I found this interesting information about bias adjustments on most of the Bryston SST series amps that I would like to share with you guys. for all of those proud owners of Bryston amps that would like to check and adjust bias on their units here it is!!!😉
http://bryston.com/PDF/Other/SST_BIASING_INSTRUCTIONS.pdf
http://bryston.com/PDF/Other/SST_BIASING_INSTRUCTIONS.pdf
No! Look at the schematics. This would not work for the 4B at all, for example. They want about 6 mV across both pins of TP1 and TP2 each (but measured separately), so that the values numerically add up to about 12 mV.
Thank you for the feedback. So means 6mV for TP1 and another 6mV for TP2 12mV in total?
English is not my native, so I am puzzleled.
So meance my bias set 12mV on each TP is twice higher than the spec!? So I need to try if the right adjustment will lower down my DC offcet, which I have 49mV and 72mV across the terminals.
Any sigestion on the possible DC leaking component on the amp board? All the elecelectrolytic caps ( aside of pwr sypply) have been replaced already.
English is not my native, so I am puzzleled.
So meance my bias set 12mV on each TP is twice higher than the spec!? So I need to try if the right adjustment will lower down my DC offcet, which I have 49mV and 72mV across the terminals.
Any sigestion on the possible DC leaking component on the amp board? All the elecelectrolytic caps ( aside of pwr sypply) have been replaced already.
Re-adjust bias to the 6.4 and 5.6 per to and tp2 on the channel, and see no influence on DC offset.
So, advice on the possible DC leakage components will be deeply appreciated.
So, advice on the possible DC leakage components will be deeply appreciated.
You do not say what model Bryston amplifier you are trying to adjust.
DC offset has nothing to do with bias current adjustment, which you are attempting.
Bryston models generally do not have an adjustment control for DC offset so to adjust it,
you would need to calculate and change certain fixed resistor(s), according to the model.
DC offset has nothing to do with bias current adjustment, which you are attempting.
Bryston models generally do not have an adjustment control for DC offset so to adjust it,
you would need to calculate and change certain fixed resistor(s), according to the model.
I believe resistors value were were calculated by Bryston at the design stage.
I bet some components went loose after 30 year that caused high DC offset, so if someone know what will be the problem, I would appreciate advice.
Ithis is rare 3B with led level indicators, which has 80v in a arm by design
I bet some components went loose after 30 year that caused high DC offset, so if someone know what will be the problem, I would appreciate advice.
Ithis is rare 3B with led level indicators, which has 80v in a arm by design
30years ago 5% tolerance resistor were very commonly used (look for the gold stripe).
And some were carbon film.
These loose tolerance and high drift resistors could very easily give output offsets that wander way off specification.
Bryston now do a 20year warranty. Was it 20years back then? If so they must have been pretty confident in the resistors they installed.
And some were carbon film.
These loose tolerance and high drift resistors could very easily give output offsets that wander way off specification.
Bryston now do a 20year warranty. Was it 20years back then? If so they must have been pretty confident in the resistors they installed.
So, do you mean sine 1988 (which is mine) they moved toward 1% and lowered down DC offset down to 4-6mV that reported by the Bryston 4b-st /sst owners in the forums?
The reason I am trying to find what should I do, as I manage to do basic repair of the non working amp that I bought and recap by my own, but to drawn through the schematic to find the root of high DC offset is above my knowledge level. Shold I send it to the doctor or with some help can manage myself as the good Bryston amp "doctor" is not common in my location.
For example my 2B from 1986 shows DC offset 4mV an 7mV on the channels, so the resistors are from the same age...., but the figures are different.
The reason I am trying to find what should I do, as I manage to do basic repair of the non working amp that I bought and recap by my own, but to drawn through the schematic to find the root of high DC offset is above my knowledge level. Shold I send it to the doctor or with some help can manage myself as the good Bryston amp "doctor" is not common in my location.
For example my 2B from 1986 shows DC offset 4mV an 7mV on the channels, so the resistors are from the same age...., but the figures are different.
By the way.
I checked the resistors on the boards, 95% of the are 6 stripes 1% 50ppm, and only 3 values are 5%.
Checked all the diodes - OK
Small transistors did not check as they need to be unsoldered from the board.
So still need advice on DC Offset improvement.
I checked the resistors on the boards, 95% of the are 6 stripes 1% 50ppm, and only 3 values are 5%.
Checked all the diodes - OK
Small transistors did not check as they need to be unsoldered from the board.
So still need advice on DC Offset improvement.
Unfortunately, I know a number of techs who would just dive in and do that 5 minute fix.
