PRR Vari-Mu help thread

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MrZpliff

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Jun 15, 2005
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401
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As Dinesh has had a few boards made and this comp probably will get built by some more people...
Couldn't we use this to thread to gather all sorts of information and questions concerning the PRR Vari-mu ?
There is the META, but I think it would be nice to have it all in one place.

For example: What meter works for you. What audio transformers do you like. What power transformers...

I'm really looking forward to start building this.

If this thread is a bad idea for some reason, mods please delete it  :)

Here's the comp:
http://www.conditionedresponse.com/DIY/C5/index.html


 
... and here is an error corrected:

There is a corrected pdf somewhere, but I can't find it...

C5-err.gif
 
Is there a part-number, or generic size designation, for the power transformer that Kent used in the layout?
 
> same ...part you called out on your page PRR.

images


Sorry to be so thick.

Digi-Key Part Number 237-1078-ND  $16.22
Manufacturer Triad Magnetics
Manufacturer Part Number VPP24-1250

 
PRR said:
> same ...part you called out on your page PRR.

Digi-Key Part Number 237-1078-ND  $16.22
Manufacturer Triad Magnetics
Manufacturer Part Number VPP24-1250

That one doesn't work for 230V primary...
I'm going to try one 230/100V that I found in my junkbox and a 230-12-12V.
Like this:
PRR_PSU_TRAFO.jpg

Should work, or ?
 
Just wondering what the total current draw of one unit is - I'm making two stereo units in one box and I was wondering if the total would exceed 24VA.

I've got one unit running allready, using bbc ll/76msc 600:600 iron and JAN sylania 6189 tubes and I'm really enjoying the sound.  Subtle compression, but overall really adds something to the mix.  I tried some Brimar 12au7s as well but it seemed to lose something in the highs.

I'm intending to try Lundahl 1517s for the iron in the second unit.

Josh
 
Hi,

I posted a couple of questions on stereo rigging on another varimu thread, before I realized this one existed.

http://www.groupdiy.com/index.php?topic=111.100

Thanks!
 
I'm glad this thread was created.

I just got my boards from Dinesh and I know I will need some help on this one.

What I have gathered so far from the various posts I have read about this build, I hear that the choice of tube and transformer really affects how this compressor sounds, especially at high GR.

Jwk1, it's nice to know that you are getting good results with the 6189. Was it a straight swap or did you have to modify the circuit?

I, myself am thinking of modifying the input and output stages to include 2 extra front panel controls for input and output gain (a little bit like the UA 175b I'm also building) and to have balanced inputs/outputs. Still working on how to do this, but will post my findings as soon as I come up with something.

I'm wondering what others are doing about transformers and tubes.

J
 
MrZpliff, I'm no expert, but I'm not to sure if your transformer hook up will work. The 100V one looks fine, but the 12V one looks wrong. I might be wrong but I don't know how you're gonna get +16/-16 from a 12V transformer. Also, it looks like your center tap needs to be connected to earth. The way you have it connected at the moment looks like they are out of phase.

But I'm new to all this, some maybe someone else can check.

Bless

J
 
Hi everyone,

This is a quote from PRR (May 2003 - http://headfonz.rutgers.edu/comp5/Comp5.html):

"You adjust the amount of compression by how much signal you push into the compressor."


Now, I was thinking of putting a line amp in the same box as the PRR comp to give it balanced +4 input and to control the amount of compression by being able to boost or attenuate the input signal.

Check out the following picture for how the idea would be implemented on the front panel.

I would also put a line output amp to control output level.

Does anyone have any advise? should I go IC or Valve for my line amps? Should I use some sort of transformer for the interstage?
Any pointers to some line amp schematics or designs?

I would much appreciate your help.

J

Front-Panel-idea.jpg

 
If it makes any difference to you guys, wouldn't it be cheaper to just use an existing preamp from your rig as a front end. I figure the impedance from the output of the preamp would be 600 ohms. Don't know if this jibes well with Bluebird's mod to put a 10K:10K as the input transformer, but something tells me it would work.

