Danish Radio stereo line amp

[lauritz]

Hi All

I'm working on 2 pieces of vintage Stereo line amps (it says "line adjustment amplifier" on the PCB) from the danish broadcasting company (DR). I have some difficulties understanding exacty how they work though. Looking at a single unit, the 2 channels seem to be linked in some way...but i do not understand what goes on so maybe someone here can help me. I have drawn up a simple schematic which is attached. Hope i did'nt make too many mistakes...

Also i'll post a couple of pictures of the unit. Beyer tranformers on the input and danish JS transformers at the output. In between there are 3 IC's pr channel and some caps and resistors. There are 4 trimpots that can be tweaked with a screwdriver from the frontpanel. Freely translated they are labelled (Recording 1, 2 and playback 1, 2)

There are 4 wires going from the board (besides the input, output and power wires) that are labelled L In, R in, L out, R out. I think that a stereopot is supposed to be attached for volume regulation, but i'm not quite sure how to...because i don't understand all aspects of the layout.

I have tried with a 2.2kOhm and a 10kOhm stereopot...different pin combinations, but i don't think any of them worked as intended. The units pass sound and the frequency response is good (flat curve from 20-20kHz), but the way i attached external potentiometer, the inside trimpot seems to affect both channels....are they supposed to do that?

Insights are greatly appreciated

best wishes
Lauritz, Copenhagen

 

[lauritz]

Top view

 

[lauritz]

and the PCB

 

[lauritz]

Been looking some more at the schematic...i understand the input stage: Transformer->inverting opamp

....then comes the trimpots....the first trimpot section has the swiper labelled R(out). Then the next trimpot -that sits between the last two opamps, has the label L(In) on one of the pins. It seems like the two channels are suppose to be linked (with a pot?) to get attenuation or something? Can't really figure it out.

Hope somene can chip in with a bit of help thanks in advance.

best wishes
Lauritz

 

[lauritz]

Oops...found a mistake in the rough schematic in first post...i've done a quick one i KICAD now and attached in original post (no component values though), so maybe it is easier to see now. Hope someone can help

best
Lauritz

 

[lauritz]

Updated schematic with values...still need help

 

[lauritz]

Still have'nt figured this one out. Would be a great help if somebody would give their thoughts on the schematic. Is it maybe supposed to be hooked up to a tape-deck or what is the purpose of the in/out taps on each channel?

Help is greatly appreciated

best wishes
Lauritz

 

[Johnblue]

Looks like an "insert" of some kind.

I think the R out is an unbalanced output feeding a tape deck and L in is the return from the unbalanced return from the tapedeck

So maybe just ignore them? Or wire them so you can use the unbalanced inputs/outputs for different sound or aux outs.

I'm pretty sure it is NOT gonna be a potentiometer though, But looks like an insert to me.  Try getting and putting signals on the ins and outs respectively

Maybe wire it as an insert with switched jacks so you can insert an unbalanced equalizer or compressor or something.
.

 

[lauritz]

Looks like an "insert" of some kind.

I think the R out is an unbalanced output feeding a tape deck and L in is the return from the unbalanced return from the tapedeck

So maybe just ignore them? Or wire them so you can use the unbalanced inputs/outputs for different sound or aux outs.

I'm pretty sure it is NOT gonna be a potentiometer though, But looks like an insert to me.  Try getting and putting signals on the ins and outs respectively

Maybe wire it as an insert with switched jacks so you can insert an unbalanced equalizer or compressor or something.
.

Thanks for the reply. I will try and hook it up again, but as far as i remember the unit did'nt pass sound without a load on the insert wires, but i have to check it again. Another thing that puzzles me is that the channel 1 has "L out" and "R in", and channel 2 has "R out" and "L in"....so they seem to be linked in some way? But i must say i don't fully understand the circuit around the two opamps to the far right in the schematic, but maybe they hold the answer....

 

[Johnblue]

Maybe Balance pot? 

 

[Michael Tibes]

To me it looks like a line amp - surprise 
I haven't tried to entirely comprehend the schematic because the pin numbering of the 5534 throws me off - they are obviously 8 pin devices, but your numbering goes beyond 10. So the first opamp seems to be a line receiver. I don't know if these input transformers are somewhat special because the circuit looks unusual. I'd expect a resistor between the transformer and the inverting input of the opamp. After that, there are 2 opamps feeding the output transformer. One of them would be inverting, so that the transformer is fed with a balanced signal in order to achive a higher or more beefy output.
You could revisit your tracing and try to create a more accurate schematic or you could just hook it up and see what happens. My guess is that RV1 sets the gain and RV2 the balance of the driver circuit. If I understand things correctly it should be set to the maximum output level.

All of that is guesswork, but maybe it helps.

Michael

 

[lauritz]

To me it looks like a line amp - surprise 
I haven't tried to entirely comprehend the schematic because the pin numbering of the 5534 throws me off - they are obviously 8 pin devices, but your numbering goes beyond 10. So the first opamp seems to be a line receiver. I don't know if these input transformers are somewhat special because the circuit looks unusual. I'd expect a resistor between the transformer and the inverting input of the opamp. After that, there are 2 opamps feeding the output transformer. One of them would be inverting, so that the transformer is fed with a balanced signal in order to achive a higher or more beefy output.
You could revisit your tracing and try to create a more accurate schematic or you could just hook it up and see what happens. My guess is that RV1 sets the gain and RV2 the balance of the driver circuit. If I understand things correctly it should be set to the maximum output level.

All of that is guesswork, but maybe it helps.

Michael

Thanks for chipping in. Yes, i realise i've been a little sloppy with the schematic, but i'm still very new to KICAD....my apologies ;-) The numbering does'nt mean anything. For each opamp there are the +/- inputs, the output and the powerrail inputs. I think the modules where costum-made for the danish broadcasting company, so it is impossible to find any information.

When i get to the studio/lab later today i wil try and hook it up again, and also double check the tracing

//Lauritz

 

[lauritz]

Maybe Balance pot?

Yes, maybe....to adjust mono/stereo imaging. That may be good suggestion, but then the middle leg of the potentiometer would have to go to ground reference i guess? I try that too....

 

[lauritz]

Thanks for the help ...it got the unit to work yesterday. Must have been half asleep last couple of times i worked on it because it was'nt a big issue.

First i tried using the the "insert" taps with a balancing pot....did'nt work.

Then i tried to short the pins and it worked as it should...i think anyway. Frequency response is great -see attached picture (mains hum not representative since the box was'nt closed by then ). I tried putting in a load as well -different pots, 10k, 22k and 220k. No matter which one i used volume seem to drop around 4 db at full load, so maybe the correct pot to put in is one with a little less resistance.

I then decided to just short the wires and adjust to unity gain for most precision. I turned the RV2 (internat adjustable pot) all the way up (volume) and RV1 all the way down. Then turned up RV1 till i reached unity. Don't know if it is the best way to adjust, but as long as the frequency response is good....maybe noise floor can be lowered by adjusting in another?

I have installed the amps in an old rack that came with the modules. Also put in a couple of pot-modules(not attached) and a phase correlation meter(no attached). Maybe i'll try to do a circuit for the meter at one point. Right now it's fine. It's on the masterbus for now and sounds pretty cool

Below i'll post a picture of the unit.

best wishes

 

[lauritz]

And the unit....

 

[lauritz]

Frequency response....sine sweep.