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Author Topic: RE: Testing Supplies Without Killing Mic Tube  (Read 4151 times)

industrial arts

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RE: Testing Supplies Without Killing Mic Tube
« on: March 19, 2011, 04:47:00 pm »

Hope I'm doing this right - can't post to the old forum.

I want to thank both Klaus and KaiS for the valuable information regarding testing a power supply with out having to use the mic as a load.  You might want to post it as a sticky.  Also, having a Variac is essential.

http://recforums.prosoundweb.com/index.php/mv/msg/33664/21732/#msg_516746

In any event, I got both supplies working.  The U47 had both a bad rectifier and first filter cap.

I did not (yet) build a regulator for the heater voltage, since both seemed to be OK when connected to the mics.  I plan to build a small pc board for a generic LM317 regulator circuit in the near future.

One supplied clocked in at about 3.6 to 3.7 volts, the other at 3.9.  Is there any problem with the lower voltage?

Thanks again

Mark Springer
industrial arts
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klaus

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Re: RE: Testing Supplies Without Killing Mic Tube
« Reply #1 on: March 19, 2011, 10:06:38 pm »

Mark,
In wise foresight by your humble moderator, your thread had already been marked as sticky, so it will continue here, once the transition finally gets on its way (can't hold my breath much longer without fainting!)

As to your approach: when I find an LM317 switching regulator in a tube mic power supply circuit, it always puts a frown on my face, as explained in the original thread, because it needs LOTS of smart filtering to stay audibly unnoticeable.
As to 3,7 VDC heater supply for an AC 701: a bit small. I fear over the long term, caking of the cathode will occur. All tubes need a minimum level of heat to prevent certain filaments from clogging up and making the transfer of electrons harder. In the case of the AC701, a minimum of 3.8VDC is prescribed by Telefunken. Minumum!

I am not sure why you cannot get into the comfortable range to regulate to around 3.9VDC.
It is not due to the regulator you are using: in the NS description of their LM317 it says:
The LM117 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to 37V output range.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

boggy

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Re: RE: Testing Supplies Without Killing Mic Tube
« Reply #2 on: March 20, 2011, 07:35:33 am »

Hello all,
I will try to contribute and help in your discussion even if I'm not familiar with microphone construction. :)

........ when I find an LM317 switching regulator in a tube mic power supply circuit, it always puts a frown on my face, as explained in the original thread, because it needs LOTS of smart filtering to stay audibly unnoticeable........
LM317 is three terminal linear integrated series regulator, not an switching regulator. It's not the best you can get for any price and complexity, but it's best you can get from three terminal regulators for its price, even for audio applications.

http://www.national.com/mpf/LM/LM317.html#Overview

Especially, for output voltages lower than 5V and down to about 1.5V, I only get (much) better Line Regulation and noise suppression/generation from two cascaded discrete linear (series) regulators. When I say "(much) better", I mean better than 100dB of line regulation in overall audible range (and beyond), lower noise and flat (constant vs. frequency, resistive) output impedance of couple of tenths of milliohms, but I pay this with about 6V (3V+3V) minimal overall dropout voltage, and I get more complicated circuitry. Bigger overall dropout voltage means bigger dissipation and bigger heatsink for same load current.

Then, it's possible to have better results than with LM317, even for low voltage applications (lower than 5V), but price and complexity is adequate. If you don't need such line regulation, low noise and flat (resistive) output impedance, and if you need a low price, low voltage output, low circuit complexity, smaller heatsink ... personally I dont know if you can find any better solution for audio applications than LM317.

Hope this helps

Best regards
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klaus

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Re: RE: Testing Supplies Without Killing Mic Tube
« Reply #3 on: March 20, 2011, 11:44:29 am »

Thanks for your thoughtful and informative reply. I was under the impression that the hissing noise (white noise) of these regulators is a direct result of their rapidly switching on and off activity?
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

boggy

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Re: RE: Testing Supplies Without Killing Mic Tube
« Reply #4 on: March 20, 2011, 12:24:45 pm »

Thanks for your thoughtful and informative reply. I was under the impression that the hissing noise (white noise) of these regulators is a direct result of their rapidly switching on and off activity?
No problem! :)
They (LM317s) don't have any switching processes in their topologies, they are truly linear series regulators, and if they "hiss", this noise is its own generated noise plus unfiltered input noise.