Regarding the interstage transformer, my shoddy research suggests that you replace the plate follower resistor with an interstage transformer. How that applies to the unit in question (PRR Vari Mu) is what befuddles me.

Regarding the input and output gain, I'm happy with just the concept of a -18db pad on the input before the transformer and one on the output after the transformer. What might be worth looking into is installing a volume pot on the pathway to the primary of the interstage transformer so you can mess around with how hot that transformer gets hit.

The line amp is a good idea, would you put it before the 600:600 output transformer? Sounds like a sexy place to put it. I'd almost be game to suggest a much more arcane solution to address this concern.  ;D

What if we created a crude open aka insert loop with a 150ohm Rshunt? I'm thinking that if there's no dangerous voltage traveling along the signal path after the tubes and the interstage transformer, that we could simply create a gap in the circuit whereas the signal leaves the system and then can be routed back in. If the signal would be stepped down after the interstage transformer to 600 ohms, then we could attach the secondary of the interstage transformers to a pair of XLR outputs aka the "insert loop outs" with a 150 ohms Rshunt inline. Then what the end user could do is take the signal (at 150 ohms/mic level) and feed it into a preamp of their choice to bump up the level and then take the output of that preamp (at 600 ohms) and feed it back into the compressor at the "insert loop ins." Since the output transformer is 600:600 ohms then it should be rather compatible.

Unless there's some issues with dangerous voltages, I'd say that this would cost us maybe 7 bucks in parts? If someone doesn't want to use the preamp method for making up gain after the interstage transformer, then if we put the Rshunt on a switch that can bypass it at the insert loop out, then the end user can just patch the insert loop with two XLR to 1/4" cables and the impedance will be at 600 ohms going into the output transformer.

For more suggestive madness.

Seeing as how Bluebird suggested the interstage transformer as 15K:600 ohm, this would actually allow us to use the mighty Cinemag CM-9600T aka the Ampex 351 block transformer clone. These are the transformers used in the JCF 8 channel D/A with all those tubes and 8 of the Cinemag CM-9600Ts.

They are rather pricey I think just under 90 a piece, but two of them combined with some crazy tube action going on around them along with a way to really feed signal to that output transformer makes for some sick compression!

Right now I've got my scheme like this for parts.

Transformers:
Edcor XSM 10K:10K (2) Input transformers
Cinemag CM-9600T 15K:600 (2) Interstage transformers
Cinemag CMOQ2S 600:600 (2) Steel core Output transformers.

EH 12AU7 (2)
3RU case to accommodate the Cinemag CM-9600Ts.
Dinesh PCB and I will probably go for the biggest knobs that will fit on the face of the case!
Everything else like the power transformer is pretty wrote and I'll probably go for decent Xichon caps etc but not go crazy with manufacturers, just what makes the most economic sense.

If you really want to go even nuttier, I'm certain that some OEP 10K:10K or 600:600 on the inputs would be pretty interesting since they have a colored sound that some folks prefer in the 1176, while others like the Cinemags.

The Edcors seem to be the order of the day though and since the ones on the input are really high headroom, I figure they make the most sense, otherwise I would go for something with more nickel to get a clearer input out of these things. However if the insert loop holds water then whatever preamp you insert in the loop offers more flavor to the mix too!

Design wise, Im leaning towards having a 5"X5" square cut in the middle of the face of the case so that you can see the tubes. I found a place that sells these really cool 6"X6" brass mesh tiles for 8 bucks plus shipping and I think mounting this on the inside of the case where the hole is would be a nice play off of the older guitar amps and tube hifi gear where you could see the tubes lit up when the unit is on, it would probably help with ventilation immensely. As far as metering, I kind of don't see the need. This is a unit that you'll use by ear anyways.

I know that PRR will always suggest the cheaper options first and he is right. My only reason for even thinking of using that Cinemag CM-9600T is because I have been searching for the right project to use it in. It just seems this project is perfect for it!

