the physical
Hi
If POT is on same PCB taking up same real estate as everything else - the act of moving the controle to one of the end stops can put a little force on the PCB transmitting some mechanical flex into the PCB - exciting an issue which is masquerading itself to look like the pot --- is this possible in your circumstance?
I cant believe you would get that much DC leakage around a pot between two caps unless it was on a floating ground.
Hi
If POT is on same PCB taking up same real estate as everything else - the act of moving the controle to one of the end stops can put a little force on the PCB transmitting some mechanical flex into the PCB - exciting an issue which is masquerading itself to look like the pot --- is this possible in your circumstance?
I cant believe you would get that much DC leakage around a pot between two caps unless it was on a floating ground.
There has to be something else going on here. You really need to take the pot out of the circuit and do some basic resistance tests on it on its own.
Cheers
Ian
You have something odd going on here.
1. The base of the transistor is AC coupled into the wiper of the pot. That fact alone means it's not possible for the pot to alter the base voltage (DC wise)... double check that result... and if it is correct is certainly why you get the thump.
2. The big question is why ? The circuit as you have drawn it can not do that unless there is a wiring error... your actual circuit is different to what you have drawn. Check the wire to the pot go to the correct points... and that you really are using a correct ground.
Can you post a photo of it ? or would that be too scary (for us
)
I'll just add to that, do you understand "why" I said the circuit can not behave as you say.
The base of the transistor is running at a fixed bias voltage.
The coupling cap on the base isolates the pot from the transistor at DC. So if the negative end of the cap were open circuit or connected to ground it can not alter the bias on the transistor.
And I take it you have confirmed by measurement that there is no DC voltage on the "top" end of the pot.
The only somewhat "weird" thing that crossed my mind was if the stage was oscillating when confronted with a cap on the base to ground... but that really is an "off the wall" thought.
The base of the transistor is running at a fixed bias voltage.
The coupling cap on the base isolates the pot from the transistor at DC. So if the negative end of the cap were open circuit or connected to ground it can not alter the bias on the transistor.
And I take it you have confirmed by measurement that there is no DC voltage on the "top" end of the pot.
The only somewhat "weird" thing that crossed my mind was if the stage was oscillating when confronted with a cap on the base to ground... but that really is an "off the wall" thought.
OK so it seems there are various earth loops. Still exist, but with some re-wiring the problem has become much, much smaller. Almost inaudible.
Definitely oscillating when the cap is grounded, that is why the base of the tranny appears as 2V below bias on my multimeter (typical bias 12 V).
If not oscillating may be the earth loop somehow affects the tranny in such a way to pronounce greatly the tiny click at the ends of the pot.
Definitely oscillating when the cap is grounded, that is why the base of the tranny appears as 2V below bias on my multimeter (typical bias 12 V).
If not oscillating may be the earth loop somehow affects the tranny in such a way to pronounce greatly the tiny click at the ends of the pot.
OK after 4-5 rewirings etc here is what I learned:
1. There is a stage with high input resistance, 1.2M, right after the Bass/Treble passive filter, and that stage is very succeptible to picking up noise and hum. Even with nothing electrical switched on around it. So I cannot simply attach a 1 metre long unshielded wires to its input, it will pick up 50Hz hum. From somewhere, I do not know where, as I switched all off, even its own PSU and fed if off a big capacitor. Hum still there! It must be picking it up from nearby electricity cables. It is like an antenna! When I attached shielded wire it is very silent, no hum. So this problem solved.
2. There is certainly something wrong with those pots, as if static slowly builds on them. And more. First, it is deadly quiet in the OFF position (hard left). I am sitting there looking at the oscilloscope without moving at all. Suddenly, with me not touching anything, it starts to buzz. Turning it right-left-right-left eliminates the buzz, after a few scratching noises of course. As if there is some energy stored and slowly dissipating only when I am turning the pot, or when the pot is NOT at the off position. Perhaps because it is made of plastic ?
Secondly, the Treble pot is weird. When you turn it it makes scratching noises. But only if there is some signal being played!!! So, I palm mute the strings on the guitar, turn the pot, dead silence (as it should be). I pluck any string, turn the pot, scratching noises while turning.
