WD Phono 3 - DC on Output?

[moley]

Hi folks,

Following checking for DC on the output of my WD Phono 3 I have found that I am measuring anything up to 250mV. The measurement is not stable at all constantly changing from a negative value of up to around -200mV up to 250mV. I did find a thread on here reporting a momentary spike of 100v at switch on but this certainly seems like a different issue. The measurement was taken with the cartridge connected to the input but not tracking the record. I measured from the hot pin of the interconnect from the Phono 3 to ground. The same occurs on both channels.

This seems quite high and I can only assume that this is not normal.

What might be the issue if this is not normal and indicative of a problem?

Although I have not had any specific issues in my current setup, a new amp that I am considering is DC coupled and specifically states that connected kit should not have any DC offset.

Any advice gratefully received, many thanks!

[Richard]

Suspect (electrically) leaky output coupling caps Moley. Are they pio? Change for new polypropylene

[moley]

Hi and thanks for the reply.

That's the 2.2UF isn't it? They are Clarity Cap MR polypropylenes and have only been in there for just over a year I think.

[bob orbell]

Hi Moley, I think that I started a thread on this problem, but can't find it , if I remember, there was / is quite high DC voltage on the output at switch on, don't know why, Colin Hepburn,( moderator ), also commented on same, the 2.2uF capacitors should not let any DC through, they are 250v rated, I did try 400v, but no change, but, it all works fine. BOB

[Phil Y]

Hi moley,
A quick question, are you measuring with the o/p of your Phono open circuit or with it connected to a power amp?
If it is open circuit, test it again with it connected to a load. A power amp or even just a resistor, say 100K.

Phil.

[moley]

Hello folks and thank you very much for your replies.

I did find the thread about the momentary high voltage at switch on, I haven't checked for that but this certainly seems like a different issue. I've now lost that thread again but recall there was a spike in the region of 100v DC being measured. In my case its a constant of -250mV to 300mV. Perhaps my test is flawed though and there is no issue at all.

Yes, I was measuring with it open circuit, I was measuring at the interconnect coming from the WD Phono 3 with this unconnected from the input of my integrated valve amplifier.

I did wonder whether or not this would give rise to the readings I was getting, I will try again with a load resistor and see what happens. In my current application I guess the first components that the signal sees after leaving the Phono 3 outputs is the 250Kohm pot in my valve amp. I have had no issues using the Phono 3 with my current valve integrated that I am aware of.

I am looking at using a class D power amp in conjunction with a simple passive volume control as an attenuator (Which I will add). The amp in question is DC coupled at its buffered input and has built in circuits for shutting down if DC is detected.

[Phil Y]

Originally Posted by bob orbell

Hi Moley, I think that I started a thread on this problem, but can't find it , if I remember, there was / is quite high DC voltage on the output at switch on, don't know why, Colin Hepburn,( moderator ), also commented on same, the 2.2uF capacitors should not let any DC through, they are 250v rated, I did try 400v, but no change, but, it all works fine. BOB

Hello BOB,
I have not seen the original thread so if the following was ruled out I apologise.
Is this not just the o/p coupling cap charging up as the PSU climbs and stabilises? A rising HT would to the discharged cap, be just the same as a low frequency A.C. signal. Once the D.C. across it stabilises the voltage at the o/p disappears.

Regards, Phil.

[moley]

http://www.world-designs.co.uk/forum...ead.php?t=1007

This is the post that I found regarding the spike at switch on. I just measured the output of my Phono 3 (no load connected) at switch on and I also get a 100volt DC spike. This drops after a around 5 seconds down to a constantly changing figure in the region of -200mV to 250mV.

I then tried connecting a load resistor as suggested, I had a 75K to hand. The DC that I measure just before the resistor is now between -50mV to 100mV, measuring after the resistor of course gives a reading under 1mV. These readings were taken around 5-10 minutes after switch on.

I guess I could live with not turning on the Phono 3 while connected to the digital amp, I note from the thread linked above there is a mention that someone killed an amp (gainclone) with their Phono 3 at switch on. I'm still a little unsure about the DC I am measuring under my dummy 75K load however.

[bob orbell]

Phil, I think you are correct about the capacitor charging and discharging, as mentioned, it has no adverse effects. Moly, this is not the post I mentioned, but it is very similar. BOB

[Richard]

Something wrong here I think -200mV to +250mV swing is a huge AC signal not DC offset. Measure again with the inputs shorted to be sure the cart isn't producing a signal.

What frequency is it Moley; how many swings per second, perhaps it is very low and not being coupled through to the speakers at the moment if you have a valve power amp with output transformers.

If it does exist I would think it will indeed be dangerous into a SS DC coupled power amp not to mention the speakers.

It could also be motorboating; very low f oscillation. Compare your Phono 3 circuit to Phono 3S which has a few extra parts which may improve stability.