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Author Topic: Neumann NN-48 power supply (unmarked caps)  (Read 838 times)

MikeCheck

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Neumann NN-48 power supply (unmarked caps)
« on: March 15, 2017, 04:53:42 pm »

Hello All!
Long time reader, first time poster.

I have a NN-48 power supply that is intended to be used with a KM56. I am checking it before using it with the mic. Plate voltage is good, but my heater was very low (~2.2V unloaded). I believe it should be 4V after loading.

I quickly traced the culprit to 3 caps underneath the board (pictured). They are not labeled clearly. Any ideas?

Note: I could not find an accurate schematic for this supply. It does not match nn48a schematic which is quickly accessible when searching.
The caps sit after R7 to ground. Heater voltage is taken from this point. There is no trim pot (R8), or diode between the caps and ground.

 Thank you all for the help!

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MikeCheck

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #1 on: March 15, 2017, 05:36:49 pm »

EDIT:
Just realized that the third component is not a diode on the schematic, it is in fact the third cap. Duh!
and...... it seems to read either 1.5 or 7.5/300.

Any thoughts?
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #2 on: March 15, 2017, 06:30:26 pm »

Looks like someone performed a previous 'repair' and replaced the original Stabilyt cells.
Stabilyt cells have a typical life expectancy of 2 to 3 years and been discontinued for nearly 50 years. These replacements (1,5/300 = 1.5V/300mA) are certain to be dead by now also.

At the time, Neumann made available for a brief transition period a solid-state replacement board SK33d, before redesigned successor supplies were introduced, among them NN48b. With modern voltage regulator devices more effective solutions are possible.

For the rehabilitation of vintage power supplies for use with the precious AC701(k) tube I have developed another 'Stabilyt Replacement' circuit with gently ramping supply which reduces stress on a cold tube filament. While working on these power supplies it does make sense to examine the high voltage plate/bias voltage stabilization and consider replacing the gas discharge tube (150B2) with a suitable 150V/1W Zener diode.

File size restrictions will result in multiple postings for the attachments.
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #3 on: March 15, 2017, 06:31:11 pm »

NN48b
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #4 on: March 15, 2017, 06:31:50 pm »

SK33d
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #5 on: March 15, 2017, 06:32:52 pm »

SK33d instructions
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #6 on: March 15, 2017, 06:33:33 pm »

Stabily Replacement
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klaus

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #7 on: March 15, 2017, 10:58:16 pm »

Thanks, Uwe.
I suggest that anyone who owns or repairs N52 or NN48 supplies copies these schematics, for future reference!
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Klaus Heyne
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MikeCheck

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #8 on: March 15, 2017, 11:54:38 pm »

Awesome info Uwe! Thank you very much.

I think I'll build and install your circuit. I'll update when done.

I've attached more pictures of the PSU for every ones enjoyment. The pics are a little dark. Sorry. Note that the serial number is "011"

Also, no markings on the power transformer. Anyone know if this can be changed to 110VAC ??
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #9 on: March 16, 2017, 12:53:23 am »

All the schematics for this family of power supplies I have do show the mains transformer to be capable to be configured for operation with either 110 to 120Vac/60Hz or 220 to 240Vac/50Hz.
Your photo of the mains transformer clearly shows the corresponding dual primary arrangement with its current implementation for 220-240V.
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Jim Williams

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #10 on: March 16, 2017, 12:56:10 pm »

The 317 regulator design is the better choice of these. I would probably pop for an LT1086 instead.

If you use the zener clamp solution, place a small film cap across it like .01 uf. Zener's generate a lot of noise and that will shunt that noise off to ground.
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klaus

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #11 on: March 16, 2017, 02:56:08 pm »

The 317 regulator design is the better choice of these.
Thanks, Jim.
That's my biggest complaint when using IC chips for regulation: switching noise. Good filtering plus your Zener suggestion needs to be implemented to avoid that.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

Kai

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #12 on: March 16, 2017, 05:13:10 pm »

... biggest complaint when using IC chips for regulation: switching noise.
The LM317 is an analog linear regulator, no switching inside. It does not generate lots of noise. What's left is filtered out by the final R2-C2 combination, no further improvements necessary.
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klaus

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #13 on: March 16, 2017, 08:12:43 pm »

Good to know. Do you have a schematic for that chip? I am curious how it regulates.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

soapfoot

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #14 on: March 17, 2017, 10:23:29 am »

Good to know. Do you have a schematic for that chip? I am curious how it regulates.

page 3:

https://www.onsemi.com/pub/Collateral/LM317-D.PDF
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Jim Williams

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #15 on: March 17, 2017, 12:07:03 pm »

Most of these older non-low dropout style regulators used a zener clamp diode with an opamp for the adjustable reference. That's where most of the internal hiss is generated.

