Author Topic: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp? More  (Read 4893 times)

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Offline t1dTopic starter

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Goal = Create a Pro-Level, XLR-Type, Pre-Amp for a Sennheiser e835 Dynamic Microphone. Use a proper THAT 1512 op amp in a true differential configuration, for active noise canceling. A-n-d, add a Passive Bandpass Filter on the front end??? I will use this to drive a simple, DIY amp.

I think I have suddenly found a problem...
My thinking was to add the bandpass filter before the op amp, so that the op amp would not be processing unwanted frequencies, needlessly. However, if I add a bandpass filter on both phases the signal, whatever mismatch there is between the component values in the two halves of the filter will be amplified. Correct? If so, what are the typical solutions, or filter arrangements.

Thank you for your help and suggestions.

« Last Edit: August 25, 2022, 02:53:03 am by t1d »
 

Offline Benta

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #1 on: August 19, 2022, 10:41:34 pm »
First, let's call things by their right name.
What you have is a high-pass (the input caps etc.) and a low-pass (the caps before the amp).

But you haven't understood the beauty of differential design. Why are your input components connected to ground?

The first four resistors can be replaced with one (prolly around 68k, I couldn't be bothered doing the calculation).
The 1n caps can be replaced by a 500p cap.
Forget the trimpots, they'll drive you crazy.

Let the amp do the rest. At most, you might have to do a bit of DC offset compensation.
 

Online David Hess

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #2 on: August 19, 2022, 11:39:23 pm »
My thinking was to add the bandpass filter before the op amp, so that the op amp would not be processing unwanted frequencies, needlessly. However, if I add a bandpass filter on both phases the signal, whatever mismatch there is between the component values in the two halves of the filter will be amplified. Correct? If so, what are the typical solutions, or filter arrangements.

You have it right.  You can filter the differential signal before the difference amplifier, but if you filter the common mode signal, then mismatch in the filters will convert common mode noise to differential noise.

The general solution is to only implement EMI filtering before the difference amplifier, and then filter the signal after the difference amplifier.
 

Offline t1dTopic starter

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #3 on: August 19, 2022, 11:53:25 pm »
Thank you, Banta, for helping me! Much appreciated. Please clear up my misunderstandings...
But you haven't understood the beauty of differential design.
No, I understand how the diff amp cancels noise, I just don't know how to combine it with the filter section. I am copying a common filter design.
Why are your input components connected to ground?
As said, I don't know how to combine it with the filter section.
The first four resistors can be replaced with one (prolly around 68k, I couldn't be bothered doing the calculation).
The 1n caps can be replaced by a 500p cap.
I could really use some details, here. How is the resistor and how are the caps arranged? You state that they should be untied from the XLR ground. But, that being so, where should they be grounded to? Each other as a virtual ground? In addition to your description of the needed corrections, a drawing would be helpful, if you have the time.
And, if the filter will still have two halves, please address my question about their differences being amplified. Edit: David answered this part.

Thank you for your effort in sharing your expertise!
 

Offline t1dTopic starter

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #4 on: August 19, 2022, 11:57:16 pm »
My thinking was to add the bandpass filter before the op amp, so that the op amp would not be processing unwanted frequencies, needlessly. However, if I add a bandpass filter on both phases the signal, whatever mismatch there is between the component values in the two halves of the filter will be amplified. Correct? If so, what are the typical solutions, or filter arrangements.

You have it right.  You can filter the differential signal before the difference amplifier, but if you filter the common mode signal, then mismatch in the filters will convert common mode noise to differential noise.

The general solution is to only implement EMI filtering before the difference amplifier, and then filter the signal after the difference amplifier.
Thanks, David. It is really great for you to join in. I don't know much about EMI filtering. Could this be as simple as a Ferrite Bead on the mic cable?
 

Offline Someone

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #5 on: August 20, 2022, 12:22:42 am »
THAT Corporation have some really amazing design resources with explanations and details, why are you not going with one of their designs?
"THAT Corporation Design Note 140"
 

Offline t1dTopic starter

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #6 on: August 20, 2022, 12:32:46 am »
THAT Corporation have some really amazing design resources with explanations and details, why are you not going with one of their designs?
"THAT Corporation Design Note 140"
Hi, Someone. Thank you for your post. I am going with a THAT design. See Figure #4, of the data sheet, for the amp circuit portion of my schematic.

Thanks for telling me of Note 140; I did not know of it. I will look it up. Cheers!
 

Offline Someone

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #7 on: August 20, 2022, 12:55:19 am »
Not really the same as datasheet Figure 4 (below). Probably best to learn how differential filters work first as suggested by Benta above.
 