Bemused
1. There is a stage with high input resistance, 1.2M, right after the Bass/Treble passive filter, and that stage is very succeptible to picking up noise and hum. Even with nothing electrical switched on around it. So I cannot simply attach a 1 metre long unshielded wires to its input, it will pick up 50Hz hum. From somewhere, I do not know where, as I switched all off, even its own PSU and fed if off a big capacitor. Hum still there! It must be picking it up from nearby electricity cables. It is like an antenna! When I attached shielded wire it is very silent, no hum. So this problem solved.
2. There is certainly something wrong with those pots, as if static slowly builds on them. And more. First, it is deadly quiet in the OFF position (hard left). I am sitting there looking at the oscilloscope without moving at all. Suddenly, with me not touching anything, it starts to buzz. Turning it right-left-right-left eliminates the buzz, after a few scratching noises of course. As if there is some energy stored and slowly dissipating only when I am turning the pot, or when the pot is NOT at the off position. Perhaps because it is made of plastic ?
Secondly, the Treble pot is weird. When you turn it it makes scratching noises. But only if there is some signal being played!!! So, I palm mute the strings on the guitar, turn the pot, dead silence (as it should be). I pluck any string, turn the pot, scratching noises while turning.
Bemused
Don't be bemused!
Get yourself some proper test equipment. i.e. a 'scope and SEE what is going on!
How anyone can rely on just a multimeter - digital at that - to measure anything other than supply voltages beats me!
Get the right tools, or you will be forever trying to fix your car engine with a shovel.
Get yourself some proper test equipment. i.e. a 'scope and SEE what is going on!
How anyone can rely on just a multimeter - digital at that - to measure anything other than supply voltages beats me!
Get the right tools, or you will be forever trying to fix your car engine with a shovel.
I did mention sitting idle looking at the oscilloscope It clearly shows the hum. But on the scratching noises and the pops there is nothing to show other than a blur.
Hum is a side-effect of long, unshielded, test leads, attached to a high input resistance, high gain input. Changing to a shielded lead immediately eliminated the hum.
Static-build up (or whatever it is) on the "master volume" pot in the off position, the diagram I showed on an earlier post, that is a mystery.
The scratching noises on the "treble" pot, but ONLY if there is some signal, that is another mystery.
It clearly shows the hum. But on the scratching noises and the pops there is nothing to show other than a blur.Hum is a side-effect of long, unshielded, test leads, attached to a high input resistance, high gain input. Changing to a shielded lead immediately eliminated the hum.
Static-build up (or whatever it is) on the "master volume" pot in the off position, the diagram I showed on an earlier post, that is a mystery.
The scratching noises on the "treble" pot, but ONLY if there is some signal, that is another mystery.
Hmm, a "blur" does indeed sound like oscillation, along with your earlier statement "the scope goes crazy for a while" as others have suggested.You could speed up the timebase so you can see a 10kHz to 1MHz signal, be sure to set the triggering to that channel, and move the trigger level around to see of you can sync on an oscillation in some ultrasonic range.I did mention sitting idle looking at the oscilloscope
Can you post a photo taken off the 'scope screen.
Hum will be either 50 or 100 hz depending on the cause ie stray pickup or PSU problem.
Anything else is more than just "hum"
The pots are easy to test... just apply a clean signal across them and see if the signal at the wiper varies cleanly. Make sure the amp is off if you try this with them in circuit.
Hum will be either 50 or 100 hz depending on the cause ie stray pickup or PSU problem.
Anything else is more than just "hum"
The pots are easy to test... just apply a clean signal across them and see if the signal at the wiper varies cleanly. Make sure the amp is off if you try this with them in circuit.
Here is a summary to avoid having to read the whole thread:
I have designed and built yet another guitar pre-amp. The "problem" areas have been:
*******************************
1. High frequency oscillations - caused by leak of output signal to high input impendance, high gain stage - fixed with a 120KHz filter at that input.
2. Spurious hum at the output - caused by 1 metre long test leads acting as antennae on a high input impendance, high gain stage - fixed with using shielded input cable and avoiding test leads with crock clips
3. Clicks and pops on the "master volume" pot when it is moving and when it reaches the far left. Resident hum, when not suppressed greatly exxagerates the pops. Hum is now gone (see 2) but quieter clicks and pops remain. Some noise also appeared while it was on the full left position, after it was idle for about 15 seconds. That noise cleared gradually when I then started moving the wiper a bit.
