[cheater]

Hi all and hi James,
I've registered this forum after seeing the excellent interaction between James and the users of Bryston electronics on this forum, which I didn't know existed. I stumbled upon it while searching for Bryston amp specs.

I've got a set of PMC speakers with PMC branded Bryston SST2 electronics. The stereo set consists of PMC MB2 XBD speakers, and the following Bryston units: 2x 10B, 3B, 4B, 2x 7B. At room listening levels, at the distance I am at, even at the low sensitivity setting selected on the back of the amplifiers there is noticeable noise produced from the 10B, which is in actuality because at maximum volume the current gain would be much too high for comfort, so the 10B is being driven much below its optimum level. So it was suggested to me that I insert pads between the 10B and the power amps.

I was going to use a "U" form balanced pad such as described here:

https://www.audiocircle.com/index.php?topic=68948.msg639349#msg639349

In another thread, James mentions that the CMRR of the SE amps is in the mid to high 70's, and for the 7B it's in the 80s. I'm not sure to what tolerance I should be matching the series resistors to match the specs. I assume specifying up to 80dB or 90dB CMRR would be a good idea, after all the parts don't cost that much, it's just a bunch of work to match that stuff up. I have a good 7 digit DMM coming in soon so I'm likely able to match down to the required value, but I don't know what the value should be, so if James reads this, would you mind terribly asking your engineers?

Also, what is the preferred input and output impedance for such a pad so as to not raise the noise floor? And is the "U" topology the best one to use here, or something else?

Next question - what's better: set the amps to low sensitivity and use a smaller pad, or set the amps to high sensitivity, and use a pad with 6dB larger attenuation?

Finally, if I wanted to make a switchable 20/30/40 dB pad - any suggestions on what topology to use? Would that still just be "U", or is something else better applicable at that point?


Thanks a lot!

[James Tanner]

Hi Cheater

I am surprised to hear about the noise issue as we have a lot of these systems out there and this is the first time I have heard of this?

Anyway I will forward this to Mike at Bryston for his thoughts.

james

[Mike Pickett]

Hi all and hi James,
I've registered this forum after seeing the excellent interaction between James and the users of Bryston electronics on this forum, which I didn't know existed. I stumbled upon it while searching for Bryston amp specs.

I've got a set of PMC speakers with PMC branded Bryston SST2 electronics. The stereo set consists of PMC MB2 XBD speakers, and the following Bryston units: 2x 10B, 3B, 4B, 2x 7B. At room listening levels, at the distance I am at, even at the low sensitivity setting selected on the back of the amplifiers there is noticeable noise produced from the 10B, which is in actuality because at maximum volume the current gain would be much too high for comfort, so the 10B is being driven much below its optimum level. So it was suggested to me that I insert pads between the 10B and the power amps.

I was going to use a "U" form balanced pad such as described here:

https://www.audiocircle.com/index.php?topic=68948.msg639349#msg639349

In another thread, James mentions that the CMRR of the SE amps is in the mid to high 70's, and for the 7B it's in the 80s. I'm not sure to what tolerance I should be matching the series resistors to match the specs. I assume specifying up to 80dB or 90dB CMRR would be a good idea, after all the parts don't cost that much, it's just a bunch of work to match that stuff up. I have a good 7 digit DMM coming in soon so I'm likely able to match down to the required value, but I don't know what the value should be, so if James reads this, would you mind terribly asking your engineers?

Also, what is the preferred input and output impedance for such a pad so as to not raise the noise floor? And is the "U" topology the best one to use here, or something else?

Next question - what's better: set the amps to low sensitivity and use a smaller pad, or set the amps to high sensitivity, and use a pad with 6dB larger attenuation?

Finally, if I wanted to make a switchable 20/30/40 dB pad - any suggestions on what topology to use? Would that still just be "U", or is something else better applicable at that point?


Thanks a lot!

Hi,

I would recommend using a U-Pad in this case, as its simplicity makes it easy to implement, and the high ratio of amplifier input impedance to crossover output impedance means that we don't need to be too concerned with impedance matching the pad.  As well, if you end up building a pad with switchable attenuation, you only need to switch one resistor value; the one between the signal legs.  You should be safe to use commercially available 0.1% metal film resistors; if you wish to match them to a tighter tolerance than this, great, but I don't think it will improve performance.

