Another update…
Removed the gain pots, cleaned up the dressing on the volume pot - board wire and the hum remains in the right channel.
Not sure if it is relevant, but the single ground cable that goes from the board to the ground bus on the RCA input/output, is going to the left channel…
Very frustrating
Removed the gain pots, cleaned up the dressing on the volume pot - board wire and the hum remains in the right channel.
Not sure if it is relevant, but the single ground cable that goes from the board to the ground bus on the RCA input/output, is going to the left channel…
Very frustrating
I built this preamp last Summer and have had a hum on one channel that I have never been able to get rid of. I have lived with it by turning the gain to 50% on my amp and that got rid of the hum. However, with the information in this string I am going to make another attempt at solving for this.
Thought had crossed my mind. Right channel is on the far side of the amp, so yes, longest wires. But shielded as they are supposed to be.
I wonder if the metal straps I’m using to hold the large filter caps in place could be the culprit’s?
I wonder if the metal straps I’m using to hold the large filter caps in place could be the culprit’s?
@perkri
First of all, really sorry that you are going through all of this. Quite frustrating I am sure as the Aikido design on its own is a good design (I'm planning on building mine from some Glassware boards) . From your spectrograph, it appears both 60Hz and 120Hz are significant. Have you checked the power supply with a scope? What is the amount of AC hum (in mv RMS) with your AC voltmeter on the output with the inputs shorted? Do you have an audio analyzer? Are all the PS capacitors functional (i.e. new, well soldered and not with poor ESR)?
Ideally you want at most 1mV RMS AC on the output of an amplifier, and about 10 times less than that for a preamp if possible. Or else, the chances that you will hear hum on sensitive speakers (like Klipsch) become high.
Any updated photos of what the inside looks like now?
Best,
Anand.
First of all, really sorry that you are going through all of this. Quite frustrating I am sure as the Aikido design on its own is a good design (I'm planning on building mine from some Glassware boards) . From your spectrograph, it appears both 60Hz and 120Hz are significant. Have you checked the power supply with a scope? What is the amount of AC hum (in mv RMS) with your AC voltmeter on the output with the inputs shorted? Do you have an audio analyzer? Are all the PS capacitors functional (i.e. new, well soldered and not with poor ESR)?
Ideally you want at most 1mV RMS AC on the output of an amplifier, and about 10 times less than that for a preamp if possible. Or else, the chances that you will hear hum on sensitive speakers (like Klipsch) become high.
Any updated photos of what the inside looks like now?
Best,
Anand.
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I hope you find a solution - and of course share it with us 🙂
The gain position didn’t have any effect on the volume of the hum for me. I just wanted to remove them to see if the wires were indeed picking up noise.
Did not remedy the issue.
I got rid of the hum on the left channel, not sure what specific action made that happen. Right channel remains a problem.
I know the instructions specify only running one cable from the RCA bus to ground, but I think I’m going to run a second to the right channel just to try.
Suspect it won’t make a difference as there is continuity from the right channel input ground to the ground bus. But what harm could it cause…
Thanks!
The PS caps are new - Solen and obligato. I never looked at the specs for the caps, but I will look online to find out. But if the caps don’t have the correct specs, it should be an issue on both channels?
Yeah, it is a 60hz problem - mostly - on the right channel.
I will check the AC on the outputs. Though last I checked, I wasn’t seeing any. Could be my meter isn’t seeing it? Seem to have misplaced the leads for my Fluke, and I’m using my Hantek handheld scope in its place. So yes, I have a scope, but have not gotten around to figuring out how to use it. Last time I used a scope was in high school electronics class, back when dinosaurs roamed the earth.
Listening to a pair of Coral Holey baskets now, and they are far more noisy than my other speakers, which are a ridiculously 79 or 80db efficient. Have not pulled the cornwalls out of the basement as they are going to be very loud from a hum perspective.
There is some hiss, but I’m attributing that to my AC in the house and the somewhat questionable tube compliment in the pre.
Need to get a fresh set, but want to sort out the hum first.
Found this, which shows the output wires grounded differently than the input.
The shields in this, are connect to the board and not to the bus, like the inputs are.
Other thing I might be wondering about, is because the build is upside down, the incoming AC is on the board side, not on the chassis side? Perhaps I should twist them and try to tidy them up a bit?
The shields in this, are connect to the board and not to the bus, like the inputs are.
