Warm Audio WA-84

[Khron]

Inadequate capacitive bypassing

Not that capacitance directly in parallel with a zener has limited effectiveness to begin with, but yeah - RC filtering downstream is to be desired.

 

[Cqwet Dbdfte]

Not that capacitance directly in parallel with a zener has limited effectiveness to begin with, but yeah - RC filtering downstream is to be desired.

yep, the capacitance with the Zener impedance would be the first RC time constant, but better add some more R before the C.

 

[Emmathom]

Hi folks

Thanks for your comments and advice. In the end I will not change the components because I don't think that the problem comes from there.
Already, in the past, I had to clean the circuits 3-4 times with IPA and the mic worked again... 'till next failure.

I think, as some of you pointed out, that the problem is at the level of the very high impedance of the capsule : the 1Go, the 470nF and the gate of the fet. This circuit is very poorly implemented in my opinion. When i build my own circuits I always make sure that the capsule coupling cap (if present), the gate leakage R and the jfet gate are connected together as close as possible and "in the air" without touching the PCB. in the WA84, the jfet is away from the capsule (2cm = quite far) and the connexion is made thru the PCB which is coated with a "resin" that melts with the solder.
The other mic works but I think it's about "luck". I'm sure to be able to build a much better circuitry than this crappy thing...


I don't want to modify the circuit knowing that I want to resell these microphones. I would do it if I wanted to keep them by redoing the PCBs on verobaord...

So I will clean the head, test continuity between capsule's peak and jfet gate, then clean again and again...

 

[jp8]

yep, the capacitance with the Zener impedance would be the first RC time constant, but better add some more R before the C.

Sure, the zener is a noise source. However, if this noise were a structural problem in this design, then any KM84 would have this problem. But this is not the case because Neumann designers were clever enough to design in an RC filter, which is R7 (10k) and C5 (4.7uF) in the original KM84 design. AFAIK, the WA-84 follows the classic KM84 schematic. Furthermore, the circuit has some PSRR, all-in-all attenuating the zener noise by more than 130 dB. I measured the noise of a 24V zener and it amounted to 28 uV. (No use trying to measure that with an oscilloscope....) Of this 28 uV, only 3.2 nV remains at the mic output, which would equal a noise level of -35 dBSPL. Absolutely a non-issue.

Now suppose C5 is open-circuit or broken. Would that cause the noise issues of that one WA-84 microphone? Without the RC filtering, the JFET amplifier + transformer would still have something like 34 dB PSRR, meaning that there would only be 0.56 uV left on the mic output. This would equal a noise level of +10 dBSPL, which, when A-weighted, would be even less. So again, much lower than the self-noise of the microphone. Again, no issue and we can rule out zener noise and a broken C5 as the root cause of the noise issue.

 

[Emmathom]

Cleaning the board, checking continuity from capsule to jfet gate... cleaning again...

 

[Cqwet Dbdfte]

I measured the noise of a 24V zener and it amounted to 28 uV.

Great! Was this the same Zener and mike with the problem? Keep an open mind when troubleshooting. Obviously, if this is a series production unit made in the thousands, this would not be a design issue. However, things can fail.

 

[Emmathom]

Great! Was this the same Zener and mike with the problem? Keep an open mind when troubleshooting. Obviously, if this is a series production unit made in the thousands, this would not be a design issue. However, things can fail.

No it's not the one with issue... Jan has not got it since it's here with me.
As I said I do beleive it's a high Z issue and the front (capsule peak, 1Go, 470pF and fet gate) must be cleaned several times

 

[jp8]

Great! Was this the same Zener and mike with the problem? Keep an open mind when troubleshooting. Obviously, if this is a series production unit made in the thousands, this would not be a design issue. However, things can fail.

No, not the same zener. The WA-84 has through-hole parts. Mine is the SMT type BZX384-B24V that I use in my KM84+/++ designs. But I don't expect a wired version to suddenly have more than 300 times higher noise, which would bring the self-noise of the head amp close to the assumed capsule self-noise.

As said, the circuit follows the classic KM84 design, so was a mass-produced mic. And still is, if you include the clones.

And yes, things can fail. But as I pointed out, even with a broken C5 you would not expect a lot of noise. And if it would be noisy, it would be kind of white noise and not as Emmathom described.

 

[jp8]

I'm just speculating now, but maybe the conformal coating applied to the PCBA in the High-Z area could be the problem, instead of the solution against moisture related issues. If the board wasn't cleaned properly before coating application, and if it was stored at very high RH, tracking could still ocurr between PCB surface and coating. Tiny amounts of ionic remnants could still cause tracking in combination with 0.1-0.2% moisture content of the PCB laminate, even if coated. I've seen this happening in field return products in my professional life. So cleaning an removing the coating could indeed be the solution.

