What "type" is this old BBC compressor??

[API]

Hi.
I got this BBC UNI 24 Compressor unit in a rack with other BBC made gear along time ago.
Is finally getting round to get this stuff up and running.
So my plan was to recap this module as it does not sound to good at the moment, the output volume is very low.

But I also was curious about the circuit since it does not look like any compressor circuit i have ever seen before.
It seems there is a diode bridge in there, and it also seems like its two channels (I thought it was mono though) with two diode bridges and 3 pairs of trafos (2 in and 1 out??).
Also has some nice Mullard OC71 germanium transistors!

Does anyone know something about it or could make an educated guess??

Thanks,
Dennis

 

[ciminosound]

I see transformers? Not familiar with diode bridge? It would be in the sidechain. Works like the 6AL5 tubes found in Altecs. Diode bridge in Fairchild 666 and many other compressors. The "two channels" would prob be audio thru and sidechain. Nice Mullard OC71 germanium transistors! Prob laid out like a tube compressor topology but with transistors. Can you draw out the circuit for us?

 

[NewYorkDave]

You're thinking of a control voltage rectifier. A diode bridge--or any other bridge with elements of variable conductance--can also be used as a signal attenuator, and therefore accomplish gain reduction.

 

[rodabod]

I'm looking for docs. for this just now.

Seemingly hard to find. I think it might be best using it for its transformers as it was probably built for a specific application, but I suppose a full description or schematic would help to decide.

UN is simply short for "Unit" whereas a standard limiter might have been an AM "amplifier", etc.

 

[API]

Hello.

Thanks for the thoughts and info!

Will try to trace out the schematic later this week, should not be to hard, even though I have never done it before!

Rodabob, would be great if you could find any docs for this!!
I also have a BBC Audio Limiter Mk 2, any chance for some info about this??
Uses the same inductor type of delay as the AM6/7 and AM6/17.
Cool unit!

Regards,
Dennis

 

[AnalogPackrat]

If you pop the board out, post a pic of the bottom of the board and we can help with the trace. Looks like a fairly simple circuit and the transistor pins are labeled, too.

A P

 

[API]

Will do tomorrow!

Dennis

 

[rodabod]

[quote author="API"]
I also have a BBC Audio Limiter Mk 2, any chance for some info about this??
[/quote]

Does it have a code? Eg. AM5, AM6, AM7, etc?

 

[API]

No, that is the weird thing.
It just says Audio Limiter Mk 2 on the front panel.
Can post a pic of it later tonight as well.

Dennis

 

[API]

Hi again,

Here is a pic of the BBC Audio Limiter Mk 2:
Does it ring any bell Rodabod?

Here is a pic of the inside (sort of):

Beside it is my old PYE compressor that i am trying to get to work!
Did a full recap but stillno signal, so i will have to dig deeper!

Here is another pic of the back side of the circuit board for the UN1/24 compressor unit.
Will start to draw out the schematic tonight.

Dennis

 

[AnalogPackrat]

Dennis,

Just wanted to let you know that I've started tracing out this mystery unit. pretty interesting. There are two identical sections each with a pair of diode bridges with transformer coupling. I'm guessing this is the sidechain detector stuff. I haven't gotten to the amp part yet. Out of curiosity--are there labels on the connector? Any markings on the transfomers?

I'll post a sketch when I finish it...maybe in a 6-8 hours (after I get home and can finish the trace).

Cheers,
A P

 

[API]

Hello AP.

I also started to try to trace the circuit yesterday, but only finished the diode bridges.
Will also try to complete tomorrow, then we can compare.

Regarding the wires, BBC had a standard in/out/power layout on their connectors.
Power is 9 and 10, input 5 and 6, and output 13 and 14.
I am holding my thumb over the input, 5 and 6 on the connector, on the backside picture.
Hope that helps.

Dennis

 

[AnalogPackrat]

OK, here's what I got. Kinda messy--need to flip-flop some sections to make it easier to follow. This has a negative supply if I'm not mistaken...PNPs, right?

A P's Trace

A P

 

[gyraf]

..at a glance, it looks much more like a clipper than like a limiter..

Jakob E.

 

[rodabod]

Ok, I had a chat with one of the older guys yesterday - he was confused when I said that the audio limiter mark 2 had no code-name.... He suggested it might not actually be BBC. Does it say it anywhere on it?

As for the UN1, I got a set of service manual books and will let you know what I find. As I said before, being a UNit, I think it may be for a specific application which could potentially be...... telephones!!

 

[API]

hi again.

Please scroll up for a pic of the Audio limiter Mark 2.
It is 100% BBC made, has the inductor board, white front and all.
It could not be that it is a Audio limiter Mk 2, ie AM/2??

Let me know if you find out anything about the UNIT, and what it coul be have made for!!

Regards,
Dennis

 

[rodabod]

Strange, I can't find any data for the "Audio Limiter". Closest I could find was an AM6/13 which is a limiting amplifier which is designed to be plugged into a desk. I could mail you a description sheet if you want it.

As for the UN*, all I can find are other UN* models, all of which as I said before are for specific applications - eg. interfacing telephone lines, etc.

If you can get model numbers for the transformers then I probably have data sheets for them. They are most likely repeater coils.

Roddy

 

[leigh]

[quote author="AnalogPackrat"]OK, here's what I got. Kinda messy--need to flip-flop some sections to make it easier to follow. This has a negative supply if I'm not mistaken...PNPs, right?

A P's Trace

A P[/quote]

Anybody ever get further with a trace or analysis of this circuit?

I'm curious but confused. It looks like the bottom 2 diode bridges (in the schematic) are being variably-biased by the V-, the biasing level being set by S1.

The very bottom diode bridge - would that be passing signal? It's not symmetric with the bridge above it (presence of C4; absence of R5), so I'm guessing not.

Leigh


EDIT: Right, diode biasing, as in a 33609 and others.

And the transistor section to the right is makeup gain, yes? Can't be sidechain-related since it's not DC coupled. But still not clear on the function of the "extra" diode bridges...

 

[rodabod]

UN1/24:

Compression unit used in AM1/6.

AM1/6: Constant volume speech amplifier for automatically balancing speech levels.

 

[Lee_M]

Hi guys,

I recently picked up a BBC rack containing a UN1/23 compressor module and an AM6/1 limiting amplifier, along with a few other modules (distribution amp, bandpass filter, PSU).
I've tracked down the BBC tech documents (with circuit description and schematics) for these modules from the BBC engineering site and am looking into mods for this comp (adding switches for ratio, attack & release and a pot or rotary switch for variable makeup gain) as it currently only has an input "range" (threshold) control.

If I'm correctly understanding what I've read, It seems that the AM6/1 amplifies the signal from the UN1/23 compressor unit back up to a reasonable level, While also catching (and suppressing) any sharp transients that slip through the UN1/23.
I'm guessing, But not having the AM6/1 limiting amplifier module may be why the OP was getting a low level output.

Has anyone else got any experience with these units or any suggestions or tips on modifying them?

Also, If anyone can help me out with any info on BBC LL/85ASC, LL/76M, LL/85A/CF or LL/87SU transformers it would be much appreciated.

Thanks.