New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

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JR.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by JR. » Sun Oct 11, 2015 2:09 pm

:lol: :lol: :lol:

Yup I remember that old thread... Too bad I didn't keep proper dated note books of my scibbling, but Wayne's publication predates the 2010 filing by years.

JR

PS: Patents are relatively easy to get since examiners are not really skilled in the art.

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Sun Oct 11, 2015 2:24 pm

I said at the time both there and here that I couldn't afford a patent attorney and was going to put it in public domain so no one else could. :lol:

BTW I have an error to correct: The total response of the preamp (no HPF enagegd) is 7 Hz.
The AC coupling of the common mode rejector/HPF dominates.

The servo tau is more like 0.235 seconds aka fc 0.7 Hz. not the 7 Hz as previously advertised.
My previous Q and fo calculations were correct (Q=0.7, fc 4 Hz) since they were based on the Tc of the servo and not what my faint memory recalled as a value.
My lack of memory and order-of-magnitude error in recollection explains why olafmatt wasn't buying my stability argument.

I used an Rsource of 1K for those calcs.
I should recalculate the fo and Q for a 100R source which as olafmatt points out is more realistic value for the Geffell and TLM-103 topologies.
I suspect 1K is the worst case assumption.

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Sun Oct 11, 2015 3:14 pm

I checked the equations and I think that with 47R sources (94R diff) I have a little low-end response peaking around 4 Hz at 60 dB gain.
It was masked by the LF roll off.

I need to do a spreadsheet to add to both THAT and my models the extra Rs that the fault protection resistors add.
The 47R is actually about 62 Ohms with Rprotect being a split 15 Ohm resistor.
The added Rs contributes to a lower Q than the equations suggest just modeling the mic's Rsource.

It appears that as Rs increases Q drops or,
Q rises as Rs drops.

To achieve critical damping it looks like the servo response tau needs to be at least 0.5 seconds maybe a little longer.
Its easily do-able by changing Cservo, Servo Rfb or Rinject.
I think a spreadsheet will make sorting it out a little easier.
This is a question of tuning.

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JR.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by JR. » Sun Oct 11, 2015 5:49 pm

mediatechnology wrote:I said at the time both there and here that I couldn't afford a patent attorney and was going to put it in public domain so no one else could. :lol:
I am working on a project right now that is probably patentable and I am not inclined to do yet another patent. I wrote most of my drum tuner patent myself, and did all the art, but paid a real lawyer to write the claims (they're the important part).

That said my new project could be a mass market product (outlet tester that actually works, as compared to the SOTA that does catch all safety hazards).

I am approaching a company to partner and will offer them assignment of my patent if they are willing to pay for the application and lawyers. I am also asking them to pay for UL approval (a similar amount to the patent), etc. Long term they would be smart to invest in a patent but short term I worry about scaring them off.

JR

PS: My friend (a pat lawyer) who helped me save money on my tuner patent is probably a patent judge by now, so I probably would have to pay more now too.

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by ricardo » Mon Oct 12, 2015 1:44 am

mediatechnology wrote:It appears that as Rs increases Q drops or,
Q rises as Rs drops.

To achieve critical damping it looks like the servo response tau needs to be at least 0.5 seconds maybe a little longer.
Its easily do-able by changing Cservo, Servo Rfb or Rinject.
I think a spreadsheet will make sorting it out a little easier.
This is a question of tuning.
Just want to point out that you get all this with transformers too :o with the added excitement of the 'primary inductance' changing with level.

I use to do a lot of this in microphones and also transformer mike preamps to build in at least 2nd order subsonic filtering. You have to choose your 'level' and also stuff like your length of mike cable. :mrgreen:

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Mon May 21, 2018 12:44 pm

The last post here was about 3 years ago...

I wanted to let people know that I have decided to retrofit this design with the ZTX851 preamp topology used in the moving coil preamp.
viewtopic.php?f=6&t=783&p=11295#p11295

I think I've proven that monolithic preamp ICs and their digital gain controllers can be adapted to "fly the rails."
Now I want to make one people can build and solder at home. No surface mount.

