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Author Topic: DIY Surface Mount Linkwitz Mic Pre  (Read 12220 times)

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Offline dshay

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DIY Surface Mount Linkwitz Mic Pre
« on: March 08, 2011, 12:13:58 AM »


Made a surface mount Linkwitz Microphone Preamp doing the mic mod technique

Sounds REALLY nice. Can't wait to see what it do on drums.
« Last Edit: March 08, 2011, 01:28:49 AM by dshay »

Offline dshay

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Re: Surface Mount Linkwitz Mic Pre
« Reply #1 on: March 08, 2011, 12:29:24 AM »
Used the schematic found here:

http://www.linkwitzlab.com/sys_test.htm

The ferrite shielding and nickel are covering a couple 32 awg jumper wires.

Used the adapter board found on the link below.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=9082CA-ND

 

Offline illconditioned

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Re: Surface Mount Linkwitz Mic Pre
« Reply #2 on: March 08, 2011, 01:08:38 AM »
Thanks for sharing.  I didn't know about that circuit board.  It is really nice for small projects.

There are a few DIY types here who appreciate this.  Myself included :).  Another good source of DIY information is at the Yahoo group Micbuilders.

  Richard
Please DO NOT mail me with tech questions.  I will try to answer in the forums when I get a chance.  Thanks.

Sample recordings at: http://www.soundmann.com.

Offline dshay

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #3 on: March 08, 2011, 01:36:29 AM »
Last time I posted on Micbuilders I was being scorned for using tantalums in the circuit like Linkwitz suggested.


Offline Church-Audio

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #4 on: March 08, 2011, 08:22:14 AM »
Last time I posted on Micbuilders I was being scorned for using tantalums in the circuit like Linkwitz suggested.

Its funny I hear all this talk about Tantalum but the funny thing is some of the most sought after vintage mic preamps used tantalum caps. I have worked on consoles worth $200,000 that had tantalum caps in the signal path the reality is if you want to build something small there are very few choices.. in larger values / small size.

And funny thing is most mics use Tantalum caps lol so much for tantalum caps being so bad.. Funny most of the guys over there spend more time bitching about stupid shit.. Wacky designs and serious misinformation. Yeah there are better caps than tantalum. There are always better parts available but how much money do you want to throw at something? lol
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Offline Chuck

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #5 on: March 08, 2011, 08:59:07 AM »
That's very cool. mshilarious on this board also does surface mount stuff like that.
Reality is merely an illusion, albeit a very persistent one.

Microphones: Microtech Gefell M300, AKG C 480 B comb-ULS/ CK 61/ CK 63, Sennheiser MKE 2 elements, CAD GXL1200 (cardioid and mod-cardioid capsule & electronics mod), Audix M1290-o, Micro capsule active cables w/ Naiant PFA's, Naiant MSH-1O, Naiant AKG Active cables, Church CA-11 (cardioid), (1) Nady SCM-1000 (mod)
Pre-amps: Naiant littlebox, Naiant littlekit v2.0, BM2p+ Edirol UA-5, Church STC-9000
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Offline Church-Audio

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #6 on: March 08, 2011, 11:24:25 AM »
Yeah, I've used tants in the signal path too!  Although only where there is no other choice due to size restrictions (X-S).  I've used them a lot as filter caps, but tantalum is getting crazy expensive so I am replacing most tants with ceramics when used as filter caps (don't use ceramic in the signal path).

You have a better option than tant here though, because you can increase input impedance higher than 10K (I would use 33K there), then you can use one of these:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PCF1132CT-ND

Careful with the iron there, those don't like getting overheated.

Also it seems you have built a two channel version? Because you have two caps vs. Linkwitz's one?  Or perhaps you have added power supply caps?  I would definitely recommend the latter, because the internal resistance of a 9V battery can be a few ohms.  A couple of 1uF ceramics right next to the opamp should do the trick.