The LT 1085/6 style low dropout regulators are much more precision and have lower output noise. I stil use a large el cap on their outputs to reduce that, 1000uf is a good starting point. Add a small film .01 uf to trap hf spikes and other noise.

Good heatsinking is equally important to reduce the noise. Cool semiconductors gererate less hiss than hot semiconductors.

Alternatively, one can add a transistor shunt after the regulator to further reduce ripple/noise. Feed the collector, run the base to ground with a cap and exit the emitter. Use a small 2k resistor from the collector to base. Observe polarity and don't let the smoke out.
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klaus

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #16 on: March 18, 2017, 12:16:18 am »

Most of these older non-low dropout style regulators used a zener clamp diode with an opamp for the adjustable reference. That's where most of the internal hiss is generated.

So is this strictly the electron noise generated by the amount of (bad) components in a (bad) circuit implementation? Or is there switching noise involved?
I was always under the impression that the constant voltage adjustment to regulate the constantly changing incoming voltage generates the audible noise in these regulators. But I don't want to perpetuate that notion if it's wrong.

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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

Kai

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #17 on: March 18, 2017, 06:04:53 am »

I was always under the impression that the constant voltage adjustment to regulate the constantly changing incoming voltage generates the audible noise in these regulators.
The key word is constantly. An analog, so called linear regulator does NOT wait for a change, then have a delay and then react.
Look at it as an analog amplifier (which in fact it is), and as every of such it regulates immediately, constantly, within its maximum speed range.
So there is no switching noise generated, because there is no switching or stepping going on. It greatly reduces the noise that is on the unregulated power supply side.
To get the best out of this a proper grounding scheme for the whole circuit is necessary. Regulation is in reference to its reference voltage input, this needs to be the center of the grounding starpoint.
A common mistake is to use a point at the unregulated side as grounding starpoint, where hum and noise currents are floating.
Further filtering (by caps) is best done close to the supplied circuit, to reduce supply impedance.
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Jim Williams

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #18 on: March 18, 2017, 12:27:42 pm »

In a good regulator design you will see around 1 mv of noise, that's a bit more than -60 db down, not bad. It's benign hiss, steadystate white noise. There are no switching artifacts as there is no switching. It shows more hiss than a quiet opamp does.

That hiss does not translate to the active element increasing it's noise as PSU filtering prevents that. With opamps you also get that PSRR ratio spec that nulls out that noise contribution another -80 db or more.

In as this is for a DC heater design, most of the audio concerns can be eliminated.
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Kai

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #19 on: March 18, 2017, 04:43:42 pm »

In a good regulator design you will see around 1 mv of noise, that's a bit more than -60 db down, not bad. ...
In as this is for a DC heater design, most of the audio concerns can be eliminated.
Some tube microphones use the DC heater for grid biasing too.
In this case additional filtering by a large cap is desirable.
Even if this is not the case, noise current introduced into the common ground connection might creep into the audio path.
A 4700uF / 16V cap doesn't cost a fortune.
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MikeCheck

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #20 on: March 21, 2017, 09:34:26 pm »

Update:
Built and installed Uwe's circuit. It's working great, and I was able to fit it in the original location of the Stabilyt cells.
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #21 on: March 22, 2017, 11:45:35 am »

Congratulations!
As I never considered this forum to be a DIY tutorial, I forgot to mention a recommendation to use a LM317-T in the TO220 package and mounting it to the metal chassis for proper heat dissipation.

As to the concerns about noise raised in previous posts to this thread:
a) the LM317 is an analog regulator with the reference far removed from the output path;
b) the filament is too sluggish to let its temperature follow mV noise variations in its supply;
c) to ameliorate any doubts, some data sheets suggest an additional capacitor of 1F or larger
    placed between the 'Adjust' terminal and ground (I never found it to make any difference);
d) anyone undertaking any equipment modifications should be qualified enough to possess a
    basic understanding about proper circuit layout and grounding principles.
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MikeCheck

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #22 on: March 22, 2017, 04:08:05 pm »

Thanks Uwe for all your help and information!

I will change to the 220 package.
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uwe ret

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #23 on: March 22, 2017, 06:25:08 pm »

As the filament current for a single AC701(k) is only 100 mA you will probably be fine with your present arangement. If it is working satisfactorily without getting too hot to touch comfortably, there is no need to replace the regulator IC.
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Jim Williams

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Re: Neumann NN-48 power supply (unmarked caps)
« Reply #24 on: March 23, 2017, 12:00:37 pm »

Use an el cap to bypass the reference/adjust pin to ground on a LM317/337/LT1085, etc. It lowers the noise. I use 47 uf here, a Panasonic FR series tantalum replacement. Those have lower impedance specs than a tantalum cap.
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