Yesterday I removed the three linked Protoboards and removed the SPI controller and THAT5173 gain controller.
I kept the original power supply and preamp Protoboards.
I now have a stack of open Protoboards some of which have been tied up for years.

I recently modified the MC preamp to check it as a mic preamp and am very confident it will be quiet enough.
The gain switching will use a mechanical switch only switching Rgain.

One of the major considerations is servo pulling imbalanced phantom currents.
A quick experiment shows that Olaf's servo used in the MC preamp will have enough range to correct mic imbalance.

Along with the MC preamp this is a project I want to get laid out on a PC board.
Losing Bruno2000 has lit a fire.
I have work to do.

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JR.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. » Mon May 21, 2018 1:38 pm

I always wanted to fly a DC coupled A/D converter up with the DC coupled amp, then optically shift the digital output down to baseline. Maybe converter technology is stable enough to consider one(or more) converter footprints now? Or not.... 8-)

BTW is the ZTX optimal for 150-200 ohm..? Reportedly some of the later low noise bipolar devices were better matches than my favorite (737/786) that IIRC were developed in Japan for MC head amps. (not just Rbb but Cob and other stuff).

Forget about populating a modern A/D converter using only through hole... As usual my apologies.... I can deal with SMD at home, while perhaps I'm not typical DIY. FWIW I don't "like" SMD for DIY, I just tolerate it because many modern parts are not available TH, while I love SMD for production.

JR

PS: Next project will be to try to find microphones that don't have capacitors inside, besides the simple dynamic mics. Maybe we need to design a DC coupled phantom powered mic. :lol:

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Mon May 21, 2018 2:25 pm

JR wrote:BTW is the ZTX optimal for 150-200 ohm..? Reportedly some of the later low noise bipolar devices were better matches than my favorite (737/786) that IIRC were developed in Japan for MC head amps. (not just Rbb but Cob and other stuff).
I think the ZTX851 will be good enough as-is but will be better with a lower Ic and low resistance value common mode stage.
Unfortunately many of those later devices aren't available any more either.
I wrote:ZTX851 as a Mic Preamp

viewtopic.php?f=6&t=783&start=200

I made some quick noise measurements at 100 and 200Ω source impedances with 1K5Ω bias resistors and gains of about 20 and 56 dB.
The high gain NF were excellent.
At low gain, 20 dB, there is 6 dB gain in the cross-coupled output with high-ish value feedback resistors.
The actual preamp gain is 14 dB.
The output stage noise dominates as expected.

Using the basic circuit as a mic preamp I would consider lowering Ic to maybe 1 mA.
The object is to reduce base current noise for the higher source impedance where it matters more than for an MC cart.
There may not be much improvement but its worth trying.

The other thing I would do for a mic preamp using this front-end would be to make the common mode rejection stages use 2K resistors to lower low gain noise.
For a MC preamp enough front-end gain is required that the output stage noise doesn't dominate and the precision of line receiver common mode rejection stages are preferred.

I did have a look at the ZTX951 datasheet and the Cob is almost double the ZTX851 at 74 pF.
The above measurments were with a 5.5 mA Ic.
I later measured the current noise in the MC Preamp configuration open circuit with no differential termination to get an idea of current noise.
The bias resistor values were 499Ω each.
None of the noise current was referred to the other input from differential termination so none of it got rejected as common mode noise.
Subtracting the voltage noise, which was about 19 dB less, I came up with 4.9 pA√Hz noise current.
I should be able to reach the 2 pA√Hz range by lowering Ic and measuring with an actual differential 200Ω termination.

BTW last night I cranked up the monitors to listen to the noise floor both open-circuit and with a 3.3Ω termination at 62 dB gain.
The open-circuit noise floor is very smooth-sounding white noise not rocky or bubbly.
The terminated noise was dominated by hum and very low level.
To be honest I wasn't comfortable messing with the Protoboard with the monitors wide open.
I like my monitors and ears and was flirting too closely with disaster straining to hear the hum.

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by olafmatt » Fri Jun 08, 2018 5:58 pm

mediatechnology wrote:
Mon May 21, 2018 12:44 pm
One of the major considerations is servo pulling imbalanced phantom currents.
A quick experiment shows that Olaf's servo used in the MC preamp will have enough range to correct mic imbalance.
Yes, that servo can probably be made to have enough correction range... I'm just not sure whether the preamp circuit topology (where the opamps source/sink the current through the input transistors, i.e. "current feedback") is the the right one to use.