More deeper analysis about LM317, 7815, discrete regulators and opamp driven discrete "super" regulators and their characteristics you can find in Walt Jung papers here:
http://waltjung.org/Regs.html
this papers are very useful to understanding (a real) boundaries of voltage regulators used for critical audio applications.
But this papers show designs for output voltages only down to 5V minimum (from 15V or slightly more than 15V). For requirements for output voltages lower than 5V you may go to fully discrete solutions or to shunt regulators (if your currents are about couple of milliamperes).... and I think that Walt Jung has papers for shunt regulators too.

Common for all of this I wrote is that if you really need very low noise supply with more than 100dB LR in overall audible range, and resistive output impedance, for your application, you may go to fully discrete design with pre-regulator (two cascaded regulators) or to low noise opamp driven discrete design (if you dont need voltages lower than 5V)... there aren't three terminal regulators in the market with reasonably better characteristics than LM317, relatively to its bigger price, for audio applications and this is an essence of my post above.

I needed regulators for (very) low phase noise (low jitter) Collpits oscillator (9V), some digital pll-s, DAC and ADC chips (1.8V, 3.3V, 5V)... and I build about six different regulator topologies and measure its characteristics, this is my experience.

But here, at PSW R/E/P Forum, we have more experienced analog electronic designers, and they may have more (different) opinions.

Best regards
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Kai

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Re: RE: Testing Supplies Without Killing Mic Tube
« Reply #5 on: April 06, 2011, 09:05:23 am »

For the tube heater supply extreme filtering or low noise is usually unnecessary.
The heater circuit isn't coupled with the audio circuit in most mic designs (except for the U47, but that's a completely different story).

Neither you do need low output impedance (resistance), as there will be an interfacing cable to the mic that destroys any effords into this direction.

Very few parts are needed to build a LM317 based regulator:
- 1 LM 317.
- 2 small tantal or foil or modern low imp. electrolytic decoupling caps.
- 2 fixed resitors or 1 trimmer or best 1 fixed resistor and 1 trimmer.
If there isn't much current needed it works without a heatsink (finger check temperature).

I don't see anything that speaks agains an LM317, it's much better then the original NC accumulator Neumann (mis-) used as a regulator in their PSU's.

Here it's more important to use a correct ground (earth) reference design to achieve low noise then using exotic "high end regulators".
Best results are achieved using a star point grounding scheme (best star point usually is the last PSU filter cap), and placing the regulator circuit close to either the filtering caps or the mic connector.
Keep in mind, the regulator regulates in reference to it's own ground connection.

In an existing PSU I would place and connect the regulator board where the original regulator had been.

Regards
Kai (TPFNAKS - The Poster Formerly Known A KaiS)
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industrial arts

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Re: RE: Testing Supplies Without Killing Mic Tube
« Reply #6 on: April 20, 2011, 11:55:31 pm »

In the end, I did not build a 317 regulator circuit for the supply since, after repairing the supply, it seemed to be working fine.  I am very much of the mind of 1) keeping vintage things as vintage as possible and 2) if it ain't broke, don't fix it.  The supply heater voltage ended up being a little low due to dialing it in with the resistor load and then having it change slightly when hooking up the actual mic.  I can experiment a little more with the series resistor to get it to ride a little higher.

That said, does the quality of the heater voltage have an audible effect on the sound of the mic?   If so - since the supply could be used on different mics - wouldn't swapping in around to other (non regulated) supplies possibly change the sound of the mic, since the voltage level and noise change slightly from supply to supply? 

I have recommended to my friend that we "marry" his supplies to the mics to be sure they are being powered correctly.  Or am I being over critical?

Thanks

Mark Springer
industrial arts

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