Warm Audio WA-67 - Teardown

[Bart_J86]

Beware that REW, although excellent, requires some knowledge of measurement theory. You need to know how the number of FFT points interferes with the LF response. Actually, as the frequency decreases, measurements are less and less reliable. Reading the REW help is not to be neglected.
OTOH, HF measurements are very sensitive to mic placement; in particular, one must clearly identify where the diaphragm is located and make sure the position is the same in each measurement. Placing two mics close to each other does not work correctly.

Thank you, I will definately keep that in mind and read the REW help!

 

[xeawr]

This is a Warm Audio WA67 thread, can you please not Hijack this thread?
Please

You can ask that in private to Kingkorg or do a new thread on that subject that is not at all related to the Warm Audio WA67

My apologies.

Regarding the dome: From my experience it makes quite a difference what's going on underneath the capsule (dome, foam etc.). I was pretty suprised the first time I tried this.

 

[Whoops]

Wouldn't be 10000% shocked if mains variations / tolerances would be involved, especially since the mains-to-secondary ratio is 1:1 or greater. Did you measure your mains voltage at the time?

Mains voltage is very stable in Portugal, there's not huge variations,
My main voltage at the workshop is always between 234V and 236V

The B+ output was set by Warm Audio or the factory producing the unit much lower than the U67 specs and that's not due to mains variations.

I'm not really seeing a reason at the moment for them to have made this decision

 

[kingkorg]

Besides the 4,7uf output capacitor,
I'm also quite curious as to why did Warm Audio set the B+ voltage for the WA67 at 190V instead of 210v.
It seems like a big difference to me.

Do you guys have ideas on what might be the reasons for it? why the 190V?

It might seem as a big difference but it really isn't. It is very unlikely that signal would swing that much to cause clipping, and even then it would be gradual and very musical type of clipping. The capsule's mechanically caused distortion would be a greater issue at those levels. There is also a lot of room for capsule pol. voltage. The variation could easily come from transformer production tolerances.

 

[Whoops]

So I did some experiments with body damping materials.
In the end I used some "felt" inside the body and foam inside the end ring cavity.





There's really not much more room to fit more felt inside.
I did a video showing the before and after, the audio in the video is from the mic itself.
There's improvement in some high end ringing, without the damping it sounds like there's a loose screw inside and that disappears with the damping materials.
Let me know what you guys think

To be honest I don't think this would present a problem in recording, a situation were no one is tapping the mic, but I had all the materials already so it cost nothing to do it, it will not harm for sure

here is the video, better to listen with headphones:

 

[Whoops]

Last week I re-did the Styrene capactors placement and soldering, while I was at it I also resoldered all components in both sides of the pcb (were it was possible), and also solder thinned the unprotected pcb traces.



Before:



After:




Soldering looks much better now and the component placing also.
But I must have made a mistake somewhere as the microphone is not working now.
I have 170V in the B+ now instead of 190V and I have no sound output from the mic.
I probably made a solder bridge somewhere while thinning the solder traces, I have to re-check everything again when I'm back at the workshop next week

 

[Whoops]

This is the PSU,
unlike the microphone it uses Chinese parts, like Power Transformer and Electrolytic capacitors.
I noticed it has a Trim, really bad placed behind the Power Transformer so it's hard to adjust but doable.
I don't know if this Trim is for the Heater or B+ rails.
I hope it's for the B+ rail, as the heater has the proper voltage but stock the B+ is only outputting 191v, I would like to adjust it to the Neumann spec of 210V (+-2%)

 

[Khron]

I'm still quite confused how and why such power supply manufacturers choose to go the passive route (RC filtering), when they could very well halve the amount of passives (400V caps can't be all THAT cheap), and just go with a capacitance multiplier, which would only add a not-all-that-exotic 3-400Vce NPN transistor... Even a heatsink-less TO220 package would be plenty, for the ~1-2mA the B+ needs.

 

[shabtek]

trying to be closer to design they copy?

 

[Whoops]

trying to be closer to design they copy?

I would guess so.

 

[Khron]

What ever happened to "a penny saved is a penny earned", then? Especially with this being a race to the bottom...

 

[Whoops]

What ever happened to "a penny saved is a penny earned", then? Especially with this being a race to the bottom...

Designing a new PSU circuit, would probably be more expensive than just copying an already made circuit that is tested for the last 60 years as working very well...

 

[Khron]

Not like it's space-age rocket science like an SMPS, but what do i know?

 

[Whoops]

Not like it's space-age rocket science like an SMPS, but what do i know?

I also don’t know but Im just glad it’s not a SMPS.

 

[Gus]

Last week I re-did the Styrene capactors placement and soldering, while I was at it I also resoldered all components in both sides of the pcb (were it was possible), and also solder thinned the unprotected pcb traces.

View attachment 88778

Before:

View attachment 88780

After:

View attachment 88781


Soldering looks much better now and the component placing also.
But I must have made a mistake somewhere as the microphone is not working now.
I have 170V in the B+ now instead of 190V and I have no sound output from the mic.
I probably made a solder bridge somewhere while thinning the solder traces, I have to re-check everything again when I'm back at the workshop next week

Check the polystyrene for shorts. The polystyrene caps are very heat sensitive.

 

[Whoops]

Check the polystyrene for shorts. The polystyrene caps are very heat sensitive.

I know Gus, but I didn't overheat any of them while de soldering or soldering,
I'll check that anyway.

Thanks

 

[Gus]

I sometimes hesitate to post things because I often do not have a clue what the other person(s) know.
I hope no one takes it as an insult.

If your up for soldering, maybe after you fix it record some samples with the 4.7uf and the stock U67 uf cap value. Try to record things with under 100Hz fundamentals or male voice 2 feet away and then 6 inches away.

 

[Whoops]

I sometime hesitate to post things because I often do not have a clue what the other person(s) know.
I hope no one takes it as an insult.

No insult taken Gus, I know you're helping and I will check the styrenes for shorts has you suggested.
I will have to trace the circuit again anyway trying to find what I may have done wrong.
I was careful when doing it but if its not working now and it was before I did a mistake for sure, we are just humans...

if you up for soldering maybe after you fix it record some samples with the 4.7uf and the stock U67 uf cap value. Try to record things with under 100Hz fundamentals or male voice 2 feet away and then 6 inches away.

I can do that for as I'm also curious about it, although I'm pretty sure it was done for a reason.
The soldering in the mic is bad for sure, so the factory Warm Audio choose is not very good in soldering and quality control from Warm Audio is not very good, but the mic is made with some pretty good parts (transformer, capacitors, Tube and capsule), and the circuit is an exact copy of the U67, so if they changed this capacitor value there was a reason for it

 

[Gus]

so if they changed this capacitor value there was a reason for it

One of the big deals about triode plate out tube microphones is the transformer.

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 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.

 

[gyraf]

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.

..ouch..