You should definitely keep the amplifier in its lowest gain mode, and use minimal passive attenuation before it.

I would also recommend checking the internal gain settings on the 10B's, as it's possible that they can be easily configured for lower gain.  Information pertaining to gain on the 10B can be found here, and page 1 is the applicable one, although the layout will vary since your units are likely the custom PMC LR version:

http://old.bryston.com/PDF/Other/10B_GAIN_ATTENUATOR_NOTES.pdf

Thanks,

Mike

[cheater]

Thanks guys.

James - regarding the "noise issue". It isn't really an issue with the hardware - just with the way it's being used. The way it all comes out of the box, it's meant for Very Loud Reproduction. So if someone wants to listen at much lower levels, they'll be surprised that there's a considerable amount of hiss. If I were in a large room, with the speakers far away from me, so that their maximum output were comfortable, the hiss would be inaudible. But as it stands, their maximum output would be deafening, because they're in a small room and much closer to me. The only good way to set up these electronics is to use attenuating pads, to essentially make the maximum loudness of the setup much lower, and also attenuate the noise at the same time. So it would make a lot of sense to include information about using attenuating pads for best reproduction in such a set up - something that kind of blindsided me. I think a lot of people in my situation are going to experience the same thing, because they don't know to do that. And they might think something's wrong with PMC or Bryston electronics just being noisy, while they're plain being used incorrectly, like I am (soon "was", as I get the pads). The crucial thing is that this is not fixed by using a preamp - which goes before the crossover. The attenuation needs to happen after the crossover. Few people will do that in their HiFi. I'm setting up a mastering suite to learn the trade so it's pretty crucial to me to eliminate all sorts of noise and distortion, and that's why I dug deep. A more competent audio engineer would have known right away, but I was missing that information

[cheater]

Hi,

I would recommend using a U-Pad in this case, as its simplicity makes it easy to implement, and the high ratio of amplifier input impedance to crossover output impedance means that we don't need to be too concerned with impedance matching the pad.  As well, if you end up building a pad with switchable attenuation, you only need to switch one resistor value; the one between the signal legs.  You should be safe to use commercially available 0.1% metal film resistors; if you wish to match them to a tighter tolerance than this, great, but I don't think it will improve performance.

You should definitely keep the amplifier in its lowest gain mode, and use minimal passive attenuation before it.

I would also recommend checking the internal gain settings on the 10B's, as it's possible that they can be easily configured for lower gain.  Information pertaining to gain on the 10B can be found here, and page 1 is the applicable one, although the layout will vary since your units are likely the custom PMC LR version:

http://old.bryston.com/PDF/Other/10B_GAIN_ATTENUATOR_NOTES.pdf

Thanks,

Mike

Thanks Mike. You are right that they're the custom PMC 10B LRs, permanently wired into the "Three-Way Mono" mode. I don't know what the inside of a standard Bryston SST2 10B LR (balanced I/O) looks like, but this one has two daughter boards, one with 15V regulators and NE5534s and one with an SMD filter module of some sort with tiny parts, what looks like 01005 metric.

The PMC 10B doesn't carry out the rotary switches to the front panel, but they're still present, just hidden behind the front bezel. Am I right that to change them you're supposed to undo the square (Robertson) screws and take off the front panel? Using the service manual I wasn't really able to figure out what those switches did - it makes sense in terms of a stereo setup, but I wasn't able to divinate what they did in three-way mono.

If I'm looking for switching attenuation then I'd like to make it something that'll be easy to switch e.g. depending on the type of material being listened to. So I think internal modifications won't be as practical for that purpose

[James Tanner]

Hi Cheater

There are small pins inside that can change the gain to each section but I would not recommend fiddling with them as they are set by PMC. 

I believe you can reduce the overall gain but I would let PMC do that for you.

james

[cheater]

Thanks! Yeah, I think I'll keep the pad external. Thanks a lot