Electronics > Open Source Hardware

Active Differential Signal (eg Audio) Limiter with Switched Attenuator

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marks2c:
Protection for audio analyser balanced input: ie 100V-Line speakers, valve HT etc to 400V (ref XLR pin 1).
Uses bi-directional depletion-mode MOSFETs to safely limit input current to below 3mA.
Uses bi-directional shunt clipper to limit analyser input voltage to +/-20V (+/-10V optionally) wrt to XLR pin 1.
CAD files are attached to the PDF (download via the attachments panel).
Attenuator SMath calculation sheet is in there too.
SMath and KiCad are free to use and awesome.
[attachimg=4]

dmendesf:
Very nice. Do you have any ideia about bandwidth? Thinking about other uses for the circuit...

marks2c:

--- Quote from: dmendesf on June 14, 2022, 05:59:38 am ---Very nice. Do you have any ideia about bandwidth? Thinking about other uses for the circuit...

--- End quote ---

Small-Signal Bandwidth
GHz BW should be possible because under normal small-signal conditions the depletion-mode MOSFETs are not 'active' (ie they are just a low-Ohm resistor, and are not switching).

As such, small-signal bandwidth is only limited by the physics of the PCB layout/substrate etc (ie should be good beyond several GHz as you can buy RF transistors good for 4GHz in the same SOT223 package).

Protective Function
Here is detail FYI (voltage is red, current is blue with current limiting of 1.5 mA):

BW=0.35/rise time. Given 2.5ns that would be about 140MHz (with XLR connectors etc...). With less stray capacitance much faster protection should be possible.

The ringing was down to non-ideal power supply leads/shunt/XLRs etc.

Other Performance
With high-impedance loads this limiter/clipper circuit is essentially transparent both distortion-wise and noise-wise.

dmendesf:
Thanks for the info. Will assembly one for testing purposes.

marks2c:
How did it go?
Did theory and practice align?

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