Originally this amp sent full rail out as soon as it powered up which I have resolved. Currently this amp has a problem with the bass boost circuit that I haven't been able to solve. This amp is similar in design as the massive, sinus live, audiopipe, and shark amps.
With it powered up and the bass boost off (fully ccw), there is ~ -7VDC at the speaker terminal, but it drops to ~ -0.500VDC with my 4 ohm dummy load connected. The amp does produce clean audio.
Where it gets interesting is when you adjust the bass boost control the DC offset goes from -7VDC (no load) to ~ +90VDC (no load) without turning it very far. It can be adjusted to achieve low offset levels at the speaker terminal and it doesn't seem to have any boosting impact to the audio signal. It behaves the same with a load connected except the current draw rises and the amp will go into protection.
Anyone seen anything similar that can help point me in the right direction?
With it powered up and the bass boost off (fully ccw), there is ~ -7VDC at the speaker terminal, but it drops to ~ -0.500VDC with my 4 ohm dummy load connected. The amp does produce clean audio.
Where it gets interesting is when you adjust the bass boost control the DC offset goes from -7VDC (no load) to ~ +90VDC (no load) without turning it very far. It can be adjusted to achieve low offset levels at the speaker terminal and it doesn't seem to have any boosting impact to the audio signal. It behaves the same with a load connected except the current draw rises and the amp will go into protection.
Anyone seen anything similar that can help point me in the right direction?
Measurements taken with boost at minimum then adjusted to maximum.
U1
Pin 1: Goes from 2.630VDC to -11VDC
U2
Pin 8: Goes from 1.314VDC to 3.8VDC
Pin 14: Goes from -9.82VDC to -10.62VDC
U3
Pin 1: Goes from .006VDC to 11.30VDC - The boost control is directly across Pin 1 and 2
Pin 7: Goes from -1.331 to 11.5VDC
Pin 8: Goes from -10.61 to -10.95
Pin 14: Goes from -6.414 to 11.7VDC
U1
Pin 1: Goes from 2.630VDC to -11VDC
U2
Pin 8: Goes from 1.314VDC to 3.8VDC
Pin 14: Goes from -9.82VDC to -10.62VDC
U3
Pin 1: Goes from .006VDC to 11.30VDC - The boost control is directly across Pin 1 and 2
Pin 7: Goes from -1.331 to 11.5VDC
Pin 8: Goes from -10.61 to -10.95
Pin 14: Goes from -6.414 to 11.7VDC
I am borrowing a smaller, but similar in preamp design Audiopipe to compare.
The Audiopipe has ~ -1.275VDC offset no load and drops to ~ -.350VDC with 4 ohm dummy load.
I measure similar voltages on all eleven pins of the driver board between the two amps.
On the three TL074's of the Audiopipe, there is negligible DC on all pins except for U3, Pin 14. There I measure ~ 1.275VDC, exactly the same as the output terminals except opposite polarity.
The bass boost works correctly and the only change on output pins while operating the bass boost control is Pin 14 of U3. It reduces to ~.850VDC.
Perry, correct me if I am wrong on the following thoughts.
I am thinking the audio driver board in the Belva is OK and not part of the problem.
In the Belva, I see ~90V square wave on one side of R5.
In the Audiopipe, I see ~70V square wave on the one side of R5. I am thinking the feedback circuit is working correctly.
The problem or contributing problem is the DC voltage present on the pins of the TL074's. I do not have more of those on hand at the moment to replace them again. Before I order more, what else should I check while I have a similar amp to compare?
The Audiopipe has ~ -1.275VDC offset no load and drops to ~ -.350VDC with 4 ohm dummy load.
I measure similar voltages on all eleven pins of the driver board between the two amps.
On the three TL074's of the Audiopipe, there is negligible DC on all pins except for U3, Pin 14. There I measure ~ 1.275VDC, exactly the same as the output terminals except opposite polarity.
The bass boost works correctly and the only change on output pins while operating the bass boost control is Pin 14 of U3. It reduces to ~.850VDC.
Perry, correct me if I am wrong on the following thoughts.
I am thinking the audio driver board in the Belva is OK and not part of the problem.
In the Belva, I see ~90V square wave on one side of R5.
In the Audiopipe, I see ~70V square wave on the one side of R5. I am thinking the feedback circuit is working correctly.
The problem or contributing problem is the DC voltage present on the pins of the TL074's. I do not have more of those on hand at the moment to replace them again. Before I order more, what else should I check while I have a similar amp to compare?
I went digging through my parts drawers and found some new TL074's that I didn't know I had.
I replaced all three again and that solved all of the DCV on the output pins except for pin 14 of U3 which is apparently normal.
The bass boost function has been restored and the DC offset at the terminal is down to ~ -4.75VDC no load, ~ -.350VDC with 4 Ohm dummy load.
Thank you again for your assistance Perry.
I replaced all three again and that solved all of the DCV on the output pins except for pin 14 of U3 which is apparently normal.
The bass boost function has been restored and the DC offset at the terminal is down to ~ -4.75VDC no load, ~ -.350VDC with 4 Ohm dummy load.
Thank you again for your assistance Perry.
I don't know if I misread earlier but I think you stated that connecting the input and adjacent ground pins on the driver board made no difference (same offset problems). Is that correct?
Yes, that is correct. I shorted those pins as result of reading some of the other threads that dealt with this design to help eliminate the preamp section which in this case didn't seem to do that.
I will return to this amp in a couple of days to do some more testing to be sure its really repaired for good.
I will return to this amp in a couple of days to do some more testing to be sure its really repaired for good.
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