LA3A step up transformer for controll path

jhaible

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Hi, I'm new here - forgive me if this has been covered before, and please point me to the right thread. A search on "LA3A" has just brought up a single hit with a short mention. I guess most people that go for the electroluminiscent type of opto compressors go straight for the LA2A - the step from using high voltage for the opto to also using high voltage for tubes isn't that big probably.
Anyway, I've made some experiments with a night light recently, and I plan to go for a transistor version a la LA3A, where a step up transformer is needed to drive the electroluminiscent foil.

So my question is: Does anybody have data for the original transformer? (A tapped inductor, actually)

So far, I've just made experiments with a 1W rated 100V ELA speaker transformer - no way to drive the capacitance of the foil with a flat frequency response.
And last night I've tried a O.E.P. Microphone transformer.
1+1:6.45+6.45, 150R primary impedance parallel wired,
secondary wired in series to get ca. 1:12 step up ratio.
The catch is that these transformers are really low power
(100mW @ 300Hz, 1mW (!) @ 30Hz), so it's probably the worst abuseto use them in a circuit like that.
However, it works remarkable well at high frequencies,
and just the low end has seriously distorted waveforms. (The latter
was to be expected.) But the night light glows just fine, and
distortion in the CV path may not be all that important.
I just fear I might destroy the little transformer.

Also, I have no idea what the step up ratio of the original transformer is.
The driver circuit is clever, as it has two feedback paths, pre and
post xformer, so a relative wide range of winding ratios maight be used without much altered closed loop gain. (?)
Anyway, knowing the original winding ratio (and power rating or core
size) would be nice.

Best Regards,

JH.
 

Mark Burnley

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Yes,

Welcome to The Lab Juergen!

I love your page- loads of great projects and information for synths and effects projects- well worth checking out if you haven't seen it :cool:

I know there was a discussion on the old forum about this transformer- did anyone get any measurements- I'm sure someone said they had access to an original and that they'd check it out?

I'm keen to have a go at this myself. I think someone mentioned using a 100v-line speaker transformer as an auto-transformer, but I see what you mean about the capacitance issue.

I'm sure we'll get to the bottom of this one!

:thumb:

Mark
 

gyraf

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The light panel drive in Urei BL40 (which has more or less the same circuit) IS a 8Ohm-to-100V transformer. But it is wired as an autotransformer, that is, pri and sec in series.

I think it's the autotransformer trick that makes the difference.

The ones that I have seen are more like 2-3W size transformers.

I don't think any of the OEP's will handle anything near these voltages.

Jakob E.
 

jhaible

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Thanks for the warm welcome!

Two people from synth-diy have pointed me to this forum, and what I've been reading so far is quite impressive!

My own experience is mostly in synthesizer circuits, and I think I have a lot to learn about pro audio processing gear, transformers etc.
When I built that 1176 clone a few years ago, that was just a happy accident (ok, I did some optimisation on the distortion levels of the
output transformer, but the transformer itself was just what I could easily get, an O.E.P. 1+1:1+1 150R). Now somewhere on this forum I read about either getting a clean sound with expensive transformers (paraphrasing) or "iron sound" with O.E.P.s.
So what I got on my 1176 clone is certainly the "iron sound" and I like it.
As I have this in my Stereo mix all the time, I want a few more compressors, mono, for individual tracks and instruments. A mono version of my 1176 clone must be made soon, but right now I want to use these electroluminiscent things to make an opto compressor. (Also to see if they differer from various Vactrol-based opto circuits.) I even got a black enclosure with relay socket from www.rs-components.de .
No idea about the right LDRs, though, to get a LA3A-like response. I know this from Vactrols, that the LDR response can make a huge difference, so it would be good to know more about the LDRs in the LA3A, too.
Also, is this right that there is one of 4 LDRs wired across the electroluminiscent foil in the LA3A (*not* in the LA2A) ? Some
auto-compression / protection measure in the CV path, maybe ??

