Introduction & First Project

[Perm0]

Some time ago I stumbled uppon this forum by accident, and I got hooked  8)

I have this "thing" lying somewhere on a shelf somewhere what once used to be an Ortofon STL 631 stereo high frequecy limmiter used (and probably modified) in the early days of Dutch FM stereo broadcasting. It appeared to have had several severe beatings somewhere in between then and the present day.

Seeing what you guys do out here inspired me to take the thing apart, clean off the several layers of paintjobs, rust and loads of dirt and have a look what we have got here, well, lucky me, all the good bits seem to be still there.

One Frame and front panel
One Power Transformer 200V 100mA rms & 6.3V 2.5 A CT
One Siemens rectifier B250 c 100 b / F1114
Two Audio transformers labelled ST3450
Two Audio transformers labelled ST3451
One Audio transformer labelled ST322
One Audio transformer labelled ST288
One single coil filter labelled ST3452

6 pots 1000 Ohm Covern ltd wirewound YL
2 pots 500 Ohm Covern ltd wirewound YL

The Sifam VU units  you see in the picture is not the original pair, but these I found somewhere and they did fit the holes in the front panel, the original pair I still have but they have broken displays.
Then I have some tube sockets en some pertinax wire boards for P2P building, there is also a stepped attenuater with two decks (channels)

So why start this thread ?

Because I don't exactly know what to do with this stuff, "you have a studio don't you ? maybe you can do something with this thing" that's how I got it... I have a home studio setup that got a bit out off hand and is now occupying half of my apartment and I work as a freelance audio / video engineer and do some installation work (lots of wiring) on broadcast systems. I have build some tube amp hifi stuff, simple designs, P2P, great sound, I also built my own nearfield monitoring speakers and a lot of hifi speakers.
But I am a novice in electronics. (simple schematics and a box of components usually end up something that looks strange but does work)

What is a wise thing to do with this stuff ?, restore the original design http://www.etec.dk/foto/ortofon/STL631_1-Schematic-StereoValveTrebbleLimiter.jpg and sell it on E bay and buy myself an island where i can sit on the beach drinking martini's all day  ;D
For me that restauration part would be somewhat too much of a challeng I think and I have no need for a high frequency limmiter.

What I would like to have is a nice stereo bus compressor so I have been going trough the meta and found a lot of stuff and even more 404's ... so far the PRR vari mu design got my attention but I think that might not be the most suiteble design for the parts that I now have, so all suggestions of designs are welcome as long as it is simple.
The front panel you see in the picture is in a mock up state, no holes driled, just some stuff I have lying around on there.
Next week I can have acces to some test equipment so I can meassure the transformers ratio's.

What would you guys do with this stuff ?

 

[PRR]

> maybe you can do something with this thing

FM broadcast transmission uses a steep treble pre-emphasis, scaled for older music. When you apply modern pop/rock music or bright compressed speech, the transmitter "splatters". A similar problem with older movie film-sound lead to "de-essers"; this is the same except adapted to FM.

In this case, program audio goes directly though, except with a passive notch-filter which is normally shorted-out by ST3550 and the tube near it. If excess high-frequency is detected (the other tubes form a peak filter and rectifier), the notch filter dips the highs.

Since current digital recordings use less or no pre-emphasis, the only apparent use is de-essing. But it is not really tuned for that specific (and fussy) chore. There's probably a protools plug-in to do that now, with more/better "knobs".

As for re-purposing.... none of the transformers really has to pass full-range audio, only treble. ST3450 does have full-range input applied to it, so it may not suck bass; the others may be many octaves short of what we want for general audio.

IMHO, you should have left it "working" and run audio through it, see what came through each stage, stock and with the bandpass filter (R19-R25 etc) thoughtfully converted to full-range. But it's a puzzle which I would not care to work on.

Is a nice case.

But nice cases don't make good music.

 

[Perm0]

Hi PRR, thanks for your reply !

> maybe you can do something with this thing

FM broadcast transmission uses a steep treble pre-emphasis, scaled for older music. When you apply modern pop/rock music or bright compressed speech, the transmitter "splatters". A similar problem with older movie film-sound lead to "de-essers"; this is the same except adapted to FM.

