Ela M251 C12 PSU

[garp]

Long time ago i build of the the Matador's C12 PSU, with great success. With this PSU mics are quiet and smooth. Then i deceided to buy Thoman's Retrotube PSU (120V / 6,3v ) for my Ela M251 clones. Compared to the Matador's PSU , Thoman's PSU is very noisy and mics sounds harsher. Do you think there is a simple way to improve filtering and noise ? Attached are views of the PCB used in the retrotube PSU :

 

[ZHOUJUN]

If a loud noise is generated, it is recommended to replace all components, especially the voltage regulator module of the heater. Transformers are also important.

 

[garp]

To be clear, the noise is not due to faulty components, just that it is noisier (by far ) than matador's PSU. The mic is hissing 6db higher than Matador's PSU , i was just asking about improvement

 

[rock soderstrom]

Do you think there is a simple way to improve filtering and noise ?

Basically, you can see what Matador does differently. I don't find the Zener diodes really convincing (Matador doesn't have any), I would take them out or at least install more filtering afterwards. Better rectifier diodes (UF4007 for example) can help, as can an additional MKP cap after the transformer or bridge rectifier. The LM317 is superior to the LM7806, Matador also has more RC filtering for the heater voltage. Possibly also elevate the heater voltage?

 

[Khron]

Long time ago i build of the the Matador's C12 PSU, with great success. With this PSU mics are quiet and smooth. Then i deceided to buy Thoman's Retrotube PSU (120V / 6,3v ) for my Ela M251 clones. Compared to the Matador's PSU , Thoman's PSU is very noisy and mics sounds harsher. Do you think there is a simple way to improve filtering and noise ? Attached are views of the PCB used in the retrotube PSU :

Is that PSU providing 120V DC on the B+, under load (ie. with the mic connected)? I was under the impression those provided 150V or more...

 

[rock soderstrom]

Is this the schematic of your PSU, Garp?

 

[garp]

It looks like , same 7806 regulator same values

 

[garp]

Is that PSU providing 120V DC on the B+, under load (ie. with the mic connected)? I was under the impression those provided 150V or more...

You are right ! (as always !) B+ is at 142v under load and there is no trimmer ! Ughh !

 

[garp]

I'll try replacing some of the 6k8 with 10k to see if i can drop the B+ to a more desirable value. Did you suggest to replace the zeners with 1N4007 ?

 

[rock soderstrom]

Did you suggest to replace the zeners with 1N4007 ?

No, don't do that. Why would you want to do that? They have a different function. The Zener diodes function here as a voltage stabiliser, they determine your output voltage.

https://www.google.com/amp/s/testbook.com/amp/physics/zener-diode-as-a-voltage-regulator

 

[rock soderstrom]

A small addition: Zener diodes tend to produce some noise. As already written, if you leave them in, you should add some filtering afterwards or take them out completely and adjust the voltage with the series resistors (like Matador)

 

[garp]

Can i replace one of them with a trimmer ?

 

[RuudNL]

I replaced the zener diodes in a Thomann PSU with 62 and 56 V. versions (~ 118V, 1.3 Watt.)
That gives me about +120 V.
No noise whatsoever!

 

[rock soderstrom]

No noise whatsoever!

Really, my experience wasn't that good. I had to retrofit RC filtering in my SELA T12 clone microphone...since then I have been somewhat sceptical about Zener stabilisation

 

[Khron]

I replaced the zener diodes in a Thomann PSU with 62 and 56 V. versions (1.3 Watt.)
That gives me about +120 V.
No noise whatsoever!

Teeeeechnically, it might be enough to even just tack on a 50-52V zener in parallel with one of the existing ones, to drag the voltage down to ~120V, wouldn't it?

Only because it might be simpler to do that without removing the whole PCB, and the two stock zeners, and replace them?

 

[garp]

What about noise ?

 

[rock soderstrom]

Test it. You can direct compare which way is better noise wise. Keep us posted

 

[HerbertR]

When modifying these China ... SCT2000 power supplies ... with 180VDC B+ I always first remove the two zener diodes or disconnect a leg from them so those two diodes can't do their "job". Then I put/set a "big" dummy resistor representing the microphone into the B+ pin and ground Pin at the 7-Pin Connector instead. Now I measure the actual B+ voltage e.g. 160VDC and recalculate the drop resistance needed down to e.g. 120V, 116V, 105VDC ... or whatever new is needed. Then I replace/increase a few resistors from R1 - R5 up to the new calculated Drop resistance until I play in the new ballpark. After that I can safely connect the microphone and fine tune the new voltage by adjusting/replacing one of the resistors. And after that as final step I put new zener diodes as voltage protection. But my experience is that zener diodes from the same batch tend to have a high tolerance. For example, 62V zener diodes showed a tolerance of 58-66V or more from the same batch. I would leave the sum of the two zener diodes a few volts above B+. Then they do their job as voltage protection in this "unregulated" circuit and are not a "direct" regulator. My impression was that too hard-working zener diodes tend to produce a little more audible noise. So I would adjust the B+ Droping Resistors typically like in "unregulated" Mic Circuits. With Zener Voltage Protection...My 49 EuroCents....

 

[lightningbefore]

Similarly, I have a Beesneez C12 PSU. Can I safely use that straight away with my Dany 251 clone mic without having to mod the PSU? As long as it's C12/251 PSU it should be ok?

 

[garp]

@HerbertR , i tried your method and reached the desired tensions by adding a 47k pot on B+, and only changed the big 5 ohms resistor on the heater (around 7v from factory) to get something closer to 6v . Now i don't have any 120v zener, can i use this PSU "as is" before getting my hands on the correct diode ? I also added additional filtering (1uf 400v) on the B+ rail, and increased the 220nf film cap to a 470nf filtering on the heater, just because i have some good ERO MKT in spare