Acetylene, noisy...?
- Thread starter jlm1948
- Start date
jesuscrisp
Well-known member
Are you sure you built it all correctly and your enclosure is grounded?
OK, found the issue.
The input stage uses voltage-to-current NFB, which reduces considerably its input impedance, to about 10kohm.
In order to get a workable gain, the input trim was at about mid-rotation, which results in 34dB attenuation of the input signal before reaching the input stage. Of course, with such a small signal, noise becomes seriously perceptible.
I don't know what the designer's intent was, but with the input trim at its minimum resistance (minimum attenuation), the signal/noise ratio is good, but the global input impedance is about 20k (half of it due to a physical 10k resistor), which makes it unusable with a passive instrument.
Since I'm gonna use the Acetylene after a bunch of active pedals, it should work OK, provided I can tame the gain.
The input stage uses voltage-to-current NFB, which reduces considerably its input impedance, to about 10kohm.
In order to get a workable gain, the input trim was at about mid-rotation, which results in 34dB attenuation of the input signal before reaching the input stage. Of course, with such a small signal, noise becomes seriously perceptible.
I don't know what the designer's intent was, but with the input trim at its minimum resistance (minimum attenuation), the signal/noise ratio is good, but the global input impedance is about 20k (half of it due to a physical 10k resistor), which makes it unusable with a passive instrument.
Since I'm gonna use the Acetylene after a bunch of active pedals, it should work OK, provided I can tame the gain.
You should watch Brian Wampler explain how it is designed to work.
It is basically a Low Gain in 30 mode.
If you want Gain, switch to 15 mode:
I don't want gain, I want get rid of noise.
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Watch the video, he refers to it as an Underdrive!
I hate to say it but, It sounds like an Component issue may be at fault????
Were all the following transistors used:
Q1 2N5089 BJT transistor, NPN TO-92
Q2 BS170 N-Channel MOSFET
Q3 J113 JFET transistor, N-channel
Q4 BS170 N-Channel MOSFET
Q5 J201 JFET transistor, N-channel
Q6 J113 JFET transistor, N-channel
Show a Good Picture of PCB & Components.
OK, problem solved.
I don't know how the original ACE something fares, but the Acetylene input stage has too much gain, so the input signal must be attenuated by about 20dB _ via the 1Meg trimmer - for it to run clean, which results in increasing the apparent noise.
I solved the problem by decreasing the gain of the stage itself, simply by decreasing significantly (factor 10) the drain resistor. The original drain resistor is 22k, I replaced it wit 2.2k - actually I installed a 2.4k in parallels with the 22k.
I was not so sure it would do the trick, but it actually works like a charm.
I don't know how the original ACE something fares, but the Acetylene input stage has too much gain, so the input signal must be attenuated by about 20dB _ via the 1Meg trimmer - for it to run clean, which results in increasing the apparent noise.
I solved the problem by decreasing the gain of the stage itself, simply by decreasing significantly (factor 10) the drain resistor. The original drain resistor is 22k, I replaced it wit 2.2k - actually I installed a 2.4k in parallels with the 22k.
I was not so sure it would do the trick, but it actually works like a charm.
Thanks!
So it seems the Acetylene is a faithful repro of the original. It would be interesting to have hands on experience of a real one, but I think being both clean and quiet is difficult.
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