Other thing I might be wondering about, is because the build is upside down, the incoming AC is on the board side, not on the chassis side? Perhaps I should twist them and try to tidy them up a bit?
Definitely twist the AC leads like Grover has advised, both primary and secondary; the toroid and the leads seem to be far away from the input wires which will help; also the purple shield should be connected to chassis ground.
You may need to rotate your AC power transformers so that the primary wires are as far away as possible from the input wires. Please braid the wires, tightly couple with zip ties and/or use heatshrink as well. Your 120V wiring is most important, then your secondaries.
I know you have done a bunch cap mods to this pcb, but I do wonder about building it just stock, getting the hum addressed and then upgrading one by one. Larger boutique capacitors can attract 60Hz hum. One needs to be careful about that.
Please use a new set of tubes especially if they are 'questionable.'
You want to preferably use a multimeter that has an RMS rating and on the millivolt setting set to AC mode.
Is there a way you can use just one input and one output for your testing and eliminate the selector switch as well as the other inputs? Again, just for testing purposes since we are trying to hone in on the problem. I need to look at the diagram that was supplied to you on the 1st page of this thread.
Is the volume control knob grounded?
Best,
Anand.
You may need to rotate your AC power transformers so that the primary wires are as far away as possible from the input wires. Please braid the wires, tightly couple with zip ties and/or use heatshrink as well. Your 120V wiring is most important, then your secondaries.
I know you have done a bunch cap mods to this pcb, but I do wonder about building it just stock, getting the hum addressed and then upgrading one by one. Larger boutique capacitors can attract 60Hz hum. One needs to be careful about that.
Please use a new set of tubes especially if they are 'questionable.'
You want to preferably use a multimeter that has an RMS rating and on the millivolt setting set to AC mode.
Is there a way you can use just one input and one output for your testing and eliminate the selector switch as well as the other inputs? Again, just for testing purposes since we are trying to hone in on the problem. I need to look at the diagram that was supplied to you on the 1st page of this thread.
Is the volume control knob grounded?
Best,
Anand.
Going to order the parts for R10 and C39 and I want to ensure I order the right thing. A few questions:
- R10 appears to be a 1/2 watt resistor based upon its size. I know the suggestion is 100K. What watt rating should this be? Any specific resistor recommendation? I assume a Vishay Dale metal film resistor will be fine?
- C39 is currently a Kemet with markings R71MKP 420 M1 19490096. The parts list says this is a .10uF 100V but the markings lead to me to believe this is a 420V. And I can't seem to find a Kemet part with this specific numbering. I see the R71 series, but that is. Recommendation for a replacement? It appears on the Tubes4hifi sight they are using a different type of resistor.
They do. They are grounded to the IEC plug.
Secondaries are twisted, just no the primaries.
Thanks!
When the hum first appeared, I had built it with all the original caps. So the hum, in theory, shouldn’t be the caps. Though I am going to swap them L/R one at a time to see if the hum switches side, now that it’s only on the right channel. Was on both originally. Other thing that has shown up in research is directionality of film caps, and which side is outer foil and how this may effect hum.
Have to look at wire length coming out of transformers, may require some mcgyvering to be able to flip them over. But I will be doing that next. Unfortunately, the chassis build does not allow for moving the transformer in any way, other than flipping them over.
And yeah, I have some extra shielded wired I could run from a set of RCAs to the volume. Should be pretty straight forward
P
From what I can tell, from the cap and the parts list, C39 is 100V. I picked up some .1’s that I’m going to drop in.
R10, as I have in mine, is 80.6 as listed in the parts BOM. I have not looked at the voltages I’m seeing around mine yet to figure out what to put in its place.
What channel is causing your problems?
I would twist them a lot tighter than that. And twist the HV and filament secondaries separately.
Thanks. I’ve pulled the transformers, and I’m redoing the wires and twisting them tighter, using heat shrink on the ends as well.
Just to confirm.
The diagram provided isn’t correct, I think?
Unless I’m mistaken, power in, should follow this protocol…
Hot - Fuse - Switch - transformers
Neutral - Transformers
Ground - chassis/neutral transformers/circuit board
The diagram provided isn’t correct, I think?
Unless I’m mistaken, power in, should follow this protocol…
Hot - Fuse - Switch - transformers
Neutral - Transformers
Ground - chassis/neutral transformers/circuit board