Preventing mic preamplifier HF oscillation

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AntK
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Preventing mic preamplifier HF oscillation

Post by AntK » Mon Mar 04, 2019 9:42 am

A long time reader, but never had the courage to participate. This is a PRO forum anyway:)

But here we go. The mic preamplifier that I recently built follows the idea of this classic circuit, except the transistors which are PNP type 2N4403.

Image

http://www.muzines.co.uk/images_mag/art ... _large.jpg

Two channels work fine, but one has unpredictable noise behaviour, which seems to be high frequency oscillation. Textbook vs. real life I think.

The board layout may not be the main problem: small SMD components, very short connections, intact ground plane and proper power decouplings.

The question is what would be the most effective way to prevent the HF problems?

There are two solutions that I remember seeing (sorry the non electronics engineer way of thinking).

1. Adding a few hundred pF capacitor from base to emitter. I guess the idea is to short the input at the high frequencies the same way as a base to ground capacitor in a simple common emitter gain stage.

2. A ferrite bead in series with the emitter. When thinking about a common emitter amplifier again, that makes sense. The ferrite acts like an emitter resistor at very high frequencies and reduces the gain at tens or hundreds of MHz where the oscillation most likely happens. There are physically large components (parasitic inductance and capacitance) tied directly to the emitters. Sounds like a good idea to "isolate" them at high frequencies.


I know that you can get lower distortion with complementary transistor pair or op-amp feedback and that there are a number of purpose made integrated circuits available. But this version is low power and good enough at the moment. And I prefer to get a simple circuit fully working first and hopefully better understand how it works (despite the non technical backgound). After that it is time for more advanced circuits.

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mediatechnology
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Re: Preventing mic preamplifier HF oscillation

Post by mediatechnology » Mon Mar 04, 2019 9:59 am

AntK wrote:
Mon Mar 04, 2019 9:42 am
A long time reader, but never had the courage to participate. This is a PRO forum anyway:)

But here we go. The mic preamplifier that I recently built follows the idea of this classic circuit, except the transistors which are PNP type 2N4403.

Image

http://www.muzines.co.uk/images_mag/art ... _large.jpg

Two channels work fine, but one has unpredictable noise behaviour, which seems to be high frequency oscillation. Textbook vs. real life I think.

The board layout may not be the main problem: small SMD components, very short connections, intact ground plane and proper power decouplings.

The question is what would be the most effective way to prevent the HF problems?

There are two solutions that I remember seeing (sorry the non electronics engineer way of thinking).

1. Adding a few hundred pF capacitor from base to emitter. I guess the idea is to short the input at the high frequencies the same way as a base to ground capacitor in a simple common emitter gain stage.

2. A ferrite bead in series with the emitter. When thinking about a common emitter amplifier again, that makes sense. The ferrite acts like an emitter resistor at very high frequencies and reduces the gain at tens or hundreds of MHz where the oscillation most likely happens. There are physically large components (parasitic inductance and capacitance) tied directly to the emitters. Sounds like a good idea to "isolate" them at high frequencies.


I know that you can get lower distortion with complementary transistor pair or op-amp feedback and that there are a number of purpose made integrated circuits available. But this version is low power and good enough at the moment. And I prefer to get a simple circuit fully working first and hopefully better understand how it works (despite the non technical backgound). After that it is time for more advanced circuits.
Thank you for joining us and posting after all of these years!
For the record I'm not a EE having only a High School education.

Your question is actually a very good one and IMHO a topic for an AES paper.

Some clues might be found here: https://proaudiodesignforum.com/forum/p ... 923#p12633

And figure 14 here: http://www.ti.com/lit/ds/symlink/ina217.pdf

If it were me I'd start by putting a couple of 10Ω series resistors directly in series with the bases so that the input to ground capacitors are on the left side.
Series resistors will lower the Q of the tuned circuit. (If there is one and there must be one somewhere or it wouldn't oscillate.)

- Or -

Remove C1 and C2 entirely and see if it stops oscillating then go from there.

I often see small capacitors (or C+R) between the collectors.

You're going to need some series R anyway for phantom protection.
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Audio1Man
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Re: Preventing mic preamplifier HF oscillation

Post by Audio1Man » Mon Mar 04, 2019 4:50 pm

A small resistor in each BASE will DQ the circuit and will add some noise. You can also add a ferrite bead to each BASE this will also DQ the circuit. C1 & C2 could be smaller. With SMD layouts the BW always is much higher and amplifiers have a better chance of becoming an oscillator. When the phantom power circuit & protection is added additional DQ may be required.
I hope this helps Duke.

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mediatechnology
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Re: Preventing mic preamplifier HF oscillation

Post by mediatechnology » Mon Mar 04, 2019 7:30 pm

+1 what Duke said.

