Gus

something different
« on: December 14, 2019, 05:17:37 PM »
I bought a tube microphone for experimenting with solid state tube replacement. It has a good size body and removable tube.
So I traced it and found something different the rest of the circuit is standard stuff
I checked my trace a few times
The part identifiers in the sim don't match the PCB markings
I made up the capsule value because I have not measured it yet.
has anyone seen a circuit fragment like this before?


RuudNL

Re: something different
« Reply #1 on: December 15, 2019, 04:35:34 AM »
Normally, I would expect R7, C7, C8 and R8 to be a single capacitor.
The same goes for R3, C3, C4 and R4.
I can't think of a reason why they used these extra components.
Would be interested in your opinion!
There is a solution for every problem!

http://www.vansteenisaudio.nl

Gus

Re: something different
« Reply #2 on: December 15, 2019, 09:37:45 AM »
I posted this because I am puzzled by what I traced

The .001uf caps are 400V

I will trace it again.


abbey road d enfer

Re: something different
« Reply #3 on: December 15, 2019, 12:09:03 PM »
It looks suspiciously like the circuit in some Neumann mics for injection of a test signal.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

RuudNL

Re: something different
« Reply #4 on: December 15, 2019, 03:12:15 PM »
If that was the idea behind it, they could have omitted R4/C4 IMHO.
A signal injected over R8 (almost) wouldn't reach the grid.
« Last Edit: December 16, 2019, 05:21:13 AM by RuudNL »
There is a solution for every problem!

http://www.vansteenisaudio.nl

moamps

Re: something different
« Reply #5 on: December 15, 2019, 06:08:42 PM »
IMHO, the network R7, R8, C7, C8 does not have any purpose (it's separated with 1Gohm from the capsule).
The upper network may be a HF boost (or not, it needs to be further analyzed).

Gus

Re: something different
« Reply #6 on: December 15, 2019, 07:35:50 PM »
I wonder if it is to mislead others or to check if someone copies this circuit.

The power supply has a HPF inside and invert switch.

The fil supply 7806 has its tab connected to PCB ground however, it has a diode in the middle ground lead to ground. So the diode is shorted out. I checked with a meter
This can be a good thing because the voltage is closer to 6VDC and not 6.6VDC(with the diode)

The high resistance parts in the microphone are PCB mounted.

I don't see an AC modulation from the capsule or gain stage(EF86 as a triode) via the parts.
« Last Edit: December 16, 2019, 05:01:01 AM by Gus »

RuudNL

Re: something different
« Reply #7 on: December 16, 2019, 06:36:24 AM »
Why would they have selected such a high value for the grid leak resistor?
With 60 pF (capsule), a 100 M.ohm grid leak resistor would result in a -3 dB point at 26,5 Hz.
Phase shift at low frequencies is the only reason I can think of.
In general, you don't want the electronics of a microphone to be 'flat' to (almost) DC...
As my old electronics teacher (a wise man!)  used to say: "never select the grid leak resistor higher than needed, because it will reduce the lifetime of the tube".
There is a solution for every problem!

http://www.vansteenisaudio.nl

abbey road d enfer

Re: something different
« Reply #8 on: December 16, 2019, 06:41:00 AM »
Why would they have selected such a high value for the grid leak resistor?
With 60 pF (capsule), a 100 M.ohm grid leak resistor would result in a -3 dB point at 26,5 Hz.
Phase shift at low frequencies is the only reason I can think of.
In general, you don't want the electronics of a microphone to be 'flat' to (almost) DC...
As my old electronics teacher (a wise man!)  used to say: "never select the grid leak resistor higher than needed, because it will reduce the lifetime of the tube".
But sure you know that moving the -3 dB roll-off also moves the noise spectrum towards subsonics, don't you?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

RuudNL

Re: something different New
« Reply #9 on: December 16, 2019, 09:47:38 AM »
Yes, but it makes the microphone also more susceptible to high humidty.
Older tube microphones never used resistors in the G.ohm range (probably because they were not available at the time) but nobody complained about noise.
I have yet to see a tube microphone that has a lower self noise than a solid state microphone!
« Last Edit: December 16, 2019, 01:09:11 PM by RuudNL »
There is a solution for every problem!

http://www.vansteenisaudio.nl


abbey road d enfer

Re: something different
« Reply #10 on: December 16, 2019, 12:04:27 PM »
Yes, but it makes the microphone also more susceptible to high humidty.
Older tube microphones never used resistors in the G.ohm range (probably because they were not availabel at the time) but nobody complained about noise.
I have yet to see a tube micropphone that has a lower self noise than a solid state microphone!
I would think the main reason is that tubes have too much grid current for operation with gigaohm grid leak, even when run at low anode voltage and reduced heater voltage, resulting in too high negative bias and tube cutoff.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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