Warm Audio WA-67 - Teardown

[Whoops]

If you look at the PS picture it looks like a 175VAC 9.5VAC transformer and full wave bridge(4 diodes for each are they 1N4007s for the higher voltage?)
1.4 x 175 = 245V This has ripple and the peak value is 35V more than 210
SO
I don't think you will get the voltage up above what it is without issues or a redesign

I’m sorry Gus but I don’t really understand why will there be any problems in achieving 210v output.
175VAC after rectification it’s aprox. 247,45VDC (175 X 1,414 = 247,45)

So I’m not seeing what’s would be the difficulty in getting 210VDC out of 247,45VDC.

maybe I’m missing something here

I can't tell what the Rs in the B+ RC string values are can't tell the color bands from the picture
Measure the voltage at the first cap in the string add up the resistor values use ohms law to figure out the current

The NU-67u has 210VAC 1/2 wave bridge and CRCRC with a 100K load

For the heater supply, is that a LM7806 with a 1N4001? I can't make out the markings.

It looks like the 4.7uf cap is rated at 160V? Think about what happens at power up with a simple supply without a ramp up circuit. B+ up before the heater.

Thanks for the analysis, I can try to trace the schematic if I have time when I’m back at the workshop

 

[Gus]

Think RMS, peak and ripple voltages
Look for the national semiconductor power supply book or app notes by other companies that sell power supply semiconductors or text books.
I gave you the important information in the post that should get you started.

I would post more about what I think but that would be giving away free design work.

EDIT I should also add what happens if the microphone is plugged in with the power supply on?

Look up how metalized film fails

 

[Whoops]

I never encountered any problem in getting 210VDC out of 247,45VDC.
If you think there will be any problem please explain why

the only thing I will do is adjust the Trim pot and check if it’s a B+ voltage adjuster or not

Thanks

 

[Gus]

Again
go the basics
you have 245 VDC with RIPPLE under load. That is why the RC filter
look up the NU-67u versions the transformer is 220VAC in one 210VAC in another you have what looks to be 175VAC so all ready you at too low a voltage
The NU-67us
The NU-67u has CRCRC
you have more RC stages

let say the WA67 works at .9mA and you have 190VDC
245-190=55VDC
55VDC / .9mA = about 61K so your supply if the transformer has a good current output should have about 61K of series resistance in the RCs filter.
Now not having the schematic or resistor color codes to trace the circuit I have no clue what WA was "designing" for.

You MIGHT be able to make the B+ part of the circuit more like the NU-67u types however the transformer might be a limitation.

That all said there should be not much difference in 210VDC or 190VDC in a circuit like that if you set the tube operating points correctly

I would care more about the transformer, capsule and grill environment

EDIT (for the next post in this thread) if they are zeners across the output you most likely are correct so the math for the resistor values would need to be redone using the higher mA due to the zeners.

 

[moamps]

Maybe I'm late to the party but isn't the B+ voltage determined/limited by a string of three zener diodes connected in series at the output?

 

[Whoops]

You MIGHT be able to make the B+ part of the circuit more like the NU-67u types however the transformer might be a limitation.

Thank you so much for your explanations Gus, if the transformer turns out to be a limitation, I can just replace it for another one with higher output voltage capability, or probably I would just rewind this transformer. Windings are wound in a split bobbin, so I can probably just re-wind the high voltage secondary.

I will do what will be needed to achieve Neumann's U67 voltage specs of B+ 210V (+-2%)

Thanks

 

[Whoops]

Maybe I'm late to the party but isn't the B+ voltage determined/limited by a string of three zener diodes connected in series at the output?

I don't have time to trace the PSU at the moment, but will try to do it at some point.
I will take some photos of the PCB for the time being.

But I can confirm there's a string of 3x Zener Diodes connected in series at the output

 

[Whoops]

LOWER B+ voltage
191V instead of 210V (with microphone connected)

So I think I discovered why the B+ voltage is lower than Neumann specs for this circuit.
I measured today the voltage of the PSU without the microphone connected and the B+ with the PSU unloaded is exactly 210V.

What I think it's happening is that the PSU's were designed to achieve 210V output but they forgot that that spec is with the microphone connected, so it's seems they design it to output 210V unloaded.
It's possible that the PSU's and the Mics are done in different factories, so they don't test the voltages with the load from the microphone.

It's just a theory of what might be happening, but it makes sense to me.

Anyway I will sort this out, and make sure in the will be up to spec.