Another thing to look for is stray capacitance from the emitters to the ground.
That area is very insensitive to capacitance and you ideally want to avoid a ground plane under them.
See the THAT paper "Analog Secrets Your Mother Never Told You." Page 16 and 17: http://www.thatcorp.com/datashts/Analog ... ld_You.pdf

How does the gain pot connect to the board?
Long traces? Shielded cable?
Are the pot traces longer on that particular channel?
Is (are) the shield(s) adding stray C to ground?
See: http://www.proaudiodesignforum.com/foru ... 3&start=63

What I've started doing for phantom protection is to split the base resistance in two (keeping the total R constant around 10-15Ω/leg) and place the protection steering diodes to the rails in the middle of the two resistors.
That places some resistance on the left-hand side to limit diode current and then protection resistance in series with the bases.
It doesn't give you any more fault protection, but what it does do is put some "DQ" (de-Q) resistance between the bases and the diode junction capacitances.
I then put the differential termination C directly across the input ahead of the protection series resistors and avoid CM termination at the bases.
See: https://proaudiodesignforum.com/forum/p ... ?f=6&t=598

The above schematic shows a 220 pF termination C at the bases. If its removed, common mode rejection is actually improved. This example is a common INA made with op amps that exhibits a similar behavior: http://www.proaudiodesignforum.com/foru ... =90#p11194

I'm not exactly sure why this is so but I've reproduced the results with 1510/1570 type preamps as well. As the differential termination C is moved closer to the bases, HF CM rejection reduces.

The moving coil preamp using the ZTX851s doesn't have the luxury of adding any series R so I used a combination of B-E capacitance (which is bootstrapped so it doesn't appear as a CM capacitance) and a large differential termination C. http://www.proaudiodesignforum.com/foru ... 200#p11373

In Wayne's Mic Preamp bruno2000 used inductors in the inputs and emitters (in series with Rg). I suggested that we try the Rg inductors and bruno sucessfully used the preamps with 500' input runs for his live recording truck. http://www.proaudiodesignforum.com/foru ... ?f=7&t=339 People at Doppler really liked the sound of this preamp.
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AntK
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Re: Preventing mic preamplifier HF oscillation

Post by AntK » Fri Mar 08, 2019 9:46 am

Came into my mind that the circuit, stray capacitances included, looks frighteningly similar to an emitter follower type Colpitts oscillator.

Image

I found an old mixer service note which recommended adding BE capacitors (470pF) to kill the possible oscillation. Those capacitors (470pF-1000pF) can also be found in many commercial designs. I think increasing Cbe lowers the Q of the oscillator.

The base resistor trick seems to be rare in commercial products. For some reason practically all designs connect the base directly to ground via a few hundred pF capacitor.

From the Colpitts oscillator point of view adding a ferrite (inductance) to the base doesn't sound like a good idea...

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mediatechnology
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Re: Preventing mic preamplifier HF oscillation

Post by mediatechnology » Sat Mar 09, 2019 5:33 am

Came into my mind that the circuit, stray capacitances included, looks frighteningly similar to an emitter follower type Colpitts oscillator.
From a thread I linked to earlier John Dunn comes to the same conclusion.

https://www.edn.com/electronics-blogs/l ... cillators-

Take a look at Cohen's OM1556 preamp and look at what he did WRT series resistors for phantom power protection. They also de-Q. The termination capacitance, which is at the bases, is a Wye network. If your base to ground caps were a single cap connected differentially, or a Wye network, it might not oscillate. See: viewtopic.php?f=12&t=1114
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terkio
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Re: Preventing mic preamplifier HF oscillation

Post by terkio » Sat Apr 06, 2019 8:40 am

The schematic in the original post shows two 100µF on +12V -12V at the op-amp :shock:
You do need two 100nF as close as possible to the op-amp pins. These are a must.
Add those, keep the 100µF, for general decoupling.
I always put 100nF Multy Layer Ceramic caps at each and every IC supply pins.
Recent story, we quickly made a preamp of this topology on a proto board. It worked right away with a TL082 and 2N2222.
Then we switched to a NE5533 ( faster ), it made a 2 Mhz oscillation over the signal.
Then, with two 100nF caps at the IC supply pins, oscillation gone, clean signal.
Just for the record: At 200:1 gain the bandwith was 100KHz, with the TL082, then 300KHz with the NE5533.

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mediatechnology
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Re: Preventing mic preamplifier HF oscillation

Post by mediatechnology » Sat Apr 06, 2019 5:54 pm

I agree there should be 100 nF at each pin. Or a single from Vcc to Vee.
The series inductance of the 100 uF at RF is so high they might not as well be there because they don't prevent oscillation.
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