Offline t1dTopic starter

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #8 on: August 20, 2022, 02:05:45 am »
As David did a great job in answering my question, I am marking this thread as solved. Thanks to everyone for joining in and helping.
 

Offline blackdog

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Hi t1d,

Not so fast!  ;)
We're not done with you yet....

Your filter at the input to limit the frequency range certainly works.
But there are and quite a few negative characteristics you introduce with those two filters.

The That corporation has done their best to make the comonmode suppression as good as possible.
With your filter, that good suppression for interfering signals is thrown right out the window!
What others also made clear, use the examples in the manufacturer's datasheet.
And also DC shifts when you adjust de Potmeters, this because the high Bias Currents of the THAT1512. (thats relativ normal for low noise components)

And then another important point, because of your extra applied filter components, the noise number becomes much higher,
this is due to the high resistance values you put in series with the microphone.
This does not have to be a problem when shouting into the microphone, but it is usually a problem with dynamic microphones due to their low output compared to the type of condenser...
With a good microphone technique, you will certainly start to hear more noise from these additional components alone.
And then there is the additional interference from the very poor commonmode suppression by the filter components.

Choose your gain wise for the THAT1512 wise (not too high, dont let the THAT1512 clip!) and then place your limiter, and then the other filter operations you want to do on the signal.
By e.g. choosing a somewhat higher supply voltage for the THAT1512 you can get more dynamic range, for example you can use + and - 19V power supply.

The best filtering effect is a bat, with which you hit the microphone user, when he screams and eats the microphone.  :-DD

One singer who had a good microphone technique was David Bowie and of course there are others.
Always sing a little further from the microphone to avoid those ugly proximity effects.
And when they start singing louder they take more distance from the microphone.
But unfortunately, that's hard to teach singers...  ;)

Kind regards,
Bram

Necessity is not an established fact, but an interpretation.
 
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Online David Hess

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Re: Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp?
« Reply #10 on: August 20, 2022, 11:28:47 pm »
Thanks, David. It is really great for you to join in. I don't know much about EMI filtering. Could this be as simple as a Ferrite Bead on the mic cable?

It could include ferrite beads, but usually amounts to RC filters but with a cutoff frequency much higher than the audio band.  Attached below is an example where the RFI filters are 100 ohms and 100 picofarads.

 

Offline t1dTopic starter

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Thank you, to everyone, for helping me!

I forgot to add to my original post that I am just a hobbyist. That means that adding details (of both why and how to do it) to replies really helps. I am fortunate to have a rather nice lab, but the equipment is way smarter than I am. So, I can probably make most any measurement that would be helpful. Although a spectrum analyzer would likely be useful, as this is an audio project, I only have a scope. But, it has FFT, which may be useful, to some degree.

David's confirmation of my concerns with the filter being before the 1512 led to
- moving the filter to after the 1512,
- adding back
   - a) Caps 1, 2 and 3 and
   - b) The Bias Resistors R1 and 2.

Regarding C1/2/3 and R1/2. I had removed these components, because I thought (incorrectly) that the filter, in its former pre-1512 position, was also fulfilling the purposes of these components.

So, here are some new considerations.…
The Data Sheet does not clearly state that C1/2/3 are simply to filter noise, as would any other decoupling cap. I believe this is their function. Do I understand correctly?

Is there significant benefit to using a precision resistor array for the biasing resistors, R1/2, to warrant its approximately $8USD price?

Figure 4, of the 1512 DS, clearly shows the use of two types of ground…
- "Rake" symbol. This ground is true Earth. It is provided by the XLR Input Connector's case. It provides the exist for (only) C3. Do I understand correctly?
- "Inverted Triangle" symbol. This is the null point of the +/-15V supply. All grounds except C3 return to this point and exist via Pin3 Pin1 of the XLR Connector. Do I understand correctly?

Benta did not expound on why he suggested that trim pots not be used to tweak the filter. I take it that there are two reasons...
- The setting is not critical.
- Trim pots are subject to thermal drift.
Do I understand correctly?

If so, I have left the trim pot footprints in place, to provide some options. They may be used for trim pots, a second fixed value resistor to be used in series with the first resistor, to give flexibility in attaining a particular value, or simply bypassed with a jumper wire, if not needed.

I would like to add David's EMI filter. See Reply #10/Figure 18.9, of the document that he provided.
- I believe it consists of the pairs C3/100pF + R7/100K(not 100R) and C4/100pF + R8/100K(not 100R). Do I have 100K, not "R," correctly?
- Which Ground does the EMI filter return to, Earth or Null? Earth is shown.
- Exactly where is it inserted into the circuit? After the biasing resistors RBIAS1/2?