I would like to ask a few questions though. Did you measure offset before replacing capacitors and are the capacitor types you fitted the same types or audiophile "upgrades"? I ask, because I have seen offset affected by components like NOS Blackgates electrolytics, for example.
Is the offset stable or slowly changing, according to hours in use?
I'm assuming of course, that there is nothing wrong with the original resistors and there is another problem, perhaps not your fault but still, as the result of changing capacitors.
I would like to ask a few questions though. Did you measure offset before replacing capacitors and are the capacitor types you fitted the same types or audiophile "upgrades"? I ask, because I have seen offset affected by components like NOS Blackgates electrolytics, for example.
Is the offset stable or slowly changing, according to hours in use?
I'm assuming of course, that there is nothing wrong with the original resistors and there is another problem, perhaps not your fault but still, as the result of changing capacitors.
Dear Ian, thank you for the feedback.
As I got the amp with pwr filter decoupling cap and resistor burnt, I replace everything at once with amp board caps. Maybe this was sutid not to do step by step? so I do not have "original" DC offset measurement.
Filtering caps on the amp boards I replaced with the same value 470uF 16V Vishay BC 056, and OOC i install same value 470uF 16V Elna Simlic II.
PWR Supply Caps just ordered awaiting to arrive.
As for the DC Offset, on pwr on multi-meter shows 49mV and 75mV after 1h shows 47mV and 69mV.
One more thing I need to add about my B3. This one with LED Output Level Meter and in a chassis of B4.
The Secondary gives 69V AC and on the caps I have 89-90 DC (between the arms) and L/R have 1-2V difference.
PWR Caps are 10000uF 80V, like the 4B models (i ordered 10000uF 100V).
Clipping protection is not on the PWR Board but on the Level board, and the schematic is a bit different from the one on the PRW board, as well as the value on the 1W resistors not 5K6 but 4K8, and they looks overheated but keeps resistance (ordered, awaiting to arrive).
As I got the amp with pwr filter decoupling cap and resistor burnt, I replace everything at once with amp board caps. Maybe this was sutid not to do step by step? so I do not have "original" DC offset measurement.
Filtering caps on the amp boards I replaced with the same value 470uF 16V Vishay BC 056, and OOC i install same value 470uF 16V Elna Simlic II.
PWR Supply Caps just ordered awaiting to arrive.
As for the DC Offset, on pwr on multi-meter shows 49mV and 75mV after 1h shows 47mV and 69mV.
One more thing I need to add about my B3. This one with LED Output Level Meter and in a chassis of B4.
The Secondary gives 69V AC and on the caps I have 89-90 DC (between the arms) and L/R have 1-2V difference.
PWR Caps are 10000uF 80V, like the 4B models (i ordered 10000uF 100V).
Clipping protection is not on the PWR Board but on the Level board, and the schematic is a bit different from the one on the PRW board, as well as the value on the 1W resistors not 5K6 but 4K8, and they looks overheated but keeps resistance (ordered, awaiting to arrive).
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OK, There are indeed variations, but standard 3B/4B models have 80 or 85V rails voltage supplies which would both require 100V capacitors, minimum. You should not fit capacitors of the same voltage rating as the unregulated rail voltages as they will be exceeded with mains power fluctuations, surges etc. They should be at least 10%, preferably 15% or higher than the nominal rail voltages so 100V rating for those rail voltages would be correct.
You say you have 69VAC across the transformer secondary but do we assume that gives you +/-89 VDC at the capacitor +terminals and power rails? Calculating, it should be (69*rt2) - 1.4V or 96V fully loaded. With no load that could easily rise beyond 100V, so the voltage rating of caps would be grossly under rated.
I think there is something wrong with your measurement there:
Models 3B,4B should have 80 and 85V rails respectively. Variations 3BIII and IV have no current limiters but only 55V rails. The AC transformer supplies for these are 114V centre tapped and 77V Centre tapped respectively, which would result in either +/- 85VDC or +/- 55VDC rails. So, what are the individual +/- rail voltages you measure with respect to ground?
You say you have 69VAC across the transformer secondary but do we assume that gives you +/-89 VDC at the capacitor +terminals and power rails? Calculating, it should be (69*rt2) - 1.4V or 96V fully loaded. With no load that could easily rise beyond 100V, so the voltage rating of caps would be grossly under rated.
I think there is something wrong with your measurement there:
Models 3B,4B should have 80 and 85V rails respectively. Variations 3BIII and IV have no current limiters but only 55V rails. The AC transformer supplies for these are 114V centre tapped and 77V Centre tapped respectively, which would result in either +/- 85VDC or +/- 55VDC rails. So, what are the individual +/- rail voltages you measure with respect to ground?
