Valve Microphone PSU noise

[zebra50]

[quote author="rodabod"]

That's a mighty high polarisation voltage.[/quote]

Yarp!

I'm struggling to understand that. Haven't come across anything north of 90 in my travels, although I have mostly looked at old LD circuits.

We're missing something - another 10M to ground to make a voltage divider and give you your 95V-ish would be my guess.

Otherwise I'm stuck.

:shock:

 

[bcarso]

[quote author="zebra50"][quote author="rodabod"]
That's a mighty high polarisation voltage.[/quote]
Yarp!
I'm struggling to understand that. Haven't come across anything north of 90 in my travels, although I have mostly looked at old LD circuits.
We're missing something - another 10M to ground to make a voltage divider and give you your 95V-ish would be my guess.
Otherwise I'm stuck.
:shock:[/quote]

Agree---but the 95V measured does pretty much agree with the 10M-ish DVM loading guess effect.

Maybe it's a rather big space behind the membrane for this particular mic.

 

[rodabod]

I know it seems ridiculously high, but the mic cable wire that goes to the mic's capsule is actually connected directly to this point.

I'll check again tonight whilst I'm re-capping it anyway.

Incidentally, the gap between the diaphragm and backplate is relatively large from what I can see.

 

[rodabod]

Ok, I checked again, and it does seem to be wired as per the diagram. With the mains here at home, I get 200V before the 10M resistor. There are no other resistors I can see attached to drop the voltage - the other end of the 10M just attaches to the 3.9K as per Stewart's drawing.

 

[PRR]

> should I raise the values of these caps?

Why?

> replace the first 33uF with a larger value since it doesn't have a series resistor?

Why?

> add local decoupling inside the mic for the polarisation voltage

Why?

> I wonder if an additional smaller cap may be beneficial though.

Why?

The mike was probably fault-free when new. Just make it As-New again. The E-caps have aged badly, replace them with similar values. Assume resistors are good. Assume moisture and contamination. You certainly do not know enough about it to assume random "improvements".

200V bias is not uncommon on measurement microphones, and these capsules are probably of that ilk.

 

[rodabod]

[quote author="PRR"]Why? [/quote]

I made the assumption that I might get a little less ripple with larger caps, and I have seen larger alues used on other typical designs. I did 'scope the B+, but admittedly it was not bad. Most of the caps seem to be reading in the range of 2/3 of their original capacitance. I belive there would be a potential drawback of larger caps though - the (I'm guessing) Germanium diode bridge might not like the surge of current draw as larger caps initially fill up.

The mike was probably fault-free when new. Just make it As-New again. The E-caps have aged badly, replace them with similar values. Assume resistors are good. Assume moisture and contamination. You certainly do not know enough about it to assume random "improvements".

True.

200V bias is not uncommon on measurement microphones, and these capsules are probably of that ilk.

I was thinking this too.

Thanks again, PRR.

I'll need to let you guys here a sample of these once I've used them on a session. I reckon they should be pretty nice.


Roddy

 

[rodabod]

Just plugged these up again since working on them as I'm planning to use them in an upcoming session.

With the PSU connected either directly into my Motu, or via other preamps, there seems to be quite a bit of hum.... with the PSU switched off! Hum increases when touching the XLR sockets of the mics. Despite this, I checked the wiring, and audio ground from the PSU PCB meets chassis ground only once. The shield on the mic cables is also grounded. Odd I thought. Also checked resistance of ground path which seems reasonable. The audio outputs from the PSU are unbalanced. Cold is tied to ground.

Anyway, just as an experiment, I tried lifting the earth at mains (I know this is unsafe and potentially stupid, but it was for a test) and the hum disappears. Same goes for with the mics switched on too.

Does this sound like an earth loop? I can't think of any other explanation, yet the earthing arrangement looks decent to me. As a solution, I had a thought that I could transformer-balance the audio outputs and then lift the earth at the output XLR. Does that seem reasonable?

Roddy

 

[BradAvenson]

If you want to avoid finding a transformer try hooking the output up to pin 2 and pin 3 to ground. 
You can float the shield on the mic end or hook it up to ground with a small cap. 

 

[rodabod]

Thanks Brad. Tried this, and of course it will work with transformer-balanced mic preamps, but the problem still exists slightly with transformerless models, for example in my Motu. I guess this is because the ground current sees a path to ground through the input resistors in the preamp, whereas the transformer-coupled preamps are isolated.

Tried balancing the mics with some 1:1s and they are now noise free.

Cheers.

 

[rodabod]

Just for the record, I tried to lower some of the "waterfall" noise of the Nuvistors by fitting some NOS Marconi 7895s. All four Nuvistors I tried were noisier than the originals and had a similar level of microphony.

I didn't realise just how noisy Nuvistors could be... Anyway, my moral of the story is, I'd be carefuly about deciding to ever choose Nuvistors over conventional valves should the chance ever arise; I think I'd rather use sub-mini valves.

 

[EvLoutonian]

Heya, chaps.

Looking at a pair of these mic's that a friend has for sale.

What is the upshot in the end? 
Are they worth doing the work to restore them, and indeed, worth making improvements?

Or just pass on them?

I know they will need some work, but is it worth it, basically?

THANKS!