[sdiy] High impedance measurement buffer for audio?

[cheater cheater]

Simplest: use a 1M tek probe with a tek scope and use "CH1 out" or
"Vert out" on the back to redistribute the signal.

More advanced: Tek AM501 or/and AM502 modules, then connected to scope
and audio interface (audio interface should have a balun or DI in
front of it to adapt 50 Ohm to hi-Z). Instead of tek you can use
Keithley 610C or 602, or PAR 113 or 120 preamplifiers. (those are good
to 1M or 300kHz, depending on what you're doing)

DIY: High impedance FET input right at the probe tip, followed by an
instrumentation amplifier in the handle, the cable goes to box which
contains a distribution amplifier network with unity gain op amp
feeding a bus that goes to multiple unity gain op amps with possible
darlington output if necessary and possibly a level switcher for each
output. I suggest also making E-field probes in multiple sizes out of
twisted insulated wire, very useful as well.

Another option: small inline resistor and tek 5000 or 7000
differential probe input module

You never mentioned the bandwidth. Bear in mind that with modern
silicon it doesn't matter what you intend the bandwidth to be (eg
audio), garbage above that limit which you're not scoping will still
mess up your circuit's function. (oscillations, EMI pickup /
transmission / coupling, shot noise, distortion).

On Mon, Oct 26, 2026 at 10:14 AM Mattias Rickardsson via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> Hi all,
>
> I often probe around in both audio circuits, power circuits and other circuits. I often want to direct the probed signal both to an audio interface and to measurement equipments like oscilloscopes. But when the test point is sensitive, I don't want to load it and affect the signal.
>
> Do you have any good recommendations for a device solving these needs:
>
> - High impedance measurement input for a +/- 20V range
> - Buffered outputs to both audio & lab equipment simultaneously
> - No gain needed
> - Very low noise and low THD needed, it shouldn't degrade the signal before going to my high-end audio interface
> - High bandwidth preferred, far higher than audio
>
> Could be a commercial product or a custom-built solution. Are you aware of any obvious solution that I've missed? Or is the low-noise low-THD requirement a tricky thing here? :-)
>
> /mr
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