In this case, program audio goes directly though, except with a passive notch-filter which is normally shorted-out by ST3550 and the tube near it. If excess high-frequency is detected (the other tubes form a peak filter and rectifier), the notch filter dips the highs.

Since current digital recordings use less or no pre-emphasis, the only apparent use is de-essing. But it is not really tuned for that specific (and fussy) chore. There's probably a protools plug-in to do that now, with more/better "knobs".

Exactly my thougts, but in better words. As I stated before I have no need for a high frequency limmiter / stereo de-esser, since I am using modern production techniques and I do not have my own record cutting machine (original design by Ortofon was build for this purpose)

As for re-purposing.... none of the transformers really has to pass full-range audio, only treble. ST3450 does have full-range input applied to it, so it may not suck bass; the others may be many octaves short of what we want for general audio.

Ah ok, and I was hoping that I had just hit the jackpot with all thast transformers in it  ;D
What I will do somewhere next week is to hook the transformers up to the input / output of my RME fireface and give them a frequecy sweep and meassure the audio responce in EaseRa.

IMHO, you should have left it "working" and run audio through it, see what came through each stage, stock and with the bandpass filter (R19-R25 etc) thoughtfully converted to full-range. But it's a puzzle which I would not care to work on.

That would have been a nice thing, but the state that this unit was in when it reached me was far from what you see now on the picture (what's lkeft of it though) No way I would have hooked this thing up on the mains supply ! To get it in a state where I could start troubleshooting on this thing was the real problem, so after doing some research I decided to take the thing apart because it was never going to function in the state that it was in. To get rid of all the dirt and rust I had to take the frame apart and to do that I had to cut some wires. Several other people had obviously been soldering around in this thing, it was far from original and totaly busted when it reached me.

Is a nice case.

Yes, it is now and it will be a lot nicer when I am finnished with this thing.

But nice cases don't make good music.

Exactly !

So that's why I will be using thuis case as a donor for some DIY stuff.
I was thinking about your PRR Vari-Mu design, I read a lot of enthousiastic reviews of this design and the basics are simple enough for me to build one, so that is my plan so far. But I would like to use as many of the old components as I can, to keep the buget low and to stay in the DIY spirit.
First I will have a closer look on the transformers, what have I got ?
And next I was planning to restore the old powersupply (with some fresh elco's) that would probably give me something like 125V,... I noticed that your vari-mu design uses a 100V power supply, the 12AU7 can handle up to something like 300V... is it a problem for the design to run on a 125V instead of a 100V ? or would that be of too much influence on the vari-mu functioning ?

So far this project has cost me a six pack of Heineken and a small can of industrial grey machine paint  8)

Kind regards, PermO

 

[Perm0]

Today I did some measurements on some audiotransformers with EaseRa (Fireface800) and two pairs of clip wires.

(The big picture; http://oi54.tinypic.com/2w2m146.jpg)

When I had to ajust level settings to compensate for ratio I also took a ref. measurement with that.
The ref file is just the two pairs of clip wires.
ST-3451 config A and Config A REF. show the amount of gain increase'decrease in the setup.

The measurement on the HQ groundloop isolator was not done here, a friend of mine did it with his Fireface and send me the file, he used a different set of in/outputs then me because I have only easy acces to the frontpanel and used the headphone out for that reason.

This week I will buy a groundloop isolator and do a measurement with this setup here.

I scored some 12au7 and 12ax7 at the army dump store and I'v got the opamp for the feedback circuit, it will have it's own powersupply and the feedbackcircuit will be build on a piece of printboard, I choose to do the tube output stage, it's easyer to adapt with the P2P plans that I have for this frame.

More thinking to do, and buy me one of these cheap groundloopisolators for testing.

 

[Perm0]

OK, that picture turned out bigger then I thought it would be  ???

At least it gives a clear vieuw on the measurements  8)

 

[gemini86]

you're only measuring up to 1.2khz?
:-\

 

[Perm0]

No, I was measuring 0 HZ - 40.000Hz. but I zoomed in on the "problem area"

Here is the complete picture;

 

[Perm0]

And here is a presentation including ST322 and ST288;

Now what's that  ???