By the way the TRIM POT in the PSU is to adjust the Heater voltage and not the B+,
like in the Neumann circuit, although in this case the Heater circuit uses an LM317

 

[Whoops]

PSU photos

 

[Whoops]

I was able to fix the microphone today, actually founded 2 Gremlins.
Of of them was a shorted out Styrene capacitor, thanks so much Gus for that, I tested them as per your advice and bingo, one of them was shorted.
Replaced that capacitor but still have problems, and them I discovered a solder bridge.
It's working now

 

[Whoops]

Was able to trace to PSU today,
I only traced the B+ part ad the Heater is based around a common adjustable LM317 regulator, it's pretty well filtered and there's nothing exotic over there.

so here is the B+ part with circuit voltages (Microphone connected)

EDITED: R3,R4 and R5 were wrongly stated has 68R, schematic was updated for the correct value that is 6,8K

 

[soliloqueen]

This may not be contributing but excellent work on all this, Whoops

 

[Whoops]

This may not be contributing but excellent work on all this, Whoops

Thanks mate,
I'm enjoying the process. If I didn't like the sound of the mic I would not have gone through all this, but I actually liked the sound a lot and think if it's improved a little bit it will be an extremely useful tool for my recording work.

I'm also sure that this thread might help in the future other WA67 owners.
Can also help any Warm Audio product owner that although bough something that looks good from the outside and it has some good parts inside doesn't know that Warm Audio units have the worst soldering job I've ever seen.
A friend of mine has a couple of API 312 clones from Warm Audio, after this thread he sent me photos of the PCB. It had good parts, Cinemag input and output transformer, Opamp looks good, PCB is fine (much better than the one on this WA67, but the soldering is also crap.

 

[Whoops]

Would like to ask you guys if I can change the values of resistors R1 and R2 in the schematic to achieve a 210V output?

Or do I have to re-wind the transformer HV secondairy for higher voltage?

Thank you so much to all the help and support you have been giving me

 

[emrr]

Yes, adjust R1/2

 

[moamps]

so here is the B+ part with circuit voltages (Microphone connected)

View attachment 88881

Old Mr. Ohm says you made a mistake either in measuring the voltage across the resistors or in measuring the values of resistors R3, 4 and 5.
If you are going to change the values of resistors R1 and 2, will you also change the values of capacitors C2 and C3?
When you raise the B + voltage to 210V, will you change capacitor 4.7uF/160V as well?

 

[abbey road d enfer]

Old Mr. Ohm says you made a mistake either in measuring the voltage across the resistors or in measuring the values of resistors R3, 4 and 5.

Agreed; these 68 ohm resistors look like 6.8k to me.
It is clear that the zeners do nothing with only 192V across them, when their cumulated nominal voltage is 212.
Apparently the current draw on B+ is about 0.9mA. Easiest way would be to decrease R1 and R2 to the same 6.8k value as the others. The zeners would operate in a sub-mA region, which does not give optimum regulation but it's not a big deal, and anyway is better than the current situation where there is no regulation at all.
The zeners would continue to provide overvoltage protection.

If you are going to change the values of resistors R1 and 2, will you also change the values of capacitors C2 and C3?

Do you think it would be necessary? I don't know. I guess only experimenting would answer the question.

When you raise the B + voltage to 210V, will you change capacitor 4.7uF/160V as well?

Excellent recommendation.

 

[Gus]

Are you sure three of the resistor are not 6.8K? I think I see blue gray black brown brown and if you use ohms law and take the voltage drop across each resistor and divide it by the resistor value you will get the current .
This is a series circuit so the current is the same(unless one or more of the caps are leaking )

did you look at the real NU-67u schematics? Your answers and a good hint are in the schematic

Also does this supply have a -6VDC output or +6VDC output. Normally +6 with the use of a 317 and you would normally use a 337 for a negative supply.
If it is +6 how are they biasing the tube? An u67 schematic has a - heater voltage.

Is the heater + supply connected to PS ground?

I could post more but why give free design work?

 

[abbey road d enfer]

Also does this supply have a -6VDC output or +6VDC output. I would think a +6 with the use of a 317 you would normally use a 337 for a negative supply.

Since it is a separate winding, anything is possible. Polarity is just a matter of which pole is grounded.

 

[Gus]

I understand about referencing the voltage note how I ask the questions

however

I never assume anything is done correctly with copy microphones.

I ask the simple questions to try to get people to think and use simple math like ohms law.
I think it is better to not always just give the answer but "point" them in a direction to think about so maybe they will learn something.