PRR Vari-Mu help thread

[Matthew Jacobs]

Hi 6t9r

Dinesh (he's on this forum & that is his username as well) is the guy to contact. I know he was making another run due to demand.


J

 

[Johan]

small Q
I've built one of these before and love it...so I thought I'd build a second one now, but this time a dual mono with separate controlls and perhaps implement a ratio controll...

..with so many reports on the radioshack thingy not being what it was a few years ago ( lower inductance appearantly?..they are now sold here in sweden as 15k:15k instead of 10k:10k). could I just raise the impedance by raising the value of the input/grid resistors to compensate, perhaps from 3k3 to 6k8 or 8k2?..or would that mess with something else?..

the resistors I'm refering to are the two 3k3 marked R4 and R33 on the schematic in the link in the first post of this thread

I want to use these groundloop transformers for a number of reasons, among the being: I allready have a few, they fit the board and I like how the one I built before sounds..

any thoughts?
thank's
j

 

[Johan]

..answering my own question here...with the ground loop transformer I can get now, I had to increase the input resistors to 4k7 to not Loose all my low end( very noticable on kickdrum..). But almost more important. These “new” ground loop transformers MUST have a ca. 10k (or the two 4k7) resistor on their output or I get a  HF rise starting around 3700Hz . I guess that is transformer ringing, but I don’t have the equipment to confirm.
That means there also must be a resistor across the secondary of the second transformer, the one before the 5532 or the sound  will get an unpleasant high end…
I don’t know, but I suspect that all low price ground loop  transformer thingy’s come from the same maker and is only labelled different depending on who is selling them.  So if you, like me, work on a budget, I hope this was helpful…if you’re using much better iron, just ignore this post, ..it was not for you..
j

 

[MrZpliff]

I was thinking of using these:

http://www.bhiab.se/perl/index.cgi but decided to go for OEP's.

600-600 ohm LF-Trafo Linjetransformator
Pris: 36.25  SEK

 

[slowbass]

Hello,

After reading the original documentation + the threads about the prr vari-mu, I'm a bit confused with the transformers and the different options people use...
Is it ok to use the same 600:600 transformers for input and output ? 
For example, the edcor wsm ? (and BTW, what is the difference between edcor xsm and wsm ?)
Any other suggestions ? All the cheap transformers I can find seem to have a limited bandwith...

Thanks !

 

[Script]

Hi,

Is there anyone with a working unit willing to run a RMAA (Right Mark Audio Analyzer) analysis, share the results, and tell us what TXs they used?

Reading all the vari-mu threads, it seems that people use all different kinds of transformers (from recommended, to reasonable alternatives and esoteric ones with highly modded schematics). Well, I'd love to test the RadioShack transformers thingies, but I don't live in the States so I'm not sure I can order them (their online shop seems to require an address in the States, hmm).

So, I tried a couple of different real cheapo ones I could find (incl. Sansui), but probably not the right stuff, because they all produce pretty uneven frequency responses (severe bass cut below 100Hz to step fq rise above 2000Hz). Of course, sound is in the ear of the "be-listener", and a slight bass roll-off with an otherwise flat fq response is not necessarily a defect

Anyone willing to share? I'd really love to get my unit back to being used again

Cheers,

Script

 

[Johan]

So, I tried a couple of different real cheapo ones I could find (incl. Sansui), but probably not the right stuff, because they all produce pretty uneven frequency responses (severe bass cut below 100Hz to step fq rise above 2000Hz). Of course, sound is in the ear of the "be-listener", and a slight bass roll-off with an otherwise flat fq response is not necessarily a defect

..see my post, three posts back..
j

 

[Sleeper]

