Warm Audio WA-67 - Teardown

[abbey road d enfer]

How much is the current in the microphone going to vary depending on signal level ?
maybe 3% of 1ma with class A ? Our zener doesnt have much work to do to compensate for current in the circuit itself ,

Agreed. The question is how much regulation is needed for proper operation?
We are not dealing with measurement microphones; a variation of sensitivity of a fraction of dB is not an issue.
So my answer is we don't need regulation, but we need some kind of overvoltage protection.

What I want to know is how to set the bias point for the zener diodes so we make best use of their abillity to regulate any small mains voltage variations ?

The higher the current the better the regulation, but one must not exceed max dissipation. For a 75V 1W, current should not exceed 13mA? but it would not make much sense for feeding a load that draws less than 1mA.

Would it be more effecient to arrange so the zener barely conducts under normal circuit opperation ,maybe 0.1mA ,

That's just about what it actully does. Simulation shows about 225 uA in the zeners, however one must take into account mains voltage variation, zener tolerance (typically 2% or more), xfmr manufacturing tolerances, resistor tolerances.

and only in the case of mains over voltage does or a disconnected load does it start to dump more current ?

That's the case with the current arrangement.

 

[moamps]

This zener diode has a dynamic resistance at that operating point (0.25mA) of 2000 ohm, and at an operating point of 3mA more than 10x less. The OP can very quickly measure the noise at the output of this power supply using an oscilloscope for different operating points of the zener diode and if there is no increase in noise by Id = 3mA, then it is certainly a better point than the one where Id = 0.2mA.

 

[Tubetec]

Im sure theres probably better software out there for modeling , but the simple block building approach of PSUD I instantly took too . Its a bit finiky at times and you can only process a 30 second segment in any one pass of the simulation , as you will see from the plots an 80 second 'non report' gap to invesigate ramp up behaviour over the only the first minute or two after power up is possible . The nice thing is it does tube rectifier sims also

 

[abbey road d enfer]

This zener diode has a dynamic resistance at that operating point (0.25mA) of 2000 ohm, and at an operating point of 3mA more than 10x less. The OP can very quickly measure the noise at the output of this power supply using an oscilloscope for different operating points of the zener diode and if there is no increase in noise by Id = 3mA, then it is certainly a better point than the one where Id = 0.2mA.

I agree, but can we also agree that regulation is not a paramount matter?

 

[moamps]

Yes and no, which I explained in my post 228. It should further be noted that Neumann defines this voltage 210V +/- 2% which means that for them the stability and accuracy of this voltage was important when designing the power supply. In the end, the noise of the zener diode is higher in the knee area, so I see no reason to place it at that operating point unless a zener compressor is designed.

 

[abbey road d enfer]

Yes and no, which I explained in my post 228. It should further be noted that Neumann defines this voltage 210V +/- 2% which means that for them the stability and accuracy of this voltage was important when designing the power supply.

Not necessarily. It could be just that it's the nominal value indicating the unit is operating as expected.
Checking the voltages was, and is still in many cases, the first step in assessing proper operation of an electronic device.

In the end, the noise of the zener diode is higher in the knee area, so I see no reason to place it at that operating point unless a zener compressor is designed.

As you mentioned, the dynamic resistance of the zener is about 2k, so whatever noise it produces is attenuated by more than 50dB@1kHz.
With an estimated noise voltage density of 0.5uV/sqrtHz, it results in a voltage that is negligible compared to the electronic noise at the tube's anode.
Of course, it's pure speculation that would need to be confirmer (or not) by experimentation.

 

[Tubetec]

Morgan Jones describes a long chain zener voltage stabiliser using 5v1 or 5v6 zeners , up to around 30 of them or more in a string depending on the voltage required . Although there is a penalty in terms of layout the cost of a bag of small voltage zeners would only be a few euros in any case , is it worth going to the extra trouble for perhaps better voltage control and less zener noise ?

In the case of the NU67 psu can we simply replace the 68k bleed resistor with a suitable string of zeners ? , only trouble I can see with that scenario is the caps have no discharge path once the voltage falls below the zener voltage after the power is switched off , could we place another resistor (220kohms)earlier in the chain so after the zeners stop conducting the voltage slowly bleeds away ?

I cant see any potential downsides with a long slow ramping up of HT , I can see a few benefits in terms of longevity of PSU components and possibly tube life compared to the 2 second ramp up with the standard value 22uf caps . I can see a big 680uf right after the rectifier bridge initially draws a large peak current after switch on possibly causing the transformer core to saturate on the peaks and extra noise in the supply until the voltage stabilises , but that all happens long before we're asking our mic to reproduce sound .

