Electronics > Beginners
Analog help: Splitting guitar signal and eliminating cross-talk
capt bullshot:
Yes, increasing R20 is the right track (kind of). To keep the ridiculously high gain (1000) you'd have to increase the feedback resistor to keep their ratio the same. So if you set R20 to 1M, the feedback resistor would be 1G (that won't work in practice).
The better way is to re-wire the first OpAmp into a non-inverting configuration, that way you can keep the 1k / 1M to set the gain and have high impedance (set by a single resistor to GND). Audioguru said the same above.
If your input is high impedance, there's no need for signal splitting / isolation, low signal output impedance vs. high input impedance allows for parallel connection of the inputs without circuits "talking back" to the source.
Scott D:
--- Quote from: capt bullshot on July 17, 2019, 07:35:26 am ---Yes, increasing R20 is the right track (kind of). To keep the ridiculously high gain (1000) you'd have to increase the feedback resistor to keep their ratio the same. So if you set R20 to 1M, the feedback resistor would be 1G (that won't work in practice).
The better way is to re-wire the first OpAmp into a non-inverting configuration, that way you can keep the 1k / 1M to set the gain and have high impedance (set by a single resistor to GND). Audioguru said the same above.
If your input is high impedance, there's no need for signal splitting / isolation, low signal output impedance vs. high input impedance allows for parallel connection of the inputs without circuits "talking back" to the source.
--- End quote ---
unfortunately, the PCBs are already fab'd. Yeah a 1Gohm isnt going to work. Using 0402 SMD too. So I cant change the design without a full re-design and re-fab, but I can easily change the values.
On hand I have 1k, 10k, 15k, 100k, 1M, 10M in 0402, So I am thinking 10k/10M as a test, and then 15k/10M thereafter and hope that solves it.
I can use a potentiometer in series with the input and measure the resistance to work out the min required value.
I may be able to get away with a lesser gain for my purposes, but its a matter of testing, as I could perhaps also increase the gain at the next stages if needed. Would be a bit of testing involved though.
I will be able to test this hopefully tomorrow; is there anything else I should consider/look for if I continue to have issues with this? - Or will it be a re-design hereafter? ;)
Audioguru again:
Don't you agree that the input resistance of an electric guitar amplifier is 1M or higher? Isn't that why the guitar's pots are as high as 500K? Then that is why you must re-design the 1st opamp to have an input resistance of 1M which is 1000 times higher than you have now. Your 1k ohms value of R20 is shorting away the output level of the guitar pickup which is why the gain of your 1st opamp needs to be as high as 1M/1k= 1000 times.
The best way to increase the input resistance 1000 times is to change the 1st opamp from its existing inverting type to a non-inverting type:
A 22nF input coupling capacitor feeds into the (+) input pin 3 of the opamp that has a 2.2M resistor to ground (its input resistance) and a voltage divider of two feedback resistors from the output to pin 2 sets the gain. The resistor from pin 2 needs a capacitor in series with it to ground. Then the gain can be much less but the circuit will have lots of output level if the load is increased from its existing extremely low resistor values to about 10k.
Without a re-design you can try changing R20 to 2.2M and changing the feedback resistor to 4.7M to 22M.
What will you use the rectified "truck horns" DC to do? Light up LEDs?
Scott D:
--- Quote from: Audioguru again on July 17, 2019, 08:45:47 am ---Don't you agree that the input resistance of an electric guitar amplifier is 1M or higher? Isn't that why the guitar's pots are as high as 500K? Then that is why you must re-design the 1st opamp to have an input resistance of 1M which is 1000 times higher than you have now. Your 1k ohms value of R20 is shorting away the output level of the guitar pickup which is why the gain of your 1st opamp needs to be as high as 1M/1k= 1000 times.
The best way to increase the input resistance 1000 times is to change the 1st opamp from its existing inverting type to a non-inverting type:
A 22nF input coupling capacitor feeds into the (+) input pin 3 of the opamp that has a 2.2M resistor to ground (its input resistance) and a voltage divider of two feedback resistors from the output to pin 2 sets the gain. The resistor from pin 2 needs a capacitor in series with it to ground. Then the gain can be much less but the circuit will have lots of output level if the load is increased from its existing extremely low resistor values to about 10k.
Without a re-design you can try changing R20 to 2.2M and changing the feedback resistor to 4.7M to 22M.
What will you use the rectified "truck horns" DC to do? Light up LEDs?
--- End quote ---
What you suggest makes perfect sense, I am also having a bit of a lightbulb moment with respect to the relationship with the output power being a product of load/vs gain (right?)
And you are right, I started with a gain of 10-100 which worked, but I found I wanted a bit 'more'.
I committed myself to the inverting design quite a while back.. so long I cant recall what the deciding factor was, perhaps it worked out to a lower part count, better or easier filtering, or it .. it worked.
I did reference a few amplifier designs etc but obviously there is a lot to consider.
so noted for Version 2.0
Thanks Audioguru
radiolistener:
--- Quote from: Scott D on July 16, 2019, 02:25:15 pm ---I have designed and built an analog circuit that will be housed inside a guitar
--- End quote ---
that's fail, electronic components, especially capacitors will be affected by guitar vibrations and will make distortions. Some times ago Dave published a video, where oscilloscope shows terrible pulses when you knock on his body. The same thing will happens with your circuit inside guitar.
So, if you're want to get best sound quality, you're needs to move your analog circuit far away from guitar.
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