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This amp came in with a problem description of humming thru each channel when plugged into the channel. I first powered up the unit without a speaker load and found about -8VDC on the speaker outputs. The positive rail was being pulled down to about +25VDC while the negative rail was good at about -45VDC. I traced that issue down to a failed U1 - 4558 opamp. I didn't have one on hand but subbed in a JRC4560D instead. That solved the DC offset and restored the power rails back to ±45VDC.
When I connected a speaker to the amp, I hear a noticeable constant higher pitched buzz that is not impacted by any of the front panel controls. Disconnecting the preamp from the power and shorting the power amp input on the power amp board still gives off this sound. With the scope, it appears to be about 250mV peak with a period of approximately 270nsec. With no load, I put my audio probe on the output transistor cases and hear the noise on all four, but the noise is barely audible on the other side of the 2.7K 1W (R8 and R20) resistors. The power supply filter caps (both the high voltage rail caps and the low voltage rails caps test good for capacitance and ESR.
With an 8 ohm speaker connected, I hear it and see it on the scope. When I parallel an additional 8 ohm resistive load to the output to give me 4 ohms, the issue disappears. Is this an oscillation/stability issue? Other?
I've attached the preamp schematic as well as two versions of the power amp schematic. My board layout matches the 300CH_Module pdf, but excludes the U2/U3 and VR1/VR2 circuitry. It generates the ±16VDC rails like the PV 300EH.pdf schematic instead. My output transistors are 8839 and 8840. The drivers and pre-drivers match the schematic.
As always, your help is greatly appreciated!
When I connected a speaker to the amp, I hear a noticeable constant higher pitched buzz that is not impacted by any of the front panel controls. Disconnecting the preamp from the power and shorting the power amp input on the power amp board still gives off this sound. With the scope, it appears to be about 250mV peak with a period of approximately 270nsec. With no load, I put my audio probe on the output transistor cases and hear the noise on all four, but the noise is barely audible on the other side of the 2.7K 1W (R8 and R20) resistors. The power supply filter caps (both the high voltage rail caps and the low voltage rails caps test good for capacitance and ESR.
With an 8 ohm speaker connected, I hear it and see it on the scope. When I parallel an additional 8 ohm resistive load to the output to give me 4 ohms, the issue disappears. Is this an oscillation/stability issue? Other?
I've attached the preamp schematic as well as two versions of the power amp schematic. My board layout matches the 300CH_Module pdf, but excludes the U2/U3 and VR1/VR2 circuitry. It generates the ±16VDC rails like the PV 300EH.pdf schematic instead. My output transistors are 8839 and 8840. The drivers and pre-drivers match the schematic.
As always, your help is greatly appreciated!
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