Or perhaps you have an output cap?  Be a bit careful there, because tants can't see reverse voltage (including from large AC signals) or they may die, which usually means failing open--maybe OK for an output cap that maybe isn't necessary, but in other circumstances potentially very bad.  I would recommend stuffing in an electro instead--you can get a very small 4.7uF bipolar electro.  Or if you are happy with 2uF, stack a couple of those PPS caps above.

While we are on the topic of circuit protection, I would always recommend a pair of clamping diodes on the output (before the 200R resistor).  BAT54S will get the job done in very little space.

For anybody building this, note that Linkwitz didn't use half of OPA2134, which wastes about 4mA.  This is probably because he already had the part laying around and didn't care about power consumption, but if you are buying parts specifically for this project, use a single opamp (like OPA134) instead.
Or use the other half for a led driver or rail splitter :)
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Offline Roger Gustavsson

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #7 on: March 08, 2011, 01:47:16 PM »
This looks like a stereo preamp and it will make sense with a dual OPA2134.

Roger

Offline it-goes-to-eleven

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #8 on: March 08, 2011, 02:24:40 PM »
Must have tube version.

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Offline Gutbucket

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #9 on: March 08, 2011, 04:44:57 PM »
Must have tube version.

I'm looking more along the lines of a no-frills but near matchbox sized 4 channel preamp.
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Offline dshay

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #10 on: March 08, 2011, 07:17:41 PM »
Roger is correct, this is a stereo unit.

Tantalums were designed into this circuit by the man who invented one of the best cross-over designs ever and used to lead teams at HP designing Microwave/RF Spectrum Analyzers.

My breadboard/ears do not lie, low ESR tantalums work well for this circuit.

I chose the finest of thin film resistors for the least amount of noise.

I would have put in .1uF power filter caps but there's not much room on this board for extra components, when I design the circuit board they'll be there.
« Last Edit: March 08, 2011, 07:25:12 PM by dshay »

Offline illconditioned

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #11 on: March 08, 2011, 08:47:27 PM »
Either way, the capsule FET is still by far the dominant source of distortion in the circuit, probably 10x the distortion of even a ceramic capacitor.
Hey there, can you answer why a constant current source (a FET with gate wired to source) is used instead of a source resistor?  I've seen this in AKG CK1x capsules, for example.  Most circuits seem to be just a single FET with a source resistor, but maybe they aren't perfect?  You piqued (sp?) my interest when you said you're working on a "cascode" circuit.  Not quite the same thing as a constant current load, but I read a nice explanation of this circuit.

OPA2134 is 8nV/rtHz.  That's the equivalent of a 5K source resistor, and given that your source impedance is much less than that, the opamp is the dominant source of noise in the circuit (and as a semiconductor, it has shot noise that dominates the <100Hz region, see its datasheet on pg. 4--you can see illustrations of both the opamp's shot noise as well as the relation between the thermal noise of the source resistance and the opamp's noise).  The good news is that nobody really cares about <100Hz noise in audio, because the source is nearly always much louder, and even if not the ear is not sensitive at all to such noise.
 
I'm suspicious of opamps, when we run them on a 5-10V swing (5V=usb power only, 10V is what we might get in a battery circuit).  Shouldn't these amps run on +/-12V or +/-15V to get their full performance?  Note that there are special low voltage opamps (a lot of JRC stuff, for example), but a lot of people are using "standard" NE5532 based designs at lower voltages.  In a cheap mixer 5532 are fine running at +/-15V.  But I would not trust those in a battery pre or a USB interface (Edirol UA5, etc).  I've started putting LM4562 in USB or battery stuff.  No measurments, but they did seem to sound more detailed.  I keep meaning to actually try one of these battery preamps and measnure distortion (eg., using RightMark or something on a PC).  To see if high-end opamps are really needed.

    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.
Yes, please share any circuits.  A few of us may experiment...

  Richard
Please DO NOT mail me with tech questions.  I will try to answer in the forums when I get a chance.  Thanks.

Sample recordings at: http://www.soundmann.com.

