Hi to everyone,

I'm trying to build a Zapnspark's generic Fet preamp, as small as possible, hence SMD components,
used with various 3-wire Primo capsules like EM200,EM273,EM182.
I want to use them mostly at live concerts, so output impedance, maxSPL-headroom, EMI/RF immunity and low noise are important factors for me.

The point of this build is to fit the PCB inside the XLR plug.The whole board i designed is 24.2mm x 12mm.
Ideally i'd like to fit also a 3pin mini-XLR on the other end(it will need some modification on the XLR's boot).

I have a few questions about this design.

1) Can you spot any error on the PCB or the selected components?
2) Would 22nf ceramic caps between pins 1-2 and 1-3 add additional EMI/RF immunity, with no drawbacks?
3) Could i replaced the 2 BC860Cs with a dual PNP like SSM2220 or relevant, in order to avoid hfe matching and reducing size? Should i expect the same performance?
4) Primo capsules seems to give the least noise and highest maxSPL figures, when biased at roughly 9v. Is it just R2 and R5 which will change the Bias voltage?

Any suggestion are welcome.

I'll post my Fusion-Eagle project for inspection and reference, Eagle Library, and my Mouser basket.
Mouser basket:

I think it would be a great simple project for this community, if implemented right.

Thanks in advance,
Raptakis Konstantinos

开云体育

2) Would 22nf ceramic caps between pins 1-2 and 1-3 add additional EMI/RF immunity, with no drawbacks?

I would consider it essential.

3) Could i replaced the 2 BC860Cs with a dual PNP like SSM2220 or relevant, in order to avoid hfe matching and reducing size? Should i expect the same performance?

SSM2220 has a significantly lower Hfe than BC860C, which would result in higher output impedance.

You have to know that CMRR performance is determined by how close the impedance at pin 2 is compared to that at pin 3, in absolute value.

With BC860C, the difference is about 6 ohms.
With SSM2220 it is 13 ohms, which means that CMRR is degraded by 6dB.

Hfe matching is largely overrated. The output impedance is roughly the impedance seen by the base divided by Hfe.
The impedance seen by the upper transistor (the one that's driven by the FET's drain) is approximarely twice that of that that drives the lower transistor.
In a typical configuration, with high-Beta transistors, the impedance unbalance is about 6 ohms. So clearly, matching Hfe is a waste of time.

Jerry thank you for your informations.

Did you spot any error on the PCB or components?

Also about the 22nf caps, i know that they should be soldered directly on the xlr pins.
Is there any difference if i add them first on the pcb?

On Mon, Dec 20, 2021 at 11:48 AM, @Par_apwn wrote:

Hi to everyone,

I'm trying to build a Zapnspark's generic Fet preamp, as small as possible, hence SMD components,
used with various 3-wire Primo capsules like EM200,EM273,EM182.

As you are intending to use 3 terminal electrets with built in FETs and access to both drain and source, why do you want to add a second FET?? ?
Wouldn't it be simpler (and have fewer components to find room for) to use this schematic instead:?/g/MicBuilders/files/Zapnspark/DIYgenericIntFET.pdf?? ?

Arjay yes, i'm going to use this with 3 terminal electrets with built in JFET. i have used only BJTs driving the balance out.
That's the exact schematic i used for desinging this PCB with SMDs for this project. even the values are the exact same for now.
I'm just considering which values should i change to control the Bias voltage.

I'm about to implement the EMI/RF ceramic caps, on the pcb.
Can anyone spot any error on the PCB?

Hi Raptakis, how did it go for you with your research? I'm learning sound oriented electronics and 3D design and think this merges both things from kicad 8.0.2 with the simulation tools to fusion. Can you provide some new insights you found or some tips, thanks a lot for sharing!