Again, I am grateful for your help, expertise and effort to reply. We have a great community.
« Last Edit: August 25, 2022, 09:27:40 am by t1d »
 

Offline blackdog

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Hi t1d,

If you do not have strong HF fields in the environment where you are going to use the microphone preamplifier then the filtering is OK.

Leave out the snubber, by the way the impedance you have chosen for this is not suitable for an opamp, it puts far too much load on the opamp.
Through RPB2 of 10K the opamp output sees no capacitive load that makes it difficult for the opamp.

If you still want some protection against e.g. longer output cables, then add directly to pin 6 of the opamp a 47 Ohm series resistor.

Your output impedance on the left side of C4 is around 10K, so the high value of C4 is not needed there at all.

Also, because of the high output impedance you may not load the circuit lower than say 150K because otherwise your tilt points will shift,
A better solution is a buffer opamp that is DC coupled and leave C4 out at that point.
Then your filter is exactly as you want it, independent of the load.

If you apply the buffer opamp, again a series resistor directly to the opamp output against generation phenomena and maybe a coupling capacitor.

Kind regards,
Bram

Necessity is not an established fact, but an interpretation.
 
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Offline t1dTopic starter

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Thanks, Bram. These are all very excellent considerations.
Leave out the snubber, by the way the impedance you have chosen for this is not suitable for an opamp, it puts far too much load on the opamp.
Through RPB2 of 10K the opamp output sees no capacitive load that makes it difficult for the opamp.
I will remove this option. It was just a consideration that I saw on a LM386 project.
If you still want some protection against e.g. longer output cables, then add directly to pin 6 of the opamp a 47 Ohm series resistor.
I will add it in.
Your output impedance on the left side of C4 is around 10K, so the high value of C4 is not needed there at all.

Also, because of the high output impedance you may not load the circuit lower than say 150K because otherwise your tilt points will shift,
A better solution is a buffer opamp that is DC coupled and leave C4 out at that point.
Then your filter is exactly as you want it, independent of the load.

If you apply the buffer opamp, again a series resistor directly to the opamp output against generation phenomena and maybe a coupling capacitor.
I like the buffer idea and I will take a shot at adding it in. But, I am sure that I will need you to guide me. I take it that another 1512 is best to use for it. I will post the corrected schematic and we will go from there. Again, thank you so much!
 

Online David Hess

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The Data Sheet does not clearly state that C1/2/3 are simply to filter noise, as would any other decoupling cap. I believe this is their function. Do I understand correctly?

Those are there to filter RF noise which could potentially be demodulated by the instrumentation amplifier.

Quote
Is there significant benefit to using a precision resistor array for the biasing resistors, R1/2, to warrant its approximately $8USD price?

Mismatch between the shunt resistors at the input and the source impedance limit common mode rejection, so the input resistors should be reasonably matched.  1% resistors are typically considered good enough, but 0.1% is not much more expensive.

Quote
- "Rake" symbol. This ground is true Earth. It is provided by the XLR Input Connector's case. It provides the exist for (only) C3. Do I understand correctly?

Correct, C3 should return to chassis ground close to the connector.

Quote
- "Inverted Triangle" symbol. This is the null point of the +/-15V supply. All grounds except C3 return to this point and exist via Pin3 Pin1 of the XLR Connector. Do I understand correctly?

Correct, and there should be a single ground connection between the chassis and that point.

Quote
Benta did not expound on why he suggested that trim pots not be used to tweak the filter. I take it that there are two reasons...
- The setting is not critical.
- Trim pots are subject to thermal drift.
Do I understand correctly?

Trim pots might be used to trim the filter if it is at the input where mismatch of the differential filter would limit common mode rejection.  Otherwise the filter cutoff is not usually that critical and adjustments only add the opportunity for maladjustment.  Do you need a filter cutoff closer than 5%?

Quote
I would like to add David's EMI filter. See Reply #10/Figure 18.9, of the document that he provided.
- I believe it consists of the pairs C3/100pF + R7/100K(not 100R) and C4/100pF + R8/100K(not 100R). Do I have 100K, not "R," correctly?

Those are 100 ohms.

Quote
- Which Ground does the EMI filter return to, Earth or Null? Earth is shown.

They RF filter returns to chassis ground close to the connector.

Quote
- Exactly where is it inserted into the circuit? After the biasing resistors RBIAS1/2?

The biasing resistors could connect either before or after, but after has some advantages.
 

