Transformerless Vari Mu Compressor build thread

[JMan]

News on my build.

I added a vumeter driver circuit + vumeters scavenged from an old sansui receiver. For now the meters are only I/O meters, but I'm planning a mod on the driver meter to add gain reduction metering on the needles.

The panel is made by a guy in my city that does plaques/stamps. He has a CNC mill and a UV printer. I brought him my DXF and the panel from the TME rack chassis.

View attachment 93110

I set this build aside for some time while waiting for the last few components to make it through backorder hell, but now that I'm starting to think about it again, I'm interested in the idea of adding VU meters like this (for me, just I/O is fine). I've done some reading on VU driver circuits, as it's not something I've ever added to a build that didn't already have them, and I think I'm sort of understanding it. I'm curious if you would be willing to share how you implemented the meter circuit in this particular build?

 

[Heikki]

This is how I would do the VU meter driver circuit.
https://ghr.fi/proaudio/6L60/meter.pdf
Then with a switch I would select if I want to monitor the input or the output.

 

[JMan]

Thank you, that’s very helpful! This just takes the audio signal directly off of the xlrs, right? (In other words, it’s otherwise entirely separate from the rest of the unit’s circuit?)

Edit: regarding component values, I understand that the diodes should be fairly well matched with low forward voltage (e.g. germanium, schottky). Anything I should consider when choosing C1?

 

[paolomunarini]

I set this build aside for some time while waiting for the last few components to make it through backorder hell, but now that I'm starting to think about it again, I'm interested in the idea of adding VU meters like this (for me, just I/O is fine). I've done some reading on VU driver circuits, as it's not something I've ever added to a build that didn't already have them, and I think I'm sort of understanding it. I'm curious if you would be willing to share how you implemented the meter circuit in this particular build?

I'm currently abroad, so I don't have access to my schematics, but my idea was a diff amp plus a precision rectifier with a RC network on the out for ballistics. As Heikki said, add a switch to select between I/O levels.

 

[Heikki]

Thank you, that’s very helpful! This just takes the audio signal directly off of the xlrs, right? (In other words, it’s otherwise entirely separate from the rest of the unit’s circuit?)

Yep, just grab the signal right from the input or output. The circuit I posted or a percision rectifier doesn't need any exotic diodes or diode matching. Rm||R6 x C1 determines the return time. My circuit doesn't have that slow rise time of real VU meter but it should be doable with couple minor changes.

Rm = meter resistance

 

[JMan]

Rm||R6 x C1 determines the return time. My circuit doesn't have that slow rise time of real VU meter but it should be doable with couple minor changes.

Rm = meter resistance

So I take it that as the value of C1 increases, so does the return time?

Any suggestions for a value to try first? I don't have a great instinct for this.

Otherwise I think everything is super clear! Thank you so much for your help.

 

[Heikki]

So I take it that as the value of C1 increases, so does the return time?

Any suggestions for a value to try first? I don't have a great instinct for this.

Otherwise I think everything is super clear! Thank you so much for your help.

Actually C1 does also affects the rise time. I have a bunch 200 uA of meters which have internal resistance around 1k and are very fast, so with them I would probably use 330 uF cap to make them behave more like a real VU meter. Most 200 uA meters probably have internal resistance somewhere between 500 ohms and 5k and some are fast and jerky and some are slower, so cap value migh be somewhere between 4.7u and 4700u.

 

[JMan]

Most 200 uA meters probably have internal resistance somewhere between 200 ohms and 2k and some are fast and jerky and some are slower, so cap value migh be somewhere between 4.7u and 4700u.

Understood - in other words, there's no prescribed value that's going to predictably work for all/most meters. I'll try starting somewhere in the middle of the range (maybe 300u or 470u) and experiment from there. Thanks for walking me through this, I know it's a bit off topic!

 

[JMan]

I've been hard at work populating my boards. There have been so many interesting modifications discussed in this thread, many of which seem to have become a de facto part of the build despite being off-board mods. I'd like to make sure that the collection of mods that I plan to implement are going to work properly together.

I'm not building the mastering version, but I understand that some of the modifications are still worth using. Here's what I plan to do and how I understand it all to work...please let me know if this looks wrong in some way.

Gain:
-Replace the potentiometers with this stepped switch for coarse steps.

Sidechain (rotary boards):
-Use resistor values for 1dB steps on threshold control (from schematic, different from silkscreen on my boards which appears to be for 3dB steps)
-Add hi/lo threshold range switch (simply adds a switchable resistor between R9 and R16 -- this is shown with a DPDT switch but I think it could be done on an SPDT as well) -- I think this is most useful with the 1dB threshold steps
-Add switchable HPF at 60 (?) Hz -- I believe that this value is at least approximately correct based on the values in Heikki's sheet, right? (680 resistors, 2u2 caps)

I'm still unclear about what the main board changes around U9 and U10 do (I know it's been explained, I just didn't properly absorb the information). I think I'll keep that as stock unless it will adversely affect the other modifications above.

I am going to see about getting a matched quad of 6sk7 tubes (if possible) as well as a quad of 6ss7, to see if I get different results from either. Is the 6ss7 just a drop-in replacement in this context, or are other adjustments needed on the main board to accommodate them? It seems like they are a direct swap, but I don't want to assume that and fry something.

