PRR Vari-mu

[living sounds]

The PRR Vari Mu was a project about building the cheapest possible vari-mu compressor. In it's basic incarnation it doesn't do massive gain reduction and has unbalanced connections. It can still sound pretty cool.

The PRR 176 was a different project, that used some of the ideas of the PRR Vari Mu but also included circuitry from the classic UA 176 compressor. You could build it with lot's of transformers and discrete op amps.

https://bottlegardenstudio.com/notes-on-the-prr-176-stereo-variable-mu-compressor/
I've build both, and they're both cool and useful compressors.

 

[living sounds]

Here are the build docs for Rev 4. I would recommend to build that one, if you have the PCBs for it.

 

[living sounds]

And here's the BOM.

 

[SIXTYNINER]

The PRR Vari Mu was a project about building the cheapest possible vari-mu compressor. In it's basic incarnation it doesn't do massive gain reduction and has unbalanced connections. It can still sound pretty cool.

The PRR 176 was a different project, that used some of the ideas of the PRR Vari Mu but also included circuitry from the classic UA 176 compressor. You could build it with lot's of transformers and discrete op amps.

https://bottlegardenstudio.com/notes-on-the-prr-176-stereo-variable-mu-compressor/
I've build both, and they're both cool and useful compressors.

Much thanks for the info , the docs , and the links !!!
(unfortunately pcbs i found on my diy corner are the "V1")

the strange thing is that the guy of the web page called "12Au7"
seems that do not had issues in bulding it (AKA "V1")
unlike the various problems described in various pages of this thread ,

so only the member "PRR" can give the required support
for complete a working unit "V1" ?

 

[living sounds]

PRR created the PRR Vari Mu, but not the PRR 176, which was created by abechap024.

I think you've got the PRR Vari-Mu. As far as I know there was only one version, and it is a clever but comparably simple circuit.

Here's what I've found on my computer.

 

[living sounds]

Everything else should be in this thread.

 

[Script]

Have the PRR176 boards and most of the components (including several sets of tubes), but I don't have the latest REV. And that's why I never sat down to actually build it. Any version prior to REV4 always read on this forum like being a PITA to build, as there seem to be various issues with the REVs like missing traces, component placement, noise, poor schematics, God-knows-what else (probably due too much excitement paired with lack of quality control at the time of shooting out those boards too quickly). So until this very day, I don't know how different it really sounds from PRR's take.

---
Built the original PRR VariMu. As 'living sounds' said, it was meant to use cheap and even leftover parts we all usually find in our drawers. No frills, proof of concept kind of thing, just basically functioning. The most expensive parts are the four(six) Xfmrs, two(four) tubes, and the PSU, oh and the case. -- I fully enjoyed building it because I (mis-)used it as a learning platform. Reading any Vari-Mu schematic I could get hold of, as well as IC based stuff for the sidechain etc etc, mine was a cable spaghetti bowl for almost a decade -- always in use, but half of the time on the bench, in a state of being changed, or having things added, or things being auditioned just to be dismissed. Now finally finished, the majority of components are off-board or on daughter boards etc. -- If you intend to go down that road, better start with a huge case. I started small and inside is tight as a rat's earhole.

Also, the biggest trouble I had with the PRR VM unit was that it was always in different states of being somewhat noisy, which gave me a lot of headache. So I read a lot and ended up trying many different things, before finally the unit quieted down sufficiently. It has to do with mixed grounds and the unit being single-ended (internally unbalanced), I think. -- Either way, the PRR PSU works (over here now) but it's not great. So my advice, if you plan on building the unit, is to separate your grounds and best ditch the PRR PSU entirely and maybe go straight for the PRR176 REV4 PSU.

 

[SIXTYNINER]

Have the PRR176 boards and most of the components (including several sets of tubes), but I don't have the latest REV. And that's why I never sat down to actually build it. Any version prior to REV4 always read on this forum like being a PITA to build, as there seem to be various issues with the REVs like missing traces, component placement, noise, poor schematics, God-knows-what else (probably due too much excitement paired with lack of quality control at the time of shooting out those boards too quickly). So until this very day, I don't know how different it really sounds from PRR's take.

