[dubkarma]

I recently replaced a battered old Crest CA4 with a 4B-ST Pro (both 250 wpc @ 8 ohms) to power the compression drivers/horns in a tri-amped TAD system (believe it or not, the 4B is the low powered amp in this set-up).

But I've got a problem with the gain controls on the 4B: they won't lower the gain enough. I thought it was pretty standard that gain controls on a power amp can lower the gain as much as required--all the way, in fact, to zero. But that doesn't seem to be the case here.

I'm wondering if there's a problem with the gain controls on this unit or did Bryston simply choose to limit the range of the gain controls?

My plan, when finances permitted, was to move the 4B to power the mids and invest in a new 3B-SST² for the highs, but I don't want to run into the same "gain control problem" all over again.

Anyone have experience with this?

[James Tanner]

I recently replaced a battered old Crest CA4 with a 4B-ST Pro (both 250 wpc @ 8 ohms) to power the compression drivers/horns in a tri-amped TAD system (believe it or not, the 4B is the low powered amp in this set-up).

But I've got a problem with the gain controls on the 4B: they won't lower the gain enough. I thought it was pretty standard that gain controls on a power amp can lower the gain as much as required--all the way, in fact, to zero. But that doesn't seem to be the case here.

I'm wondering if there's a problem with the gain controls on this unit or did Bryston simply choose to limit the range of the gain controls?

My plan, when finances permitted, was to move the 4B to power the mids and invest in a new 3B-SST² for the highs, but I don't want to run into the same "gain control problem" all over again.

Anyone have experience with this?

Hi,

The ST can only go from full gain to -13dB.  The newer SST amps have a sensitivity switch as well which would allow for a further 6dB of gain reduction. Are you using the Balanced or RCA inputs?

I know there was a technical reason why we limited the gain which I believe we did to prevent the input stage from being overdriven - I will check on that.

james

[dubkarma]

The ST can only go from full gain to -13dB.  The newer SST amps have a sensitivity switch as well which would allow for a further 6dB of gain reduction. Are you using the Balanced or RCA inputs?

I know there was a technical reason why we limited the gain which I believe we did to prevent the input stage from being overdriven - I will check on that.

james

James,

Thanks for the clarification, bad news though it be. It would take some experimentation, but it may be that even the maximum 19 dB attenuation possible with SST/SST2 series amps may not be enough flexibility for running a 110-112 dB compression driver with direct radiator mids and bass.

I'd ask you about the possibility of customized gain controls (for an SST2 purchase), but if it's a matter of not overloading the input stage, that may not be an option.

I am using the balanced inputs: the DAC and the electronic crossover use balanced I/O only. Preamp is BP-26MC.

C'est dommage; I thought Bryston was my solution to those coarse PA-type amps, but maybe not.

Regards,

Joel.



 

[b5pt9]

I was just experimenting with connecting source outputs direct into 14B Pro balanced inputs (bypassing preamp).  As expected with only 13db of attenuation available it was tremendously loud even at full CCW on the trim pots.  I'm thinking that adding 10db inline cable attenuators is relatively cheap and might put the amp gain in a usable volume range (20db would probably be better).

[dubkarma]

I was just experimenting with connecting source outputs direct into 14B Pro balanced inputs (bypassing preamp).  As expected with only 13db of attenuation available it was tremendously loud even at full CCW on the trim pots.  I'm thinking that adding 10db inline cable attenuators is relatively cheap and might put the amp gain in a usable volume range (20db would probably be better).

Thanks for the thought: can you explain what "inline cable attenuators" are? Possibly some kind of resistor inserted at the input of the amp? Is this something one makes oneself or are they commercially available?

Cheers,

Joel.

[b5pt9]

Here's a link to some attenuators I found this morning, I saw these on Audiogon and Ebay.  I assume it's just a resistive voltage divider that reduces the signal strength.

http://www.rothwellaudioproducts.co.uk/html/attenuators.html

[dubkarma]

Thanks, b5pt9; I've gone ahead and ordered a pair of the Rothwell 10 dB XLR in-line attenuators. That'll give up to 23 dB attenuation on the 4B-ST and up to 29 dB on SST/SST2 models, which should be enough. The question remains as to how sonically transparent they are, but at the worst, they should be a viable short-term solution, enough to decide whether I wish to keep the speaker system in question.

Cheers,

Joel.

[James Tanner]

Hi dubkarma,

Engineering tells me:

Hi James;
 
You are correct in thinking that the gain controls are limited in range to prevent input stage overload.  The only way to circumvent that is to move the gain controls to a circuit location prior to the input stage.  That in turn would require different pots with a higher value.   

We can modify his amp(s) either before or after he buys them, to drop the gain.
 
cwr

[dubkarma]

You are correct in thinking that the gain controls are limited in range to prevent input stage overload.  The only way to circumvent that is to move the gain controls to a circuit location prior to the input stage.  That in turn would require different pots with a higher value. We can modify his amp(s) either before or after he buys them, to drop the gain.

James,

Many thanks for following up on this. I'm in Toronto. If I want to have my 4B-ST modified or if, when I order the 3B-SST² I'm planning to order through my dealer and want it customized, should I contact Mike Pickett? Or should I be talking to someone else?

Thanks again,

Joel.

[James Tanner]

You are correct in thinking that the gain controls are limited in range to prevent input stage overload.  The only way to circumvent that is to move the gain controls to a circuit location prior to the input stage.  That in turn would require different pots with a higher value. We can modify his amp(s) either before or after he buys them, to drop the gain.

James,

Many thanks for following up on this. I'm in Toronto. If I want to have my 4B-ST modified or if, when I order the 3B-SST² I'm planning to order through my dealer and want it customized, should I contact Mike Pickett? Or should I be talking to someone else?

Thanks again,

Joel.

Hi Joel,

Contact Chris Russell at Bryston.

james

[dubkarma]

James,

A belated thanks for your input on my question about attenuation range of Bryston Pro amps. I spoke to Chris Russell as you suggested.

I'm now waiting to take delivery on a new 4B-SST SQ which I've ordered with a 0 to -30 dB attenuation range--and as an interim measure, I purchased some 10 dB Rothwell in-line attenuators for my 4B-ST. Eventually, I'll want to move the 4B-SST SQ to replace the 4B-ST and replace the 4B-SST SQ with a 3B-SST SQ, but...well, you know, the economy and all that, it'll be a while.

In any case, your help--and Chris's--is much appreciated.

Regards,

Joel.