[Sebastiaan de Vries]

Dear James,

Looking in the schematics again of the 7 and 14BSST I was wondering something. If we look at the input stage, we see two (discrete) op amps. One as audio receiver, and the second as inverter for the negative amplifier. I my opinion this is not that elegant in an real differential design. The disadvantages are.

-Positive and negative are not 100% equal so common mode noise reduction will be reduced
-The negative signal will pass through two op amp stage (first the receiver then the inverter) in comparison to the positive side that only sees one op amp (the receiver)

Till my supprise when I looked in the schematic of the 28BSST, I see bryston did it all the way I ever wished for! putting the two op amps in opposite phase from each other. Both act now as line receiver and because they are out of phase from each other we get and equal plus and minus 180 degrees signal.
Now we have an 100% real differential working and an equal positive and negative.

I always felt sad already from the beginning my 7BSST's where connected like that internally because this isn't he most elegant way in an real differential design.

My question is can we see it this way also in the 7 and 14BSST in the future? It won't cost any extra I think. The two op amps are already there. They only have to get connected in an different way and a few resistor values have to get changed.

Thank you in advance.

With kind regards,
Bas

[Sasha]

Sebastiaan,

What is your opinion on using transformer in place of op amps to split the signal?

Sasha

[Bob Reynolds]

-Positive and negative are not 100% equal so common mode noise reduction will be reduced
......

Till my supprise when I looked in the schematic of the 28BSST, I see bryston did it all the way I ever wished for! putting the two op amps in opposite phase from each other. Both act now as line receiver and because they are out of phase from each other we get and equal plus and minus 180 degrees signal.
Now we have an 100% real differential working and an equal positive and negative.

Here's a quote to consider from Bill Whitlock of Jensen Transformer:

The purpose of a balanced line is to transfer a "signal" from one
place to another while rejecting "ground noise", which is not white
noise or hiss, but power line related hum and buzz. To accomplish
this noise rejection, two signal lines are used and the IMPEDANCE
of the two lines to ground must be equal or "balanced". Since
anything connected to the line affects its impedance, always consider
the "system" consisting of the driver, the line itself, and the receiver.
It is a popular belief that the signals must have opposing polarity and
equal amplitudes, or symmetry. Signal symmetry has NOTHING
to do with noise rejection. The system must reject noise even
when there is no signal and, in fact, this is usually how system noise
testing is done.


Source:
http://www.jensen-transformers.com/an/an002.pdf

[NewBuyer]

I'm very curious then: Why did Bryston implement different balanced (xlr) input designs in the first place, between their various amp models? I'm sure there must be a good reason

[1oldguy]

Wondering if the 7B sq versions are now wired like the first poster is talking about.Naturally this is way over my head but why not ask right?Anyone?

[Mad Mr H]

Is this to do with the 7 & 14 (I dont know the 28) being a permenant dual & quad channel amp in fixed bridge mode......?

The 14 is really four amps, the 7 is really 2 - In fact the 7 is really a 4B in constant bridge mode, the 14 is really 2x4B in a single box (still with me!)

So the outputs are +ve and -ve from a bridge mode amp and NOT 0v and +ve from a single amp ???



Im NOT an electronics engineer so this is only a suggestion......And the output state may not be related to the input stage design.....

Andy.

[1oldguy]

Maybe James can shed light on this?

[James Tanner]

OK I will get enginering to give us a detailed explaination.

james

[Mad Mr H]

Hi,

I 'think' you answer this here

http://www.audiocircle.com/index.php?topic=68948.0

You put it much better than I did 


It appears that the 7,14 & 28 input stage is to do with the Bridge mode design of these amplifiers,

AND that is actually offers LOWER CMRR than the 2,3,4,etc series.

(CMRR - Common Mode rejection ratio see here for details http://en.wikipedia.org/wiki/Common-mode_rejection_ratio)



So my trip to PMC,UK last week did teach me about the 7,14,28 design.
Pete (Service) and Andy (R&D) spent time to go through the design of the monoblock bridge design with me to improve my understanding of the product.

Like I say I am not an electronics engineer but I do like to understand how things work . A little knowledge can also be dangerous  hence my suggestion rather than statement in previous post.