 

[Whoops]

I'm having a lot of mixing work at the moment and I'm also teaching so I didn't have time recently to get back into this project. Will get back to it as soon as I have time

But in the meanwhile I sent an email to Warm Audio inquiring about the capsule,
asking why they say in their website "That meant not only ensuring that the hole pattern was identical" when the capsule they supply has a completely different hole pattern than the K67 and why they state "we went to great lengths to recreate the capsule from the original ’67, making sure that ours performs the same way, has the same frequency response and contributes to the same chewy character as the original"
when the capsule provided has less high end boost than the K67 so it's actually darker so has a completely different frequency response of the original.

After some weeks I got a reply saying that the capsule I have was not a factory mistake and that is the capsule they supply with the mic and that "we crafted the capsule with the vintage '67 sound as top priority, and massaged the original design."
Well they didn't "massaged" the original design as they didn't develop this capsule it was actually 3U Audio that developed it, also if a vintage 67 sound was really their top priority they would not have used a Darker capsule than the original one (while maintaining the same circuit)
No explanation was given as to why they are lying in their website, but I just noticed that they changed the original text and removed the part were they said "that the hole pattern was identical" after my email inquiry.

It's too much bullshit, marketing bullshit, I have this WA-67 mic, I will improve it a bit and use it a lot for sure but to be honest I will never buy anything in the future from Warm Audio neither I will recommend it to anyone.

 

[soapfoot]

Yeah, that's not good business.

 

[Whoops]

Had some spare time today to realx and work on the PSU.

I did as Abbey suggested, changed the Zener string to before C5 and added a Zener, I didn't have 7,5V or 8,2V zeners in my stash but had a 9,1V one so I tried with that.
All done, I'm getting between 209V and 210V at B+ so it's pretty good and within Neumann's specs.

So I will close the PSU adventures and leave it as is, as I'm happy with the result.

here is the PSU schematic after the mods




So after working on the PSU there's only 2 things I will do.
I will try the microphone with a standard K67 capsule and compare the sound to the stock 3U Audio modified K67 capsule (modified for a flatter frequency response).
I will also try 0,5uf and 1uf cap values (Neumann values) for the output capacitor instead of the stock 4,7uf, I have to replace this capacitor anyway since it's only rated at 160V so i might as well try the original U67 values. It seems the 4,7uf value would increase the response in the very low-end but it interacts also with the transformer and this mic uses a Lundahl transformer that it's probably not a BV12 exact replica.
But to do this 2 changes and tests I will need to get hold of some original Neumann U67 mics so that I have something to compare against with, I have to ask in some friends studios to see if I can make the tests there and use their U67s as a reference.

Thank you Abbey and thanks everyone that helped.

 

[Ricardus]

This might be of general interest to some people.

 

[Tim Campbell]

I'm having a lot of mixing work at the moment and I'm also teaching so I didn't have time recently to get back into this project. Will get back to it as soon as I have time

But in the meanwhile I sent an email to Warm Audio inquiring about the capsule,
asking why they say in their website "That meant not only ensuring that the hole pattern was identical" when the capsule they supply has a completely different hole pattern than the K67 and why they state "we went to great lengths to recreate the capsule from the original ’67, making sure that ours performs the same way, has the same frequency response and contributes to the same chewy character as the original"
when the capsule provided has less high end boost than the K67 so it's actually darker so has a completely different frequency response of the original.

After some weeks I got a reply saying that the capsule I have was not a factory mistake and that is the capsule they supply with the mic and that "we crafted the capsule with the vintage '67 sound as top priority, and massaged the original design."
Well they didn't "massaged" the original design as they didn't develop this capsule it was actually 3U Audio that developed it, also if a vintage 67 sound was really their top priority they would not have used a Darker capsule than the original one (while maintaining the same circuit)
No explanation was given as to why they are lying in their website, but I just noticed that they changed the original text and removed the part were they said "that the hole pattern was identical" after my email inquiry.

It's too much bullshit, marketing bullshit, I have this WA-67 mic, I will improve it a bit and use it a lot for sure but to be honest I will never buy anything in the future from Warm Audio neither I will recommend it to anyone.

Well Warm approached me initially to design a capsule for their 414 clone. They wanted me to come up with a prototype that would be mechanically similar to a real CK12 but that they could build in China and their criteria was that it couldn't cost more than 100 USD. When I told them that I wasn't really motivated by that kind of business model they said it was ok because 3U Audio had sent them a real CK12 capsule that they could buy for 50 dollars. He showed me their capsule. It was 3U's K67 type edge terminated capsule. I told Bryce that wasn't really a CK12 capsule and he told me it looked like one to him.

 

[Whoops]

I told Bryce that wasn't really a CK12 capsule and he told me it looked like one to him.

Well in the WA67 mic the capsule fitted doesn’t even look like a K67 at all, clearly seen to the naked eye that the hole pattern is no the same, that didn’t seem to bother them and even write on their website that the “hole pattern was the same”

thanks for sharing that story Tim, it’s a cheap company that invested in image and marketing and that are full of crap

 

[TheJames]

I told Bryce that wasn't really a CK12 capsule and he told me it looked like one to him.

Well...There ya go. That's all you need to know folks.

