MAB
Major Contributor
I recently reviewed a stack of classic Bryston gear, including a 3BST amplifier. I was pleasantly surprised at the good results. I have the big brothers of that 3BST amp, the 7BST monoblocks. I also recently built a resistive load with enough capacity to test such a powerful amp. Here is a test of this classic amp.
I got the amps used years ago. They have powered a range of speakers. They have been reliable. Mine were manufactured in 1999 according to the date code. I have never opened them. I have no idea what the previous owner did with them, except they appeared well-cared for when I bought them 12 years ago.
It has balanced inputs using TRS or XLR, and unbalanced RCA. It also has 'parallel' and 'series' modes. The amp is actually two modules, which can be operated bridged, or in parallel. It can drive very difficult loads in the parallel mode.
5 Watt 1 kHz Performance Dashboard
Following usual ASR practices, using the amps' balanced inputs with a QA403.
4 Ohm Parallel Mode
96dB SINAD lands it at the bottom of the excellent category. This is a pretty good result despite the mains noise. I wonder if that is age-related.
4 Ohm Series Mode
SINAD degrades to ~90dB in series mode. Much of it is increased noise. Which makes sense since the amp is bridged in this mode.
8 Ohm Parallel
8 Ohm Series
Again, excellent performance with degradation in series mode compared to parallel. We will later see how much power is available in each configuration.
Lastly,
2 Ohm Parallel
2 Ohm Series
Excellent performance at this moderate power level into 2 Ohms.
Power Output
Here is the THD+N vs. Power for 8, 4, and 2 Ohm loads. Parallel and series modes were measured except for 2 Ohm load which I only measured in parallel. I also didn't quite make it to 1% distortion on each run, call me a chicken!
In any case the amp exceeds the following power figures at 1% distortion:
2Ω Parallel: 775 Watts
4Ω Parallel: 475 Watts
8Ω Parallel: 250 Watts
4Ω Series: 850 Watts
8Ω Series: 610 Watts
I didn't want to redo the runs to 1%, suffice to say they have a ton of power.
The amp has clipping meters incorporated into the faceplate's LED:
Onset is about 2% THD, the LED start turning amber. By 3% distortion the meters are lit up solid red. I tested a second amp and it has the same clipping threshold for the LED. They provide very useful feedback. They look tasteful, yet appropriately urgent when they turn red.
Here is the THD alone across power for the various loads and mode:
The amp did get warm doing these high power sweeps, but didn't complain, protect, or blow a fuse.
Distortion and Nose across frequency
Looking at THD+N at 4Ω in serial mode across a range of power from 130mW to 200W, the amp shows little dependence of distortion and noise across frequency.
4 Ohms in parallel mode, even better performance.
Here is THD alone into 4 Ohms in parallel mode, the amp is well controlled across frequency:
IMD / Multitone distortion
Here is the intermodulation distortion at 5 Watts, 19 + 20kHz
First into 4 Ohms in parallel mode:
And into 4 Ohms in series mode:
Both are great results. I can see some noise at 35kHz that goes away in series mode. Looking at Bryston's description from the owner's manual:
Is this out of band noise at 35kHz getting nulled in bridge mode? Or did something get turned off in my house? Not sure, I can't hear it, but interesting.
Here is multitone into 8 Ohms, parallel mode:
19 bits of distortion margin, 16 bits including the noise.
And serial mode into 8 Ohms:
18 bits of of distortion margin, and 14 bits of noise margin. The amp is noise limited, still excellent performance.
Frequency Response
Here is the frequency response into the three loads, in parallel and series modes:
-3dB down at 60kHz or higher depending on load and mode. The parallel mode has the most extended response at a given load resistance, which isn't surprising.
Matching
Here is the performance of the second 7BST compared to the one I just tested.
Except for some slightly questionable noise the second amp (right channel, red trace) is matched to the first, after 35 years of use.
I'm glad I measured these, and didn't blow anything up.
I now have a better understanding of the parallel and series mode. I also wondered if the amp was as clean as I thought it was into low impedance loads, which it is. I've driven Infinity Kappa speakers years ago, and even more difficult speakers with the previous version of the 7B, and it worked well never overheating or shutting down, sounding great. This confirms my impressions of it's performance.
