Transformerless Vari Mu Compressor build thread

[TomKelly]

Voltage changes with RV1 at R48, but not at R37 (both against ground)

 

[Heikki]

Measuring either of the points on the picture against ground the voltage should be the same.

 

[Heikki]

All of these three points should have nearly identical voltage as the ones in the earlier picture when measured against the ground. if not then it's a clue where to look next.

 

[TomKelly]

Ah - my misunderstanding. I thought you meant R37 and R48 on the mainboard.

R10, 27, 37 & 48 are all about 6.7V. R8 is about 6.4V. Faulty Q3?

 

[Heikki]

Q3 is a possible suspect. Check that you have correct transistor as Q3.

check Q2 too and check that Q1 is oriented correctly.

 

[TomKelly]

Will do. Why would suspect transistors there mean that changing RV1 doesn't change the voltage at the test point?

 

[Heikki]

Diagnosing from a distance is often not better than guessing.

Another measurement that would be useful to know is measuring voltage from R5. Put attack to slow position and see is voltage measured from R5 is different than voltage measured from R8.

 

[TomKelly]

Understood - I was wondering more generally why changing RV1 wouldn't have any effect. With attack in slow position R8 is 6.47V and R5 6.41V.

Q3 is a BC547 and is orientated correctly, as is Q1. Q3 seems to check out with a multimeter.

Thanks for the remote diagnosing - it is appreciated! It's strange that everything did work as expected for a little while, and then this problem occurred after I started adjusting the trimmers.

 

[scott2000]

If not making any progress, maybe some various pictures of the boards and wiring would be helpful in case there is something that stands out as suspect?

 

[Heikki]

One thing that would make the * voltage not change is that if Q1 is oriented the wrong way. But then I would expect the voltage to be nearer 4V.

Could you do the following thing. Turn RV1 to completely clockwise position and measure the voltage between ground and these points on the picture. Then turn RV1 completely counter clockwise and measure the voltage.

 

[TomKelly]

Huge apologies (cautionary tale to anyone reading this). I was trying to do this in between other things, and was adjusting the wrong RV1.... Just shows I can put these things together, but can't stop myself making stupid mistakes. Everything is now working, voltages are doing exactly what they ought to, and the unit sounds fantastic. Thanks to Heikki for the design and the boards, and to everyone who's contributed. Will take some pictures asap

 

[scott2000]

Awesome! I had quite a few goofs that Heikki had to deal with. Having meter on AC is always a classic too..lol
Thanks for updating.

 

[manulaudic]

Hi guys, I have an issue with pots on the rotary version. The Lorlin have a "D" profile and the pot screw is placed on the edge of the "D" profile and I can't get the pots secure.
Have you guys figured out some good pots for this? Or then do these Lorlin exist with round profile?
I didn't really check that on the mouser cart and I thought maybe these would be oriented properly

Especially Scott2000, I can see you have used the same Lorlins with the same profile

Manu

 

[manulaudic]

Yeah. Thanks to Heikki being patient with my goof up. Had miswired the threshold switches to the wrong resistor on the sidechain board.

Room is limited on the front panel so I opted to mount the threshold switches and output trimmers/pots under the meter boards....
Awesome..

Can I ask you which knob did you use for this?

 

[scott2000]

Can I ask you which knob did you use for this?

For the rotary board Lorlins?
These. Cool that there is no set screw. And they are impressively snug.

https://www.electronicsurplus.com/3...MI8aqon4nq8wIVE4dbCh344gqJEAQYAyABEgJNhvD_BwE
Can't remember what knobs were used for the output trimmers. Just some I had. Probably from Love My Switches.

 

[Heikki]

do these Lorlin exist with round profile?

CK1059 can be used instead of CK1054. There should be a round replacement for CK1034 too.

 

[manulaudic]

CK1059 can be used instead of CK1054. There should be a round replacement for CK1034 too.

I should have checked that before...

I'll find the proper knobs, thanks everyone

 

[Heikki]

CK1039 should be round shaft replacement fot CK1034

 

[living sounds]

Is the compressor capable of over-the-top compression (including distortion) like the Sta-Level?

Also, would it be possible to build a non-mastering stepped version that otherwise works like the pot version in terms of levels and GR selection?

 

[Heikki]

It depends on how much compression is over the top. For me often more than 1 or 2 dB of compression is over the top but the compressor can do 20 dB of compression fairly clean.

The functionality is pretty much the same whether pot or rotary sidechain is used. If the use is not mastering I wouldn’t use the 0.5 dB step volume control. For most uses potentiometer or a more coarse stepped attenuator will do fine for the volume control