I don't have a rightmark and mine fits in to the category of highly modded with expensive Jensen transformers...
But then again the mods on mine cost about 2 dollars in parts and I needed to make 1 modification to Kent's board.
Those parts plus 2 edcor input transformers, 2 jensen interstage transformers, and 2 neve output stages (with 2 transformers) totaled about 302 dollars.  My labor was about the same as it would have been to make the stock Vari-mu.  My unit easily rivals and uses exactly the same gain reduction tube-stage as a UA175 (minus the tube sidechain amp) and since that unit is more than 8000 dollars used (at least through vintage king) I've rationalized my labor pretty well.  My point here is that PRR has come up with a very awesome platform here. He designed it with supercheap transformers because he's PRR and he wanted to show us that it could be done and done well on the cheap.  That's how PRR rolls. 
do yourself a favor and spend at least edcor money on your transformers. It's worth it, especially if the radioshack one's suck now.
Sleeper

p.s. I did A-B Edcors vs. the Jensens in the interstage and I could hear the difference well enough to go Jensen, but the edcors were still pretty good in the interstage slot.  Tried some utc and triad stuff on the input and it didn't make much difference there.  Edcors performed very well there.  Confirms what Bluebird did.

 

[sevenfive]

"Those parts plus 2 edcor input transformers, 2 jensen interstage transformers, and 2 neve output stages "

Hi Sleeper, which Jensen transformers did you use for the interstage?

 

[Matthew Jacobs]

These are the transformers I used:
http://www.amazon.co.uk/Skytronic-Ground-Loop-Isolator-201-086/dp/B000NVWB9O/ref=sr_1_1?ie=UTF8&s=electronics&qid=1260444887&sr=8-1

I guess they're the "new breed" of ground loop transformers. Hope they work. I soon as I have my unit built I will report on how they sound. I have decided to build this stock first (cause it's so cheap) and then mod it later. I'm also thinking of using the 6BC8 tube in my mod.

Hi Sleeper, which Jensen transformers did you use for the interstage?

+1 on that question?


J

 

[tweedclassic]

i have cinemage transformers in mine and the thing rocks. the goal was pure intimidation. and the thing is pretty friendly to the mix when i have a run away vocalist. 

 

[Sleeper]

Hi Sleeper, which Jensen transformers did you use for the interstage?

I used the Jensen JT-10KB-D
Model # Impedance Ratio  Turns Ratio  Max 20Hz Input  THD 20Hz  THD 1kHz  20Hz/20kHz

JT-10KB-D  40k:2.4k                 4:1                     +21dBu     0.015% <0.001%    -0.04dB/
The 6bc8 seems very happy with 40k to look at and this steps down the levels to the sidechain and my neve output stage to the point that I can actually use some of that neve gain for flavor.
Because of this I changed the gain resistor on the 5532
(the 5532 has to provide more sidechain gain than it did in PRRs version so  actually I did a tiny bit of photoshop surgery to the board before I etched it and made the gain resistor into a trimpot, so I could control how much sidechain gain I wanted as part of the calibration/listening tests... this has the added benefit of letting me calibrate Left and Right channels more accurately with less perfectly matched tubes.)

 

[Matthew Jacobs]

Hi sleeper

Thanks for the info on the TX

The 6bc8 seems very happy with 40k to look at

Did you connect the tube heaters in series? Is the board easy to mod? Did you also modify the PSU? I think the 6BC8 draws a little more current, doesn't it?

J

 

[Sleeper]

Hi Matthew,
Yeah I think I may have connected the heaters in series.
What made everything work so easily is that the PRR and the UA175 have the exact same B+ voltage...
I'm away from home at the moment. Ic ouldn't tell you much more in detail, but I've posted a lot in some of the other varimu and varimu mod threads.

 

[Script]

Hello,

Thanks, Johan.
I read your post, actually several times
Read it again, had a good night's sleep and tried different resistor values across the secondary of the isolated TX yesteray. Then I figured that it (a real cheeseburger-priced-but-doesn't-have-a-name-printed-on-it TX, and most probably an insult to the entire circuit may need a resistor across the primary. Tried it and and it flattened out the sharp fq rise that I had before (had used it at one point in tracking as a natural exciter -- haha, eh?). Test square wave (1000 Hz) in Visual Analyzer looks fairly good too (for that no-name thingy, that is). However, it also gives me a slight high fq roll-off starting at 12Khz to -2.5db at 20Khz (maybe not a bad thing for tracking) and an overall drop of 2.5db in volume as well, hmm. I'm learning here. Need to test with TX in circuit next, and might try a Zobel network (resistor plus capacitor) for better results...

