Modifying PRR Vari-Mu Compressor for +4dB Operation

Hello,

I found the PRR project quite interesting for mastering applications. Although, it is rated for -10dB signals.

Is it possible to change that to+4dB? I really would like that feature because I am dealing with +4dB equipment for mastering.

Is it really that hard to get Veroboards here in the U.S.? If so, where can I get those the underground way? :grin:

Thanks

Maybe an step down trafo at the input??? like 4:1 / 20k:1k ???

Lets wait and hear the professor. He always knows what to do. :thumb:

Seems to me a 4:1 at the input should do quite nicely. Or you could use a balanced pad.

This was more an easy-to-build proof-of-concept than a full-Pro idea.

Conversion from/to Pro levels, impedances, and standards is a trivial add-on detail that you can do any way you want.

The input will accept "+4" levels if you don't turn the source up to meter-banging. If you can't control the source, use a 10K:10K input transformer and a 10K pot in front of that. Or if you gild sows-ears, use an old-Pro type 600 ohm step attenuator and 600:600 input iron.

The output needs more muscle to drive true 600 ohm balanced loads at nominal +4dBm. There are many -10:+4 converter boxes. Or use two op-amps in any of several standard ways. Or add NY Dave's hollow-state line amp with a gain-pot between.

Well I'll move over to this thread. Odd I was thinking about the same things.

Regarding the tube output stage, there was a comment made that the output was weaker than the opamp. I suppose that makes the -10:4 conversion more difficult?

I have three other questions: would keeping the bias pot as a front panel control provide any interesting sound effect, or does it need to be set & left?

Also with respect to the +140V supply to the tube output stage, what modification should be made to the power supply?

What difference would 12AX7s make? I have a bunch of those laying around.

Many thanks for this project.

Regards

Jon

If you can stand eating two channels on your console just bus the signal out to the comp and then back in to the channels. I get plenty of control that way. I suppose it's not exactly the "purest" signal path but then I was never a purest. Dan Kennedy suggested a 4:1 tranny on the front end as well a while back to step it down but I haven't tried that. Honestly, the output is pretty hot so if the piece of gear you have after it has any gain make up at all it should work OK. That does make it a little difficult on an insert though.

cheers,
kent

> What difference would 12AX7s make? I have a bunch of those laying around.

Suck bad. Overload on a whisper. This ISN'T a straight amplifier: many things that are "good" in plain amplifiers are BAD for vari-gain operation.

12AU7 isn't expensive; that's why I picked it. It isn't an ideal tube for the chore, but is VERY available, both under that number, the older 6SN7, military numbers, and in larger derivatives like 12BH7.

If you must experiment: 6BQ7/6BC8/6BS8 is similar but -maybe- better, being intended for vari-gain duty in TV tuners. It isn't as dirt-common as 12AU7, but hardly rare, and 6BC8 is not at all fashionable. $5 at thetubestore.com or tubedepot.com The voltages are similar enough to 12AU7 that it should be possible to get it to work without radical change. And it may give less grunge in deep GR.

> the +140V supply to the tube output stage

Sorry. PS plan modified. Don't worry if it isn't exactly 140V, it isn't fussy.

*** My 12AU7 Limiter page ***

> bias pot as a front panel control provide any interesting sound effect

If you mean the balance pot between the cathodes: offsetting it will give a little even-order distortion and a LOT of thump. Unless you want thump (and it is usually a very bad sound), set R32 for minimum thump when going in and out of limiting, then leave it alone (unless you change the tube).

If you mean the trimmer near the output: that really needs to be set so with heavy gain reduction the output level is 2V RMS (2.8V peak). What you are really doing is setting the signal level in the 12AU7 plate circuit: too high will distort bad, and too low puts you in the noise. It does not have to be 2VRMS exact, but within say 3dB or 1.4V-2.8V RMS. And for Stereo, equal both channels.

Thanks for the help.

There's just one thing. I am an ABSOLUTE beginner. I have started messing around with electronics a month and a half ago

So, can I use a breadboard and jumper wires for this project? I know this seems a dumb question but can those jumper wires handle that type of voltage? I am thinking of the breadboard solution because I can mess up without the soldering.

Which sounds best for the +4dB output, the op-amps (and which one?) or a conversion box?

Thanks

Excellent! Thanks for the info. Looking at my collection of 12AX7s, I see that none of them match anyway, so it's off to shop for some vintage 12AU7s!

Regarding the pots, yeah I meant R32. It actually works better for me if only the attack & release pots are useful on the front panel, because I'm planning to use FunkLogic's wonderful Digilog Dynamicator and I wanted the third pot (the Digicator) as a treble bypass.

One last question: Should both sides of the tube on the output circuit be 12AU7s or 12AX7s? There's one of each there.

ok so ill use a 10K:10K input transformer and a 10K pot in front for the input

how about a 600:10k on the back (1:4) to add gain to the signal for 2buss applications? seems like a 10k OP impedence would be too high? not sure why though. i thought the rule of thumb was high imp input low imp output. (i.e. 10k in 600 ohm out)

thanks!!!!

that´s the standard, but if you are really only going to feed it to 10k or higher impedance inputs, a 1:4 600:10k at the output should work.

About breadboarding, go for it, no problem at all. Just make sure you do not get any shorts the high voltage supply. With some breadboards and beginner, yes, it should be a little dandgerous. Did you read about safety practices when dealing with HV tube circuits?

regards, and do not get hurt!

ok for my +4 attempt at a stereo vari-mu i will use..

edcor

XSM10K/10K INPUT (with 10K audio taper pot)

XSM600:600 OUTPUT (before the opamp)

XSM600:10K OUTPUT (after opamp to provide additional gain)


any reason why this wouldn't be a good idea? xfos are 13 a piece so any advice before the 80+ investment is appreciated.

I wouldn´t really use a 600:10k at the output. I mean, low impedance output is always a ggod thing. You never know where you will need to feed it to...

Add a 5532 on each channel. A little gain for convertion from -10 to +4. And one 5532 only can also give you balanced output with good drive.

well i guess i could add another 5532 after u3 but that would mean another board and power supply routing. an extra edcor seemed quick and dirty. as far as what it gets plugged into, it would only ever be my console (toft) or interface (ensemble) both are pretty high imp >10k.

Did anyone try doing what ebartlet suggested way back when?

http://www.groupdiy.com/index.php?topic=3438&postdays=0&postorder=asc&start=36

read down just a bit further from the link you posted. Svart did the +4 mod and liked it.
KM

Yeah, that was helpful. :roll: I did read that thread.

My take was that my question implied that I was asking if anyone else other than Svart tried it, like maybe even those asking about it in this thread...

PRR said:

The output needs more muscle to drive true 600 ohm balanced loads at nominal +4dBm. There are many -10:+4 converter boxes. Or use two op-amps in any of several standard ways. Or add NY Dave's hollow-state line amp with a gain-pot between.

Click to expand...

any links to the 'standard ways'?  i assume the goal is to  provide gain to +4db and low impedance?

I added a 5K 10turn trim pot in place of the 2.2k gain resistor on the board (referring to Kent's board)
I can get quite a lot more gain out of the 5532 I have in there right now.  I'm doing that because I'm driving a different tube that seems to like a little more sidechain oomph.  I'm not sure if I'm quite up to +4 but I think it's close. 
you could do that AND change the little voltage divider after the amp - monkey with the 1k threshold potentiometer and resistor so you can throw off a little more level as you head down into the sidechain.
That should work. 

I'm also finding that the trim pot on the 5532 lets me adjust a tiny bit more for tube imbalance Left to Right.

great idea! thanks