Mic capsules distorting at low level in binaural dummy head. Why :( - Page 5 - diyAudio
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Old 1st October 2012, 10:46 AM   #41
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Although now I'm completely puzzled, yesterday the 33k and 22k in series gave me a reading of 6.4v... today without changing anything it's reading at 13.1v.
What's going on....!?
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Old 1st October 2012, 10:50 AM   #42
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Quote:
Originally Posted by Mr Chris View Post
Hi,

I have done the Linkwitz capsule mod and used his dual 9V battery powered OPA2134 Preamp. It works very well.

The main point to note is the modded capsule supply comes from the -9V rail.

I don't think your circuit is compatible with the modded capsule?

Some active D.I boxes use a SMPS from the +48V phantom to supply +/- rails of an opamp.

Just a thought.

Chris.

Huh... that's interesting. Bill Wall of this site told me the circuit would be compatible with the modified capsules.
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Old 1st October 2012, 11:06 AM   #43
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Poxican View Post
I'm trying all sorts of values here but can't seem to hit a sweet spot. I'm thinking it's best to maintain a .5mA supply since we don't have a graph of how the voltage/current correlates.

I'm wondering if maybe the 6v/.46mA will prove to be the best result.
I really don't know what to advise for the best on this. You have to run the capsule at or within its recommended voltage range. The current it draws will be determined by that voltage... that's something you have no control over. The current will be whatever the capsule takes at that supply voltage. If all that is correct then I guess performance limitations come down to either the capsule of its implementation. I just wouldn't like to say... it's not something I have practical experience of.
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Old 1st October 2012, 11:16 AM   #44
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Poxican View Post
Although now I'm completely puzzled, yesterday the 33k and 22k in series gave me a reading of 6.4v... today without changing anything it's reading at 13.1v.
What's going on....!?
That needs investigating. First be absolutely sure the meter leads and connections are good. Flux and oxidation can give rise to inconsistent readings.

If your sure your readings were reliable then recheck them again. Confirm that the 48 volts is constant on the board. If the 48v is constant and the reading really does alter then you have either a construction problem/fault or (unlikely) a problem with the capsule.
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Old 1st October 2012, 11:32 AM   #45
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Ah, I was at fault there I think. I was using a set of test leads with croc clips to attach the probes to the points to measure the voltage across the mic, and they seem to give much lower readings. Placing the probes directly onto the same points gives a consistent 13.2v on the circuit using the 33k/22k in series.

Sorry about that, a stupid time-wasting mistake.

I fear that I've wasted even more time if it turns out this circuit is in fact not compatible with the modified capsules.

Last edited by Poxican; 1st October 2012 at 11:36 AM.
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Old 1st October 2012, 11:58 AM   #46
Mooly is offline Mooly  United Kingdom
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Originally Posted by Poxican View Post
Ah, I was at fault there I think...........
Not at all

These are practical problems that catch out even experienced engineers.
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Old 1st October 2012, 12:57 PM   #47
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I use a linkwitz modified WM-60AY capsule with the Eric Wallin preamp II preamp. I've not had any problems with it, and I did not modify the circuit in any way. I believe it feeds the mic capsule with ~4.2V (it was designed for use with the WM61A capsules). The resistor "feeding" the capsule is 3.3k (so that can provide around 1.2mA)

I actually don't really understand the working of the circuit but here is the schematic and explanation --> Eric Wallin's Internet Homepage

I've really only used it for speaker testing. Any problems I have had, have been when I have clipped the digital input on my sound card.

Tony.
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Old 1st October 2012, 05:18 PM   #48
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I put this circuit to one side this afternoon, and built two of Scott Helmke's designs. The both seem to work beautifully, though I haven't tested at high SPLs. I'll let you know how I get on.

I still want to get this circuit working though, and will keep working on it. I'm SUPER grateful for all the time you guys have generously invested trying to help me to get it working. I've learnt loads, it's been invaluable.
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