I think the summary would be, Warm Audio makes decent to good sounding microphones at a target price-point. However they mis-label and incorrectly market the microphones as copies of legendary microphones of yesteryear when they make obvious shortcuts and use incorrect parts and materials to produce said microphones. In addition, the microphones are sometimes poorly assembled and subject to failure.

Built to price-point, not to a quality standard.

 

[TheJames]

Well Warm approached me initially to design a capsule for their 414 clone. They wanted me to come up with a prototype that would be mechanically similar to a real CK12 but that they could build in China and their criteria was that it couldn't cost more than 100 USD. When I told them that I wasn't really motivated by that kind of business model....

Just curious Tim....What kind of capsule would you find interesting and motivating? If you could design a microphone capsule that would get you excited, what would it conceptually look like?

(If you chose to entertain this topic should probably be it's own thread. If you're interested in thinking out loud, please do start a new thread with your musings. I'm sure many of us would be interested in your thoughts.)

 

[Tim Campbell]

Well I like the capsules I currently build but I have a few unique designs and am just about to start the prototyping of a completely new LDC capsule.
Old designs have really been done to death and there is a lot of room for innovation.
Just start with what you find the shortcomings of current capsules to be.

 

[Whoops]

Built to price-point, not to a quality standard.

That's right, but I'm fine with that but I don't think it's fine the lies that they don't have remorse in marketing, it's just bullshit.
I think company Integrity is really important, one can sell cheap stuff and have integrity, Warm Audio doesn't have any.

 

[Briain]

This might be of general interest to some people.

Hi Folks

Yes, I subscribe to his channel and I rather enjoyed that mini-tour of the innards (unusual to see Mr Carlson doing an 'unboxing video', but good to see that he 'unboxes' it more than most unboxing videos; unboxed right down to the PCBs) and I'll be interesting to see his next video showing what he'd modified in his existing WA microphone.

I also recently watched a couple of interesting videos from a channel I subscribe to called 'IMSAI Guy' (he's an ex-HP optical chap, but though he does occasionally cover optical, he mostly covers non-optical things including test equipment and radio / amateur radio related equipment) who is - it would appear - pondering a new microphone for his channel. He walks through the circuit design of a Rode vs a Neumann TLM 103 (and it's also interesting to see the very cool PCB in the TLM 103; now we can see where some of the money goes).



There's a second video, but for some reason YouTube is acting up at the moment and I cannot get the link for it (though it should automatically pop up after the first video, I'd expect).

Briain

PS Sadly, after looking at the picture at Neumann stage microphone KMS 105, the PCB in my KMS 105 looks far less exotic than the one in the TLM 103 (but more standard though its PCB might be, it still sounds jolly nice, to me). ;-)

 

[kingkorg]

Hi Folks

Yes, I subscribe to his channel and I rather enjoyed that mini-tour of the innards (unusual to see Mr Carlson doing an 'unboxing video', but good to see that he 'unboxes' it more than most unboxing videos; unboxed right down to the PCBs) and I'll be interesting to see his next video showing what he'd modified in his existing WA microphone.

I also recently watched a couple of interesting videos from a channel I subscribe to called 'IMSAI Guy' (he's an ex-HP optical chap, but though he does occasionally cover optical, he mostly covers non-optical things including test equipment and radio / amateur radio related equipment) who is - it would appear - pondering a new microphone for his channel. He walks through the circuit design of a Rode vs a Neumann TLM 103 (and it's also interesting to see the very cool PCB in the TLM 103; now we can see where some of the money goes).



There's a second video, but for some reason YouTube is acting up at the moment and I cannot get the link for it (though it should automatically pop up after the first video, I'd expect).

Briain

PS Sadly, after looking at the picture at Neumann stage microphone KMS 105, the PCB in my KMS 105 looks far less exotic than the one in the TLM 103 (but more standard though its PCB might be, it still sounds jolly nice, to me). ;-)

He has made quite a few bloopers there, and mixes up things. He doesn't claim to be a mic expert, so i guess it's ok. Your KMS is not less exotic or worse by any means than TLM103.

The difference in price is due to the fact the Neumann pretty much invented what we today regard as condenser studio microphone (not exactly but bear with me). 99% of the people picture a Neumann in their heads once studio is mentioned. You pay for german manufacturing, QC, taxes, hourly rate, and R&D both historical and future. Neumann is not made by low wage immigrants, or OEM but by well educated professionals. You pay for the badge as well. Others more or less make stuff based on what Neumann or similar companies had to come up with in the past. Røde is based on automated process, and pretty much one design Doug Ford came up with. I love Røde, don't get me wrong, but it's a different approach. Not much R&D going on.

 

[Delta Sigma]

Well I like the capsules I currently build but I have a few unique designs and am just about to start the prototyping of a completely new LDC capsule.
Old designs have really been done to death and there is a lot of room for innovation.
Just start with what you find the shortcomings of current capsules to be.

Any SDC plans???