Looks like I might be right 

[95Dyna]

Wondering if the 7B sq versions are now wired like the first poster is talking about.Naturally this is way over my head but why not ask right?Anyone?

Like oldguy (from another oldguy), the technicalities of Mr. de Vries qustions are over my head but I do know that two of the major improvements to the 7/14 SST2 happen to involve the input stage and the performance of the bridged mode.  Bryston has positioned this list of improvements below as trickle down from the 28B SST2 thereby making them essentially the same amp with lower power output (if you can consider 1K WPS into 4 ohms low output!).




New power supply transformer design
New push on/off power switch
New chassis design
New Soft-Start Circuit
New Balanced input stage
New Power Supply board
Bridging mode performance improvement
Reduced point to point wiring
Reduced circuit board count
Redesinged output chokes

[1oldguy]

Hopefully when the time is right James will give us the scoop straight from the source.
I am more curious as to how it's made as opposed to how it sounds for now of course,that will change when I start using them.

[James Tanner]

Hopefully when the time is right James will give us the scoop straight from the source.
I am more curious as to how it's made as opposed to how it sounds for now of course,that will change when I start using them.

Hi 1oldguy,

Here's what engineering said in another post which does answer the question: http://www.audiocircle.com/index.php?topic=68948.0

james

[1oldguy]

A quote from the first poster:"Till my surprise when I looked in the schematic of the 28BSST, I see Bryston did it all the way I ever wished for! Putting the two op amps in opposite phase from each other. Both act now as line receiver and because they are out of phase from each other we get and equal plus and minus 180 degrees signal.
Now we have an 100% real differential working and an equal positive and negative."

I would venture that I have the least amount of knowledge here.But I guess that's how one learns by asking.I also take pride in the Bryston amps,after all I did buy 5 of them. aa.......So If I were to ask the question on a more basic level(I did read the link),Is the 7B SST Squared "Now"made the same way as the 28B is?If not,why not?I am just curious as to anything I can learn about my chosen amps.

[95Dyna]

Dear James,

Looking in the schematics again of the 7 and 14BSST I was wondering something. If we look at the input stage, we see two (discrete) op amps. One as audio receiver, and the second as inverter for the negative amplifier. I my opinion this is not that elegant in an real differential design. The disadvantages are.

-Positive and negative are not 100% equal so common mode noise reduction will be reduced
-The negative signal will pass through two op amp stage (first the receiver then the inverter) in comparison to the positive side that only sees one op amp (the receiver)

Till my supprise when I looked in the schematic of the 28BSST, I see bryston did it all the way I ever wished for! putting the two op amps in opposite phase from each other. Both act now as line receiver and because they are out of phase from each other we get and equal plus and minus 180 degrees signal.
Now we have an 100% real differential working and an equal positive and negative.

I always felt sad already from the beginning my 7BSST's where connected like that internally because this isn't he most elegant way in an real differential design.

My question is can we see it this way also in the 7 and 14BSST in the future? It won't cost any extra I think. The two op amps are already there. They only have to get connected in an different way and a few resistor values have to get changed.

Thank you in advance.

With kind regards,
Bas

To summarize then, if Mr. de Vries would inspect the schematic of the 7B SST2, which is currently not on the website, he would see the input stage and the bridging method to be identical to the 28B SST2?

[James Tanner]

A quote from the first poster:"Till my surprise when I looked in the schematic of the 28BSST, I see Bryston did it all the way I ever wished for! Putting the two op amps in opposite phase from each other. Both act now as line receiver and because they are out of phase from each other we get and equal plus and minus 180 degrees signal.
Now we have an 100% real differential working and an equal positive and negative."

I would venture that I have the least amount of knowledge here.But I guess that's how one learns by asking.I also take pride in the Bryston amps,after all I did buy 5 of them. aa.......So If I were to ask the question on a more basic level(I did read the link),Is the 7B SST Squared "Now"made the same way as the 28B is?If not,why not?I am just curious as to anything I can learn about my chosen amps.

Hi 1oldguy,

Sorry I realize that some of this stuff can get a bit out there.  Anyway the 7B and the 14B SST2 are now identical to the 28B.

james

[1oldguy]

In  a Word .....Sweet.