No matter which servo arrangement you use with that preamp circuit, it boils down to this: When you servo so that the output has 0VDC offset, then there is 0VDC across Rgain. Since the inputs sit Vbe below (or above, depending on transistor's polarity) the emitters/Rgain, you basically servo away any DC offset on the inputs (assuming identical Vbe for the two input transistors).
My personal rule-of-thumb is to never "leak" out any DC servo correction voltage/current through the input. But without DC blocking caps this circuit does. Of course, in this case it is meant to do so, more on this below. In a line-level circuit I'd put a unity gain buffer (non-inverting opamp) in front of it to stop my servo's correction action being visible through the input. With a microphone input that's of course not a good idea.

If we assume that the phantom resistors (the 2x 6k8) in the preamp are perfectly matched, then any DC offset on the input (before our servo corrects it) is caused by inequal current draw of the microphone. For whatever reason that is. Once our servo is used, it artificially changes the voltage on the input, which means that the microphone operates under different DC conditions now.
I'm not saying it does not work (remember, I also build a DC-coupled micpre with a flying frontend), just that it changes operating conditions for the microphone. And from preamps with phantom blocking caps and servos we know that we can get interaction between those phantom-blocking caps and the servo. Basically peaking in the frequency response or worst case even osciallation. Now we don't have caps of kown value in the preamp but persumably caps of unkown value in the microphone.
Again, it does work... usually. I haven't found any microphone that seemed to care. But that doesn't mean they don't exist. And "conceptually" it is wrong to alter the DC voltage on the input that the microphone created there.

The only advantage my servo circuit brings to the table is that it's Fc doesn't vary with gain (as long as the transistors have high enough beta and you pick the resistor values wisely). In order to not affect the microphone with your servo, you need a different preamp circuit.
Normally (i.e. with phantom blocking caps or if there is known to be 0VDC offset on the input) that preamp topology you are suggesting to use has many advantages. Just in this case it has one big disadvantage, in that it doesn't allow you to "decouple" the input DC offset from the output DC offset. At some point I have thought about using a folded cascode on the input, differentially modifying the current through the cascode transistors in order to DC-servo... but as long as the opamps source/sink the current through the input transistors you will end up in the exact same place.

From building my own preamp with a "flying" front-end, I know that it is easy to just take an existing preamp circuit and "fly it". But what's normally best is not the best circuit to "fly". - Sorry, I have no suggestion which preamp circuit to actually use... because all of them that have a DC gain of one (which should be fine for normal use, i.e. unless someboy shorts one input to ground) have caps in them.

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology » Fri Jun 08, 2018 6:48 pm

Thanks for the follow-up. It's good to hear from someone who has built a similar circuit and actually recorded something.
olafmatt wrote:I haven't found any microphone that seemed to care. But that doesn't mean they don't exist. And "conceptually" it is wrong to alter the DC voltage on the input that the microphone created there.
Well that's good news. I'm pretty sure your mic closet is fuller than mine and you got to test it with a broader number. My current thinking was is that if a mic produced such a huge amount of imbalance one could switch in capacitors for those rare occasions.
The only advantage my servo circuit brings to the table is that it's Fc doesn't vary with gain (as long as the transistors have high enough beta and you pick the resistor values wisely).
I think the improvement offered by your servo is significant given the consequences of a varying Fc causing peaking (due to input capacitor interaction) and a slow response at low gain.

I'm not so worried about injecting common mode current back into the microphone since that's what phantom resistors already do: The differential imbalance its correcting is likely due to internal component tolerance. It seems like the majority of these would be the ones where the outputs are AC-coupled and the DC-imbalance wouldn't significantly affect its AC characteristics. It seems like its putting current into the leg that needs it more. (BTW the current imbalance spec is +/-4% IIRC so its not a lot.)

The point to me seems to be that you've done it, it works, and you haven't seen any harm from it.

From experience, what value do you see in the technique? Lower noise, increased transparency? What are the audible benefits realized in practice?

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