MoFo

[little eddy]

I've been struggling with one channel of this amp, (the other channel works fine).

I have 2 inputs. When using 'In 1' wired directly across the 50k load resistor (taking the bias voltage to the pre transformer output), the audio sounds fine. But when I wire to 'In 2' via the dc decoupling cap, there's noise/cracking that I can't fathom or resolve.

I was thinking it was a poor connection somewhere but have even hardwired most of the 0V line and 'In 2' input circuit but it still persists.

So I am admitting defeat and ordered another set of parts and will build another!

Question for you boffins. The amp/output is silent when there is no input connected. Then when I connect the pre, although there's some hum from the pre that I need to sort out, this is louder when using 'In 1' as opposed to 'In 2'. Any ideas? Might this be because there is additional noise on the bias voltage coming off P1 and therefore might a cap between the bottom of R3 and 0V help? p.s. using a laptop power supply.

[little eddy]

Might have found the issue - a noisy laptop supply!

Will try and get another to test my theory but if not, although a lot more work have the parts for a CRC supply.

Just one question. I was looking at an F5 power supply schematic and it has a CL-60 thermistor between 0V and the chassis ground. Was going to fit a thermistor to the tx primary but what does this second one do? And if I am going to use one, should I use two to 'separate' the 0V of the 2 channels as I will have a dual mono power supply?

[Nick]

It works as a highish resistance link between signal and chassis ground (household earth) under normal circumstances, but under fault conditions it will allow current from signal to chassis earth to flow.

[little eddy]

Slow (& very cold going).

Hopefully can get both channels working tomorrow and fingers crossed that the culprit was the laptop supply.

22mF/2R5/44mF/1R0/22mF. Voltage down a bit from the SMPS so around 10.8W. If it works will tidy up and add the various snubber caps. Might also add some additional drain resistors as 88mF takes some discharging.

And room to use the small Hammond chokes in a CLCRC power supply if I get some beefier ones for the amps.

Thinking of supplementing the heatsinks by bolting additional ones to the back. Hopefully will allow me to get to around 2A Idss and 20W.

[little eddy]

Well I wasn't prepared for that.

Wired up the MoFo with CRC supply and it sounded so different to when fed with the laptop supplies.

I feel assaulted and it didn't sound as if it was fed with a valve pre-amp. The detail was astounding with so much grabbing my attention. Bass was great and taught. And the sound stage - at one point I felt I was being flanked and surrounded.

The downside was it was just too 'detailed' especially in the higher octaves, with many of the audible little dings, taps etc on some recordings distracting me from all the positives. I'm hoping that some of this will mellow with further use and there being so many large electrolyics - is this normal and might things settle down? Perhaps some silver interconnects needed at least in the short term to tame things a little!

The other good thing is that at around 12W, I find myself questioning whether I need to strive for any more power.

But got to get my priorities right so first thing must be to sort out the hum on my 41MP pre-amp.

[little eddy]

Phase 2 might be happening sooner than I thought, and the amp will put on a bit of weight.

I've been looking for some suitable microwave transformers to use as chokes as employed by several DIYAudio builders. Found a guy in Swinton with a number of different models for sale that I could go and test. Picked these up for £10 each.

Only issue is a high Rdc of 2.1R, higher than many dedicated chokes so this will limit my max power to around 12.6W. For some reason when people have used MOTs in the US, the primary winding resistance has been much lower! Good news is that they are >150mH so no excuse for bass.

Ed's 60mH Hammond ones will move into the power supply.

[IslandPink]

Very interesting on the effect of the conventional power supply. It should fill-out in tone as the electrolytics form up, at least for 10 hours or so. The HF detail - I would have expected the SMPS to be more 'detailed' in a bad way - ie. strident, so that's a bit odd. I wonder if the resistors in the PS ( the big blue ones ) are having an effect - are they metal films or metal oxides ?

[Nick]

Also try adding film bypass caps on the electrolytics.

[little eddy]

Also try adding film bypass caps on the electrolytics.

What sort of size? I have some 4u7 Vishay MKP1839.

There is also a 0.1uF in series with a 1R0 3W resistor to be added in parallel to the cap bank, (I'm following the DIYAudio universal PS guide).

Target schematic as attached.

[little eddy]

I wonder if the resistors in the PS ( the big blue ones ) are having an effect - are they metal films or metal oxides ?

They are MO but they should be going once I reuse the Hammond chokes to make CLCRC.

[IslandPink]

OK, see how that goes .

Also try adding film bypass caps on the electrolytics.

Aaarg ! Get outside and thrash yourself with a birch branch !

[Nick]

What sort of size? I have some 4u7 Vishay MKP1839.

That will be fine, if its not fitted yet, try it.

[Nick]

OK, see how that goes .

Also try adding film bypass caps on the electrolytics.

Aaarg ! Get outside and thrash yourself with a birch branch !

Built many high current solid state amps lately?

[IslandPink]

I sense your question is in some way rhetorical ?

[Nick]

I sense your question is in some way rhetorical ?

You think?