Preamp for electret capsule in XLR connector

[Fastunov]

I have been looking for a preamp circuit for an electret capsule for a long time (with a built-in fet, primo) that would fit into an XLR connector.
I found a LOM Audio preamp, and then I came across an old Alice - ScottHelmke circuit.
At first glance, the board and the circuit are very similar. But I can't figure out what the 6-legged 4g element is

 

[cyrano]

An electret requires one resistor and one cap. Look for "SimpleP48", by Richard Lee, if my memory isn't failing yet again. I believe Richard is a member here, but you'll find lots of "preamps" on the Micbuilders group:

https://groups.io/g/MicBuilders/topics

 

[Fastunov]

4g - PNP Silicon AF Transistor BC857...-BC860 Is it really him?

 

[abbey road d enfer]

I have been looking for a preamp circuit for an electret capsule for a long time (with a built-in fet, primo) that would fit into an XLR connector.
I found a LOM Audio preamp, and then I came across an old Alice - ScottHelmke circuit.
At first glance, the board and the circuit are very similar. But I can't figure out what the 6-legged 4g element is

Only one side is populated, right?
Then I reckon it's not an Alice...

 

[Drosselmeier]

But I can't figure out what the 6-legged 4g element is

standard chinese Schoeps

 

[kingkorg]

Just to pick your brain, you can take this route.

https://naiant.com/custom_audio_reproduction_equipment/product_spec/msh-4-specification/

 

[abbey road d enfer]

View attachment 99482
standard chinese Schoeps

Abusive use of the Schoeps name IMO. Of course there are similarities, but this chinese circuit has enough significant differences to warrant another name.

 

[Fastunov]

Just to pick your brain, you can take this route.

https://naiant.com/custom_audio_reproduction_equipment/product_spec/msh-4-specification/

Is it really possible to fit a 6418 lamp in an xlr connector?

An unpleasant feature of this lamp. The microphone body is very sensitive to touch, the lamp resonates and emits a sound similar to "feedback". Calms down for 5 minutes

 

[Fastunov]

Only one side is populated, right?
Then I reckon it's not an Alice...

Yes, the elements are on the same side.

 

[Fastunov]

I found this version.
https://groups.io/g/MicBuilders/topic/zapnspark_generic_fet_smd_for/87854418?p=

 

[rogs]

I have been looking for a preamp circuit for an electret capsule for a long time (with a built-in fet, primo) that would fit into an XLR connector.

Depends on which Primo capsule you are describing? ... but if it is the EM258, and you choose to go down the 'SimpleP48' route you might like to double check to actual voltage being supplied to the capsule.
I have lost a couple of these capsules when they were fed with a supply of 8 volts....
Also it's a bit fiddly to isolate the negative connection of the capsule from ground, which you need to do with the Simple 48 supply to keep it 'balanced'.
I've found the attached circuit works well for this kind of application - as long as you use a screened cable to the 3.5mm jack socket fitted to the XLR, and don't run it for more than around 20 feet.

 

[Solomute]

I designed an electret preamp that occupies about 1 cm^2 of board space, with single-ended output. It was for an aviation headset application. Would probably not be too hard to modify for a recording application.

The big thing keeping schoeps-style circuits from being viable in super-miniaturized applications is the enormous electrolytic coupling caps they require. My design has zero coupling caps, just one low voltage (but large) ceramic cap for a bias servo. If anyone is interested I can dig up the schematic.

 

[Fastunov]

I designed an electret preamp that occupies about 1 cm^2 of board space, with single-ended output. It was for an aviation headset application. Would probably not be too hard to modify for a recording application.

The big thing keeping schoeps-style circuits from being viable in super-miniaturized applications is the enormous electrolytic coupling caps they require. My design has zero coupling caps, just one low voltage (but large) ceramic cap for a bias servo. If anyone is interested I can dig up the schematic.

I am sure it will be interesting to many and perhaps contributes to the development of the scheme

 

[Solomute]

Here's the basic topology. Again, this was for an aviation headset application with single ended output. The idea to try doing this was inspired by this Texas Instruments application note, and functions on the same principle.

J1 is the fet inside the electret capsule.

R2+R3+Q3 makes a vbe multiplier, Q1+Q2 buffers it. C1 "slows down" the vbe multiplier and turns it into a bias servo. Because C1 can't possibly see more than 0.7 volts, it can be 100uf even in an 0805 ceramic package. C1+R3 sets the low frequency response.

J1 "steals" base current from Q1+Q2. This current varies with the sound hitting the capsule, causing the amount of collector current drawn by Q1+Q2 to vary. This varying current is drawn across R4 and turns into the output voltage.

R1 is a feedback resistor and sets the transimpedance of the circuit. May require a coupling cap.

The electret I was using will happily run on 1.5V of bias voltage, which results from the vbe of Q1+Q2.

There are many reasons I wouldn't use this particular topology for a phantom-powered electret buffer, but one could definitely play around with the basic principle and come up with something more suitable.