M31 Preamp

[oneoldude]

Anybody got an idea of what the "Hybrid Preamp" circuit in the link below might look like?

It is certainly no tube circuit but I can't find anything about it on the net.

Here is the URL:

http://www.linearx.com/products/microphones/m31/M31_4.htm

Thanks

 

[Frank Nash]

https://en.wikipedia.org/wiki/Electret_microphone

**broken link removed**

 

[oneoldude]

That is the "Electret Element" (ECM) in the leftmost pink box. I am after the "Hybrid Preamp" which is the rightmost pink box in the URL:

http://www.linearx.com/products/microphones/m31/M31_4.htm

Thanks

 

[ronsimpson]

The way I see it is the preamp is the J-FET.

 

[oneoldude]

No the JFET is part of the MIC capsule and that is followed by the preamp. Start here under "Microphone" https://www.linkwitzlab.com/sys_test.htm and check out the various links. Linkwitz shows his preamp. I just wonder what the M31 preamp looks like.

ronv, where are you when I need you?

 

[audioguru]

Your link does not say anything about what is a hybrid preamp circuit for their M31 microphone.
Why not modify your electret mic as described by Linkwitz and make his preamp instead?

 

[oneoldude]

Hello McDuck,

I see you are at it again.

What makes you think I do not have the Linkwitz preamp and modified capsules? I pointed to Linkwitz!

I do have the Linkwitz preamp (as well as others) and I have pro audio gear as well. Also I have both modified and unmodified capsules. None of that stops me from trying to learn if something else exists that might be useful to me. Now that you know would it have changed your question?

If you choose to respond again with an, "I didn't know," type answer, that is exactly the point.

 

[audioguru]

Hello McDuck,

I see you are at it again.

Instead of being nasty to me, why didn't you ask the manufacturer of the M31 mic what is the hybrid preamp they talk about?

 

[ronv]

It may look like this:

**broken link removed**

Is it the DC coupling that intrigues you?

How did your preamp turn out?

 

[Frank Nash]

LinearX has free support. Have you asked them? They sell the 'hybrid' with the LEAP software so I would guess they don't welcome DIY. If it was me I'd put an audio frequency band-pass filter in it. Also correct any distortion from the electret, but those are usually pretty flat. The cable rules out a balanced connection. So a single supply quad op amp band pass filter?

 

[audioguru]

If it was me I'd put an audio frequency band-pass filter in it.

Why do you like "telephone quality" sound with no deep bass and no sizzling highs?
I enjoy wideband sound (20Hz to 20kHz plus and minus 1dB).

 

[Frank Nash]

Would those not be in the audio frequency? Reg Williamson's audio window comes to mind.

 

[audioguru]

Some people think that voices have a narrow frequency response of 300Hz to 3kHz like a telephone. Then it sounds like the grunts and groans from a pig.
The low frequency sounds and the high frequency consonant sounds are gone.

 

[audioguru]

Deleted a duplicated post.

 

[Frank Nash]

In the 1980's I bought a bunch of those capsule Panasonics #P9959-ND Linkwitz talks about. I went out to record thunder storms. Got home and during playback got glorious deep in the gut thunder-rumble. But.... also, unmistakably, 18-wheeler rubber-on-highway noise from miles away.

4th order Butterworth highpass 20 hz filter and now there is inky-black silence before thunder boom.

If you get a chance read Reg Williamson's Audio Amateur article.
"Open the window only enough to keep the garbage out."

 

[oneoldude]

ron,

I could not physically get the circuit in the wand or in an XLR connector. So I did my take on the Linkwitz circuit instead. Neat feature is to offer switchable reference from ground to V+ or Ground to V- so that both cut and uncut two wire capsules can be used and gain is adjustable from 10 dB to 40 dB. It seems to work ok but I have not wrung it out yet with cut capsules or formally tested it yet. Here is the circuit: http://www.htguide.com/forum/showthread.php?41165-Linkwitz-Pre-Amp-modification

If you look close at the M31 pages you will see that simply because they use and XLR3 connector does not mean the circuit is balanced. As Frank says it cannot be balanced because it is a two wire circuit on the output. That means unbalanced. They use the XLR and an outboard ps to feed 9V in through the XLR in non traditional fashion and from the XLR they go two wire down the line. If you read through the couple pages on the M31 you will find a downloadable PDF that describes their wiring scheme so that you can use a variety of PSs of your own choosing. Nice.

