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What is the role of the capacitor in Simpl48 circuit?

 

Dear All,
My questions are, In the simpl48 circuit, what is the role of the capacitor? What happens if it is not used? What are the results at different values?
Many thanks.
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The capacitor blocks the 48V to your mic. The 48V via pin 3 is limited to a safe voltage by the resistor. Smaller values will make a 6dB/octave HP filter in the audio range. Mostly nor desired.
 

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Le 05/02/2025 à 07:31, Adam Jo5eph via groups.io a écrit?:
Dear All,
My questions are, In the simpl48 circuit, what is the role of the capacitor?
Tj=he role of the capacitor is to conduct AC signal whilst isolating DC.
What happens if it is not used?
Not used, as in "not there" or as in "replaced with a short"?
In the former case, very low signal, in the latter case, too low DC voltage on the capsule, so no signal.

What are the results at different values?
The capacitor acts as a high-pass filter, in conjunction with the mic preamp input impedance.
Small cap-> no bass.

Many thanks.
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A couple of years ago, Martin Kantola shared his simple P48 without a capacitor. Pure genius!
Do a search for:
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One resistor P48 electret mic
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Different pros and cons compared to SimpleP48.
 

In order to work, it takes disconnecting one of the internal 6.81k resistors inside the preamp, or to isolate it DC-wise, which is exactly what teh capacitor does.

Le 05/02/2025 à 13:34, renthomsen via groups.io a écrit?:
One resistor P48 electret mic
 

Got it, thank you much.
Then, what should be the capacitor value to cut off audio at frequencies lower than 70Hz?
Thanks again.
 

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It sepends on the actual input impedance of the mic preamp.

For a typical 2kohm input impedance, the capacitor should be 1.15uF for 70Hz cut-off.
The actual response would be first-order, i.e. only 6dB/octave.

Much less efficient than any active high-pass filter in the preamp.

Le 06/02/2025 à 03:11, Adam Jo5eph via groups.io a écrit?:

Got it, thank you much.
Then, what should be the capacitor value to cut off audio at frequencies lower than 70Hz?
Thanks again.
 

It sepends on the actual input impedance of the mic preamp.

For a typical 2kohm input impedance, the capacitor should be 1.15uF for 70Hz cut-off.
The actual response would be first-order, i.e. only 6dB/octave.

Much less efficient than any active high-pass filter in the preamp.

The actual effective resistance for LF roll-off is 1k5 (input Z of the preamp) + Zo (of the mike)? So a much smaller capacitor is needed for a 70Hz cut-off.
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But Zo of the mike(s) varies a lot so, like Jerry, I wouldn't use this to set the LF limit but only to get rid of subsonic noise.
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Much of this is on page 2 of the latest version of SimpleP48.pdf? dtd. 29sep24
 

That depends on your capsules impedance and the impedance of the following circuit. Trial and error is a fast way to discover what works for you. 2,2uF? Lower higher? It costs near to nothing to try this.


Op do 6 feb 2025 03:11 schreef Adam Jo5eph via <adam.jo5eph=[email protected]>:
Got it, thank you much.
Then, what should be the capacitor value to cut off audio at frequencies lower than 70Hz?
Thanks again.
 

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Le 06/02/2025 à 21:35, Richard Lee via groups.io a écrit?:

It sepends on the actual input impedance of the mic preamp.

For a typical 2kohm input impedance, the capacitor should be 1.15uF for 70Hz cut-off.
The actual response would be first-order, i.e. only 6dB/octave.

Much less efficient than any active high-pass filter in the preamp.

The actual effective resistance for LF roll-off is 1k5 (input Z of the preamp) + Zo (of the mike)? So a much smaller capacitor is needed for a 70Hz cut-off.
I would say slightly larger.
With Zin= 1.5k and Zo=150r, the capacitor must be 1.35uF for -3dB@70Hz
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But Zo of the mike(s) varies a lot so, like Jerry, I wouldn't use this to set the LF limit but only to get rid of subsonic noise.
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Much of this is on page 2 of the latest version of SimpleP48.pdf? dtd. 29sep24
 

Thank you so much everyone.
Is there any formula/equation to calculate resistor and capacitor values to getting cut frequencies below 70Hz and 8.5v output voltage?
By the way, I wan to use PUI5024 capsules.
Many thanks.
 

