[sdiy] High impedance measurement buffer for audio?

[Mattias Rickardsson]

On Mon, 26 Oct 2026 at 10:29, <rburnett at richieburnett.co.uk> wrote:

> Almost anything you connect in the path is going to degrade the signal
> to some extent compared to going straight into your high end audio
> interface.
>

This is what I'm expecting, and some of those issues might be acceptable in
the end.


> How high an impedance would you consider as "not loading" for your
> purposes?
>

Similar to an oscilloscope probe would be good enough, meaning 1 Mohm. This
makes a big difference compared to 9 kohm input impedance of the audio
interface. I expect that 10 Mohm would be hard to do without sacrificing
the low noise... but could be handy to have as an option when noise is not
the primary issue.


> For instance you could use a x10 or x100 scope probe and then amplify
> the signal back up afterwards.  Will definitely add some noise though.
> Some old scopes used to have a Ch.1 output via BNC on the back that you
> could feed to other test and measurement gear.
>

I don't have that option on my scopes but would assume that it might be
quite noisy when passed on to audio gear.

Anyway, I'd assume that a voltage-follower high-impedance buffer could do
the full trick, but not sure how easy it is in terms of signal fidelity.
:-)

/mr



> On 2026-10-26 09:04, Mattias Rickardsson via Synth-diy 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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