Offline srb1954

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The output coupling capacitor should connect to pin 3 of RV2 for best filtering at higher frequencies. The way that it is currently drawn the HF filtering will plateau at higher frequencies if the RV2 is adjusted to anything other than 0 \$\Omega\$.

The output coupling capacitor C4 is probably not necessary as any DC out of U1 is already blocked by Cbp1. C4 would only be required if the input of the subsequent stage is biased to something other than 0V. To avoid loading the output of the band-pass filter the subsequent stage will have to have a high input impedance, say > 100k \$\Omega\$, relative to the resistor values used in the band-pass filter. With this high input impedance the value of C4, if required, can be made much smaller, say 470nF, and still obtain adequate bass response. There is no point having the inter-stage coupling designed to pass extremely low frequencies <1Hz if the preceding band-pass filter has already substantially cut off everything below 50Hz.

« Last Edit: September 05, 2022, 10:27:24 pm by srb1954 »
 

Offline t1dTopic starter

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These are really great responses, David. Clear and concise. Thank you!

Quote
Is there significant benefit to using a precision resistor array for the biasing resistors, R1/2, to warrant its approximately $8USD price?

Mismatch between the shunt resistors at the input and the source impedance limit common mode rejection, so the input resistors should be reasonably matched.  1% resistors are typically considered good enough, but 0.1% is not much more expensive.
I matched a pair from my stocks and they are nearly dead on.

Quote
- "Inverted Triangle" symbol. This is the null point of the +/-15V supply. All grounds except C3 return to this point and exist via Pin3 Pin1 of the XLR Connector. Do I understand correctly?

Correct, and there should be a single ground connection between the chassis and that point.
I thought that the circuit ground was to be fully isolated from Earth ground. So, I am going to repeat back to you your instructions, for verification for certain.

At this location,
1) the PCB Board's +/-15Vsupply Null Point Return,
2) the Aluminum Case Project Box Chassis,
3) Pin#1 of the XLR connector
4) and the Metal Case of the XLR Connector
are all to be tied. Do I understand correctly?

Do you need a filter cutoff closer than 5%?
No, not at all, for my application.

Again, David, just excellent. Thank you.
 

Offline t1dTopic starter

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If you still want some protection against e.g. longer output cables, then add directly to pin 6 of the opamp a 47 Ohm series resistor.
Kind regards,
Bram
Thanks, Bram. My thinking is that you suggest the 47 Ohm series resistor as a typical low impedance termination. If I have that correctly, would not the more common value be 50 Ohms? From what I can tell, whatever value I have that is closest to 50 Ohms is okay, as the value is not critical. Even though this is a low amperage preamp circuit, I take it that the resistor's wattage rating needs to be considered.
 

Offline t1dTopic starter

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At this point, I am taking a side-step and building out the DS Figure 4 circuit, with the addition of Bram's resistor. I will play with the assembly as a proof of concept. I am bodging the (errant) original board that included the filter in the wrong position. The input caps and resistors can be fitted nicely.


The board needs a cleaning and the camera really shows it. Bram's output resistor will go in the C4 location.




« Last Edit: August 27, 2022, 03:34:45 am by t1d »
 

Offline t1dTopic starter

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A linger question about the needed value for the gain pot... I stuck a 1K multi-turn trim pot on the board, sort of as a place holder. I will have to learn what math to do to calculate the correct value, but I am shooting for an output of 0-1.5V. IIRC, 1.5V will be close to what is considered "Line Level" for pro audio equipment, with just a touch of extra headroom. Any advice in this regard is welcome.
 

Online David Hess

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Mismatch between the shunt resistors at the input and the source impedance limit common mode rejection, so the input resistors should be reasonably matched.  1% resistors are typically considered good enough, but 0.1% is not much more expensive.

I matched a pair from my stocks and they are nearly dead on.

Going by memory, 1% resistors yield a worst case common mode rejection of 54dB which in my experience is more than good enough.  Better resistor matching quickly reaches diminishing returns because capacitive effects begin to dominate, and then the capacitance needs to be trimmed to get better performance.

The resistor mismatch *includes* the source impedance mismatch, which is why differential oscilloscope probes have as high an impedance as possible, and why an instrumentation amplifier with very high input impedance performs better with mismatched source impedances than a difference amplifier with lower input impedance.

In audio applications this comes up when an unbalanced output is connected to a differential input.  One side of the differential input sees the remote ground point, presumably with an impedance of zero, while the other side sees the single ended output which typically has an impedance of a few hundred ohms, although it is possible to design a line level signal output with an impedance of zero which avoids this problem.  This mismatch also degrades common mode rejection, but usually not significantly.

