Topic: DIY Surface Mount Linkwitz Mic Pre

[illconditioned]

To: mshilarious

OK, it sounds like you've got a handle on the circuits .  I'd love to get a set of collettes when you get the actives working.  I've been experimenting DIY (right now, Studio Projects C4 capsules, just cut off a bit of tubing + capsule socket!) , but I'd like to get a set for AKG as well.  Hopefully without destroying any more innocent gear.

  Richard

[it-goes-to-eleven]

If you like, I'll share a circuit that is balanced from capsule to preamp, that will give you a very nice CMRR figure that will reject what can otherwise be very audible interference (you often don't even need to rely on shielding if balanced)

I am interested in any designs you'd care to share.  In particular, I am interested in ultra-low noise, very high gain pre-amps for nature recording.   Maybe something with multiple stacked gain stages that can be enabled via switches or jumpers.   I'd like to keep the cost down via DIY so I can feel a bit more comfortable leaving gear unattended for long periods of time.

Similiarly, I am looking for low noise capsules.  Some higher end, and some cheap enough to leave out in the weather.

[kirk97132]

I'd also like to see anything you want to share.  From anyone

[illconditioned]

To: mshilarious

I'm still curious about using a constant   current source (such as a FET) to replace a source resistor.  This is   supposed to increase linearity, but I don't understand why.  I don't   expect you to analyze circuits for me, but I'm wondering if you have   come across any pointers/analysis online.

This seems to be the   state of the art in several mic designs I've seen.  The FET stage has no   voltage gain, but seems to have a constant current source.

Anyway, good luck with your designs.  I'm looking forward to the AKG stuff...

Thanks,
  Richard

[illconditioned]

^^^ Yes, I guess I did ask that question before.  Never got an answer, except some hand waving...

OK, it seems like we can view the FET on a 3d plot, with Id as a function of Vds and Vgs.  So Id= f(Vds, Vgs) would be a "surface plot", the height of which is the current, Id.  If we bias the FET so it has say, 1/2 Vdd on the gate, and put a current source as the source resistor, this will put us at a particular point on the surface.  If done properly, Vgs will be approximately -1/2 Vco (cut off voltage), so the FET is approximatly "half on".

Ok, here is the claim: If we take a slice with Id fixed, Vds should be a (locally) linear function of Vgs.  So, taking an (AC coupled) output from the source of the FET will give a signal (locally) linearly varying in Vgs.

Why is it linear?  I need a mathematical model of the FET to figure this out.  Note that if we fix Vds, the model Id vs. Vgs is *quadratic*.  So it seems that (roughly speaking) for a fixed source resistor, we're taking a local linear approximation to a quadratic, so that will introduce distortion.  The above will be an improvement if we can show it is really a linear curve, not a local linear approximation of a quadratic.

Let us all know if you (or anyone else) get mathematical formulas to justify the above reasoning.  It should be possible to analyze the above cases with some simple algebra.

Well, that said, I just epoxied "collettes" for my Studio projects C4 mics.  I used a single (self-biased) FET (K596 C0J, extract from Transsound TSB-120A capsules).  I used a zener diode to drop 6.8V off the 40V or so input from the phantom adapter.  A very simple circuit, but should do for now.

  Richard

[it-goes-to-eleven]

Thanks for the suggestions, MS!

[dshay]

When you have a high sensitivity capsule you don't need a particularly low noise amp.  There are many instrumentation amps that will work fine, the very low noise ones are THAT1512, INA217; but I have used lower-power inamps like INA111, INA128, INA118, AD620, LT1167, etc. successfully in my amps.  Gain is set with a single resistor, so it's very easy to implement with a switch--harder to do with a pot, you need a reverse-log taper pot, unless you don't mind your pots working counterclockwise.

If you don't need a balanced  input, then an opamp circuit as above in the OP works fine, make the shunt resistor in the feedback loop switchable to change gain.  If you need lower noise, try OPA2227 or OPA1612.

The datasheet for THAT1512 is better than many companies design/app notes.

http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf

[sasa999]

If you like, I'll share a circuit that is balanced from capsule to preamp, that will give you a very nice CMRR figure that will reject what can otherwise be very audible interference (you often don't even need to rely on shielding if balanced), a much more useful concern than worrying about flicker noise of carbon resistors in a circuit that has <10mA current (much less through any of the resistors).  I will also add that Linkwitz-modded capsules are slightly more susceptible to interference above 10kHz, even when balanced and shielded.  But for drums or loud concerts, that's probably a good trade.

Can I see this circuit? Right now I am building mic preamp