Zobels are explained in this thread:  "Damping Transistor Ringing" (Zobels):
[http://www.groupdiy.com/index.php?topic=32441.0]
A true hardcore read, but very interesting. Not sure though it can be done with RMAA and Visual Analyzer only.

And thanks, Sleeper.
I read about your mod, as well as the "bluebird" one elsewhere, many times too Sounds very promising and I have had a look at the UA175/176 schematics already (mod scheduled for the future or second unit). And thanks for sharing the TX serial numbers, as I don't usually have TXs lying around. Too bad you don't have the free RMMA softo (WIN only, I think) for measuring

Happy so far, since the unit is off the shelf again

Script

 

[sevenfive]

Hi Sleeper, which Jensen transformers did you use for the interstage?

I used the Jensen JT-10KB-D
Model # Impedance Ratio  Turns Ratio  Max 20Hz Input  THD 20Hz  THD 1kHz  20Hz/20kHz

JT-10KB-D  40k:2.4k                 4:1                      +21dBu     0.015% <0.001%    -0.04dB/

Hi Sleeper, thanks for the information, it's much appreciated.

 

[skipwave]

I built up a channel of PRR VariMu from junk drawer bits, and all I can say is "Wow!" In keeping with the spirit of the project, I used only stuff I already had lying around and a few bits picked up at RattyShack. The input transformer is a vintage Triad JO-12, because it had a center-tapped secondary for the CV feed. I won't reveal the output transformer type just yet because I don't want the evilBay price to skyrocket before I try to win another.

Many thanks for PRR for contributing so much to this forum, and continuing to put up with the inane questions and incoherent ramblings of newbs and knows-enough-to-be-dangerous-types like myself.

Now I've got to try and scrape together enough pennies to build a stereo version for my mix bus.

Back to tapping out programed computer drums. Boo. At least I get to run them through my new compressor when their all manually pseudo-quantized.  ;D

 

[mhelin]

Regarding the 230V line power supplies is it OK to use a transformer with dual 115V primary and 6V secondary voltages (like this one: http://fi.farnell.com/multicomp/mcta015-06/transformer-15va-2-x-6v/dp/9530231 ) to get 230VAC and 2x12AC  secondary voltages when 230V line is connected one of the 115V primaries like in PRR's original PSU design?  Obviously it will work but is it safe (will the inductance of the primary winding be too low for 230VAC)? Also the 230V output has to be regulated to 100V or the input gain has to be increased by using 1:2 input transformer if needed, maybe not needed with +4 dBu levels?

 

[PRR]

> 230V line is connected one of the 115V primaries

NO! Do not connect higher voltage than the transformer is rated for. There's a range: you can put 125V on a "115V" winding, but NOT 230V on a "115V" winding. It will smoke.

Use two transformers-- 230V:24VCT(2*12V) and (backward) 24V:120V.

 

[mhelin]

> 230V line is connected one of the 115V primaries

NO! Do not connect higher voltage than the transformer is rated for. There's a range: you can put 125V on a "115V" winding, but NOT 230V on a "115V" winding. It will smoke.

Use two transformers-- 230V:24VCT(2*12V) and (backward) 24V:120V.

Thanks,

What differences excactly there are between say 115V:12V and 230V:24V mains transformers? At least the DC resistance I was looking for was the same for both. Anyway, I will propably get an AC wall wart with 12-15VAC output and get the opamp and heater power from that an add a small 2x115V:12V backwards for the anode voltages. For DIY projects that should be also safer than DIY wiring the mains supply.

Found these from Farnell:

FIN:
http://fi.farnell.com/ansmann/8161024/adaptor-mains-ac-ac-12vac/dp/1453783
http://fi.farnell.com/myrra/45015/transformer-3va-115-230v-2x-12v/dp/1777927

UK:
http://uk.farnell.com/ansmann/8161024/adaptor-mains-ac-ac-12vac/dp/1453783
http://uk.farnell.com/myrra/45015/transformer-3va-115-230v-2x-12v/dp/1777927

Guess the primaries and secondaries of the latter can be connected parallel (in phase obviously).