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

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

Post by ricardo »

Don't think you need SC protection on P48V. Although its not in the P48V spec. we used to sneer at desks which wouldn't give good P48V on one channel even with all 71 other mike inputs shorted to earth. Some early famous Name UK desks were notorious for wimpy P48V :mrgreen:

It's only 14mA per channel.

48V ripple eater on the preamp sounds good from that standpoint.

I think ECM8000 imbalance is worst case.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Thanks ricardo.

My thinking was I'd take 66V to the actual preamp board and put the 48V ripple eater there so there is very limited opportunity for it to see a dead short. Glad you agree that it would not be required.

I'm looking at what it would take to use a 1570/1583 in this application. I think I've come up with a "differential" Deboo servo. I need to test that on a separate board.

My girls are coming by for Thanksgiving today. I might be able to talk my daughter Robin into cutting a vocal with the prototype.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

The more I look at using the THAT1570 or THAT1583 in this project the more I like it.

The 1570 has the performance improvements THAT write about in the datasheets but in this circuit the 1510/1512 have one thing going against them that require a work-around.
The 1510 and 1512 realize common mode rejection.

The DC-coupled input-capacitorless preamp's flying rail generator provides three outputs: One of them is the Vcm "center tap" artificial ground.
Vcm is required for two things: The differential servo reference and the 1510's ground reference, pin 5.
The 1510 reference pin is the "ground" connection for the differential to single-ended stage.
When the rails are flown, pin 5 needs to fly with them to provide a "pseudo" ground and center the output between the flown rails.
The problem with pseudo ground is that it's well, pseudo.
In order to have full common mode rejection, the "recovery" stage on the right-hand side of the film capacitor(s) need to be differential and sense not only the 1510's output but it's pin 5 Vcm AC potential.
The output needs to "see" the difference between Vcm and Preamp Output.

A 1570/1583 has a few extra things going for it in this application:

A THAT1570 has no "ground" pin and does not realize common mode rejection.
Its differential Deboo servo has no ground connection.
It has a common mode gain of "1."
The 1570 does not require a low-gain Vos trim because it has no added offset of the 1510's output amplifier.
The 1570's low gain offset appears differentially and is thus servo'd.

It just makes more sense to use a 1570/1583 because it requires less "stuff."

Sure an Ac-coupled differential amp is required in either case, but a whole op amp is eliminated.
Plus, the Vcm servo no longer has to drive a direct load: It drives only the flying rails to bracket the supplies around the input.

Test circuit for a non-flying, non-phantom powered, 1570 preamp used to check the servo:

Image
THAT1570 Servo Test Circuit with A Differential Floating Capacitor Deboo Servo

All resistors in the above are 1%.
Note that the differential Deboo is non-inverting: The servo correction outputs cross-couple to opposing inputs to provide the proper polarity.
The noise performance is virtually identical when Rcm = 0 Ohms or 100K.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

I rebuilt the DC-Coupled Input Capacitorless Preamp with a THAT1570.

The output is recovered by the AC-Coupled high impedance line amp followed by a double-balanced differential amp.
All I can say is wow. This thing rocks.

Here is spectrogram of the noise floor with the circuit built on an open protoboard. 0 dB = 0 dBu.

Image
DC Coupled Input Capacitorless Preamp Noise Floor THAT1570 60dB Gain 154R Source Impedance

It should be noted that the 1/f noise below 10 Hz is present at about the same level with the inputs to the analyzer shorted.
The noise floor rises a tad, maybe 3 dB, with phantom on.

FWIW the Beringher ECM8000 still requires about +/-7V of servo correction to satisfy it's quiescent current imbalance.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

I've rebuilt the prototype and have a bunch of updates to provide soon but stumbled upon an interesting thing about microphones while poking around this circuit with the 'scope:

Some phantom-powered mics have a significant low frequency common mode component due to internal dynamic signal currents. Other's don't.

Handling noise in particular increases current draw significantly in some mics to the point you can see it modulate the Vcm servo slightly.
Both phantom legs get pulled closer to ground together - in common mode - as signal current increases.
It does seem to follow...

It's of absolutely no consequence in this circuit since Vcm doesn't do anything now and the LF common mode rejection is as good as it is at mid-band.
The Vcm servo only drives the floating rails and the ICs riding them have significant PSRR.
As to CM rejection I'm seeing typically -80 dB at 40 Hz low gain and I suspect it can be better on a real PC board.
CM signals are passed through this preamp at unity gain up to the cross-coupled differential output where common mode rejection is realized in a "double-balanced" configuration.

But I have to wonder if active Mic preamps with capacitor inputs that have either low-value or mis-matched input capacitors convert some of this mic-induced LF common mode signal to differential.
It appears to mostly be sub-sonic in the sample I have that produces it.
ricardo
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by ricardo »

mediatechnology wrote:[bSome phantom-powered mics have a significant low frequency common mode component due to internal dynamic signal currents. Other's don't.
Wayne, will you share which mikes do this?

Gotta admit us evil mike designers sometime ASSume supa dupa preamps ... though not usually as evilly as ECM8000 :o

We gotta assume supa dup transformer inputs too which would be seriously upset by mismatched currents on pins 2 & 3
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Ironically the ECM8000 doesn't do this.
Though it does pull unequal static current the dynamic current looks fairly constant.

The Audio Technica AT3525 does have significant CM "wiggles."
It has a low DC resistance from pins 2 to 3 and appears to have a transformer-coupled output.

I suspect the the lower the DC output resistance the more the dynamic current appears in CM.

I need a full Mic closet to check this. I may have one...

Makes the case for Mic Preamps with excellent LF CM rejection.
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JR.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. »

Interesting... I used to do a lot of tone burst testing, because it would reveal things that static testing didn't. For a mic/preamp i can imagine other obscure things like popping plosives, wind noise, etc.. but these are quickly getting esoteric and arguably gray area off-sheet for expected performance. Better is always better tho.

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

Post by emrr »

ricardo wrote: We gotta assume supa dup transformer inputs too which would be seriously upset by mismatched currents on pins 2 & 3
Geez, yeah, old school phantom through the transformer center tap versus pair of 6K8's. Very different end result.
Best,

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

Post by mediatechnology »

My thinking is that mics that have internal transformers, and thus a low DC resistance between pins 2 and 3, are more likely to show this effect in common mode since the winding equalizes the LF currents between legs.
Also, these mics may tap phantom off of their output transformer center tap as well (producing CM modulation) rather than resistors to each leg.

I hope to post pics today of the preamp protoboard.

Not including output drivers (2X THAT1646 in the dual class-A) there are only six ICs.
The signal passes through only two 470 nF PP film caps; four total with 40 Hz -12 dB/oct HPF engaged.
The preamp is fully-balanced input-to-output.

THAT1570
5534 Vcm Servo
OP2277 Differential Deboo Servo
LME49860 AC-coupled line receiver/HPF/Opt post-preamp +10dB "ribbon" gain stage
2X THAT1246 Double-Balanced Common Mode Rejector

I figure the 5534 and HV complimentary pair are about the only gizmo that's being added to eliminate input caps.
So it's really not a complicated thing...
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