 

[JMan]

Am I correct in understanding that the HI-LO switch is in "HI" position when off (the added resistor is not in circuit) and "LO" when on (resistor is in circuit)? Or do I have this backwards?

 

[scott2000]

-- this is shown with a DPDT switch but I think it could be done on an SPDT as well)

Not sure about this

I think this is most useful with the 1dB threshold steps

Only useful with 1dB steps really

Add switchable HPF at 60 (?) Hz -- I believe that this value is at least approximately correct based on the values in Heikki's sheet, right? (680 resistors, 2u2 caps)

I think you have to also take into consideration the 330r resistors as well as the transformer's inductance. So an RL filter .. I believe it's 120hz in mine iirc... It's mentioned in here somewhere . Guess your transformer choice would affect this some?

I'm still unclear about what the main board changes around U9 and U10 do (I know it's been explained, I just didn't properly absorb the information). I think I'll keep that as stock unless it will adversely affect the other modifications above.

Pretty sure it's only useful for when you use the mastering input switches with the 21 steps .But can't remember. Something about being able to have unity at center position but idk. I really need to start messing around more.

Am I correct in understanding that the HI-LO switch is in "HI" position when off (the added resistor is not in circuit) and "LO" when on (resistor is in circuit)? Or do I have this backwards?

Hmmm....With the resistor in, you have more output before compression starts so...


I wonder if the output trimmers (I used 10turn pots) would be useful outside of the mastering version too. You can fine tune your levels. since your considering things to add.

Hopefully Heikki will jump in and clarify or correct anything.

Tubes were pretty easy to work with in regards to getting them to work well in this. I did try a couple that I think were just goofy and it made it difficult to get the speced results but even then, it was hard to notice. But I'm a fan of the Sylvania gt or gty for sure .

 

[Heikki]

Scott pretty much answered everything, but I would use the 1dB threshold steps always if you have the rotary version of the sidechain. I think it's useful with the coarse stepped gain too.

When the resistor is in circuit in the hi - lo switch higher output level is needed before the compression starts. Wire the switch which ever way makes more sense to you. With the 1dB steps I recommend always adding the hi - lo switch.

If I remember correctly the HPF is around 120 Hz. I must have mentioned it somewhere.

Changing U9 & 10 to INA2137 drops the gain -6dB

 

[JMan]

-- this is shown with a DPDT switch but I think it could be done on an SPDT as well)

Not sure about this

In fact, I think one could even get by with and SPST. Here's how I'm understanding it -- maybe I'm missing something, but I would think that this should work...



In this picture from Heikki's diagram, it seems that the switch's only function is to connect or disconnect the added resistor between R9 and R16. At minimum, that connection only requires two contacts, and a non-connection requires, well, zero. So presumably the following configuration would work just fine, although it might be less elegant in terms of mounting the resistor:



Not that this is an especially important detail of the build, obviously! But assuming I'm not missing something, perhaps this expands the options for folks who might have some single pole switches lying around and want to save a few bucks.

 

[JMan]

Also, thanks to you both for your replies to my rather long post above!

 

[JMan]

I'm thinking of using a tap off of the +15v rail of the power supply to feed some 12v relays. I was planning to just toss an LM7812 in between to regulate the voltage, along with the capacitors indicated in the attached circuit from the datasheet. Any reason that this would cause a problem? I'm not too clever when it comes to power circuitry, so while I don't see an issue, I figured I might as well ask the experts...

 

[paolomunarini]

I'd tap it off one of the 24V regulators before the dc-dc converters. Power draw from the dc-dc is limited to 100mA.

Another option (maybe better) would be using the 36V DC in and a LM317 with fixed resistor network (max Vin for the LM317 is 40V)

Third option: 24V relays and another LM7824

Last, more modern and efficient, a dedicated dc-dc converter to make the 12V

 

[JMan]

I'd tap it off one of the 24V regulators before the dc-dc converters. Power draw from the dc-dc is limited to 100mA.

Another option (maybe better) would be using the 36V DC in and a LM317 with fixed resistor network (max Vin for the LM317 is 40V)

Third option: 24V relays and another LM7824

Good call. I hadn't considered the limited current output of the dc-dc converter. I think I'll go the LM317 route as I've already got 12v relays on hand.

 

[paolomunarini]

Good call. I hadn't considered the limited current output of the dc-dc converter. I think I'll go the LM317 route as I've already got 12v relays on hand.

Account for an heatsink because you will be dropping 24V on that LM317. It will get hot very quickly.

Linear regulators sweat when they do big voltage drops

 

[JMan]

Account for an heatsink because you will be dropping 24V on that LM317. It will get hot very quickly.

Linear regulators sweat when they do big voltage drops

Indeed. That does seem like a really big voltage drop, though seemingly well within the capability of the lm317. I'm thinking I'll mount the regulator to the case as a heatsink, which I'm already doing with a couple of the others in this build.

 

[Heikki]

Not that this is an especially important detail of the build, obviously! But assuming I'm not missing something, perhaps this expands the options for folks who might have some single pole switches lying around and want to save a few bucks.

Your plan works too. I used double switches because it's convenient to hang resistors there.