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  #31  
Old 25th August 2021, 01:16 PM
MGCRead MGCRead is offline
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Location: Malmesbury, Wilts, UK
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Default Re: Building a 300B PSE

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
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  #32  
Old 26th August 2021, 07:08 AM
bob orbell bob orbell is offline
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Default Re: Building a 300B PSE

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
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  #33  
Old 29th August 2021, 05:02 PM
MGCRead MGCRead is offline
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Default Re: Building a 300B PSE

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
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  #34  
Old 7th September 2021, 02:59 PM
MGCRead MGCRead is offline
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Default Re: Building a 300B PSE

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
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  #35  
Old 7th September 2021, 04:02 PM
willslenco willslenco is offline
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Default Re: Building a 300B PSE

Probably mains transformer (is it original specification?) is creating a power surge on switch on, I'd try a 2amp slow blow.
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malcolm ("You can't shine if you don't burn" - Kevin Ayers)

If what I'm hearing is colouration, then bring on the whole rainbow
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  #36  
Old 7th September 2021, 05:48 PM
MGCRead MGCRead is offline
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Default Re: Building a 300B PSE

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.
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  #37  
Old 12th September 2021, 11:23 AM
MGCRead MGCRead is offline
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Default Re: Building a 300B PSE

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
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  #38  
Old 12th September 2021, 05:31 PM
John Caswell John Caswell is offline
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Default Re: Building a 300B PSE

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

John
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  #39  
Old 12th September 2021, 06:58 PM
MGCRead MGCRead is offline
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Default Re: Building a 300B PSE

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
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