Breakthrough with WDPre3

[Richard]

Interesting, the power amp I mentioned having fun with was a 909 (a QSP without a party frock). These two do not have a mains earth (unlike their daddy the 606) and use a 2 pin kettle plug.

For signal input there are phono inputs but they and the multipin inputs are via a Quadlink bus and input pcb. Removing that means arranging a signal earth connection for Pre3 otherwise the power amp signal is floating (as no earth connection is made via the interconnects to Pre due to the OPTXs).

Is yours standard or has it been modified?

[Riggers]

My QSP is Bog Standard. I did a little research and found a guy on the WD forum a few years ago that had a similar issue as myself
http://www.world-designs.co.uk/forum...ead.php?t=4783

Another really minor issue is that since I have had the QSP I occasionally get a little minor RFI breakthrough. I did read somewhere that the speaker cables could potentially act as an aerial and take a weak signal to the output transistors. I was thinking of getting some fancy-schmancy shielded 'speaker cable (Like one of the Chord items) to see if this would help, but the Quad has these annoying 'speaker binding posts that apparently only accept bare wire.

I wonder if the WD88VA would be less or more prone to this issue as I live less than 1km from the hill in Brisbane where all the TV towers transmit from.

[bob orbell]

Hi Riggers, I think that you can remove the little red and black blanking caps from the speaker outputs on your amp. they are there to stop some country's mains plug being inserted, they are even fitted to amps. for this country. BOB

[Richard]

Yes the blanking plugs prise out so you can use 4mm plugs.

RF might be coming down the speaker leads but may also get in the speaker voice coils. It may also be via the mains and it's worth clipping ferrite chokes on your mains leads.

The RF problem could be related to the problem you have with the cdp. Your power amp is "safe" in as much as it's classed as "double insulated" but it doesn't have any earth connection at all when you are using it with Pre3.

It would have a signal earth connection when used in a Quadbus system or with a pre which made a signal earth connection via the interconnects. Pre3 doesn't do that as it uses an output transformer.

I believe the original idea in not connecting ground across the transformer was to allow a balanced connection option. You may want to investigate connecting ground across inside Pre3. John Caswell may advise as I'm not familiar with balanced operation but cannot see (if you don't want balanced) why this cannot be done for regular phono plug connection.

To check if this is the issue;

Pre3 case earth post is signal ground. PSU3 case earth post is mains earth. Try connecting a wire between Pre3 case earth post and touch it (or wrap it tightly around) the outer bare shell of one of the input phono plugs going into the power amp.

Also try similar with a wire from PSU3 case earth post to see if either kills the cdp breakthrough, and if it does, which is better. The connection will provide an earth connection for the power amp and may also stop your RF problem.

[Riggers]

Hi Richard et al
I tried the connection of an earth wire from the earth post on the pre3 to one of the phono plug outer shells going to the power amp. No improvement in the breakthrough. I did not notice any RFI when testing this but this is only an occasional issue.

[Richard]

Hi Phil, so no easy fix there then. I never had your problems with my Pre3 but I used it with 2 or 3 power amps and with one in particular found reduced hum using an earth wire between the two.

It shows how fickle these things can be I suppose and it would be worth trying a different power amp if you can lay hands on one. In my case the 909 in standard form was fine despite its 2 pin mains but I never liked the idea. I presently use a 606 Mk2 (3 pin mains) which is lovely and quiet as the grave with Pre3 and its own pre.

I used my Pre3 with 300B stereo, WD88, and Glasshouse 300B and it behaved perfectly also.

Where to look next I don't know but if the inputs are truly shorted I cannot see it being conducted. Did you construct your Pre3? Did you use any unshielded or twisted pair wiring? Is the OP transformer wiring well away from input wiring etc?

[Riggers]

Hi Richard
I constructed the Pre3 myself as well as the PSU and Phono3. I did try to construct as per pics in the manual, and to be honest I am very happy all round with the performance of the gear. It may well be that the power amp is a compatibility issue, ( I had no idea that there was only a 2 pin mains connection to the power amp). I'm happy to keep playing with my gear as is, certainly I'm not keen to go pulling apart relay boards etc.

Phil.

[John Caswell]

Hi all,
The CD player still has not been proved/disproved as the problem. You really need to borrow another CD to see if the problem still exists.

John

[bob orbell]

Hi all, my thinking points to the Pre input relay board, because Riggers built the kit, he may have a wrong connection on the relay board, Phil, have you tested as I said? do you have 0 ohms between signal input centre spigot and its outer connection (not the earth post) on all 5 inputs, left and right, with the Pre switched off? now with power on, starting with Phono input and the selector switch at Phono, (leave the power amp OFF) you should have 50k or 100k (depending on the pot value) with meter at the same test as above, check all 5 inputs in turn, they should all be the same, the 4 remaining inputs must read 0 ohms. If all is OK them I am stumped. BOB

[Richard]

Yes check as Bob says but also do a simple check first to be sure your relay pcb is actually connected to ground;

Pre can be off, then put a probe on any input phono outer shell and the other probe on the case earth post looking for 0 ohms.

Also check inside that the bare wire links between the phono socket ground shells and Pre ground (shown connected at G in the pic below) haven't been missed. Check that they then run unbroken to the pcb earth connections (one of the 2 is seen near the L in WORLD in the pic) via the pot,