4. Scratching noises on the "treble" pot ONLY when I am turning it and at the same time if there is a signal on it. No singal, no noises, no matter how hard I am turning it. A small singal, at any frequency, causes scratching noises to appear. In contrast, the "bass" pot is virtually silent in operation.
******************************
I have a bag of those pots not soldered onto anything and I will test a signal on them to see if they produce a clean output on the wiper while moving. They are plastic Tyco 100KHz linear types.
I have designed and built yet another guitar pre-amp. The "problem" areas have been:
*******************************
1. High frequency oscillations - caused by leak of output signal to high input impendance, high gain stage - fixed with a 120KHz filter at that input.
2. Spurious hum at the output - caused by 1 metre long test leads acting as antennae on a high input impendance, high gain stage - fixed with using shielded input cable and avoiding test leads with crock clips
3. Clicks and pops on the "master volume" pot when it is moving and when it reaches the far left. Resident hum, when not suppressed greatly exxagerates the pops. Hum is now gone (see 2) but quieter clicks and pops remain. Some noise also appeared while it was on the full left position, after it was idle for about 15 seconds. That noise cleared gradually when I then started moving the wiper a bit.
4. Scratching noises on the "treble" pot ONLY when I am turning it and at the same time if there is a signal on it. No singal, no noises, no matter how hard I am turning it. A small singal, at any frequency, causes scratching noises to appear. In contrast, the "bass" pot is virtually silent in operation.
******************************
I have a bag of those pots not soldered onto anything and I will test a signal on them to see if they produce a clean output on the wiper while moving. They are plastic Tyco 100KHz linear types.
That would mean that the pot presents a parasitic capacitance while it is reaching the far left position. The wiper maybe climbing over onto different material and there may be a small resistance still before it reaches the stop, and at that point there is also a capacitance generating oscillations?
Is there an industry standard way to "silence" pots as they are mechanical devices subjected to all sorts of stresses and wear over time and having your few millivolts of precious signal flow through them could be asking for trouble?
That would be a big help to solving your problem... however all the other 4 points you make, I'm afraid point to a combination of circuit shortcomings and wiring problems.
You also ask "is there an industry way way to silence pots..." and the answer is yes.
1. The pot has to be reasonable quality.
2. The circuit must not be affected by the change in resistance as the pot is turned... so if it bursts into oscillation it's not the pot at fault but the circuit design.
3. All pots are noisy with DC on them, so you either AC couple or use something like FET devices with negligable input currents, which allows you to get away without using caps.
4. Hum... has nothing to do with the pot. If the hum alters in a strange way as you turn the pot then again, there is a problem with the circuit.
If you post some photos we might have a better idea... and a circuit diagram of it all.
If you're oscillating with the pot turned all the way down (input shorted), try putting a couple of k ohms between the output of the pot and the input to the stage it drives. Increase the value until the problems go away.
If your construction technique or layout are less than optimum, and the circuit has a lot of gain somewhere, you'll likely chase a lot of these before it's all over.
If your construction technique or layout are less than optimum, and the circuit has a lot of gain somewhere, you'll likely chase a lot of these before it's all over.
I checked a few pots, all 100KOhm. Fed 15-30mV into them, checked to see what's coming out on the scope. It seems it is ok except at middle/low settings they pick up high frequency oscillation from my fingers, especially if I touch the back of the pot. The whole pot is made of plastic. Only a few mV but it is there, T=1.5 at the 20uS setting, so that should make it 33.3KHz? Where is this 33.3KHz coming from? This is me standing up, holding pot on left hand and turning wiper with right hand. Tone generator feeding the pot and oscilloscope probe on middle leg. The more I touch the back surface the higher the amplitude. I switched the fluorescent lights off, just in case, still there. Hmm, no idea what's happening here. May be an artifact of the scope or the tone generator?
In addition I seem to remember in equipment I have seen, pots are metal and earthed. If a test lead can pick up noise and hum then so can a plastic pot which is not earthed in any way? So should I replace plastic pots for metal pots and make sure their casing is earthed? Or perhaps surround the ones I have in aluminium foil ?
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