DC voltage when switching to phono

[colin.hepburn]

What is happening I just re tested my phono2 and this time it climbed steadily to 90v falling to 0v

[bob orbell]

I bet you wish you had not started this thread Colin, I am completely puzzled, I wonder if it is pulses of HF DC, hope to get my new scope soon, once I get the hang of it, it may shed some light about what's going on. playing a record now, sounds good.

[colin.hepburn]

Originally Posted by bob orbell

I bet you wish you had not started this thread Colin, I am completely puzzled, I wonder if it is pulses of HF DC, hope to get my new scope soon, once I get the hang of it, it may shed some light about what's going on. playing a record now, sounds good.

Hi BOB
i wonder have i discovered a common problem here with the WAD/WD phonos
I’m lost to was hoping you could get to the bottom of it
Maybe Dr John will come in here
Ho and yes sound is good too

[Greg.]

All very interesting. When the Mrs is out tomorrow, I'll run tests on mine for comparison.

[A Stuart]

Originally Posted by colin.hepburn

What is happening I just re tested my phono2 and this time it climbed steadily to 90v falling to 0v

That makes sense.
Pardon me if I you have already excluded this explanation.
I understand you are metering the Phono2 output relative to signal earth.
When it is switched off (and has been for a time eg minutes) it is clearly zero.
When the output valve starts conducting the voltage across R34 rapidly rises, presumably over a second or few seconds. The voltage across the output capacitor is near zero, then increases as it charges up through R38&R36.
The ouput voltage across R38, plus a small fraction across R36, plus the (initially near zero) voltage across the capacitor must by definition equal the anode voltage - (Kirchoff's law? - or is that the currents one?). So the as long as the Capacitor voltage is near zero, the output voltage nearly equals the cathode voltage.

The charge which is being sucked through R38 accumulates in the capacitor, remembering V = Q/C , eventually causing the voltage across the capacitor to equal the cathode voltage. Then there is no more capacitor-charging current causing a voltage to appear across R38. Or if you want to look at it this way, the Cathode voltage is negated by an opposing voltage across the capacitor.

As the valve starts up, changing over a length of time rather than instantaneously, the following will not be precise, but roughly, this drop should have a half life of 2.2 seconds . That means for example 90V, then 2.2sec later 45V, 2.2sec later 22.5v, then 11.25V, 5.6V etc. All on account of the CR constant being 2.2 microFarads x 1MegOhm = 2.2 seconds.

If the above is true, you should see a similar but mirror image Negative voltage pattern on switch off.

So, to get to the point - If you were to switch inputs to Phono during this voltage swing, before output settled to near-zero, you would get a click as the contact was made.

On further thought, if the relay board shorts the unused inputs to ground while not selected, which I was under the vague impression that it does, I would struggle to see that any of the above is relevant after all. So I will shut up now!

Alastair

[Richard]

Ah I think I see what's going on.

I measured mine again as Bob and Colin had, watching the volts from cold switch on, during running, and then I continued watching at switch off.

I set the meter to the 200V range and from cold they rose to +105V then fell back to 0V.

Switching across to mV range there was a plus and minus fluctuating reading up to a few tens of mV as reported before.

Switching back to 200V range it showed 0V again so I switched off and watched. Over a few seconds they rose negatively to a max of -17V then fell away to 0V.

Looking at the output circuit we are metering across the 1M and seeing any voltage developed across it. We are seeing the cap charge at switch on via the 1M resistor (and 10M meter). Then, when charged to the cathode voltage it stops charging so the voltage shows 0V. During running we see the varying voltage developed across the 1M resistor caused by the cap charging and discharging as the voltage at the cathode rises and falls minutely, probably due to HT or mains fluctuation.

The meter isn't analogue and checks voltage (according to my instruction book) with a response time of <1 second. So we see a series of sampled voltages. On an analogue needle meter we'd just see a smooth progression. So although the cap doesn't pass DC it does pass on a replica of it's charging and discharging, at switch on and off, and when running, due to the fluctuation of the cathode voltage. Any mistake perhaps is to think we are measuring voltage leaked by the cap when really we are just seeing the effects of it charging and discharging across a load.

[bikerhifinut]

Originally Posted by Richard

Ah I think I see what's going on.

I measured mine again as Bob and Colin had, watching the volts from cold switch on, during running, and then I continued watching at switch off.

I set the meter to the 200V range and from cold they rose to +105V then fell back to 0V.

Switching across to mV range there was a plus and minus fluctuating reading up to a few tens of mV as reported before.

Switching back to 200V range it showed 0V again so I switched off and watched. Over a few seconds they rose negatively to a max of -17V then fell away to 0V.

Looking at the output circuit we are metering across the 1M and seeing any voltage developed across it. We are seeing the cap charge at switch on via the 1M resistor (and 10M meter). Then, when charged to the cathode voltage it stops charging so the voltage shows 0V. During running we see the varying voltage developed across the 1M resistor caused by the cap charging and discharging as the voltage at the cathode rises and falls minutely, probably due to HT or mains fluctuation.

The meter isn't analogue and checks voltage (according to my instruction book) with a response time of <1 second. So we see a series of sampled voltages. On an analogue needle meter we'd just see a smooth progression. So although the cap doesn't pass DC it does pass on a replica of it's charging and discharging, at switch on and off, and when running, due to the fluctuation of the cathode voltage. Any mistake perhaps is to think we are measuring voltage leaked by the cap when really we are just seeing the effects of it charging and discharging across a load.

Aha! So my suspicions were heading in the right direction?
I never gave it much thought but mine does the same, not a massive thump but it's there nonetheless.
That's also probably why the old troughline does it too.

[bob orbell]

Thank you Alastair and Richard, I understand now. BOB (happy again)

[colin.hepburn]

Yep thanks Alastair Richard
I think I follow what you are saying
But why does this switching crack still happen after the valves have conducted I mostly have my phono switched on and warmed up when I have a listening session
So the phono can be ready to use at any time over the evening so when switching from CD say to phono I still get this crack if I forget to turn the volume off first