Building a 300B PSE

[MGCRead]

Yes I understand that, it is the various earth/0V lines on the power supply circuit diagram that confused me. They show the earh symbol ( as does the feedback switch ) and I was confused about which "earth" that meant. However studying the layout diagram it is clear that the PSU earth is the 0V signal earth, not the chassis.

None- the-less I have far too many 0V wires going to R24 and it would be neater to link them together on the tag board and take only one cable to R24, so I will re-wire accordingly. Then I hope a photo of the underside might help members spot where I am picking up the hum. At the moment the wiring is too chaotic.

Cheers, Malcolm

[bob orbell]

Malcolm, don't get too paranoid about cable neatness, yes of course it looks so much nicer, but too many cables bunched neatly together can present it's own problem. With regard to HT 0 volts, it is exactly that, not earth, I keep the two completely separate on all my kit and all is almost totally silent, you could try removing the earth lift resistor from 0 volts to earth ( chassis ) to see if this helps your problem, but DO keep mains earth connected to the chassis. Bob

[MGCRead]

An update. I found a couple of earthing errors: the white lead ( 0V ) on the second secondary supply wasn't connected and the regulator for the V2 heater was inadvertantly earthed. I still get a little mains hum but the whistling has stopped. I have put a few sine waves in and so long as I keep the input signal very low all seems OK. I am not sure the feedback circuit is working properly yet so will check that out tomorrow. Nor have I injected a square wave. So far so good.

Thank you everyone for your help. I will keep you updated. And I will have to take a lot more care when building the second monobloc !

Cheers Malcolm

[MGCRead]

I have been listening to the amp ( mono only ) for a while now. I can't say I am particularly impressed. The bass is rather reveberative but the treble seems fine ( at least as far as my hearing goes ). No audible mains hum. I would very much welcome any ideas for taming the bass.

On the plus side I am well chuffed at having got it to work at all

One other problem: the 1.6A slow blow fuse ( as specified ) blows when I switch it on unless the power is brought up via my variac. I changed the reservoir caps to 82uF but to no avail. I am assuming a soft start circuit will solve this - but which one? There seem to be a lot to choose from. Have others had this problem ?

Many thanks for all your help so far, Malcolm

[willslenco]

Probably mains transformer (is it original specification?) is creating a power surge on switch on, I'd try a 2amp slow blow.

[MGCRead]

Thanks, I'll try that. I guess I bought the transformer the best part of 20 years ago, but it is the genuine WAD product.

[MGCRead]

A 2A fuse did the trick except when I switched it off a few times in farly quick succession.

Does anyone have ideas about the reverberation at low frequency ? I note my 400V and 350V lines are about 15% low although the other voltages are OK. Perhaps I should restore the 120uF reservoir caps to replace the 82uF ones ?

Cheers Malcolm

[John Caswell]

Sorry Malcolm,
Don't understand "reverberation at low frequency'.
Please explain further with an idea of the frequency

John

[MGCRead]

Hello John

it sounds are if there are some lower harmonics trying to get through. I have built the Wilmslow Audio TL12 speakers:

https://www.wilmslowaudio.co.uk/wa-tl12-2101-p.asp

and it claims the Volt bass can manage down to 23Hz ( hmmm ! ) but it sounds as if it is struggling to handle an even lower frequency. The CD in question ( Keiko Abe playing the marimba ) sounds extremely good through my normal set up.

In summary very low notes sound muddy. So far it is the only CD that gives a serious problem but I haven't tried other marimba CDs yet.

The scope doesn't show a problem with a sub 20Hz sine wave although I notice the tops of the cycles are more rounded than the input cycles.

Hope that helps. I wonder if its just the case that the amp isn't powerful enough to cope ? After all it only claims to be 20W ( which I assume means 10W at best RMS ).

Cheers Malcolm

[MGCRead]

I have now built the second amp. Still a lot of distortion with bass notes (much more than first amp). Unfortunately my scope has packed up (I can't complain, I bought it over 20 years ago second hand; pity as it had a nice CRT screen ), so I can't investigate further. Can anyone recommend a cheap oscilloscope ? My requirements are not demanding as I only do basic electronics - mostly audio.

Thanks Malcolm