JH.
 

gyraf

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Look here for a nice pic of a La3a T4b Opto..:

http://img56.photobucket.com/albums/v172/gyraf/T4b_05.jpg



Jakob E.
 

jhaible

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[quote author="gyraf"]The light panel drive in Urei BL40 (which has more or less the same circuit) IS a 8Ohm-to-100V transformer. But it is wired as an autotransformer, that is, pri and sec in series.

I think it's the autotransformer trick that makes the difference.

The ones that I have seen are more like 2-3W size transformers.

I don't think any of the OEP's will handle anything near these voltages.

Jakob E.[/quote]



Hi Jakob,

thanks for the info!

Let's try the math: 8R @ 2W means 4V, 100V/4V means 1:25 ratio.
8R @ 3W means 4.9V, or 1:20 ratio.
Auto transformer connection just means making 1:26 from 1:25,
doesn't it? And less power loss from stray inductnace - maybe
this makes the difference! (?)

Yes, I was more than surprised to see this OEP mic transformer
spitting out ca. 150Vpp - it's even pretty much a sine wave
above a few hundred Hz. Maybe it's much better than specified ??

I'll try the autotransformer connection next, give that 100V trafo
another chance.

JH.
 

jhaible

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[quote author="gyraf"]Look here for a nice pic of a La3a T4b Opto..:

http://img56.photobucket.com/albums/v172/gyraf/T4b_05.jpg

Jakob E.[/quote]

Nice! The colour looks familiar, too!
But shouldn't there be more than 2 LDRs ?

JH.
 

jhaible

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>[Yes, I was more than surprised to see this OEP mic transformer
>spitting out ca. 150Vpp - it's even pretty much a sine wave
>above a few hundred Hz. Maybe it's much better than specified ??

BTW, what is the nominal level of the control voltage across the foil, at full compression ?

JH.
 

chriss

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Hi Jürgen,

I had the same idea: Do the Audio-Path in tube-style, but the driver for the T4B with the transistor-circuit of the LA3A. Makes not much sense to do a functional-module like this with tubes these days, does it?
And I wondered if the frequency-response of this transformer matters, but I think it does, though not as much as it should be necessary to use high-quality audio-transformers here, am I wrong?
And depending the size: EL-foil is usually used with DC-to-high-voltage-convertors. Take a look at RS-Components 'Optoelektronik'-'Zubehör für LED-Anzeigen'. And these convertors use very tiny transformers to get up to 150 Volts!
I wonder if it is possible to use such a transformer for a LA3?!... I have one that I had to take out of the remote-control for my Roland R880-Reverb, because this made such an annoying high-freq.-noise....

regards

Chris
 

jhaible

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Hi Chriss,

there is a little misunderstanding: I plan to do everything with transistors, just like the LA3A. If I were using tubes in the audio path, I would certainly also use tubes in the CV path.

Step-up converters can use tiny transformers when they work at a rather high frequency. Covering the whole audio range is a lot more difficult.

No precision needed as in the audio path, though: You won't hear a little distortion from the CV path, as the LDR's tome constants will filter it out. But a drop in volume will surely alter the compression courve. If you have a 6dB drop at 5kHz in the CV path, a 5kHz signal will come thru much louder as it will be compressed later!

I was wondering if such effects are part of the original's charm, but I severly doubt it. I suppose they were aiming for a rather flat frequency response in the CV path, and succedded in doing so (?). But that's only a guess from my side, and I'd love to see it confirmed or corrected.

JH.


