[LAU] Re: Guitar to USB devices can be quite good

On Wed, May 21, 2025 at 08:23:05AM -0700, Len Ovens wrote:

> Why do I think they have likely all done them wrong? The Application notes
> say: "Please follow the reference schematic for microphone section design."
> So not only is it easy but it is recomended. I am sure it works well for a
> computer mic.

Many manufacturers will just copy the application note circuit.
The reason is that good analog audio designers are rare (and
expensive).

Even as mic input that circuit is strange. R10 (8k2) in series with
the input ? It may be there for protection (together with the varistor),
but otherwise it just adds noise.

> Anyway, for those who want a good quility direct to usb guitar cable here is
> my journey to fix a toy into something better.
> 
> https://www.ovenwerks.net/hardware/GuitarUSBhack/index.html

Removing the DC bias is of course a good idea. But I don't think
the resulting input impedance will be anywhere near to 1M. Together
with C11, 1M would form a 1.6 Hz highpass. In other words, if the
chip input impedance were really around 1M then C11 could (and
should) be smaller.

Now even if the resulting input impedance is lower than what 
a typical guitar amp input would provide, that shouldn't be a
big problem. You will just need some EQ to compensate, and the
volume control on the instrument may 'feel' different.

Then there's C18, no value given. It will resonate with the
pickup inductance at some frequency, so it may be interesting
to experiment with.

Ciao,

-- 
FA

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