Their pre amp must in the wand because ECMs cannot drive very long cables. My guess is that they end up with around a 10 dB gain and low output impedance so they can drive decent cable lengths. That is in essence what I want to do. But my guess is that they are doing it with surface mount components because their M31 wand is only 1/4" in dia. I won't be able to compete with that but I would like to know what they are doing because the purpose of that MIC is exactly what I need. It is a measurement MIC for loudspeakers.

 

[oneoldude]

Frank,

Don't listen to McDuck. All he wants to do is insert negative comments. In prior posts he chided me for attempting to pick up earthquakes (he thought my bandwidth was too wide) now he is chiding your bandwidth for being too narrow. And he is doing it without knowing how wide your bandwidth actually is! Amazing! The guy won't quit! And he never includes helpful information.

It is true that in certain recording situations you might want to limit your bandwidth to only what you want to hear. But the MIC we are talking about here is a measurement MIC for measuring loudspeakers. Its FR needs to be straight as a string from at least 20 Hz to 20KHz. In order to do that you must keep the 20-20K bandwidth away from the stop bands of the upper and lower freq roll offs. That means your 3 dB down points need to be below 7 Hz on the low side and above 55 KHz on the top side. If you do that, and have equally good amps, etc., then the only non-linearity you need to worry about comes from the MIC which you can correct.

As I understand the specs for the M31 (and other measurement MICs as well) the manufacturers typically do not straighten out the MICs frequency response with electronics. That would be very difficult and expensive to do requiring different circuit components for each capsule because no two capsules are identical in FR. What they typically do is measure the MIC after it is built and give you a calibration file that you import into your measurement software and the MICs FR is corrected there. My measurement software and all competent measurement software has that capability.

Your 18 dB filter at 20 Hz will be down perhaps 6 dB at 20 Hz and that is unacceptable for a loudspeaker measurement MIC. But it might be just fine for what you are doing.

 

[oneoldude]

Deleted Duplicate Post

 

[oneoldude]

ronv,

I may be dead wrong that the M31 has gain in the MIC circuit. They may only have a BJT or FET buffer to lower output impedance so longer cables can be used. But they do warn that relatively high voltages may come from the MIC with high SPL inputs. A simple buffer, even with a little gain might only require a transistor and a couple resistors. Such circuits might fit into a wand or XLR3. Any thoughts?

BTW since you use LTSpice here is a working spice model for the WM61A. It is a bit clunky but it does work and reasonably portrays the FR of the WM61A MIC capsule in circuits. It does make a couple error calls but they can be worked around.

**broken link removed**

Also, here is a very fine article on the use of the WM61A capsule (indeed any modern ECM) that includes a circuit to electrically correct the capsules FR. It is worth reading. But the circuits are quite complex and too large to use in a wand or XLR3. See what you think.

http://www.johncon.com/john/wm61a/

 

[audioguru]

Don't listen to McDuck. All he wants to do is insert negative comments. In prior posts he chided me for attempting to pick up earthquakes (he thought my bandwidth was too wide)

The preamp circuit you recently posted a link to also picks up earthquake frequencies. Linkwitz used a 2.2uF coupling capacitor from the mic to the preamp with a 10k input impedance but your cap is 50uF (!). -3dB will be at 0.27Hz.
The value of your output capacitor is also too large.

Your preamp is missing the 10k resistor that feeds the mic and a filter should feed this resistor so the preamp does not motorboat.

Your 18 dB filter at 20 Hz will be down perhaps 6 dB at 20 Hz and that is unacceptable for a loudspeaker measurement MIC.

No.
It is a 4th order Butterworth highpass 20Hz filter which is 24db per octave and is -3dB at 20Hz. 10Hz is -24dB.

Here is your preamp: