Gain mismatch on Audio + Design Stereo Limiter - can I adjust gain internally?

[Farringdon]

Hi I've got one channel louder than the other one on my Audio Design F601-RS stereo limiter, which as you can imagine is a problem for stereo limiting. 

Does anyone know if it's possible to adjust the gain internally so that the two channels are balanced in volume again?

Thanks in advance.

(note: After replacing some capacitors the gain went up on that channel, but it's still quieter than the "good/louder" channel).

 

[SSLtech]

Oh lordy...

Those things are not at ALL simple to adjust. They're not as predictable or linear as VCA based units, so the gain-reduction tracking is invariably all over the place between the two channels.

It's NOT going to be a single adjustment, you have to go through a COMPLETE adjustment/calibration, and BOTH channels will likely sound different afterwards.

I do like them, but as mono units, since those and the Compex are the devil's own job to set up... and then they drift with age...

Keith

 

[fazer]

ADR had pretty good Alignment Instructions in the manual.  I had a vocal Stressor Years ago that would need adjusted.  I always followed the manual.  I guess it is a 760 compressor.

 

[Farringdon]

I see... so this is not an unusual problem then

Any of you had volume/gain balancing problems because of bad capacitors?  I'm just worried that the volume won't stay the same even if I manage to adjust it, but it can't be that inconsistent if all the radio stations in the UK used it?

 

[Marcocet]

Anybody have a link to the manual? I'm also looking desperately for one for the ADR express limiter

 

[Farringdon]

I have a manual for the F601-RS somewhere.  Would that be helpful?

Will look for it.

 

[Marcocet]

Anything would help! Thanks

 

[Farringdon]

Try this for manual

http://www.sendspace.com/file/57sm4m

 

[Farringdon]

Does this (below) look like the right instructions for adjusting the two channel's gain so that they match?  Or do I need to do more?

Sorry I'm a bit of a newbie, still learning.  Channel 2 is a a couple DBs quieter than Channel 1 now so it's not a huge adjustment.

 

[fazer]

I looked at the Manual you provide.  The VCA adjust is a possible problem in cal but you need to start on page 17 and follow to the end.. The adjustments are interactive on my S01 comp, so you need to go through all adjustments  otherwise you could fix the balance Left to Right but have the limiting gain reduction act differently from left to right.    Start on page 17.  And follow to end.  Also ADR uses Dbm values for some of the adjustments.  Keep a Calculator handy with a log conversion to change to a voltage reading.  (Just in case your meter does not support Db readings).  Make sure you have a dual trace scope as well as the meter.  You need the scope for Symmetry adjustments.  The Distortion Analyzer is to confirm the quality of the signal.  I think you can fix your problem without it.

 

[Farringdon]

Thanks for the advice.  I'm new to this though, what equipment do I need to make these adjustments?   At the moment I don't have any "testing" equipment (i.e. meters), though I do have a calculator

 

[fazer]

you need a 2ch oscilloscope (says 16 meg or more bandwidth in your manual) a oscillator with sinewaves, and a volt meter or dvm .  Also a distortion Analyzer but you can do what you want to do without it.  You may want to find a tech to do this if you have not done it before.  The test gear you need is possible to do with a usb scope plugged into a computer.  There are no high voltages to worry about like tube gear would have.

 

[SSLtech]

but it can't be that inconsistent if all the radio stations in the UK used it?

I don't know where you heard that "all the radio stations in the US used it"... but it's blatantly untrue.

Yes however, they were most certainly prone to drift.

Keith

 

[Rob Flinn]

but it can't be that inconsistent if all the radio stations in the UK used it?

I don't know where you heard that "all the radio stations in the US used it"... but it's blatantly untrue.

Yes however, they were most certainly prone to drift.

Keith

I think the ADR Gemini Easyrider was a bit more common for broadcast, but has little in common with the F601 or F760, since it's full of op amps & chips.

 

[SSLtech]

Darn... I just noticed that I typed "US" when I really meant "UK".

The Gemini was what we used on the New Order "Brotherhood" album; three stereo pairs of them in a multi-band arrangement, as a matter of fact.

As the name suggests, the two channels were 'twinned' and tracked MUCH better than FET-based Compex type units. -I want to say that they might have been Blackmer-cell type VCA-based units, which are of course inherently more predictable in terms of gain behaviour, though I can't say that with certainty.

Keith

 

[Farringdon]

Do any of you guys think the F601-RS might be better used as two mono limiters for tracking and mixing?  The internal adjustments sounds like it might be too much hassle.

If it tends to "drift" then I'm not too confident about using it as a stereo mix buss limiter unless there are some gain matching tricks I don't know about...

SSLtech - the one I bought was from a broadcast station. I guess I was misinformed (not by the seller though) when I was told that it was a popular limiter for broadcasting in the 80s.

ps:  I also have a Compex 2 (mono) which looks similar, but it has more variables and sounds more clean to my ears.

 

[Rob Flinn]

Having got off my ass & looked at the Gemini easyrider diagram it is in fact a fet compressor & is completely discreet in the audio path.  LM339 comparators are used for the metering, but  they are the only chips I can see.

Note to self ...... Look at diagram before making comment.

 

[topcat]

The F601 service manual is posted at the gmail acct.

I used thes units for years as FM modulation limiters at transmitter sites, they proved to be very reliable and stable.....much nicer technical solution than the Gemini or 760... but the sound is 'less colourful'

They are VCA based ....dbx IIRC the VCA is refered to as a VNG101 which also appeared in the 'Compex Mkii'...the VNG101 is very similar to the ntp vca board (may have a schematic somewhere...will check) from memory.....the N in VNG refers to Napier the Scottish mathematician who calculated base e logs....and so to VCA theory

I have the gemini and 760 compex schematics (and if I can find it to scan the compex Mk2) if anyone is interested.

TC

 

[Farringdon]

topcat,

With the F601-RS, did you ever have problems with gain differences between channel 1 and 2?  Or dying capacitors that cause volume drops?

Many thanks!

 

[Rob Flinn]

Topcat, do you have any info on the 3u early version of the F760X-RS  ?  I've been trying to get some service info on this one for ages so I can calibrate mine.  Some people say they're the same as the 2U version, but the circuit boards are different.