Hope this adds some fodder for brainstorming and discussion!
Bluebird, where you at??

Peace
Illumination
 
Yes!!! Nice!!!

During mixdown I always have a bunch of pre-amps sitting around not doing much... In fact my latest craze is sticking my G9 on the master of pretty much everything. I've recently used it on the master of a Innovason digital desk at a live event and it sounded great. Lots of harmonic sweetness going on there... nothing like digital...

Driving this comp with a pre-amp is a good idea and the insert you suggest is good. I can imagine the crazy tones coming from that setup...

A part of me would still like everything contained in one box. This would allow me to take the compressor around to various locations and setups. I guess as I'm building 2 of these, I could have one of each.

I have dropped the idea of a valve line stage, as I want this comp to remain cheep and cheerful. I am now looking at using the Hamptone JFET gain stage for my input and output amps. I think the combination of JFET, Vary-Mu and solid state rectified sidechain will sound very interesting and still remain cheap. If we come up with a solution using external pre-amps, I can always substitute them with internal ones based on the Hamptone JFETs. I'll have some schematics ready in a few days...

Thanks Illunination for your suggestions. Are you gonna use the 12AU7 for this. I though there was a better tube around that could work with this design?

J
 
Well if Bluebird can chime in with how things are getting placed in the circuit along with any changes he made to voltages to work better with different tubes then we might be getting somewhere.

Realistically, I always hate using expensive preamp designs as utility pieces.

I would hate to be the guy that has API preamps inside a compressor but no api preamps to track drums or vocals with LOL.

I think for your purposes if you want to install the JFET pre or line stage in your unit, the sky is the limit.

I figured that putting a crude insert which requires 2 TS jacks and 2 XLR jacks is probably a hell of alot cheaper and less time consuming (we do want to have the time to use this stuff right?? LOL) than building another piece of gear that you have to trouble shoot, recal and also make fit inside a case.

I've seen inserts on tube preamps before, so I have no problem believing that an insert on the PRR Vari Mu wouldn't work for boosting the signal.

I ain't the greatest with schematics or reading them, but I do have a good imagination lol.

I just want to see how crappy the compressor sounds with the EH12AU7. They might not be that bad ya know. I have heard that people complained about the some 12AU7s not giving them a good high end on the processed material. Some guys aspire to use this compressor during mastering I'd wager and that's why they are nitpicky about freq response. The interstage transformer should address some issues with low end freq response, this may actually be what was causing problems in other builds. If the low end comes thru too strong during compression then the source can come back duller due to the low end dominating the highs and people call it muddy.

I have been sitting here the whole time, pondering if we should be using something like EF86 instead of 12AU7? However PRR knows his stuff. Don't forget that there are different 12AU7 all over. We only need two of them! It might be worth a shot to get some Amperex or Telefunken 12AU7 (didn't Kent use Telefunkens eventually?) and see how they sound. It would possibly give you what you're looking for and free you from further tweaking power supply values and the circuit etc.

One other thing that's been bouncing around in my head is that I would love it if we could have a sidechain filter in this thing, like the SSL compressor and the Manley Vari Mu. It would be so cool if its possible for us to tap into the detection section of the circuit (if there is one!) and hook up something like a sidechain with passive filters. Maybe a 80 hz 120hz 180hz with capacitors (and resistors?) on a 3 position rotary switch, along with a short throw switch to bypass the sidechain. This way the compression action would bring up the mids and highs alot more since it wouldn't trigger on the bass as much. At least this is what I'm thinking.  ;D

Peace
Illumination
 
Just a quick update for the weekend.

I changed my choice of transformers for the input and the interstage. I spoke with Dave at Cinemag and he recommended a different transformer for the interstage and the input.

