewhere else.

This is the type of tube socket I recommend using.


Some hints and tips

C9 on the power supply board can be omitted

C29, C30, C31, C32 on the main board can be replaced with jumpers. Read Post #84 and Post #86.


Read post #30, how to balance the channels.

Read Post #80, modifications for mastering use

Round shaft Lorlin rotary switches
Post #156
Post #158



Some components needed seem to be nearly ot of stock and have long factory lead times. Get them while you can.

Recom R24-150B

DCWN06B-15

GE40I36-P1J


Alternative power supply here!


In this post some gut shots from Manu Laudic’s build.

In this post pictures from paolomunarini's build.
 

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Heikki

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I have been asked a couple of times via email if there are any alternatives for the wall wart. Best to answer it here too. Almost any wall wart/AC adapter with 36V output and at least 800mA current capability should work. There are cheap 36V led power supplies and 36V printer AC adapters that will work perfectly. I think Scott used a Kodak 36V/3A power brick.
 

manulaudic

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Hi, I am currently building the main board and in the place for R31 R32 R33 and R34 on the schematic it is 4,99k but on the board there is 8 resistors slots. Is it to get the proper 4,99k with a combination of resistors in parallel or is it to be inserted with 8 4.99k which would bring the total resistance off from the schematic.

Thanks
Manu
 

T.LO

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Hi, I have a question to the zener diodes in the BOM. Does a zener from 1N53series (200V) work too? I mean as a substitute for the 2M200Z R0 in the BOM...
 

Heikki

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Scott has been building another compressor for himself and this time with a new sidechain board which has rotary switches. Attached is a picture of his new build. Anyone wanting to build this compressor in the future there will be option to use the rotary switch sidechain.


Schematic for the sidechain with rotary switches
Sidechain schematic
 

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scott2000

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Not sure if this answers anything but here's the theory of operation for the sidechain I believe.

The audio voltage is applied between the pins 5 and 8 of the transformer T1. Diodes D2 and D3 convert the audio voltage into negative pulses with respect to the voltage V +. When the negative pulses exceed the threshold voltage of the base of the transistor Q4, the Q4 signal is amplified and the pulses are turned positive. Potentiometer RV4 can be used to change the gain of transistor Q4. Transistor Q3 acts as a buffer. The pulses charge the capacitor to the positive voltage of capacitor C2. R6 + RV2 determine how fast C2 charges and RV3 + R8 + R6 + RV2 determine how fast C2 discharges. Transistors Q2 and Q1 form a darlington circuit from which the control voltage is applied to the cathodes of the tubes.
 

Heikki

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Why is the release control not straddled across the 2.2 uf cap?

We wouldn't want the release resistor/pot to be connected to V+. But perhaps you are asking why the release control is not after the attack control? The way it is arranged now, the attack time always adds to the release time. I try to limit the possibilities of the user adjusting the controls to make it sound bad.
 

rankot

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Is it absolutely necessary to use those power bricks, or I can use some other kind of power supply with additional filtering?
 

Heikki

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Is it absolutely necessary to use those power bricks, or I can use some other kind of power supply with additional filtering?
Doesn’t matter what kind of power supply is used. The compressor needs around 200V DC for the B+, around 25V DC for the tube heaters, sidechain and the led boards, +15V and -15V DC for the INA chips. In the first version of this compressor I built I used meanwell SMPS for the 25V and linear power supply for everything else.
 

Heikki

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Couple pics of Scott’s build with the rotary sidechain board.
 

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manulaudic

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Hi, I am wondering if i could use a different value cap on C9 of the power supply. it's given 10uF 250V but i am having a hard time sourcing it fast and i have some 3,3uF 250v lying around.
 

Heikki

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Hi, I am wondering if i could use a different value cap on C9 of the power supply. it's given 10uF 250V but i am having a hard time sourcing it fast and i have some 3,3uF 250v lying around.
Yes you can use 3.3u/250V for C9 or you can leave out the capacitor altogether. C9 on the power supply board isn't really needed.
 

manulaudic

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I am having trouble understanding the testing, I have the right voltages around except for 182v which is 180 and 12.6v on F which is 12.4

My question is with the signal feed. I feed 50hz with REW at -3dbfs but I need to raise my channel input level all the way to be able to see changes in the leds with threshold... Am I suppose to set the input gain at a certain position? how much voltage should the 50hz be sent at?

I have also tried with the file but I am not sure if I need to adjust both trims for less thump. Also channel 1 is louder than channel 2

in my voltage checkup and setup of side chain rv1 trim pots. With fully clockwise trim I have a difference in voltage between both boards 3.89v and 4.15v before adjusting. Should they be same voltages with trim fully clockwise?

Also, if someone can explain to me what the link does? Which board becomes the controlling board, ?

Otherwise, it's very clean, no noise, compression is there. If I noodle with some audio file I need to raise the input fairly loud to hear and see some compression and thus increasing the overall volume, which makes it a little hard to compare when bypassing as the loudness is affecting the judgement

Hope you can help me get this sorted :)
Manu
 
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