Douglas Self wrote an excellent book called Small Signal Audio Design on this subject, although I do not remember if he covers common mode rejection as thoroughly as I have described.

Quote
Correct, and there should be a single ground connection between the chassis and that point.

I thought that the circuit ground was to be fully isolated from Earth ground. So, I am going to repeat back to you your instructions, for verification for certain.

At this location,
1) the PCB Board's +/-15Vsupply Null Point Return,
2) the Aluminum Case Project Box Chassis,
3) Pin#1 of the XLR connector
4) and the Metal Case of the XLR Connector
are all to be tied. Do I understand correctly?

Ideally there is a single point ground where the chassis ground and circuit ground meet.  In practice this becomes a problem when multiple external connectors are present, so some compromise has to be made.  Having the chassis completely separate from the signal ground is not something I would recommend, but maybe some audio equipment does it?  Maybe someone in the forum knows.

Where trouble starts is how the XLR ground and shield are connected.  One recommendation is that the shield be connected to the chassis, and the pin-1 ground be kept separate until it attaches to the single point ground at the chassis.  This helps keep currents in the shield from affecting the signal ground as much.

I do not know if any audio equipment does it, but in test instrumentation where ground loops are a problem, the input may be completely galvanically isolated from the circuit and chassis.  An instrumentation or difference amplifier with a differential input is almost as good in an application like audio, and of course a transformer could be used but that is less practical these days.
« Last Edit: August 27, 2022, 04:45:37 pm by David Hess »
 

Offline t1dTopic starter

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Quote
Correct, and there should be a single ground connection between the chassis and that point.

I thought that the circuit ground was to be fully isolated from Earth ground. So, I am going to repeat back to you your instructions, for verification for certain.

At this location,
1) the PCB Board's +/-15Vsupply Null Point Return,
2) the Aluminum Case Project Box Chassis,
3) Pin#1 of the XLR connector
4) and the Metal Case of the XLR Connector
are all to be tied. Do I understand correctly?

Ideally there is a single point ground where the chassis ground and circuit ground meet.  In practice this becomes a problem when multiple external connectors are present, so some compromise has to be made.  Having the chassis completely separate from the signal ground is not something I would recommend, but maybe some audio equipment does it?  Maybe someone in the forum knows.
In my original design, which had the bandpass filter on the inputs before the 1512, I had mistakenly tied the filter to circuit ground. We know that the input caps of a properly designed input (DS Figure 4) should be tied to Earth ground. We can infer from Benta's reply question ["Why are your input components connected to ground?"/Reply #1] that he does not think the two grounds should be tied. I will send him a PM and ask him to provide some insight.
 

Offline t1dTopic starter

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Just to recap where we are, these are the/some/all? open questions...
- If Earth Ground and Circuit Ground are ever to be tied (at the chassis.)
- Calculation of the resistor value of the Gain Pot.
- The purpose and value of the resistor that Bram suggests should be added to the output. Or, the use of an output buffer op amp and its circuit design.

I have sent PMs to Benta and Bram asking them to join back in, particularly regarding the above issues upon which they have previously commented.

I am learning a lot and having much good fun. Thank you to everyone!
 

Offline t1dTopic starter

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Just to recap where we are, these are the/some/all? open questions...
- If Earth Ground and Circuit Ground are ever to be tied (at the chassis.)
- Calculation of the resistor value of the Gain Pot.
- The purpose and value of the resistor that Bram suggests should be added to the output. Or, the use of an output buffer op amp and its circuit design.

I have sent PMs to Benta and Bram asking them to join back in, particularly regarding the above issues upon which they have previously commented.

I am learning a lot and having much good fun. Thank you to everyone!
I did not receive any replies to the above. I am posting an updated rough-draft of the corrected schematic. In addition to the above, I could use help in determining how to terminate the circuit's output, in order to have it at the proper output impedance. Please and thank you, as always.

It was suggested that I not use trimmers on the pass filter. I have left them, just to have options to tweak the resistor values, with pots, or splitting the total resistor value between two resistors, if a single common resistor value will not do the trick.
« Last Edit: October 21, 2022, 06:27:50 am by t1d »
 

Offline jonpaul

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Just joining this very long and old topic.

Application is studio dynamic mic?

Never saw a RC single pôle bandpass needed in any mic preamplifier

Just RF EMI beads or caps.


Best to follow the excellent Les Tyler app notes from THAT

Suggest that you Omit bandpass and output elcap

Set gain as THAT spec sheet reccomended

best mic preamplifiers use a matching transformer.

Just the ramblings of an old retired EE

Jon

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