So now my inputs will be the Cinemag CMLI-1515B and the interstage transformer will be the Cinemag CM-9589L (Nickel/steel). The output transformer will still be the Cinemag CMOQ2S.
He recommended a different interstage transformer because the CM-9600T has feedback winding and the 9589L would be a better fit. It has a slightly different ratio than the 15K:600 transformer that Blackbird advised, instead it is a 9600 ohm: 600 ohm. Dave said it should still be pretty good as a 4:1, instead of a 5:1. He said this should be a pretty colored and yet fast sounding unit if the tube section is done right. He also confirmed that the interstage transformer goes where the plate follower resistor goes. So thats two folks addressing it in that manner!

PRR can you confirm the way we'd add the interstage transformer? Anything to look out for?

Hope we keep things going!

Peace
Illumination
 
Yo bro... I like your ideas...
I'll look into the sidechain filter thing. I had a look at the schematic and it looks quite easy to do.
How we can mod the board is another matter...

The insert thing I best leave that for you to sort out... cause I haven't gotten my head around it yet. Would you be able to post some diagrams of how you would put inserts on the PCB board?

BTW, it does not need to be a JFET line stage. Any line amp will do. I just couldn't find any other schematics is all my searches. Does anybody have any? I was thinking, a circuit based on the 5532 would be cool, that way we could just use the same power supply. All I'm looking for is basically any line amp with adjustable gains. I was thinking of using Gyrafs GSSL input/outputs design for Balanced/Unbalanced connections. can this design provide any gain as well?

Personally I think I'll be using this compressor on sub-groups and possibly more for character than accuracy, so if the only drawback of the 12AU7 is reduced top end then I can live with that.

I like your design ideas, I've had many people comment that they would like to be able to see the tubes. I've been thinking of using LEDs to illuminate the inside. This could look really cool with the valves glowing as well... NICE!!!!

I'm also wondering where exactly in the circuit do we put the interstage TX? Sorry, I just can't work out what exactly is the plate follower resistor...

J

 
You took the led thing right out of my head lol. I kind of let it go because I decided to put more holes in the face of the unit.

You know like a pattern of holes in various shapes and have that grille material behind it. If you do the singular hole you might have adjust how you orient the pbc to see the tubes. Multiple holes in the face with the grille would definately let you see the tubes. The LEDs would be pretty sweet too!

Since you can use pretty much any type of heavy gauge grille material, I was thinking that if we had something that we could paint on, then it would be pretty sick if we could have like a 3RU black case and then have the mesh be a blood red like those Crowley and Tripp microphones! Then if you had red LEDS inside the case to illuminate it even further that would be rather insane!

Until PRR chimes in about how the unit detects, we can't sort out how to install a sidechain. You could always just eq the source material with a low shelf cut at the desired frequency and then boost it back up post compression. It probably wouldn't sound the same as a true side chain but it would lower the compressors reaction to bassy material.


And once we sort out where the interstage transformer goes, then that's a good step in the right direction for finding a point to tap the insert. As long as no lethal voltages are passing at that point then an insert of some kind would be pretty simple to implement.

We just need the other folks to chime in that know what they did or what they'd recommend!

Peace
Illumination
 
Hi guys,
I do not know about the mods of the board you are acvtually using (is it kents pcb?  if yes it's pretty original)....but I just thought about all those things you are actually planning to change and want to ask some questions...and thoughts

Wouldn't it be easier to first use a simple way of retrofitting it to +4db balanced in-out operation?
I did not built prr's comp yet but reading his original notes, he said the input can be handled full floating (don't connect in- to ground, xlr/trs ground pin to chassis). Then you have a balanced in at ~-10db. Assuming you feed it with +4db pro level gear you may just pad the signal passively down stepped before the tx_in with a rotary switch and some resistors to have a compression control.
For the output you may just use something like a mini-pcb containing a THAT balanced line driver + opamp on a very small pice of perfboard after the single line out and make -10unbal /+4bal operation switchable at the backplane maybe? Must have a readily made perfbord layout somewhere on the harddrive i made some time ago....I can post if there is interest. I think it's Rob's circuit originally (btw he sells readily assembled nice mini xlr-module if I recall it right).
Just thought that would fit the bill nicely, keeps it clean, simple and elegant and doesn't cost much...?

Kind regards,
Martin
 
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