TapeOp Omnis and Phantom?

[nacho459]

So I want to build half a dozen of the TapeOp omni mics, but I want to be able to run them off regular +48v phantom power instead of the 9v batteries. Have any of you done this before? I'm guessing I would need some type of step down transformer or attenuator to knock it down, but then how do I keep it from also affecting the sound?

 

[smilinfu]

I am of no assitance, but what transducer are you using? I could not find the one mentioned in the article anywhere.

 

[Scodiddly]

There's a fairly common circuit for this, originally taken from the Radio Shack PZM phantom mod.

My version, which requires the 3-wire "Linkwitz mod" (try that in Google) can be found at:
http://www.scotthelmke.com/phantom-power.jpg

It's a pretty typical version, and can be easily adapted for non-modded capsules. But the mod is really worth learning and doing, since it makes the capsules capable of handling much higher SPL.

 

[moosapotamus]

There's a big thread about this over at TapeOp (well, where else?). Do a search on "DIY mics," and it should turn up.

No trafo needed, btw. The discontinued capsule in the article can be replaced with WM-61a (Digi-Key).

~ Charlie

 

[alk509]

[quote author="Scodiddly"]It's a pretty typical version, and can be easily adapted for non-modded capsules. [/quote]

Hey Scott, I remembert somebody saying that using two 10K resistors in place of the 2k2 ones yielded better distortion figures... do you know anything about that?

Peace,
Al.

 

[nacho459]

Scott, What is that little mic pictured at http://www.scotthelmke.com/mic-other.html Is it just a WM-60 connected to an XLR? I would imagine that there is some sort of circuitry inside the XLR connector, right?

 

[Scodiddly]

I'm in the process of breadboarding some variations on my circuit to try to lower the noise floor a bit, so maybe 10K is better than 2.2K. But we'll see...

Nacho, that mic is indeed just a WM-60 capsule on an XLR. It has to be plugged into a special power supply box with the phantom circuit from above, or a battery box like the TapeOp omni. I built a couple of dual-channel phantom circuit boxes to use with various capsules, easier than building a phantom circuit for each one.

 

[moosapotamus]

[quote author="alk509"]...I remember somebody saying that using two 10K resistors in place of the 2k2 ones yielded better distortion figures...[/quote]
I've tried both 2k2 and 12k, and found lower noise with 12k resistors.
But, I'd be interested to hear what Scott finds regarding distortion.

~ Charlie

 

[Scodiddly]

[quote author="moosapotamus"][quote author="alk509"]...I remember somebody saying that using two 10K resistors in place of the 2k2 ones yielded better distortion figures...[/quote]
I've tried both 2k2 and 12k, and found lower noise with 12k resistors.
But, I'd be interested to hear what Scott finds regarding distortion.

~ Charlie[/quote]

Since I don't have any good way of measuring mic distortion, I'm going for "sounds good" and low noise. The choice of resistor should really hinge on what current the capsule FET can handle and what the supply voltage is used. I've been quite happy with the sound of 2k2, but had to ditch my mics for a recent piano recording because the noise floor was too high.

 

[moosapotamus]

Scott,
"Sounds good" is my main mode of operation, too. I notice that you have 0.47uF caps in there, while I've been using 1.0uF. I guess that's going to impact the frequency response, but what about noise/distortion? Any ideas? Maybe I'll just try it out.

~ Charlie

 

[Marik]

[quote author="Scodiddly"]There's a fairly common circuit for this, originally taken from the Radio Shack PZM phantom mod.

My version, which requires the 3-wire "Linkwitz mod" (try that in Google) can be found at:
http://www.scotthelmke.com/phantom-power.jpg

It's a pretty typical version, and can be easily adapted for non-modded capsules. But the mod is really worth learning and doing, since it makes the capsules capable of handling much higher SPL.[/quote]

Scott,

The "Linkwitz mod" to my ears improves the overall sound, as well.
When I was working on my ECM8000 mod, I also tried the schematics you posted (Schoeps circuit never dies :grin: ). For me it did not sound very good, as it lacked bass, and also was kinda dry. I came up with the following schematics, I posted on HR forum:

http://homerecording.com/bbs/attachment.php?attachmentid=8978&stc=1

 

[Scodiddly]

Huh... I went back and checked some of my own responses on the TapeOp thread... here's the result of my testing back then:

I sat down and did some testing. I hooked up a 100 watt amp into a
10" guitar speaker, and used a specific spot on the grill cloth as my
reference point. I measured both mic output (from a mixer channel direct
out) and speaker drive signal, both in dBV. Distortion by visual
observation on oscilloscope.

I tried all three setups: unmodded, half-modded (my version), and the fully
modded, impedance balanced version posted by ljudatervinning. I did two
sets of tests, one with 2.2k load resistors and one with 10k load resistors.
Panasonic WM60-AY capsule (same capsule for all tests).

