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Old 17th October 2007, 05:16 PM   #1
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Default Wiring PSU and Aikido PCB

I am sure that there is something fundamental about the hum in my Aikido. Before going exotic, I want to be sure that the basics are correct (as far as is possible).

I experimented with a cheater plug on the Aikido and then on my main amp. The level of hum was further diminished by doing this becomming barely audible from the nearly barely audible prior. OK, this implies that I have a ground loop that is having an effect.

My concern lies with ensuring that my PSU and PCB are connected correctly.

My current configuration is to take the PSU GND and connect it to the GND pad of the PCB. From their I connect to chassis earth - thereby connecting both PSU and PCB to earth.

This is shown in the first diagram. In the PCB instructions John Broskie says that the PCB can be floated, jumpered to the chassis via J7 and the mounting bolt, or floated using a small value cap. However, in my current configuration, floating the PCB means that the PSU is also floated and that is not good, is it?

So, should things be wired as in my second diagram?

Thanks,
Charlie
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Old 17th October 2007, 08:21 PM   #2
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I like neither of your solutions. Since I obviously assume that Broskie's PCB is designed properly to avoid ground loops, I would attach the PSU B+ and ground wires to the PCB, and then bond that PCB to the chasis via either the jumper and bolt or a separate wire from the PCB grounding land.

EDIT: stupid me, that is option #1, didn't see it right away
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Old 18th October 2007, 03:59 AM   #3
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Leadbelly,

I am glad that you agree with one of my wiring diagrams, so my current configuration is probably correct. I do notice that both the filament transformer and the B+ transformer (I have separate ones) do begin to buzz when power to the latter is applied. It is not clear exactly which one of the two transformers is actually buzzing, but the transformer buzz is equally present when I put my ear to either trafo. I expect that it is this buzz that is being transmitted on the signal.

When power is applied to only the 6.3V filament trafo, it does not buzz at all. Only when B+ is applied. This leads me to think that the B+ trafo (or B+ PSU) is causing the buzz, and that it is being passed onto the filament trafo via the 1/4 B+ voltage reference (subject of an earlier post).

What I can do is to replace the 125-0-125 with a 275-0-275 and a quick rewire to use full-wave tube rectification. My 275-0-275 is one of the 300 series from Hammond rather than the slightly lower quality 200 series which is my 125-0-125. The 300 series can also be wired for 120V, which my current trafo can't and it tends to see 124V on the 115V rated primaries.

You know, I should simply stop worrying about all of this. I have a great sounding Aikido preamp. The truth is that the hum is only audible when everything is really, really quiet and my ear os close to the speakers. At some point, I'll rehouse my Aikido and use aluminum for the chassis plate rather then the steel used at present. That'll also be the time to change out the trafos.

Charlie
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Old 18th October 2007, 05:52 AM   #4
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Have you considered emailing John with the question directly? I've always found him to be extremely pleasant and helpful with respect to either the Aikido or anything else on tubecad. John Broskie is one of the great guys in our hobby.
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Old 18th October 2007, 10:44 AM   #5
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If your residual hum is 120Hz, try diddling the nulling resistors. In the one on my workbench at the moment, I put in a trimmer to adjust it and get the last bit of ripple out. It will need to be readjusted as the tubes age or get replaced.
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Old 18th October 2007, 04:24 PM   #6
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I did have some communication with John Broskie and he was very helpful in suggesting that hum was a bane for many designers and that he has also had some problems with it in other projects.

Sy, Excuse my ignorance, but what are the "nulling" resistors? Are these resistors R15 and R16 in Broskie's design? They are in series between B+ and GND in the following configuration: B+ - C6 - R15 - R16 - GND. I cannot remember my exact values, but I think I have C6 = 0.1uF; R15 = 82K; R16 = 100K.

If these are the actual values would trimpots work here?

Charlie
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Old 18th October 2007, 04:43 PM   #7
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Yes, R15 and 16. I subbed a trimmer combo for R15. You could just use a trimmer rated at something higher than the optimum value, but to minimize the sensitivity of the adjustment, you'll probably want to have a series resistor make up a portion of that- if your range goes to 20% over and under the theoretical value, that should cover most tube variations.

For example, let's say that R15 is theoretically 90K. I'd use a 75k resistor in series with a 25k trimmer.
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Old 18th October 2007, 11:36 PM   #8
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Thanks Sy,

I'll do as you recommend. However, I'll first go and check my actual values against the newer version of John Broskie's PDF instructions as he gives firmer values for these resistors for specific tubes.

I really cannot tell the frequency of the hum anymore, it is really that quiet. My gut feeling is that it is 60Hz. BUT buzzing transformers will likely do so at 120Hz. Either way, replacing R15 with a resistor/pot combo is hardly going to break the bank!

Charlie
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Old 19th October 2007, 03:17 AM   #9
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Kind of a stupid question, how to tell a 60hz hum from a 120Hz?
Sorry for thread jack, Charlie. I am assembling my Aikido, your threads are great references to me!!
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Old 19th October 2007, 04:34 AM   #10
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Fred,

As I have read, 60Hz sounds pretty low and dull, while 120Hz sounds more raspy. One thing I did was to download a freeware signal generator for my Mac and played sine waves at these frequencies. The problem with the hum from my Aikido is that it is so quiet that I really cannot tell which of the two frequencies it most resembles! I mean, everything has to be dead quiet - no AC, ceiling fan or cycling fridge in order for me to hear the hum from further than 8 inches from my speakers. BUT I know it is there! It was worse when I used a voltage divider on my attenuator to give more useable steps. When I moved the divider into my main amp, the hum was significantly less and I have no idea why!

I am very flattered that my blundering about with my Aikido has resulted in useful threads. I would really recommend care in placing of your components, especially your signal wiring and ground wiring. When I finally get around to re-housing mine, I'll pay lots of extrat care to component placement.

Good Luck.

Charlie
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