All OpAmp mic design (no FET at first stage)

[homero.leal]

I think putting a different OpAmp (low power and low noise) will be better than using OPA134.

If you could suggest one, which one will be... ADA4610 ? Hope it's available on the Multisim component database.

I'll try that this afternoon, After returning home from the office...

Regards Khron!

HL

 

[Khron]

No idea if they have spice models for these, but i found a couple more that might be suitable:

http://www.mouser.fi/ProductDetail/Texas-Instruments/OPA2141AIDGKT/?qs=sGAEpiMZZMtCHixnSjNA6FLu9XBbepgu91vhsJaTJxM%3d (0.8fA/sqrt(Hz) low enough current noise @ 1khz?)
http://www.mouser.fi/ProductDetail/Texas-Instruments/OPA2131UJ-2K5/?qs=sGAEpiMZZMutXGli8Ay4kIKSYmYm6QhIizTFFEKniV8%3d (a bit noisier, but 2mA max quiescent current, and cheaper )
http://www.mouser.fi/ProductDetail/Texas-Instruments/TLE2072ACD/?qs=sGAEpiMZZMtCHixnSjNA6J5DQWv82hgv%2f94TwBMZ8m4%3d (same ballpark specs as the 2131)

 

[homero.leal]

I used TLE2072CP, and it looks nice!

Also, added a small 40pf cap from first OpAmp output, to (-) input.

Regards!

 

[Khron]

I was just thinking that, since some of the more "fancy" opamps are available in SOIC (or smaller SMD) packages, the input pin may very well need a guard trace/ring around it, to minimize current leakage (and thus, noise).

http://cds.linear.com/image/resized/81_blog_7.jpg
http://cds.linear.com/image/resized/81_blog_6.jpg

 

[ricardo]

Well Ricardo, I was thinking on using two OpAmps... something more or less like this...

This is not a LN circuit.  It has 6dB more gain but more than 6dB more noise.  You can get extra gain simply by making C1 in Zap's Occam-2d.pdf smaller.  The resulting S/N performance is nearly 2x worse than if you used a single OPA134.

THD is worse too but you are unlikely to notice the difference.  There is no advantage in overload.

There's loadsa stuff filtering the power rails and other important points that are VITAL for LN.  The exact method you use is important.  eg you can get rid of the Zener.

Doubling the current consumption and nearly doubling the complexity to get worse performance is a poor trade-off.  This will be even more true if you find a better FET OPA than OPA134.

TLE2072's noise is about the same as TL071 which is unacceptable in a high quality mike.

The simple charge amp (as in Zap's Occam-2d.pdf) is in fact, one of the classic ways to achieve best possible performance.

Anyone have Guru Wurcer's Linear Audio articles?

 

[homero.leal]

:-[

I looked for Occam-2d.pdf on Zap's folder, but I only found Occam-2e.pdf. Is this is the one you mean?

And... is it really balanced? It has just one OpAmp, but uses 1N4148 diodes for XLR2. I don't see how signal from OpAmp output could go thru diodes, and then to 47uf cap... It's some kind of strange.

Best regards!

HL

 

[ricardo]

I looked for Occam-2d.pdf on Zap's folder, but I only found Occam-2e.pdf. Is this is the one you mean?

Yes.  Occam-2d.pdf is the version on my hard disk.  The only difference seems to be R9, 13 are 150 instead of 47.  Probably means he found ADA4841-1 was less stable than he anticipated into long mike cables.  OPA134 should be a lot better and can use 47R.

And... is it really balanced? It has just one OpAmp, but uses 1N4148 diodes for XLR2. I don't see how signal from OpAmp output could go thru diodes, and then to 47uf cap... It's some kind of strange.

This is known as "Impedance Balanced".

It is explained by many true gurus in many articles eg http://www.uneeda-audio.com/zbal.htm  ...  but see also Self & Rod Elliot.

Neumann used it in their TLM mikes IIRC.  I used it in the Calrec Mk4 Soundfield from 1980.

 

[Khron]

The Oktava MK-012 also uses an impedance-balanced output arrangement, and so does the t.Bone SC140 (which might just be a direct knockoff of the Rode NT-55 - at least the capsules fit, that much i know).

 

[homero.leal]

Ok... Filter caps added. Only one OPA134. Charge Amplifier. Impedance balanced.

Better now? Hope so...

Regards!

HL

 

[ricardo]

Looks much better. 

You don't need the Zener D1 or R4.  Just adjust R10, 11 to give you the voltage supply you want.

C5 needs to be a nice electrolytic.  10u or so.

Let us how you get on and if you can compare the noise performance with a Schoeps type circuit

BTW, OPA140 is 'on paper' a very good choice (better than OPA141) but I don't like recommending stuff I've never tried.

I mentioned Eni (current noise) as something which is important but hardly ever spec'd.  This is because it depends on loadsa stuff including exact temperature.

What it IS correlated with is input bias current.  You are looking for low single figure pA or less.  Remember this is also very temperature dependent.

 

[homero.leal]

Looks much better. 

You don't need the Zener D1 or R4.  Just adjust R10, 11 to give you the voltage supply you want.

C5 needs to be a nice electrolytic.  10u or so.

Let us how you get on and if you can compare the noise performance with a Schoeps type circuit

BTW, OPA140 is 'on paper' a very good choice (better than OPA141) but I don't like recommending stuff I've never tried.

I mentioned Eni (current noise) as something which is important but hardly ever spec'd.  This is because it depends on loadsa stuff including exact temperature.

What it IS correlated with is input bias current.  You are looking for low single figure pA or less.  Remember this is also very temperature dependent.

It Works!!! Need to do some more tests, but seems to work really nice!