---
Built the original PRR VariMu. As 'living sounds' said, it was meant to use cheap and even leftover parts we all usually find in our drawers. No frills, proof of concept kind of thing, just basically functioning. The most expensive parts are the four(six) Xfmrs, two(four) tubes, and the PSU, oh and the case. -- I fully enjoyed building it because I (mis-)used it as a learning platform. Reading any Vari-Mu schematic I could get hold of, as well as IC based stuff for the sidechain etc etc, mine was a cable salad bowl for almost a decade -- always in use, but half of the time on the bench, in a state of being changed, or having things added, or things being auditioned just to be dismissed. Now finally finished, the majority of components are off-board or on daughter boards etc. -- If you intend to go down that road, better start with a huge case. I started small and inside is tight as a rat's earhole.

Also, the biggest trouble I had with the PRR VM unit was that it was always in different states of being somewhat noisy, which gave me a lot of headache. So I read a lot and ended up trying many different things, before finally the unit quieted down sufficiently. It has to do with mixed grounds and the unit being single-ended (internally unbalanced), I think. -- Either way, the PRR PSU works (over here now) but it's not great. So my advice, if you plan on building the unit, is to separate your grounds and best ditch the PRR PSU entirely and maybe go straight for the PRR176 REV4 PSU.

Everything else should be in this thread.

Very much thanks Living Sounds and Scripts for all the infos and docs !!!

so checked about 3 threads about PRR Vari Mu
(this one , that about the V4 -176 , and another )
unfortunately much confusion on all... , because various customized versions ,
apart that about the V4-176 , that have bom and other docs like the 1 to 7
also posted above ,

unfortunately the pcbs i found are not the V4 but the V1 (check attached image)
but there is any chance to know info/specs about the parts required like :
audio transformers type
tubes type
pots type
vu meter
psu transformers from 220/230V to the required voltages
wirings
any other more as well ...

or is the other attached image of pcb with parts values from the original pdf
the reference about ?
more thanks.

 

[living sounds]

Very much thanks Living Sounds and Scripts for all the infos and docs !!!

so checked about 3 threads about PRR Vari Mu
(this one , that about the V4 -176 , and another )
unfortunately much confusion on all... , because various customized versions ,
apart that about the V4-176 , that have bom and other docs like the 1 to 7
also posted above ,

unfortunately the pcbs i found are not the V4 but the V1 (check attached image)
but there is any chance to know info/specs about the parts required like :
audio transformers type
tubes type
pots type
vu meter
psu transformers from 220/230V to the required voltages
wirings
any other more as well ...

or is the other attached image of pcb with parts values from the original pdf
the reference about ?
more thanks.

Again, these are two very different units. What you call V1 is the PRR Vari Mu, whereas the other is the PRR176. You can forget any docs, BOM etc. for the PRR176 since it does not apply.

Here is more info:

https://groupdiy.com/threads/prr-vari-mu-help-thread.33625/https://web.archive.org/web/20040330042729/http://music-club.rutgers.edu/headfonz/comp5/Comp5.htmlhttps://groupdiy.com/threads/prr-vari-mu-compressor.1164/https://groupdiy.com/threads/spaghetti-bowl-prr-vari-mu.43587/

 

[SIXTYNINER]

Again, these are two very different units. What you call V1 is the PRR Vari Mu, whereas the other is the PRR176. You can forget any docs, BOM etc. for the PRR176 since it does not apply.

Here is more info:

https://groupdiy.com/threads/prr-vari-mu-help-thread.33625/https://web.archive.org/web/20040330042729/http://music-club.rutgers.edu/headfonz/comp5/Comp5.htmlhttps://groupdiy.com/threads/prr-vari-mu-compressor.1164/https://groupdiy.com/threads/spaghetti-bowl-prr-vari-mu.43587/

Thanks again for info !!
and also for the links , that have already checked .

So , if not wrong ,
the PRR Vari Mu V1 have same pcbs as the project called "12AU7" ,
first version , "lovely" single side pcb , few parts,
defined "easy" to build by various .. ,
unfortunately less documented : ( ,

then there are V2 , V3 (...somewhere i found V3 do not exist..)
but no sense to build ,
as seems the V4 is better ,
and the most documented V4 by abechap024 ,
but no more pcbs about ,
as also probably the most important doc called :
PRR_REV4_BUILD_MANUAL2.pdf
(anyone have this doc ?)
at the first post of the 32 pages thread at this link :
https://groupdiy.com/threads/prr-176-rev-4-build-thread-manual-up-1-16-14-chk-1st-post.54563/
about the V1 pcbs are them trash
because no build support ?