I hope this is interesting, and encourage questions and suggestions.
I got the amps used years ago. They have powered a range of speakers. They have been reliable. Mine were manufactured in 1999 according to the date code. I have never opened them. I have no idea what the previous owner did with them, except they appeared well-cared for when I bought them 12 years ago.
It has balanced inputs using TRS or XLR, and unbalanced RCA. It also has 'parallel' and 'series' modes. The amp is actually two modules, which can be operated bridged, or in parallel. It can drive very difficult loads in the parallel mode.
5 Watt 1 kHz Performance Dashboard
Following usual ASR practices, using the amps' balanced inputs with a QA403.
4 Ohm Parallel Mode
96dB SINAD lands it at the bottom of the excellent category. This is a pretty good result despite the mains noise. I wonder if that is age-related.
4 Ohm Series Mode
SINAD degrades to ~90dB in series mode. Much of it is increased noise. Which makes sense since the amp is bridged in this mode.
8 Ohm Parallel
8 Ohm Series
Again, excellent performance with degradation in series mode compared to parallel. We will later see how much power is available in each configuration.
Lastly,
2 Ohm Parallel
2 Ohm Series
Excellent performance at this moderate power level into 2 Ohms.
Power Output
Here is the THD+N vs. Power for 8, 4, and 2 Ohm loads. Parallel and series modes were measured except for 2 Ohm load which I only measured in parallel. I also didn't quite make it to 1% distortion on each run, call me a chicken!
In any case the amp exceeds the following power figures at 1% distortion:
2Ω Parallel: 775 Watts
4Ω Parallel: 475 Watts
8Ω Parallel: 250 Watts
4Ω Series: 850 Watts
8Ω Series: 610 Watts
I didn't want to redo the runs to 1%, suffice to say they have a ton of power.
The amp has clipping meters incorporated into the faceplate's LED:
Onset is about 2% THD, the LED start turning amber. By 3% distortion the meters are lit up solid red. I tested a second amp and it has the same clipping threshold for the LED. They provide very useful feedback. They look tasteful, yet appropriately urgent when they turn red.
Here is the THD alone across power for the various loads and mode:
The amp did get warm doing these high power sweeps, but didn't complain, protect, or blow a fuse.
Distortion and Nose across frequency
Looking at THD+N at 4Ω in serial mode across a range of power from 130mW to 200W, the amp shows little dependence of distortion and noise across frequency.
4 Ohms in parallel mode, even better performance.
Here is THD alone into 4 Ohms in parallel mode, the amp is well controlled across frequency:
IMD / Multitone distortion
Here is the intermodulation distortion at 5 Watts, 19 + 20kHz
First into 4 Ohms in parallel mode:
And into 4 Ohms in series mode:
Both are great results. I can see some noise at 35kHz that goes away in series mode. Looking at Bryston's description from the owner's manual:
Is this out of band noise at 35kHz getting nulled in bridge mode? Or did something get turned off in my house? Not sure, I can't hear it, but interesting.
Here is multitone into 8 Ohms, parallel mode:
19 bits of distortion margin, 16 bits including the noise.
And serial mode into 8 Ohms:
18 bits of of distortion margin, and 14 bits of noise margin. The amp is noise limited, still excellent performance.
Frequency Response
Here is the frequency response into the three loads, in parallel and series modes:
-3dB down at 60kHz or higher depending on load and mode. The parallel mode has the most extended response at a given load resistance, which isn't surprising.
Matching
Here is the performance of the second 7BST compared to the one I just tested.
Except for some slightly questionable noise the second amp (right channel, red trace) is matched to the first, after 35 years of use.
I'm glad I measured these, and didn't blow anything up.
I now have a better understanding of the parallel and series mode. I also wondered if the amp was as clean as I thought it was into low impedance loads, which it is. I've driven Infinity Kappa speakers years ago, and even more difficult speakers with the previous version of the 7B, and it worked well never overheating or shutting down, sounding great. This confirms my impressions of it's performance.I hope this is interesting, and encourage questions and suggestions.