[quote author="chriss"]Hi Jürgen,

I had the same idea: Do the Audio-Path in tube-style, but the driver for the T4B with the transistor-circuit of the LA3A. Makes not much sense to do a functional-module like this with tubes these days, does it?
And I wondered if the frequency-response of this transformer matters, but I think it does, though not as much as it should be necessary to use high-quality audio-transformers here, am I wrong?
And depending the size: EL-foil is usually used with DC-to-high-voltage-convertors. Take a look at RS-Components 'Optoelektronik'-'Zubehör für LED-Anzeigen'. And these convertors use very tiny transformers to get up to 150 Volts!
I wonder if it is possible to use such a transformer for a LA3?!... I have one that I had to take out of the remote-control for my Roland R880-Reverb, because this made such an annoying high-freq.-noise....

regards

Chris[/quote]
 

chriss

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Ahh - I see,

but I like the idea of only haveing the tubes in the audio-path. I feel for the control-path it's the same like using tubes in the power-supply, but anyway this is a question of taste.

What I'd like to know is the two voltages coming out of the LA3As power-supply. Does anybody know them? Jürgen have you already built this? I'd like to use my workbench-supply first...

best regards

Chris
 

jhaible

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> but I like the idea of only haveing the tubes in the audio-path.

I also like tubes, but so far using tubes for me means also having a huge and humming power supply. I want my stuff quiet (speaking of mechanical hum, not what may get into the audio path), so I'm always using toroidal transformers. If I had a source of affordable toroidals with "typical" tube voltages, I'd build more tube stuff.


>I feel for the control-path it's the same like using tubes in the power->supply, but anyway this is a question of taste.

I think once you go into the trouble with all the PSU issues, a couple of extra tubes for the CV path will hardly make a difference. Especially if you don't need the LA3A's step up transformer anymore!


>What I'd like to know is the two voltages coming out of the LA3As power->supply. Does anybody know them? Jürgen have you already built this? >I'd like to use my workbench-supply first...

I've never had a real one in my hands. But the Schemos show a 27V zener at the base of Q12, so the stabilized supply voltage is expected to be around 26.3V . (Unless the put the zener in just for protection, setting an upper limit to a normally lower voltage, but this is very unlikely IMO.)
I don't know how big the voltage after the bridge rectifier is. Something between 35V and 45V I guess.

JH.
 

jhaible

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> Jürgen have you already built this?

No, just made some experiments so far.
And last night I built my own photo coupler, follow the link for first pictures:

http://www.oldcrows.net/~jhaible/opto_compressor/jh_opto_comp.html

(I'm using a 11-pin socket because I want 4 LDRs without any pre-wired connections.)

JH.
 

Kev

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Good stuff JH :thumb:

have you seen CJ's T4B page ?
Might be good for a different point of view.

Perhaps some pins ala 990 style Gain Blocks may be useful to set up a new pin arrangement standard for stereo optos and/or trimmable opto's

just a thought :roll:
 

jhaible

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LDRs are NORPS-12, bought from www.rs-components.de , order code 651-507.

Please note that I have no idea if these even roughly fit the time constants of the original LDRs or not. It's just try and see if it works. I don't even have a LA3A to compare. My criterium is if I like the end result, not if it's a perfect clone. I'm genarally using the word "clone" to give credit to what inspired me, not to make any claims of similarity. (I still don't know if my 1176 compressor clone is anything close to the original - I just know I like it a hundred times better than the cheap Behringer Composer I had bought some years ago.) I say this just in order not to raise false expectations.

But I'd like to know: What are the turns ratio of the LA3A's input- and output transformers?

And, are there any pictures of the interior of a LA3A on th eweb?

JH.
 

Kev

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[quote author="jhaible"]
And, are there any pictures of the interior of a LA3A on th eweb?[/quote]

it's not much but it's all I have ,
http://recording.org/users/kev/esoteric.htm
scroll down to LA3.
:roll:
I probably do have the original pictures on my drives somewhere.
Can't be sure if they offer any better resolution.
 

jhaible

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it's not much but it's all I have ,
http://recording.org/users/kev/esoteric.htm
scroll down to LA3.


Thank you!

JH.
 

Gus

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I have been thinking about a la2 with a poweramp IC sidechain like one of the TDAs or NS stuff driving a PA type line transformer.
 

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