Finished my chipamp kit using a 160v 18+ 18+ Avel torodial. The amp boards are bolted to an aluminum heatsink whick is in turn bolted to the base of the enclosure (thermal paste at both locations). Sounds great but I do have a "hiss" when no signal is being sent to the amp. I only have the AC inlet earth and the power supply board ground connected to the chassis ground. Should I also ground the RCA inputs to the chassis? The enclosure is from ebay and just barely fits everything.
You shouldn't hear any hiss unless you put your ear near the speaker. Better double check for HF oscillation. I agree that you need a larger heat sink. If the black part of the case is aluminum, I'd put the sink into good thermal contact using an insulator if V- is on the sink.
+/- 15 volts and 8 ohm load is as hard as I'd push it with that heat sink.
Don't connect the inputs to the case. They should be returned to their own small signal ground point.
+/- 15 volts and 8 ohm load is as hard as I'd push it with that heat sink.
Don't connect the inputs to the case. They should be returned to their own small signal ground point.
Your probably good with the heat sinks. Monitor the heat with use to be sure. With the supply voltage in use, don't use less than 8 ohm loads even with good sinking.
Without a volume pot on the input, it is like running the amp wide open with only the input bias resistor as the primary input impedance. The amp will likely pick up noise or self feedback causing oscillation. Try shorting the input to input ground to see if the noise stops. (I'm assuming the amp is not connected to anything.)
If this works, you can do a couple things. First make the input impedance is as low as possible for your application. If the amp is for portable music players, for example, you can solder a 1k ohm resistor across the input jacks to short out the weak signal of the noise. (Don't fiddle with the input bias resistor) The audio from the player won't be affected.
The engineers here will tell you to filter HF. This is the most correct answer, but may take some rework to that amp to be proper.
If the cable made the difference, your amp is probably on the verge of being unstable. I would not adjust the negative feedback. Increasing R3 could make the gain too low for your input signal (don't know much about iMacs) and too low can cause instability. Take care of the input situation as described previously.
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