Mic Preamp DC-Servo: The "backwards" way

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olafmatt
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Re: Mic Preamp DC-Servo: The "backwards" way

Post by olafmatt » Tue Feb 24, 2015 6:03 pm

Sorry for getting this thread back to live.... but it was really worrying me that reality worked better than simulation... :roll:

For reference I'll link in the image from my first post again:

Image

My proposal was to inject the DC servo's (differential) correction voltages into the bias voltages (non-inverting inputs of the opamps) to "indirectly" set the relative collector current for the transistors. So in effect instead of connecting R7 and R9 to gound they get (differentially) moved up and down by the servo.

Then we went on to discuss how the effective frequency of the DC servo changes with gain. The two "traditional" ways of attaching the servo are:

A) sample DC offest across the gain set resistor and inject servo voltage into the bases -> the "gain" the servo sees does not change with gain changes, but we amplify any remaing DC offset by that amps gain

B) sample at the output and inject into the bases -> gain changes change the servo's frequency, so at low gains it takes ages for the DC offset to get servoed away

Now I finally saw the real difference in my approach: When altering the gain by changing Rg we also change the gain of the transistors (because Rg is the emitter resistor for the transistors). So in other words, the higher we set the gain, the higher the gain of the transistors gets (and the "remaining" gain is added by the opamp). The crucial thing is that the gain of the transistors does not stay constant. What follows is that any voltage change injected into the non-inverting inputs does not see the full gain of the complete composite amp.
To sum up, when sampling the output DC offset and injecting the servo voltages into the bias voltage nodes the servo's frequency will not change as much as it would when injecting into the bases. Advantages are that we still sample at the very output, so the amp will not further amplify any remaining offset left by the servo (so no need for super special low-offset opamps or manual trimming). And of course we don't have to mess with the bases, so the common-mode bootstrapping (not shown above) does not get compromised.

Olaf

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mediatechnology
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Re: Mic Preamp DC-Servo: The "backwards" way

Post by mediatechnology » Thu Feb 26, 2015 9:01 am

Have you tried it?

olafmatt
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Re: Mic Preamp DC-Servo: The "backwards" way

Post by olafmatt » Thu Feb 26, 2015 12:22 pm

Yes I did try (and it did work). That's how I realized that my lazy assumption that the "new" injection nodes see the same (overall) gain applied as the bases do is wrong. - At low gains I was getting way faster reaction of the servo than I thought I would.

Olaf

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mediatechnology
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Re: Mic Preamp DC-Servo: The "backwards" way

Post by mediatechnology » Thu Feb 26, 2015 2:42 pm

Cool!

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Re: Mic Preamp DC-Servo: The "backwards" way

Post by olafmatt » Thu Feb 26, 2015 3:27 pm

mediatechnology wrote:Cool!
Indeed.

It basically goes like this:

Image

Just insert your favourite differential servo (but keep in mind that it needs to remove the CM component). When not using T-bias one could use a non-differential servo, just monitoring the voltage difference between the outpus and generating one single correction voltage.

One could also play with the values of R4 and R5 (in relation to Rf) in order to control how much gain the transistor contributes and how much it's gain varies when changing the "overall" gain. - Actually the fact these circuits can be made stable over a 60dB (or more) gain range is based on the change in gain of the transistors. At lower gains the available loop gain decreases (because of the decreasing transistor gain) and thus helps stability.

Olaf

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mediatechnology
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Re: Mic Preamp DC-Servo: The "backwards" way

Post by mediatechnology » Thu Feb 26, 2015 4:35 pm

Thank you for posting this and following up.
I know that I never tried this because, using the 1510/INA217, 1570 etc., there is no access to that node.
With a discrete front end it seems very simple to do.
So you going to share the final schematic when you're done?

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Re: Mic Preamp DC-Servo: The "backwards" way

Post by mediatechnology » Wed Apr 25, 2018 5:36 pm

olafmatt wrote:
Thu Feb 26, 2015 3:27 pm
mediatechnology wrote:Cool!
Indeed.

It basically goes like this:

Image

Just insert your favourite differential servo (but keep in mind that it needs to remove the CM component). When not using T-bias one could use a non-differential servo, just monitoring the voltage difference between the outpus and generating one single correction voltage.

One could also play with the values of R4 and R5 (in relation to Rf) in order to control how much gain the transistor contributes and how much it's gain varies when changing the "overall" gain. - Actually the fact these circuits can be made stable over a 60dB (or more) gain range is based on the change in gain of the transistors. At lower gains the available loop gain decreases (because of the decreasing transistor gain) and thus helps stability.

Olaf
I just got around to testing this method and it works very well with the ZTX851 MC phono preamp test circuit. viewtopic.php?f=6&t=783
I used a Differential Deboo servo sampled at the INA output.
Since my bias resistors were originally referenced to the negative rail rather than ground I injected, through resistors, directly into the non-inverting input rather than at the lower end of the string.
I may need to tweak that approach.

The servo works as intended and the servo frequency doesn't change with gain.
Input coupling capacitors (if used) also don't have the potential to produce a resonant servo response.

This approach works very well to minimize both output offset and Rgain offset in circuits where Rfb is not switched.
In Olaf's preamp circuit shown above, as well as the ZTX851 MC preamp, the op amps supply emitter bias current.
There is DC current in the feedback resistors.
Gain is switched by changing Rg.

This approach can't be used to servo preamps using THAT's 51XX-series.
The 51XX gain controllers require that there be no DC current in the feedback resistors. (DC messes with the internal distortion reduction.)

For preamps using 51XX devices a servo similar to Olaf's can be used to modulate the emitter current sinks differentially.
(See THAT's DN-109 http://www.thatcorp.com/datashts/dn109.pdf)
DN-109 and similar topologies can have their emitter current sources split with both emitters sinks' bases driven by the sum of the common mode servo (to eliminate Rfb current) and a differential servo to eliminate output Rgain or output Vos.

I like this circuit. Sorry it took so long to try it Olaf.

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Re: Mic Preamp DC-Servo: The "backwards" way

Post by olafmatt » Tue May 15, 2018 1:24 am

mediatechnology wrote:
Wed Apr 25, 2018 5:36 pm
I like this circuit.
Glad it works not just for me.

In the thread about the MC preamp you mentioned the reduced power supply rejection caused by the fact that the two non-inverting inputs of the opamps are not connected together. This is basically something I was already mentioning as a drawback of this approach in my initial post.

But since we are only interested in having them not connected at lower frequences and don't care about higher ones, one could come up with some RC combination between these two opamps (or maybe even just a C). But I've never tried that, instead went with separately sub-filtered power supplies (using cap-multipliers and/or shunt regulators) for various sections of my mic preamp. Additionally I have large-ish caps to ground on the non-inverting inputs of the opamps. But that makes finding a stable combination of component values a little more tricky, since you are creating a second-order DC servo response.

Olaf

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Re: Mic Preamp DC-Servo: The "backwards" way

Post by mediatechnology » Wed May 23, 2018 5:46 am

I opted for a shunt regulated supply for the collector loads and servo bridge but did place a small 100 nF HF bypass between the two arms.

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