This is the PCB, for BM700/BM800 mics:

And this is a screenshot of a test recording. Noise is mostly from PC and room fans:

I had some problems at first, but luckily I  could find cause. One of the 47uf elecrtrolytics had a bad soldered pin.

I thought it was a problem of to much current being drawn by the OPA134, but it's working fine now.

This is a pic of the actual mic:

As you can see on the pic... I air floated OPA134 pin 2. Commonly I do the same with the gate pin when using FETs, so just followed the same philosophy to protect this critical junction.  If somebody could recommend a more elegant way to do it, I'll appreciate!

On monday I'll send it to a friend for a studio test... will keep you informed.

Regards!

HL

 

[homero.leal]

Thank you very much Ricardo, Khron and all of you guys for your help!

Hope it keeps working as of now!

Best regards!

HL

 

[ricardo]

Homero,

if you can post the final circuit, it would be useful to lots of other DIYs ... especially if you have some voltage measurements on your working mike at important points.

This type of circuit is likely to become more popular as new OPAs meant for good performance with low consumption / voltage become available.

 

[homero.leal]

Homero,

if you can post the final circuit, it would be useful to lots of other DIYs ... especially if you have some voltage measurements on your working mike at important points.

This type of circuit is likely to become more popular as new OPAs meant for good performance with low consumption / voltage become available.

Of course Ricardo, this is the final schematic of the circuit:

I redrew the schematic on Eagle CAD, since I like PCBs from Eagle, so that's what I used.


I could not find an OPA134PA on the component database, so I used a generic DIP-8 layout and then changed the label to OPA134PA.

All of the polarized caps are electrolytics. All of the non polarized caps, are ceramics.

Resistors are precision 1% 1/4W.

One curious or strange detail. I bought some special low impedance electrolytics. As I read that these are the best for singal coupling, I used this caps for signal coupling on XLR 2 and XLR3, and they worked... but after doing a test recording, I saw a strange signal behavior on the DAW track. It seems as a very low frequency oscillation:

After replacing this two caps for a pair of nichicon audio grade electrolytics, everything worked fine as on the previous DAW image.

Regarding voltage measurements, I got:

XLR2 18.95V
XLR3 18.97V
Zener(+) 11:31V
OPA134 PIN 2: 0.577V
OPA134 PIN 3: 5.54V
OPA134 PIN 6: 5.57V
OPA134 PIN 7: 11:14V

I was hoping that OPA134 PIN 2 will be also aprox 5.5V, but that was not the case, don't really know why. Anyway, the mic is working.

Thank you again Ricardo for your advice and help!

HL

 

[ricardo]

Regarding voltage measurements, I got:

XLR2 18.95V
XLR3 18.97V
Zener(+)  11:31V
OPA134 PIN 2: 0.577V
OPA134 PIN 3: 5.54V
OPA134 PIN 6: 5.57V
OPA134 PIN 7: 11:14V

I was hoping that OPA134 PIN 2 will be also aprox 5.5V, but that was not the case, don't really know why.

The reason for this is the impedance at PIN 2 is 1G

Only an electrometer (special voltmeter with loadsa G impedance) will give accurate results at that point.

The important voltages are

PIN 7 : the 'rail' voltage which tells you how much current the OPA is taking
PIN 6: the OPA output.  Should be 1/2 pin 7 if everything OK.

If you try different OPAs in this circuit, please post what voltages you get.

Dunno about the LF oscillation but it could be cos you still have the Zener in place  8)

 

[Khron]

I could not find an OPA134PA on the component database, so I used a generic DIP-8 layout and then changed the label to OPA134PA.

You could've used any other single opamp (in the "linear.lbr") component - the pinout is pretty standardized

 

[mhelin]

This type of circuit is likely to become more popular as new OPAs meant for good performance with low consumption / voltage become available.

OPA164x in now popular OPA which has been discovered lately even though it's quite old (datasheet dated DECEMBER 2009–REVISED APRIL 2016) actually.
https://www.ti.com/lit/ds/symlink/opa1642.pdf
There are many designs based to this chip family:
http://www.opic.jp137.com/https://www.instructables.com/True-Condenser-OPA-Mics
TI should again change the OPA on this application note:
https://www.ti.com/lit/an/sboa320a/sboa320a.pdf

 

[rogs]

OPA164x in now popular OPA which has been discovered lately even though it's quite old (datasheet dated DECEMBER 2009–REVISED APRIL 2016) actually.
https://www.ti.com/lit/ds/symlink/opa1642.pdf

I've used both the OPA1641 and 1642 in my various projects here: OPIC Impedance Converter
Unlike the design linked to in the 'instructable' above, I don't regulate the supply to the op amp, and get a little extra headroom as a result.
Using the single amp 1641 draws half the current, so that helps a bit as well!
The audio output is single sided - although the output is still impedance balanced - so there is no additional noise introduced by the resistors in the inverting second amp used for a differential audio output.
I've found the various configurations I've tried all work pretty well .. low distortion and low noise....

 

[mhelin]

Any ideas what the current load of 1641 or 1642 could be at full level into 600 ohms load? CAD E300(s) uses the batteries to get better transient response:

During transients, these OpAmps may require more current than typical phantom power supplies can deliver. The extra current demands are accommodated by our unique power supply design. Instead of using phantom power to operate the microphone, we use it to charge a pair of rechargeable NiCad batteries. This system creates a huge current reserve for the microphone’s electronics, yet there is no maintenance involved because the microphone automatically keeps the batteries charged during use.

Some kind of current reserve could be also created by using (extra) large capacitors as well.

 

[mhelin]

Here an E100 with the original op amps replaced by OPA1642's:

cad equitek e-100 condenser repair/upgrade


Regarding the E300 schematics, do you see any errors there, like are the non-inverting inputs on the first stage floating really?