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Author Topic: regulated power for tube mics-- impact on sound?  (Read 9981 times)

soapfoot

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regulated power for tube mics-- impact on sound?
« on: January 18, 2011, 11:54:13 am »

Is regulated power as a positive or not in tube microphone power supplies?

How much does the load resistance presented by the microphone vary during normal operation?  Is it small enough to be negligible?

Is there any benefit (or downside) to a regulated supply?
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brad williams

ssltech

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Re: regulated power for tube mics-- impact on sound?
« Reply #1 on: January 18, 2011, 12:04:38 pm »

Can you be more specific?

Regulated AC supply into the tube mic PSU, or regulated DC PSU?

Keith
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

soapfoot

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Re: regulated power for tube mics-- impact on sound?
« Reply #2 on: January 18, 2011, 12:09:26 pm »

Sorry, regulated DC PSU.
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brad williams

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Re: regulated power for tube mics-- impact on sound?
« Reply #3 on: January 18, 2011, 01:22:57 pm »

Ah, gotcha.

The dominant consumption in terms of current is usually the heater. Since this doesn't change much with signal, I'd imagine that it's largely insignificant. -That doesn't preclude regular checks though.

For my own microphone, I'm building a slightly 'improved' power supply in that it will have -among other things- a relay in the HT line BEFORE the C/R/C/R/C ladder, energised by a current sensing circuit in the HEATER feed.

That way, if the mic is unplugged, the PSU voltage won't rocket upwards. -Since that's incorporated, it may as well be regulated and adjustable, since that adds very little to the difficulty, with modern designs.

I'm also contemplating a constant CURRENT drive for the heater, which is much kinder on startup.

Of course in this design, the heater is being fed along a separate conductor. -If you think about how the heater is fed in a U47 circuit, significant variations in AC voltage can alter the way that everything is balanced, so that particular design is likely to be 'differently-sensitive' compared to a design with the heater fed separately.

A regulated HEATER voltage with unregulated HT is a third combination to consider.

Keith
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

soapfoot

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Re: regulated power for tube mics-- impact on sound?
« Reply #4 on: January 18, 2011, 01:54:03 pm »

I have a lot to learn about power supply design.  I know the basic building blocks, but I need to learn more about regulation and how to work toward a target voltage.
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brad williams

David Bock

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Re: regulated power for tube mics-- impact on sound?
« Reply #5 on: January 18, 2011, 02:49:52 pm »

when you're dealing with the most common single stage single ended tube mic amp, everything you do or change is audible. This includes capsule, tube, xfmr, AND PSU's.
In short, topology and implementation matter.
There are a few different  B+ reg techniques, and they do sound different from each other.
There are also very expensive vintage mics that get used by pro's everyday that have NO B+ reg.
Remember that your decisions about what psu type to use will have to take into account it's effect on sound, as well as all the usual specs (hum, noise, heater/B+ ramp up, impeadance vs. frequency, shorts capability, cost, size, power draw, field generation). Looking at the specs side only might provide functionality, but not necessarily an ideal sonic signature (please don't get the idea that there's no such thing as a sonic signature- every piece of gear has one by definition).

soapfoot

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Re: regulated power for tube mics-- impact on sound?
« Reply #6 on: January 18, 2011, 02:58:10 pm »

David B.,

Would you be willing to share how you power your 47-style mics, or is that proprietary information?

I'd love to get more thoughts also on the way these things affect the subtle (or not-so-subtle) changes in the sound of the microphone.
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brad williams

ttown

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Re: regulated power for tube mics-- impact on sound?
« Reply #7 on: January 18, 2011, 10:59:47 pm »

In my limited experience with tube mic PSUs, I have to say that the PSU has a rather remarkable effect on the final sound.

I have taken the same mic (e.g., Neumann KM54/56/64 and Schoeps m221a/b) and powered them from different PSUs and the most audible effect is on the noise floor.

Although uncertain, I would guess that the issue is more to do with the quality of the op-amps in the PSUs vs. the actual voltage applied to the mic, on the H+ and/or on the anode side.

We tend to think of tube mic PSUs as simply supplying voltage, but they also have op-amp circuitry of varrying quality - which can have a rather profound effect on the final sound of the tube mic!
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zebra50

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Re: regulated power for tube mics-- impact on sound?
« Reply #8 on: January 19, 2011, 03:44:53 am »

Quote:

We tend to think of tube mic PSUs as simply supplying voltage, but they also have op-amp circuitry of varrying quality


Hi!

Can you give us an example of this?

Thanks!
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Stewart Tavener
Xaudia - studio, ribbon mic repairs
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ttown

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Re: regulated power for tube mics-- impact on sound?
« Reply #9 on: January 19, 2011, 12:10:16 pm »

Here's a good read on op-amps and PSUs:
http://tangentsoft.net/audio/opamp-ps.html

The take-home message is that poor regulation of DC power typically results in higher (audible) noise levels.  

I can offer an anecdotal example.  Up until a month ago I had a fully functional aftermarket PSU for my SM69 tube mic (I fried the PSU late one night during a session by plugging it into a step-up transformer by mistake - but that's another story!).  That PSU had a higher noise level with the same mic than two other AC701 PSUs that I have, being especially notably noisier than a PSU from Peter Drefahl, so I wondered why?

I took apart the now fried SM69 tube PSU only to discover crumby Russian-made op-amps/DC voltage regulators that were likely the source of the higher noise floor!
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David Bock

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Re: regulated power for tube mics-- impact on sound?
« Reply #10 on: January 19, 2011, 12:43:21 pm »

Quote:

Here's a good read on op-amps and PSUs:

I think it's safe to say that since we're discussing B+ and heaters we're talking about tube mics,  the overwhelming majority of which do not contain op amps. So too for the power supplies, the majority of which do not use op amps or voltage referenced comparators to provide B+ or heaters. The most common heater circuits use three terminal regs or a single xistor, common for B+ to have single pass xistor, zeners, or even none regulation. This op amp reg B+ is mainly an aftermarket phenomena which does not yet affect the majority of tube mics, thankfully. Just to keep it modern, the recent Neumann mics are use switching psu's, which is a whole other discussion.

soapfoot

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Re: regulated power for tube mics-- impact on sound?
« Reply #11 on: January 19, 2011, 01:38:38 pm »

In a circuit where the bias is tapped off the heater and applied as a positive voltage to the cathode do you think a heater supply with a linear regulator like a 7806 would be too noisy?
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brad williams

David Bock

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Re: regulated power for tube mics-- impact on sound?
« Reply #12 on: January 19, 2011, 02:07:10 pm »

I can guarantee it.
BTW, about that whole opamp psu link that was provided: bear in mind that high voltage, high impedance tube stages are completely different things than low impedance  opamps.

soapfoot

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Re: regulated power for tube mics-- impact on sound?
« Reply #13 on: January 19, 2011, 02:11:53 pm »

in such a circuit, would one be better off with no regulation at all.. a bunch of Pi filters and maybe a choke?

Do you see any benefit to fast recovery epitaxial diodes in the bridge rectifier?
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brad williams

David Bock

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Re: regulated power for tube mics-- impact on sound?
« Reply #14 on: January 19, 2011, 02:21:54 pm »

BTW I appreciate Brad using his whole real name, but who is "ttown"?
Quote:

in such a circuit, would one be better off with no regulation at all.. a bunch of Pi filters and maybe a choke?
them's gonna be some big chokes.
Quote:

Do you see any benefit to fast recovery epitaxial diodes in the bridge rectifier?
a basic fast recovery helps but you don't need to go hexfred.
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