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From: Phil Allison on 6 Apr 2010 02:31

"Gone Fishin'"

>> Change both R values and both C values together and do not go under 1kohm
>> for R.
>>
>> Many other op-amps could be used, but the NE5534 is a good choice.
>>
>
> So changing both R to 50pF and leaving both R at 100K will give 31.8KHz?


** You must reduce the two R values to a few kohms at most and then pick C
to suit.

2.2 kohms and 2.2 nF is about right.

Fine adjustment can be done with a pot of about 10% the value of R in series
with one of them.



..... Phil



From: Gone Fishin' on 6 Apr 2010 02:49
> ** You must reduce the two R values to a few kohms at most and then pick C
> to suit.
>
> 2.2 kohms and 2.2 nF is about right.
>
> Fine adjustment can be done with a pot of about 10% the value of R in series
> with one of them.

Thanks Phil. Much appreciated.

From: Sjouke Burry on 6 Apr 2010 13:01
Gone Fishin' wrote:
>> The opamp needs to be able to feed the lamp, so you need a
>> high-output one. Then bild it, check frequency against the value
>> of C, change C accordingly.
>> The lamp makes sure of a stable output without distortion.
>
> I calculate 50pF with 100K gives 31.8KHz.
>
> But which of the C do I change? The one connected to ground? Which of the R
> is part of the RC circuit? (The circuit is a bit confusing because it uses 2
> identical R & C values.)
>
> If I want to make the RC variable, which R do I replace with a pot (or fixed
> + pot)?
>
> Also, this op amp (LF155) is specified as dual-supply type. I need a single
> supply op amp. Will the R-C connection to ground of this circuit be connected
> to -Vcc regardless if it is a single or dual supply op amp?
>
> Thank you.
>
You change them both, both circuits have to balance at the
wanted frequency.
Changing only one will degrade oscillator quality.
If you can lay your hands on a tuning cap, they often contain
two sections, and make testing rather easy.
From: Gone Fishin' on 6 Apr 2010 14:45
I need some advice regarding the grounding of the parallel R & C.

<http://tinypic.com/m/9quype/3>

The LF155 is a dual-supply amp. I need a single-supply amp.

I presume that in this circuit the ground is chosen because it is the
mid-point between the dual supplies (ie, +/-15). With a single-supply amp,
this point is not mid-point but rather it is the "negative supply" point.

How should this circuit change for use with a single-supply amp?

Thanks.

From: Baron on 6 Apr 2010 16:28
Gone Fishin' Inscribed thus:

>> The opamp needs to be able to feed the lamp, so you need a
>> high-output one. Then bild it, check frequency against the value
>> of C, change C accordingly.
>> The lamp makes sure of a stable output without distortion.
>
> I calculate 50pF with 100K gives 31.8KHz.
>
> But which of the C do I change? The one connected to ground? Which of
> the R is part of the RC circuit? (The circuit is a bit confusing
> because it uses 2 identical R & C values.)
>
> If I want to make the RC variable, which R do I replace with a pot (or
> fixed + pot)?
>
> Also, this op amp (LF155) is specified as dual-supply type. I need a
> single supply op amp. Will the R-C connection to ground of this
> circuit be connected to -Vcc regardless if it is a single or dual
> supply op amp?
>
> Thank you.

Sounds like home work !

--
Best Regards:
Baron.
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