2.2k load resistors:
unmodded mic output -10, speaker at 2.3, mic almost distorted
half-modded: mic output -3.5, speaker at 10.5, mic almost distorted
fully-modded: mic output -6, speaker at 10.5, mic almost distorted

10k load resistors:
unmodded mic output +5.1, speaker at 8, mic almost distorted
half-modded: mic output 0, speaker at 12, amp at limit
fully-modded: mic output -4.3, speaker at 12, amp at limit

10k load resistors, capsule almost touching speaker dust cap:
half-modded: mic output +4, no visible distortion, amp at limit
fully-modded: mic output -2.5, no visible distortion, amp at limit

Results? I think I'm right. Wish I could get it loud enough to see the two
different mods into mic distortion, though. It also looks like the classic
PZM phantom power circuit would better match the Panasonic capsules if
implemented with a higher load resistance. It's worth pointing out that the
unmodded half-modded versions have twice the load resistance of the
fully-modded version. Hmm...

So why have I been using 2.2K resistors? I dunno, must have started working off an old schematic after taking a long (several weeks) break from building these mics. If the ideal resistance is 1.2K per volt on the capsule, then 12*1.2 = 24.4, and divided by 2 gives roughly 12K ohms.

D'oh.

 

[moosapotamus]

[quote author="Marik"]I came up with the following schematics, I posted on HR forum:
http://homerecording.com/bbs/attachment.php?attachmentid=8978&stc=1[/quote]
That looks interesting.
I've been tinkering with WM-61A capsules and might like to try it out.

Can anyone suggest some possible replacements for the 2SK389GR?
I can't even find a datasheet.

Thanks
~ Charlie

 

[Marik]

[quote author="moosapotamus"]

Can anyone suggest some possible replacements for the 2SK389GR?
I can't even find a datasheet.

Thanks
~ Charlie[/quote]

Datasheet is here:

http://www.pcpaudio.com/pcpfiles/transistores/2SK389.pdf

Kevin Carter of K&K audio has them:

www.kandkaudio.com

Or you can use two 2SK170GR's, instead.
Please, pay attention that capsule is "floating" (it was PRR's idea), and its body is not grounded.

 

[moosapotamus]

Thanks, Marik!

[quote author="Marik"]Please, pay attention that capsule is "floating" (it was PRR's idea), and its body is not grounded.[/quote]

Yes... sort of a twist on the Linkwitz mod? In other words, still need to cut the trace from pad2 to the case on the capsule. But, the case connects to the junction of those 10K resistors instead of ground, yes?

Thanks, again!
~ Charlie

 

[moosapotamus]

Hey Marik - I finally got a chance to put your modified ECM8000 circuit up on my breadboard and I have a question about the transistor connections... I've got the source pins connecting together through that 10ohm resistor in the middle. Is that right? Or, do the drains connect together, or drain on one to source on the other? (I can never figure out which is the drain and which is the source unless they are labeled in the scheme. )

I had to use 100uF caps on the output 'cause all the 47uF's I have are too small (35V or less). But as it is, I like it much better than the Schoeps-type circuits that I have tried, too. They seem somewhat harsh in comparison. This seems to be much lower noise, too. Overall, warmer and more natural sounding to me.

Thanks for sharing!
~ Charlie

 

[Marik]

I drew all the sources on the bottom, so 10 Ohm from ground goes to source of the lower FET, then from drain through another 10 Ohm resistor to source of the upper FET.

Yes I also like this schematics better than Schoeps (the reason why I used this one :grin: ). To me it sounds more neutral and smooth. Now my main objection is the capsule--that's why I don't play with the ECM8000 anymore. Although Panasonics are good for what they are, there is no comparison to a "real" thing. I have some ideas about 1/4" DIY omni caps, and will try them as soon as I have some time.

 

[moosapotamus]

Cool, thanks! I'm not hearing or seeing any noticable difference on my meters. But, I'm willing to go with the sources on the bottom. I'll probably make myself at leat two of these to use for general purposes.

I hear what you're saying about "the real deal." But, bang for buck, it's well worth it to me to have a few of these to kick around. :green:

Thanks, again!
~ Charlie

 

[PRR]

> I can never figure out which is the drain and which is the source

It often does not matter one bit.

Most JFETs are symmetrical, or nearly so. The breakdown voltages are pretty sure to be the same either way. What sometimes varies, and makes a JFET asymmetrical, is the capacitance: they can shift the Gate over closer to the Source to get less Drain-Gate capacitance. But the difference may be like 5pF versus 7pF: sometimes helpful in radio stages where total capacitance may be around 10pF, but not a big deal in audio where even a condenser capsule is 30pF and most other audio much higher.

So yeah: unlike BJTs, which mostly care which is Collector and which is Emitter, JFETs mostly don't care which way. (The Gate is unique and does have to go the right place.)

 

[moosapotamus]

Good stuff, PRR! Thanks!

~ Charlie