World design 36 Watt Class A circa 1994

[Turbs]

I am powering the above amp with 22v 0 22v 600va each channel unloaded voltage
What is the max safe voltage i can apply to this board 24V 0 24v unloaded voltage ??

Any help would be valued

[Richard]

Hi Turbs and welcome,
I can't remember many posts about this amp but have had a look through the supplement article.
It says 36W is achieved using a 22V-0-22V 160VA transformer and that the WAD custom designed transformer gives in excess of 40W which suggests it was slightly higher voltage.
It says specs are achieved using 160VA but 120VA could be used giving lower 4 ohm power output.
Offset is adjusted at R3 and bias at R13; suggested 1.6A.
The only circuit voltages shown are 22-0-22 which are loaded so I wouldn't think 24-0-24 unloaded would be a problem. Your existing 22-0-22 unloaded 600VA TXs will have stiffer regulation than 160VA TXs so if they are giving close to 22V on test all should be well.

[Turbs]

Hi Richard
Thanks for looking at my Amp

I have had trouble earthing, now well (m8 bolt star washers ) star earthed to the chassis

No hum when not connected, hum when connected to pre amp, mobile phone any source !!

The 30 * years old transformers buzz quietly if you put your head in contact with amp.
No audible noise coming from trans.in normal operation.

I have the output earthed and the input also earthed.
Is it possible safe to remove all amp earths to stop the hum.??
The chassis will earthed for safety

Your time is valued thank you

[bikerhifinut]

Sounds like a classic "earth loop".
How is the amp earthed to its chassis and mains earth?
If they are all at the same potential then I would suggest an "Earth lifting" resistor between signal (0V rail) earth and the chassis at the main earthing point. 10 ohms should be sufficient, use a 2W or better resistor in case it does get a fault and the resistor needs to pass current to earth. I'd also put a high voltage capacitor of about 47nF in parallel with the lift resistor, a ceramic will be fine in this position.
That may be enough to break the loop.
Check that your input socket (I shall assume you are using standard RCA "phonos"), is of the sort that has an isolation washer(s) so that the ground or outer of the socket does not contact the chassis electrically. This ground must be made at the INPUT of the amplifier. Another good practice is to use "Pseudo balanced" interconnects. This involves the use of a twin core screened lead per channel, e'g. microphone cable. the screen should be conected to the earth return wire in the cable at the input socket, but not at the other end where only the inner wires should be connected. This effectivley breaks the "loop" where a signal can be induced by small differences in potential at each end.
I doubt your problems are serious as you get no hum when the amp is disconnected, which is a Good Thing.
Whatever you do, DO NOT under any circumstances break the connection between the case/chassis and earth, for your own and others safety. And I would heartily recommend keeping the circuit to Signal ground/ 0V by using a low ohmic value high wattage resistor.
Hope I haven't confused you.

Andy

[Richard]

Hi Turbs,

Looking at the Oct 1994 supplement circuit; the way I read it circuit ground 0V is “lifted” from mains earth by 100R.

The chassis is solidly bonded to mains earth. Then the circuit ground 0V is isolated from the chassis and connected to it only at one point via R33 100R // C20 0.1uF film cap shown.

That would be similar to many WAD/WD valves amps and SS amps such as Quad 606.

(Do you have the article, I wonder if R33 is referred to by mistake in the text btw?)

Anyway, yes, to do what you suggest,
make sure the input RCAs and speaker posts are all isolated from the chassis.
Isolate your circuit star ground point from the chassis.
Connect mains earth directly to the chassis only.
Connect the circuit star ground to that chassis earth point via the 100R // 0.1uF.
Check with a meter to confirm the 100R “lift” is present by using one probe on any circuit ground point (RCA shell, speaker black post, etc) and the other on the chassis earth point.

Re. grounding of the various circuit points. Treat the junction of the 2 PSU caps C15/16 as the earthiest point. The speaker black posts should have a good direct low impedance connection to that point which helps stop high output currents from modulating the sensitive input.

[Richard]

Ah, Andy writes faster than me

[bikerhifinut]

Looks like we both arrived at similar conclusions Richard, your description is better I think.

A.

[Richard]

I had the benefit of the article in front of me Andy

Here's a bit of a drawing of how the circuit ground looks "lifted" inside the mains-earthed chassis.

Circuit ground is drawn here as a star arrangement with the various stages connected to the earthiest point PSU 0V. The 1st is the input ground and the 4th is the output stage. In a valve amp the 2nd point may be the input valve and the 3rd the splitter/driver.

In a traditional bus bar arrangement the red line between input and PSU caps 0V would be the bus bar. The middle 2 connections would then go vertically onto that bus bar so the pecking order of the stages would be kept and the small difference in impedance would prevent ground loops.

In practice many layouts are a mixture of the two arrangements.

[Turbs]

Hello Gents

Thank you for your comments, most of which i have tried.

In the end I used two 5 amp diodes head to tails (e.g passing current when faulty.)
A 5 watt 30 ohm resistor and a ceramic cap
One per input Earth. this setup works a treat

I am very very very happy to report that the hum i have lived with for years is no more.........

Low noise floor......... at long last

THANK YOU

I plan to recap all electrolytic,and the 1uf-4off each board, They where the demo version pre assembled (one diode on the copper track side of the board)
There are 4 off 1uf caps two wima one from RS and one solen.
I will replace with ( German Audiophile Make) polypropylene
And Japan made Nichicon Bt 125c caps

I assume that this will be a upgrade

Any comments Gents

[Turbs]

Also the output transitors SM0718 20ff
and SM7401 2off
Do anyone no of a source to buy them ?
Or a suitable replacement ?
Any thought's

THANKS