Allison Research "Gain Brain" FET-Limiters / mods

[matthias]

hello!

I recently got some allison gain brain limiter modules and before I put them into
a rack enclosure I'd like to fix a small problem with the led meter and apply some mods.

1) nearly all modules have a problem with the led-bargraph:
the last or the last two leds are always illuminated,
even if there's no gain-reduction nor a signal at the input.
all other leds are working fine.

I think this is a very common problem with the gain brains.
There are only transistors and resistors in the meter circuit
Do you have an idea which part has gone bad?

2) do you have an idea if it's possible to add a pot for "attack control" ?
if yes, is there a resistor that is responsible for the "attack time" of the
limiter?

3) What would be your choice of transformers to add balanced
in- and output?


here's the schematic

pics of the module:

pic1
pic2

thanks for your help.
matthias

 

[analogguru]

Hallo, zuerst einmal einen schönen Gruß aus Wien.

Then you should learn to use the search button of this forum. with this you will find a link to the manual which can be found here:

http://www.cmaudio.net/gainbrain_manual_and_schematic.pdf

Then print page 5, the schematic. At the bottom you will find the trim adjustments for R 73 and R 76.

1) nearly all modules have a problem with the led-bargraph:
the last or the last two leds are always illuminated,
even if there's no gain-reduction nor a signal at the input.
all other leds are working fine.

I think this is a very common problem with the gain brains.
There are only transistors and resistors in the meter circuit
Do you have an idea which part has gone bad?

IMHO: nothing, only R73 needs to be readjusted.

Follow the steps described under: "METER ZERO ADJUST"

2) do you have an idea if it's possible to add a pot for "attack control" ?
if yes, is there a resistor that is responsible for the "attack time" of the
limiter?

Idea? mechanically ? no, not really.... only beside the LED´s seem to be a place for a trim pot.

Electrically: cut the track between D13/Q23 an the capacitor C22.
Insert a 1k (or 5k) (trim)potentiometer.

3) What would be your choice of transformers to add balanced
in- and output?

People here will hate me, but my choice would be no transformer at all, where it is not absolutely necessary. Balancing can also be done without transformers.

analogguru

 

[emrr]

I used some older Jensen output wired 150:600 for the step up voltage they give, along with the balancing and isolation. My inputs are still unbalanced. I've said it a million times before, balancing is over-rated.

 

[matthias]

thanks for your help!!

Then you should learn to use the search button of this forum. with this you will find a link to the manual ...
Then print page 5...
At the bottom you will find the trim adjustments for R 73 and R 76.

I scanned and uploaded this schematic, so I don't need to print it

of course I tried to calibrate the meter with the trim pot, but that didn't solve my problem. it is so that the last led is illuminated (-24 db) , not the first led at the zero/-2db mark.

I'll try the attack mod this day..

 

[analogguru]

I scanned and uploaded this schematic, so I don't need to print it

Ok, you win....

Good so if it is the 24db LED you can try following:

Solder a wire between the base of Q49 and ground. If the LED ist still lightning, Q 49 is broken.

Q49 is a MPS A13: 30V/500mA/625mW ß>10000/10mA

You can try to use a BC 517 as a replacement, but it has a different pinout- C and E are exchanged.

With no input signal measure the voltage at the collector of Q 34 and Q 35 - it should be near 0 volts. Measure the voltage at the colector of Q 36. it should have also about the same voltage near 0 volts - if you read more (about 24 V) then Q36 will be broken.

analogguru

 

[dale116dot7]

I dunno about balancing being overrated.... it sure gets rid of buzzes in my studio, and is much more tolerant of plugging in new pieces of gear without adding buzz everywhere. It depends on how things are grounded though. I find balanced inputs and unbalanced outputs work well for me.

 

[emrr]

balancing is unneeded if there's no problem. usually there isn't. people just rush to balance things for no definite reason.

 

[matthias]

thank you.

the leds are working now.

I tried the attack mod, but if I increase the pot the signal
gets more and more distorted. I think I can live without an
attack knob, because the "function-control" also affects
the attack time of the circuit.


there's still one thing that I found a bit strange:

I connected the unit to my converter (rme multiface)
to get more limiting and a louder signal into the unit,
I shorted R3 (10K), the resistor connected in series with
the input pot wiper. that works very well, but if the input signal
gets louder, it sounds more and more as if the frequency
curve is showing a boost in the bass range.
this doesn't sound bad at all, but not as you would
expect from a limiter device.

do you think that could be a problem of impedance "mis"- matching ?
(my converter and the unbalanced transistor input)
or are there any components that could cause the
amplification of the bass range?

thanks,
mat.

 

[Rob Flinn]

I have apair of these on the bench at the moment. What is the most likely culprit for an led not lighting in the GR meter ??

 

[matthias]

if the led is NOT lightning, check the led itself first...

 

[desol]

Does anyone know where i can get the audio connector for a gain brain?...I have one that i need to rack up...

Thanks

D.

 

[barclaycon]

I've got a few for sale if you can't find any.
I salvaged them from a Kepex rack some years ago.
I take it we are talking about the 10-way connector for the original Gain Brain.

 

[desol]

Yep...those are the ones...

I'll send a pm...

D.

 

[BrianJSiegel]

I've got a few for sale if you can't find any.
I salvaged them from a Kepex rack some years ago.
I take it we are talking about the 10-way connector for the original Gain Brain.

I have been looking everywhere for an extended card for these guys! Any chance you can let one go?

 

[Rob Flinn]

I have been looking everywhere for an extended card for these guys! Any chance you can let one go?

You will notice that you are responding to a post that is 13 years old.  Haven't seem Martyn (AKA Barclaycon) round here for probably at least 5 years probably more. 

 

[PRR]

> Haven't seem Martyn (AKA Barclaycon) round here for probably at least 5 years probably more.

Last post last year. Last visited last month.  Yes, not as active as before but not vanished.

 

[BrianJSiegel]

You will notice that you are responding to a post that is 13 years old.  Haven't seem Martyn (AKA Barclaycon) round here for probably at least 5 years probably more.

Ok well,  I still need it. Even if I’m nearly clueless as to how posting on blogs works. Any chance of me finding one anywhere? Suckers are hard to find

 

[pvision]

Looking for an edge connector for a Gain Brain Mk 1 if anyone has one or a part number

Happy New Year everyone, especially those living in the UK and "enjoying" our new life as the pariahs of Europe

 

[Rob Flinn]

Looking for an edge connector for a Gain Brain Mk 1 if anyone has one or a part number

Happy New Year everyone, especially those living in the UK and "enjoying" our new life as the pariahs of Europe

Difficult to get. I just tack wires on the circuit board behind the edge connector fingers. That way if a connector surfaces & can still use it.

 

[analogguru]

.... as the pariahs of Europe

Pahriahs ?  I would describe you as lucky FIFO's (First In, First Out) 

BTW, a 3,6mm edge-connector normally should do the trick.