 

[Emmathom]

I'm just speculating now, but maybe the conformal coating applied to the PCBA in the High-Z area could be the problem, instead of the solution against moisture related issues. If the board wasn't cleaned properly before coating application, and if it was stored at very high RH, tracking could still ocurr between PCB surface and coating. Tiny amounts of ionic remnants could still cause tracking in combination with 0.1-0.2% moisture content of the PCB laminate, even if coated. I've seen this happening in field return products in my professional life. So cleaning an removing the coating could indeed be the solution.

How to remove the coating ??? Isopro Alcool does not...
- Acetone ? (good against all glue)
- Trichloroethylene ?
I'm 99% sure it's in the high Z area... The components here (thru-hole) should be connected "in the air" > 1Go + 407pF + jfet gate (but the latter is 2-3cm away from front and I don't want to modify so much the circuitry since I want to resell asap this crappy pair)

 

[jp8]

Maybe by heating up the coating, then remove mechanically.

Or use special coating removal chemicals. Cheapest that I could find: https://www.digikey.com/en/products...IgTCBcDaIMIHsCGAXAlgOwOYAIBOApgLYIBuBeIAugL5A

Mind you: remove polystyrene capacitors as they will be destroyed by IPA, Aceton or this cleaner. Having said this: I now see the WA-84 has such capacitors, so perhaps this already happened... I'd replace them by C0G ceramic capacitors. I'm about to test microphonic behavior of capacitors and allegedly, polystyrenes are sensitive to microphonics. Add to this the chemical sensitivity and there is no reason to use polystyrene caps. Have you tried replacing the polystyrene caps already?

 

[Emmathom]

The Digikey link does not open...

I did cleaned the PCB with polystyren caps on

I did not yet try to replace the poly. caps

 

[wafelki]

Polystyrene caps destroyed by IPA you say? Anybody seen one?

Stupid question but - its not the capsule, right?

How many times cleaning helped this problem before? And for how long?

Have you tried the hair dryer?

Here in Auchans there is that "Pikko Universal Diluter", 500ml plastic bottle for like 1.50 euro. A mix of 10 different solvents that you smell 3 floors down but yeah, you may call it hi-z low noise magic elixir And thats what you need to save your polystyrenes from!

 

[ricardo]

Mind you: remove polystyrene capacitors as they will be destroyed by IPA, Aceton or this cleaner. Having said this: I now see the WA-84 has such capacitors, so perhaps this already happened... I'd replace them by C0G ceramic capacitors. ... , allegedly polystyrenes are sensitive to microphonics. Add to this the chemical sensitivity and there is no reason to use polystyrene caps. Have you tried replacing the polystyrene caps already?

Not allegedly but in reality. You can test this for yourself by using a polystyrene in place of a capsule. I found this circa 1980 and Guru Wurcer did so more recently. I think he mentions it in his Linear Audio articles.

And polystyrenes often die when flow soldered too. These days, there's no reason to use polystyrenes when COG/NPO ceramics are cheaper, more robust & quieter.

But if you have an existing mike that's OK, just leave them in. The microphony may be part of the sound you like

 

[Emmathom]

Hi wafelki

Polystyrene caps destroyed by IPA you say? Anybody seen one?

Stupid question but - its not the capsule, right?

Nop, swap with the other one (which works in the other body) and still failure

How many times cleaning helped this problem before? And for how long?

2-3 times and 'till the next time... I mean before going recording > test > issue > cleaning > recording ok > back home - mics back to storage. Then next time 50% ok 50% issue. The mics are stocked with silica gel close to the capsules (as any of my mics)

Have you tried the hair dryer?

Nop

Here in Auchans there is that "Pikko Universal Diluter", 500ml plastic bottle for like 1.50 euro. A mix of 10 different solvents that you smell 3 floors down but yeah, you may call it hi-z low noise magic elixir And thats what you need to save your polystyrenes from!

I can give it a try (and change that polystyren caps for C0G/NP0)

 

[Emmathom]

And polystyrenes often die when flow soldered too. These days, there's no reason to use polystyrenes when COG/NPO ceramics are cheaper, more robust & quieter.

Hi @ricardo , do you think the ones in pict attached would do ? (470pF & 5pF) Mouser charges 20€ for 4 caps (2€ total) when Banzai music only charges 9€...

But if you have an existing mike that's OK, just leave them in. The microphony may be part of the sound you like

Knowing the issue is certainly in the high Z area (and that I already changed the 1Go to ground) I must change the 2 polystyren caps to pursue "my procedure"

 

[ricardo]

do you think the ones in pict attached would do ? (470pF & 5pF) Mouser charges 20€ for 4 caps (2€ total) when Banzai music only charges 9€...

If a reputable supplier advertises them as COG/NPO, they will be OK. I think both Mouser & Banzai are in this category