Ian, agree with you.
The inlet is ~110V, turns to 69V AC from the 2 secondaries with common tap, which should turn into the (69*1.41)-1.4=96V DC which is 48V DC in the rail (Bryston exactly use 55V caps at 3B and 80V at 4B, my model come out from factory with 80V caps instead of 55v).
But what I have is 89V DC between -Vcc and +Vcc (~44.5V across rails and ground), which turns to be slightly below the calculated 96V.
The problem I see is ~7V under voltage from the caps, were did they lost? Leakage?
Also maybe oscilloscope can show exact picks but I do not have any.
I also check the voltage from the bridge without pwr caps plugged, same 69AC and 89V DC, which make me worry about undervoltage.
The amp board version is 3B IV
The inlet is ~110V, turns to 69V AC from the 2 secondaries with common tap, which should turn into the (69*1.41)-1.4=96V DC which is 48V DC in the rail (Bryston exactly use 55V caps at 3B and 80V at 4B, my model come out from factory with 80V caps instead of 55v).
But what I have is 89V DC between -Vcc and +Vcc (~44.5V across rails and ground), which turns to be slightly below the calculated 96V.
The problem I see is ~7V under voltage from the caps, were did they lost? Leakage?
Also maybe oscilloscope can show exact picks but I do not have any.
I also check the voltage from the bridge without pwr caps plugged, same 69AC and 89V DC, which make me worry about undervoltage.
The amp board version is 3B IV
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Tomorrow I will try to remove both channels and to see what would be the voltage, to check if the channels might influence undervoltage somehow.
Also need to remove LED Level board and to see the voltage.
TO all: Sorry for my English it is not my native.
Also need to remove LED Level board and to see the voltage.
TO all: Sorry for my English it is not my native.
It is a good plan to disconnect the load from the transformer secondaries. Be careful you can't accidentally short out any tapping.
Measure the unloaded Vac from each winding. And measure the input voltage.
Measure the unloaded Vac from each winding. And measure the input voltage.
If your transformer is 120V rated, but your local supply is 110V, you would be 9% low already. The secondary voltages, if intended to be 77VAC total, would only be 69V and the rectified rail voltages +/- 50V.
Dodging the matter, Bryston nowhere state line voltage or variable performance ratings on schematics or in the user manuals for legacy products. So it could be officially 110-120V if in Ontario, Canada and transformers should have been designed, I think, to deliver correctly and safely at the highest expected supply voltage rating, or 120V there.
That's a good reason to consider cap ratings and product specifications with suspicion until you know the supply voltages at which the tests are carried out and recommendations refer.
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I think the transformation ratio keeps the same whatever 105 or 125V are on the primary.
As per the schematic, the primary is 120V the secondary is 77V, means transformation ratio is 0.642, means ~110V inlet will turns to the ~70V secondary. Yes, this might be not the the best conditions for the transformer but will be working conditions.
The other think that I see is the AC to DC conversion is below the calculation, as I wrote, 69AC measures 89DC on the bridge, while it should be turned to the (69Vac*1.41)-1.4=96Vdc (as Ian recall the right formula).
So this is 7Vdc undervoltage on the bridge ratifier, which is really might be a leakage.
Anyway, DC offset on the terminals is still in //
As per the schematic, the primary is 120V the secondary is 77V, means transformation ratio is 0.642, means ~110V inlet will turns to the ~70V secondary. Yes, this might be not the the best conditions for the transformer but will be working conditions.
The other think that I see is the AC to DC conversion is below the calculation, as I wrote, 69AC measures 89DC on the bridge, while it should be turned to the (69Vac*1.41)-1.4=96Vdc (as Ian recall the right formula).
So this is 7Vdc undervoltage on the bridge ratifier, which is really might be a leakage.
Anyway, DC offset on the terminals is still in //
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You need to allow for the transformer regulation value.
If we assume the regulation is 6%, then the output voltage is Vrated * 1.06
then a 120:77Vac rated transformer will have an open circuit measured ratio of 120:81.62Vac and that would give a rectified and smoothed voltage of ~114Vdc when no current is drawn.
As you draw more current the smoothed (averaged) voltage drops and the ripple voltage increases.
If we assume the regulation is 6%, then the output voltage is Vrated * 1.06
then a 120:77Vac rated transformer will have an open circuit measured ratio of 120:81.62Vac and that would give a rectified and smoothed voltage of ~114Vdc when no current is drawn.
As you draw more current the smoothed (averaged) voltage drops and the ripple voltage increases.