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-   -   Psu iii mod (http://www.world-designs.co.uk/forum/showthread.php?t=7868)

bob orbell 22nd March 2020 10:13 AM

Psu iii mod
 
Hi all, read here for something I found interesting. Many of you will know my basic system but here is a reminder, two WD PSU III's, one Phono III, one Aikido CF Pre, WD KT88 all are as basic kits apart from KT88, this has the better power supply parts, OK, for quite a few years I have been powering the Phono and Pre from two separate power supply's, both supply's have high voltage regulators (home made ) the Phono runs at 250 volts the Pre 200 volts, for a long time I have not been too happy with the sound of vinyl, so I did a few tests on the scope and found the HT reaching the Phono had small spikes on the line, at around 100 Hz :(, a test with the pre was just a straight line :confused:, so I removed the snubber capacitors that I had fitted to the choke ( two 220uF 1000 volt back to back with CT to 0 volts ) no change, I then removed the choke and bridged the circuit, the scope now showed a straight line, so connected back into the system and vinyl now sounds bloody good, smoother more detail and better separation of instruments, even sibilance had been reduced, so why ?, I have read that a choke needs to draw a min of current before the power supply woks correctly, otherwise it reverts to pulse charging of the following capacitors, so is this the reason my vinyl has not sounded good, the Phono draws very little current. If anyone ells is using a phono on its own, it may well be worth trying the power supply with the choke removed. Comments please. Bob

bikerhifinut 22nd March 2020 09:08 PM

Re: Psu iii mod
 
Looks a bit like the choke "ringing" do you think? Or if its a "spike" at regular 100Hz intervals could it be switching noise from the bridge rectifier not being attenuated by the choke? And then exciting it at that frequency? One for John C or another with the necessary technical knowledge?

If its the PSU needing a minimum current draw then maybe trying a resistor across the unused HT output would help? Use ohms law and work it out as the value obviously depends on the voltage you are applying across the resistor. Not forgetting to do the ohms law power calculation for power rating of the R.

Might be worth a shot Bob?

What have you done instead of the choke between the caps on the C L C filter? I'd personally recommend a 150 ohm or if that's different to the DC R of the choke, a preferred value close to it to make a C R C filter which will reduce ripple far better than just a capacitor. For those not using regulated HT.

You shouldn't need a choke anyway if using a regulator on the HT as the regulator will ensure a steady, smooth voltage if its working correctly.

In the absence of a WD official kit for a regulated PSU, I'd recommend the Glassware (Broskie) Audio PCB kit, not cheap but its all top quality parts and a cinch to construct, adjustable output voltages using the table and resistor kit supplied. Assuming that in the current situation its available and there's still a transatlantic freight service.

Andy.

bob orbell 23rd March 2020 09:25 AM

Re: Psu iii mod
 
Thank's for reply Andy, I changed R1 for a 180 ohm resistor ( should be 100 ) after removing choke, that was all I had most suitable, did think about the Glass wear PSU, two would be a bit of a price, but very happy with what I have done with the WD PSU's, I hope JC posts, stay safe all. Bob

bikerhifinut 23rd March 2020 11:42 PM

Re: Psu iii mod
 
For what its worth Bob, the glassware board has plenty of capability for both Pre3 and Phono3, I use just one and I'm running the pre3 circuit with a resistor to get the volts to 250, and run the pre3 bit at the 270 I set it to. No problems and all stays nice and steady.

A

Stay safe and sane.


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