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Regarding "cut frequencies below 70Hz " see /g/MicBuilders/message/37143

Regarding "8.5v output voltage": I suppose you mean DC voltage across the capsule Why specifically 8.5?
DC voltage depends very much on the characteristics of the built-in FET. FET's have quite a large tolerance. Put in the same conditions, two capsules may show differences of 50%.
You don't mention the vboltage of the power source, which directly affects the voltage across the capsule.

Le 08/02/2025 à 02:33, Adam Jo5eph via groups.io a écrit?:

Thank you so much everyone.
Is there any formula/equation to calculate resistor and capacitor values to getting cut frequencies below 70Hz and 8.5v output voltage?
By the way, I wan to use PUI5024 capsules.
Many thanks.
 

The gentle LF rolloff set by the choice of C value is not going to "cut off" anything. Please read up on single pole filters. If you need a hard cutoff below a specific frequency, the easiest place to get that is the highpass filter available on most field recorders, or by filtering in an audio editor afterwards.
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Re resistor values, the documentation of the SimpleP48 circuitry is superb, and covers all of this quite well. I did a little experimentation with my PUI AOM5024 capsules, but mostly to ensure that I wasn't exceeding the voltage and current ratings for the capsules. I ended up choosing 82k for resistor. I've built two simpleP48 pairs so far with AOM5024 capsules. In one pair, I used 4.7uf caps. In the other pair, which is now semipermanently installed outside, I used 2.2uf. There's still more than enough low-end there.?
 

So what difference will be expected in the recording between a simpleP48 and a simplestP48 mic?
I've been using Martin's resistor only simplestP48 for a while and just assumed I would be missing any low frequency cut off.
I've looked for the YouTube clips for the simplestP48 but they are no longer there.
 

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Are you sure that SimplestP48 works with an unmodified preamp?

Le 09/02/2025 à 22:41, Fuzzyface via groups.io a écrit?:

So what difference will be expected in the recording between a simpleP48 and a simplestP48 mic?
I've been using Martin's resistor only simplestP48 for a while and just assumed I would be missing any low frequency cut off.
I've looked for the YouTube clips for the simplestP48 but they are no longer there.
 

Yes, working ok with my standard Zoom F3.?
I'll check the setup tomorrow though as you have me doubting myself now....... just cautious as I can't find any references online anymore.
From memory the resistor value is different to that when using a capacitor. I'm using AOM5024 capsules.
 

Thank you Jerry,
I want to use PUI5024 capsule with Evo4 Audio interface, and the xlr output voltage is 47.6V.
I thought 8.5v output voltage is best for long term use/recording and maximum SN ratio, but I could be wrong...
I have experimented with different Simple48 output voltages (10v, 9.5v and 9v) and I did not see a serious change at voltages above 8.5v.
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There's some confusion about what a "simplestp48" is. Can you post a schemo?

Le 09/02/2025 à 22:52, Fuzzyface via groups.io a écrit?:

Yes, working ok with my standard Zoom F3.?
I'll check the setup tomorrow though as you have me doubting myself now....... just cautious as I can't find any references online anymore.
From memory the resistor value is different to that when using a capacitor. I'm using AOM5024 capsules.
 

This is what I have.
In this particular instance I'm doubling up the capsules but for a single capsule I'd have used a resistor of about 20k.
I think the resistor values should maybe be slightly higher as 9V at the capsule is pushing it a bit.
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Sorry to jump in on your post Adam but this info may be relevant.

IMG_20250210_164910873_MFNR~2.jpg

 

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OK thanks.
My document was with a 147k resistor, which implied modifying the preamp (deleting one of the phantom resistors).

Le 10/02/2025 à 18:00, Fuzzyface via groups.io a écrit?:

This is what I have.
In this particular instance I'm doubling up the capsules but for a single capsule I'd have used a resistor of about 20k.
I think the resistor values should maybe be slightly higher as 9V at the capsule is pushing it a bit.
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Sorry to jump in on your post Adam but this info may be relevant.

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