 

[Script]

What you call V1, the PRR VariMu, is in no way related to PRR176 (REV 1, REV2, REV3?, REV4). The only thing they have in common is the sidechain, used at the time by PRR (Paul Rutger, I think) and maybe the tube B+ voltage. Print out the schematics and compare.

The PRR '12AU7' VariMu (-- what you called V1 --) is easy to build. Back then, I was able to do it with hardly any background in electronics. There's a PCB, an overlay, and also an easy enough schematics somewhere. Not much else needed, really. There are some pitfalls, possibly, but threads are not many and not long. And quite a few people here built it.

Only thing I'd build differently today is the PSU, as already mentioned, and probably swap order of ATT and REL. Might still do.

 

[SIXTYNINER]

What you call V1, the PRR VariMu, is in no way related to PRR176 (REV 1, REV2, REV3?, REV4). The only thing they have in common is the sidechain, used at the time by PRR (Paul Rutger, I think) and maybe the tube B+ voltage. Print out the schematics and compare.

The PRR '12AU7' VariMu (-- what you called V1 --) is easy to build. Back then, I was able to do it with hardly any background in electronics. There's a PCB, an overlay, and also an easy enough schematics somewhere. Not much else needed, really. There are some pitfalls, possibly, but threads are not many and not long. And quite a few people here built it.

Only thing I'd build differently today is the PSU, as already mentioned, and probably swap order of ATT and REL. Might still do.

Again thanks Script !!!

As posted above i found the old V1
reason why here to ask about)
because seems easy to build , single side pcb , not much parts ,
but what audio transformers , potentiometers , wirings ,
and other required for a working unit ?

also checking (and post) on the V4 thread for all required docs and info ,
but seems that also there not all required is easy to get ,
mostly for broken links ,
unfortunately ... : (

 

[Script]

PRR VariMu -- Not a BOM though

tubes type

2x12AU7 -- preferably used -- to keep it in the spirit of the project

vu meter

Cheap cookie-cutter 100µA work perfectly fine on mine. Changed or added a resistor on the PCB for this though, IIRC. You'll have to experiment a bit. Before that I used old 'leftover' VUs from an old Sony tape deck, worked fine but ultimately didn't fit on the faceplate. The metres are not there for precision anyway, it's a soft knee unit after all, they're just indicators that 'something' is happening.

psu transformers from 220/230V to the required voltages

You in 'infinity' country ? PSU as had been envisioned (the power PCB) is for 100/110 mains only !! That circuit with mounted 2x110V/2x12V tranny hacks one of the two primary windings to derive 100V B+. This works in 100V/110V countries ONLY, like Japan, but not ideal even here. Anyway, better built a proper PSU for this (look at PRR176). If you have some unused 2x110V/2x12V around, use it by all means (just run wires to PCB, but be careful and know what you do !!).

For 220V country you need either a dedicated multi-winding power tranny (expensive) or at least two trannies. For a quick, near-stock build first, could plug that power PSU as is intended into a 200 to 100 step-down converter. That's what I did when I used my unit with 220V/230V mains plugs.

wirings

Look at overlay. Almost self-explanatory. You have the schematics ? Notice that wiring between ATT and REL is off-board, IIRC.

any other more as well ...

- for 1uf caps poly.
- resistors can all be 1/4W -- although 22K and 1K on 100V B+ better be higher specc'ed (1/2W or more).
- audio TXs, for schematic and PCB as is, requires four (2xIN, 2xINTER). You have any audio TX sitting around unused `? Why not try those first.

(If you get circuit basically running, might consider a proper balanced output stage (i.e. another 2xTXs or any IC-based output driver solution.)

 

[fazeka]

Hi all,

Many years ago I built a PRR vari-mu (I guess we're calling that V1 now?!?) with PCBs based on kent's layout. It used the Radio Shack transformers. I eventually moved it on at the time as I had moved on to other designs and builds.

15 years later this thread has sparked my interest. I have a quad of UTC O-30 "ouncers" that I am keen to use in a new point-to-point version but am uncertain if they will be adequate despite not quite being 600:600 (they are 500:500). What would be the consequence in using these lower impedance transformers in the PRR vari-mu?

Thanks,
Chris