ST288 and ST322 are not present in the original schematics I don't know what I have to think of this results / measurement not ok for this transformer ?  

 

[Perm0]

Now here's something that might explain that "typical sound" of these old transformers...

I used ST3451 confic A as it had the best frequencyresponce and now let's have a look at the THD.

 

[abechap024]

Sweet looking case!

Sometimes transformers that don't pass the full frequency range can be really useful, just as long as you know what the do pass and when to use them.

I would say that case is dying for 2 channels of nice vari-mu compression.



I would say use the transformers and then if you find that your lacking anything buy some full frequency ones...

but as far as I can tell from your graphs they look pretty decent....

 

[Perm0]

The plan so far is to go for a version of PRR's vari-mu compressor, I searched for some schematics and did some copy pasting in paint.
This is the result;

Does this make any sense ?

The tube output stage will be easyer to adapt to the frame, the opamp feedback loop will be on a small printboard.
I could use "Baco cheapo trafo" from the measurements as a sidechainfilter sort of thing on that, maybe a bypass switch ?
External side chain input would be nice too.
In use it would probably sit on my unballanced masterbus insertpoints, these are pre-fader (so you don't take your music out of compression during fade-outs) so I would like some form of input attenuation to create some headroom, the abillity to give it a push when I want to without having to set up a new mix.
I have some Lorlin switches  that I will put resistors on for the attack / release controls.
Also scored some cheap GE 12au7 from a dumpstore  http://www.hifitubes.nl/weblog/wp-content/general-electric-12au7.pdf
And I have a Sovtek 12ax7LPS for the output stage.
The Jensen power transformer that came with the frame has the option to use 6.3 CT (12.6 in series) is there any advantage in doing that instead of a normal 6.3V heater supply ?
Resistors and some caps I have plenty, I ordered a small print transformer and some stuff do to a +15 -15V powersupply for the opamp, I was thinking about using a 1.5V AA battery for the ofset.

Does this make any sense ?

 

[abechap024]

For the 1.5v bias, you might want to experiment with an LED, many of the exhibit a voltage dorp around 1.5v just connect that in series with a current limiting resistor from one of the rails and your in business....and don't need to mix your master piece for 10 hours - finally get it perfect and then have the battery go out on you!

 

[Perm0]

Hi Abechap,

That might not be a bad idea, the guy who suggested the battery to me sayd it's a nice clean powersource and it will last for 10 jears or so.

Here's my plan for the opamp powersupply;

I'm just thinking out loud here, please tell me if I make stupid mistakes everyone !  :-*

 

[Perm0]

Updated schematics...

 

[gemini86]

This has worked with great success... no need to go over-engineer something if not needed, but you can still add in the regulation if you feel it's a must. Although I would go 317/337 so it can be dialed in with trimmer, since you're going to the trouble anyhow.

 

[Perm0]

Hi Gemini,

Thanks for your reply.
If I had a powertransformer like that I would have done it like that.
I have a 6.3 2.5A - 200V transformer, I also have the stuff to build a regulated supply for the tube output stage (that will do it no harm I guess)
I think I'll take the 100V ref. with a zener from that.
The problem was that one opamp, so I asked someone who knows a lot more of this stuff then I do to come up with a stand alone small powersupply for the opamp, this will be a small print transformer that will sit on a print with the regulator, opamp and the feedback circuit.
I will adapt the LED 1.5V supply on that board and drop the battery.
This board will then sit in place where the trimmerboard was placed, the rest of the design will then be p2p build, in th frame, I only have to change one tube socket to make this all work, so that seemed like a good idea to me.

I'm trying to adapt this design to the stuff that I already have as much as I can and fit it all in this box, and make it sound good enough to sit on the masterbuss.

I have little understanding of electronics, so any comments or suggestions are very much appreciated  8)

 

[Perm0]

Hi folks !

I rearranged some bones in a mountainbike accident (being stupid) so building stuff was out of the question for a while.
Some guy was so kind to put back my bones in the right places with some screws and metal plates  ;D

I'll start working on this soon and I will keep you guys posted  8)