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Old 1st June 2013, 05:00 PM   #21
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Anything between 470K and 1M is a good value to use for the input impedance of a guitar circuit. You can go lower and higher, but this is the best range to go for with utility circuits like this. If you start getting close to 100K and below, it will begin to noticeably load the pickup down and decrease the high frequency response. Sometimes this can be exploited if a certain effect is desired, but for utility circuits, best to just stick to the 1M range.
For the input opamp, I'd use a TL062. Because it works well with the 1M impedance, and it's very low current draw. It's noisier than a TL072, but since it's being used to drive and LED circuit, I don't think you'll notice the noise.
I would strongly consider reducing the input impedance of the subsequent stages, especially if you'll be using 4558s for them. 1M is good for the first stage, but I'd initially start with around 100k for the subsequent ones.
Probably a good idea to try loading the output of the 10uF coupling cap with a 10k to ground before the rectifier diodes.
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Old 1st June 2013, 05:02 PM   #22
Mooly is offline Mooly  United Kingdom
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The 1nf cap will shunt a lot of the higher frequencies (reducing their amplitude), how much depends on the source output impedance. By all means try reducing or removing it.

If the guitar output impedance is 10K then that's not all that high in relation to your circuit. The 10K series input resistor isn't doing much tbh as it is swamped by the 1Meg and its really that 1Meg that is setting the input impedance of the first stage. You could even short the 10K out.

I'll see what I can come up with but it won't be today
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Old 1st June 2013, 05:03 PM   #23
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Quote:
Originally Posted by swissCHEESER View Post
not even sure if i`m spelling that right...but if i wanted to raise the input impedence of my "amp" to allow for a better acceptance of the signal.... would it behoove me to swap out the 10k resistor on the input with a 100k..?
No, leave it 10K. The 1M going to the virtual ground divider is what sets the input Z of a non-inverting amp as it is arranged here.
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Old 1st June 2013, 09:05 PM   #24
Zero D is offline Zero D  United Kingdom
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@ swissCHEESER

I wondered where you were !

There's a Lot wrong with your latest circuit. But rather than go into it, try this one i've simmed. The LED is on with no signal, & flashes with more signal.

If your not happy after you've tried that, then i think the way to achieve what you want is 2 circuits working together. One would be an OpAmp configured as a comparator which would turn off an LED when it's triggered by a signal. Another OpAmp would flash in sympathy with a signal. Probably a dual OpAmp, or 2 x single, would do it.
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Old 2nd June 2013, 11:24 AM   #25
Mooly is offline Mooly  United Kingdom
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Here is a .avi file that will play straight off in media player. Is this what you want
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Old 2nd June 2013, 01:31 PM   #26
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Default Mooly

Yes, that is exactly what I want! How did you do it?
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Old 2nd June 2013, 06:06 PM   #27
Mooly is offline Mooly  United Kingdom
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I'll draw it tomorrow for you... seem to be running out of time today

Its little different to what have actually.
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Old 2nd June 2013, 07:21 PM   #28
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Default Thanks a Million!

Wow Mooly, I can`t wait to see it! Thank you so much for taking the time to build a working prototype. Let me know how I can repay you for the drawing. swissCHEESER
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Old 2nd June 2013, 07:23 PM   #29
Mooly is offline Mooly  United Kingdom
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If you wait a minute I'll send it
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Old 2nd June 2013, 07:34 PM   #30
Mooly is offline Mooly  United Kingdom
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Here we go, Hope it makes sense. Opamps are 741

This is the exact circuit. Some values could be optimised but this works...

The 2v7 zener simply allows the green LED to extinguish fully (due to the 741 not being able to swing fully to each rail)

The 270K applies a little forwar bias to the opamp/rectifier output and improves sensitivity and response.

The 4.7uf cap (it is 4.7 the dots faint on paper) on the rectifier output can be as low as 0.1uf. Experiment to suit taste.

The two 6K8's generate the 6 volt reference. They can be a lot higher in value to save a bit of current.

No coupling cap between opamps 1 and 2 because the DC voltage on opamp output and the input of the following stage are essentially the same.

The 82K feedback resistor on opamp 3 was put there with the idea of adding gain but it didn't need it... but the option is there.
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