Pictures of oscillation for a newbie

[markiemrboo]

Originally posted by lndm
When I said interference, that is normal, we all need to keep it out. It can get in through your inputs, or directly to your circuitry if it is not shielded, or through your speaker cables and the feedback loop.

Oh right. Well I have heard tinfoil going to ground wrapped around the wire acts as a good shield. Seems kind of difficult to do and quite fragile though. Might give it a go.

As for ferrite bead on the speaker cable, I assume I would be able to put this around the internal speaker cable runs instead of the run from the amp to the speaker (which is what i've heard people saying).

BTW, I don't necessarily think this is causing your heating issue, however it appears to be at the forefront obscuring your investigations.

I did change the insulator to mica today but it made no difference... of course

I did attempt some calculations to explain the rather large difference in DC offset between the two chips. The calculations seemed to suggest the... bias current (I think) on the chip with the higher DC offset was about twice that of the other chip. I don't quite know what bias current is yet, but might this mean that one chip would be drawing more power at idle than the other? (apologies for my non understanding & usual guessing)

If your brain tells you it is the same signal, it should be the same signal. We humans can recognise these shapes so trust your instincts. That said, there might be more than one source and you can usually find out by changing the time base as this tends to focus the shapes around a band of frequencies and blur the rest (not literally though).

Ok.

Is your signal ground lifted from the chassis with some components? You should measure from your signal ground and not the chassis.

Yes it is, sorry. What I meant was that I was measuring from the star ground side of the disconnecting network, which connects to the chassis. Bad wording

Basically, it ought to be satisfactory. Have you checked your DC rails for cleanliness and compared what you see to the outputs?

If I understand correctly, yes. The display I was seeing at the output looked very much like an amplified version of the display I saw when measuring the power rails, even ground to the "0v". This is what made me think that it was maybe just external noise / non amp related noise bought in by the 1X probe and amplified.

You might try this, but might be more interested in what it looks like with the amp on.

Ok

[quote]
You may want to try both. The cable shielding might come under scrutiny this way. Interference from that could be of a greater level than the hiss.

You may need to check your components and joints and you may have a bad chip. (This is not very helpful but it may need to be checked)

I assume that the only real way to test for a bad chip would be to replace it with another? Would a bad chip actually play music fine? On neither chip did I see any protection kicking in at any time, and after running for the whole day they did all seem about the same temperature to me...

Hmm. I think maybe I should also probe the speaker outputs of my current integrated amp to see if this has the same sort display.

EDIT: Oh, these is a slight difference between the two power supply boards by the way. One has Panasonic FC main filter caps, the other has...."R.M" caps.


EDIT: I seem to be seeing the exact same wave pattern appearing on the output on my current integrated amp.

[markiemrboo]

I managed to get a temperature probe to *just* reach the amp chip heatsinks. The 'hot' one is 43C, and the 'cold' one is 38C. I can't measure the regulator heatsinks as the probe lead doesn't reach. But by touch, it seems like the difference in temperature is probably about the same as with the chip's.

Perhaps I just shouldn't be worrying

[lndm]

Nothing useful to say other than I feel that such a difference is probably not a problem per se, but possibly a mismatch. For one thing, are your chips mounted identically in orientation, componentry, and the amount of thermal grease (not too much)? You mention different capacitors and to a small degree these may affect the level of the rails.

I would probe the components on the boards and compare the voltages between the two in an attempt to find a lead.