Bryston 3Be Power Amplifier - Adjusting and Minimizing DC Offset

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John Tulett

  • Jr. Member
  • Posts: 19
  • Fascinated by vintage audio equipment
Purpose:
* To expand on the previously shared knowledge restoring my Bryston 3Be Power Amplifier.
* To share my experience measuring and adjusting this power amplifier's DC offset voltage.   

Equipment:
* Bryston 3Be stereo amplifier, serial number 330033.  Purchased new in 1992.


Figure 1. Bryston 3Be front view after repairs - photo from January 2023.

Background:
* While the repair and restoration of this 3Be power-amp is documented in the report, "Bryston 3Be Power Amplifier repair - Hints and Lessons Learned", what I neglected to document in this report was how I measured and adjusted the 3Be's DC offset voltage, mainly because there was nothing wrong with the offset voltage and it did not need adjusting. https://www.audiocircle.com/index.php?topic=184960.0
* With that said, I was pleasantly surprised by the large number of views and interest in this report and its related report, "Bryston 2B Power Amplifier repair - Hints and Lessons Learned". https://www.audiocircle.com/index.php?topic=184949.0
* Typically, with modern DC coupled class A/B amplifiers, a DC offset voltage in the circuitry will shift the output power transistors' bias point, leading to distortion.  Furthermore, a DC offset voltage will pass through to the speaker terminals and if the voltage is large enough could trigger a shutdown of the amplifier's loudspeaker protection circuity.
* As a rough guideline, less than 10 mV offset can be considered negligible and very good, while more than 50 mV offset is undesirable leading to distortion and should be investigated.
* Therefore, I believe it is worthwhile to share my experience of measuring and adjusting the 3Be's DC offset voltage.

Tools and Test Equipment:
* Fortunately, both the Bryston 2B and 3Be power amplifiers are straightforward to work on using common tools and common sense.  For the 3Be power amplifier a description of the hand tools and equipment used for measuring and adjusting the DC offset voltage is shown in Table 1 and Figure 2.


Table 1. Suggested hand tools and test equipment for measuring and adjusting DC offset voltage.


Figure 2. Rear of 3Be amplifier with; Robertson screwdriver, trimming screwdriver, analog inputs shorting cable.

Method 1, no adjustment required:
* With the analog inputs shorted, and nothing connected to the speaker terminals, power on the amplifier and allow it to warm up for at least 1 hour.
* With a suitable DVM having millivolt range, measure the DC voltage present at the left side, and right side set of speaker terminals, and record these values.  Example of my measurements are displayed in Table 2.
* If the measured DC voltages are smaller than +\- 10 mV this is considered negligible, and no further action is necessary.  Note the polarity of the measured offset voltage is not important, what is important is to achieve the lowest absolute value of offset voltage. 


Table 2. Initial DC offset voltage measurements after restoration and repairs, but before adjustments.

* If the measured DC voltages are larger than say +\- 10 mV, or if you wish to further reduce the offset voltage, proceed to method 2 (as I did).

Method 2, adjustment required:
* Please understand no adjustment of the DC offset voltage can fix broken circuitry, if the amplifier needs to be repaired this must be remedied first.
* Safety. Ensure unit is powered off and the AC power cable is fully disconnected.
* Remove the fastening screws holding the top cover in place (there are thirteen Robertson screws retaining the top cover on this 3Be amplifier).
* Familiarize yourself with the location of the trim pots and the correct type of screwdriver needed to adjust them.  There should be one trim pot for each channel (Fig 3.). 
* Suggest marking the initial position of each trim pot set screw, in case there is a problem with adjusting the offset value or something goes wrong, you can always return to the initial setting.  Gently rotate the trim pot set screw by a small amount to ensure it is not stuck.


Figure 3. Location of trim pot - left channel shown with top cover removed.

* Replace the top cover, without fastening screws.  Even with the top cover loosely in place this will allow the circuitry to warmup as normal. 
* As described in method 1, with the analog inputs shorted and nothing connected to the speaker terminals, power on the amplifier and allow it to warm up for at least 1 hour.
* With a suitable DVM having millivolt range, measure the DC voltage present at the left side, and right side set of speaker terminals.
* Carefully remove the top cover, being extremely careful that AC and DC high voltages are live within the chassis when power is on.
* While observing the DVM, slowly rotate the trim pot set screw to minimize the measured DC voltage.  As mentioned, the polarity of the measured offset voltage is not important, what is important is to achieve the lowest absolute value of offset voltage.
* With the top cover removed you might see some fluctuation in the measured DC voltage, this could be due to noise pickup from lack of shielding without the top cover.   Gently replace the cover and record the final DC offset voltages.  In my case replacing the top cover provided a more stable reading.  Example of these final measurements are displayed in Table 3, Figures 4 and 5.


Table 3. Final DC offset voltage measurements after adjustment.

* While your results may differ, the results shown in Table 3 were achieved by turning each trim pot set screw approx. 2 turns clockwise (when viewed from above) from their initial positions measured in Table 2.


Figure 4. Final DC offset voltage after adjustment, left channel.


Figure 5. Final DC offset voltage after adjustment, right channel - note top cover in place but not fastened.

* Finally, to turn power off, disconnect the power cord, and reattach the top cover with the retaining screws. 

Final notes:
* Bryston 3Be.  Interesting the DC offset voltages measured for the first time in 30-years were still within specification (+5 mV left channel and +7 mV and right channel), indicating the circuitry in this Bryston 3Be amplifier is quite stable.   Although I could have left things as is, I pursued optimizing these offset values and hence my findings being documented in this report.
* Bryston 2B.  Another interesting observation with the related Bryston 2B amplifier restoration, here the DC offset voltages are non-adjustable but being measured for the first time in 44-years and documented in the report showed a respectable -4 mV for left channel and -6 mV for right channel.  Presume these low offset values were obtained by careful selection and matching of the input differential transistor pairs when the unit was manufactured in 1978.
* Lastly, pleased to report since their restorations both amplifiers continue to perform faultlessly and sound great just like they used to!