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From: JosephKK on 16 Jun 2010 01:53
On Mon, 14 Jun 2010 15:03:01 -0700 (PDT), «Leo» <leo2100(a)gmail.com>
wrote:

>On 12 jun, 14:09, «Leo» <leo2...(a)gmail.com> wrote:
>> I need to make a high input impedance amplifier for a mic preamp with
>> a single supply. I was thinking of using a TL071 (non-inverting) opamp
>> with high resistance bias resistors (to bias the opamp to half the
>> supply voltage) so the input impedance results is  aprox. 1MegOhm. But
>> I don't know if such a scheme would work. I figured since the input
>> bias currents in the JFET input opamps are very low (max 200pA @ 25ºC,
>> 7nA full range) I could bias it with two 2.2MegOhm resistors. Also I
>> think adding high value resistors increases noise at the input...and
>> the overall gain of the circuit would be kind of large (~1000, or
>> perhaps larger), so it would give me a large noise at the output. The
>> input from the mic is in the micro-volt range.
>>
>> I don`t know if a discrete bipolar solution would be better...or any
>> other clever circuit configuratios for that matter...
>
>To all.
>
>So I guess I'll use high enough imput impedance, like 10 times or
>more...The application is neither critical nor Hi-Fi, but it would be
>good to make the best of the components I have available...and that
>excludes any of those JFETs mentioned before. So long as the circuit's
>noise isn't audible enough compared to the actual signal it's OK. the
>idea is to plug it in a 100W guitar power amp for band rehersal, and
>mix it with the guitar signal...I know it's not the best solution but
>I can't afford another Amp.

Ah, that information brings up another solution. You should be able to
get low-Z to high-Z in-line transformers for a reasonable price. Try
rooting around in school kid band garage sales. Easy, reliable, works.
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