Offline dshay

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #12 on: March 08, 2011, 10:24:05 PM »
Dunno why the tude, I was trying to give useful practical advice.  Tants do have higher distortion than polys, see Cyril Bateman's very careful work for evidence.  Does that make them unusable for audio?  No, as I said, I use them myself.  But you do have another option here.

Either way, the capsule FET is still by far the dominant source of distortion in the circuit, probably 10x the distortion of even a ceramic capacitor.

You also might consider that resistor noise above, say, 100Hz, is almost purely thermal and thus only a function of temperature and resistance.  I use the fancy resistors too, but it makes a 0% difference in noise in the audible spectrum.

OPA2134 is 8nV/rtHz.  That's the equivalent of a 5K source resistor, and given that your source impedance is much less than that, the opamp is the dominant source of noise in the circuit (and as a semiconductor, it has shot noise that dominates the <100Hz region, see its datasheet on pg. 4--you can see illustrations of both the opamp's shot noise as well as the relation between the thermal noise of the source resistance and the opamp's noise).  The good news is that nobody really cares about <100Hz noise in audio, because the source is nearly always much louder, and even if not the ear is not sensitive at all to such noise.

The next question is what is feeding the circuit, and if it's a Pana capsule as with Linkwitz's schemo, you have a noise floor of about -106dBV, or the equivalent of a 68K resistor (not exactly, since the Pana noise is mostly 1/f up to 10kHz or so).

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.

My apologies.

Jaded from years of reading forums where audiophile amatures poke around under the hood demanding cotton insulation wire and wooden potentiometer knobs.

I would also welcome any designs centered around battery operated field recording. 
« Last Edit: March 08, 2011, 10:29:26 PM by dshay »

Offline Chuck

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Reality is merely an illusion, albeit a very persistent one.

Microphones: Microtech Gefell M300, AKG C 480 B comb-ULS/ CK 61/ CK 63, Sennheiser MKE 2 elements, CAD GXL1200 (cardioid and mod-cardioid capsule & electronics mod), Audix M1290-o, Micro capsule active cables w/ Naiant PFA's, Naiant MSH-1O, Naiant AKG Active cables, Church CA-11 (cardioid), (1) Nady SCM-1000 (mod)
Pre-amps: Naiant littlebox, Naiant littlekit v2.0, BM2p+ Edirol UA-5, Church STC-9000
Recorders: Sound Devices MixPre-6, Tascam DR-680, iRiver iHP-120 (Rockboxed & RTC mod)

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Offline dshay

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #14 on: March 08, 2011, 10:49:23 PM »
I'm eagle'ing this board up but it's taking me extra time trying to unlearn the microcontroller boards I've designed and put in a star ground instead of planes.

Offline illconditioned

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #15 on: March 08, 2011, 11:07:07 PM »
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
Please DO NOT mail me with tech questions.  I will try to answer in the forums when I get a chance.  Thanks.

Sample recordings at: http://www.soundmann.com.

Offline it-goes-to-eleven

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #16 on: March 09, 2011, 11:09:12 AM »
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.

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #17 on: March 09, 2011, 12:12:32 PM »
I'd also like to see anything you want to share.  From anyone ;D

Offline illconditioned

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #18 on: March 09, 2011, 02:47:46 PM »
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
Please DO NOT mail me with tech questions.  I will try to answer in the forums when I get a chance.  Thanks.

Sample recordings at: http://www.soundmann.com.

Offline illconditioned

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #19 on: March 09, 2011, 09:56:43 PM »
^^^ 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
« Last Edit: March 09, 2011, 10:02:22 PM by illconditioned »
Please DO NOT mail me with tech questions.  I will try to answer in the forums when I get a chance.  Thanks.

Sample recordings at: http://www.soundmann.com.

Offline it-goes-to-eleven

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #20 on: March 10, 2011, 11:50:02 AM »
Thanks for the suggestions, MS!

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Offline dshay

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #21 on: March 13, 2011, 08:55:14 PM »

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




Offline sasa999

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Re: DIY Surface Mount Linkwitz Mic Pre
« Reply #22 on: October 10, 2011, 05:52:24 PM »
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 :)

 

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