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In Reply to: RE: ok, so now I am really confused..... posted by Triode_Kingdom on November 20, 2016 at 11:55:39
I am pretty sure the negative input is not grounded, but i don't think I am going to be able to see anything else the way this thing is put together. Not much room between the board and the bottom. Since it appears there is now not much difference between the two channels sound wise now, I am going to assume for the time being that the switchers are ok and try to get the 26 and the preamp squared away and then listen again. I have attached two scope shots. The first was taken at the outputs of the switcher with a 1x probe, volts/div 50m, sec/div 10us. The second shot was taken with at the outputs of my filter, volts/div 20m, sec/div 10us, 1x probe.
Edits: 11/20/16 11/20/16 11/20/16Follow Ups:
It would be very rare to find an OEM power supply with either the negative or positive grounded. The reason is the manufacture has no idea of how the power supply is being used. You may want a positive ground.
Consider a bipolar +12,G,-12 power supply made from two identical units. One unit would have it's negative grounded and the other it's positive grounded.
The filament that the supply is going to power will be grounded through a resistive CT (or hum pot) and cathode resistor.One wouldn't want the supply to be otherwise grounded.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 11/21/16
"I am pretty sure the negative input is not grounded..."
Do you mean the negative output? All you need is an ohmmeter to see if the 12V negative output is connected to the case.
"...but i don't think I am going to be able to see anything else the way this thing is put together. Not much room between the board and the bottom."
Please don't try to make live measurements anywhere inside this unit. Most of it is tied directly to the AC mains. This is an electrolution hazard.
"I have attached two scope shots. The first was taken at the outputs of the switcher with a 1x probe, volts/div 50m, sec/div 10us."
The wave there is 50kHz, and amplitude 50mV. Just to be sure that's all there is, you should reduce the sweep speed to see if there's any 60/120Hz energy also. The residual 50kHz can be mostly filtered out, but you should wait to see if you're actually going to use this supply.
"The second shot was taken with at the outputs of my filter, volts/div 20m, sec/div 10us, 1x probe."
What filter is this? That's a 25kHz wave.
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Buy Chinese. Bury freedom.
I just checked and there is no continuity between the negative output and the case, so it must not be connected. Don't worry, I won't be messing around in there when it's hot.
The filter is my home made one I was using with the 10 volt trans. It hs a .2 ohm resistor between the + terminals of two 10k caps and the - terminals tied together, and has a dual winding common mode choke with positive going to one winding and negative to the other. The outputs of the windings go to the + and - of the filament.
Does it matter if it drops the frequency to 25kHz in this application?. It's not my design, I borrowed it from a design for an 845 filament supply that someone had posted on the asylum several years ago.
PS Just remembered the output is adjustable.... boy it is ever nice to be able to adjust the voltage to the nearest 1/100 volt by turning a screw. Now set at 10.00 vdc, if my fluke isn't lying to me.
Edits: 11/21/16 11/21/16 11/21/16
I'm not sure how 25kHz is being created from what appears to be a 50kHz triangle. Maybe the filter is ringing. The second trace does look more like a sine wave.That aside, you're not using the right type of filter for this. The 50kHz wave at the output of the switcher isn't common mode. It's differential between the (+) and (-) terminals. In fact, the power supply already has two devices rejecting high frequency common mode energy between the AC mains and the 12V output. One is the input noise filter, the other is the output transformer. The latter isolates common mode audio signals at the filament from the active circuitry in the supply, and it precludes the need for anything else in that regard.
To reduce the remaining 50kHz energy, you'll need to add two chokes, one in series with each output terminal. The chokes should have a value of about 50uH and be rated for 5A or more (813 filament). The (+) and (-) voltage at the output side of the chokes should be shunted with a 220uF cap. This design will attenuate the 50kHz energy about 60dB, or 1/1000. Based on your measurement, the result should be about 50uV of HF noise.
Incidentally, did you have the supply loaded when you measured the 50kHz energy at the output? That's a necessity in order to obtain an accurate reading. Output noise will be much less when the supply is loaded, due to attenuation by the chokes in the internal filter.
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Buy Chinese. Bury freedom.
Edits: 11/21/16
Ok, before we go any further I need to bring you up to date on what I got done this morning. I would have posted it sooner, but my honey do list got enforced priority about mid morning. : )
Anyway, I was working at tweaking things a bit and noticed that the hum pots were no longer doing anything when I had the meter on the wings of the pot and set to mV. I got the lowest reading when I had it cranked hard to port or starboard, and I was still getting about 25 mV. Not good. So I decided to try taking the pots out of from 813's and replacing them with a couple of 50 ohm resistors tied to the filament pins and each other with a wire going to the cathode cap and resistor. Well how 'bout that sports fans, the ripple dropped to 1.7mV at the filament pins! So then I change the filter on the 26 to match the filters on the 813's, and danged if that ripple doesn't drop to 1.7 mV too. Haven't removed the hum pot on the 26 yet but will try that next. I was feeling pretty cocky by now, so I checked the ripple at the speaker terminals and get slapped in the back of the head. It was 105 mV. Phooey says I, and decide to put a shorting plug in the amp input, and when I check it this time at the terminals I get 0.5mV! Couldn't hardly believe it. I double checked everything to make sure it was correct, and it was. So that means I have a problem with my preamp, but I will work on that later. Just to be sure, I turned the volume up and put my ear right next to the speaker with the shorting plug still in and it was dead quiet.
So to get back to the rest of your post, I am not sure why, but my filter seems to be working pretty well, at least as far as hum goes. What would be the symptoms of the 50kHz if it is still there? Right now it sounds as if all my troubles are gone with this right channel and it sounds good to boot.
Edits: 11/21/16 11/21/16 11/21/16 11/21/16
Great, sounds like you've made a lot of progress!
"What would be the symptoms of the 50kHz if it is still there?"
You'll have a 50 kHz wave at the speaker terminals. Check for this with your scope. It can be nulled with the hum pot, just like 60 Hz.
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Buy Chinese. Bury freedom.
Where would the hum pot need to be placed?
Wire it just like you normally would. The only thing different here is that the AC isn't 60 Hz. On this point, you may not be getting any appreciable high frequency "bleedthrough" at the secondary of the ampifier's OPT. Take a look with your scope - if it's only a few millivolts, I'd let it go for now.
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Buy Chinese. Bury freedom.
But then why is the speaker dead quiet as it is now?
Because you can't hear 50 kHz. It might mix with something else though, so it's important to attenuate it as much as possible.
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Buy Chinese. Bury freedom.
Hate to dog this, but would the mix be audible, and if so in what way? If not, I am not sure why we need to try and get rid of it. Right now I don't hear anything I think I need to get rid of.
Edits: 11/21/16
The potential issue is IMD. When two frequencies mix in a not entirely linear device (like a tube), sum and difference frequencies are created. Those frequencies can mix again in never ending combinations with harmonics of the original frequencies. These spurious frequencies are known as intermodulation distortion products. Their audibility can vary considerably with different source materials, speakers, specific output tube, etc. Regardless of whether you hear it now, it's good practice to minimize the potential for this effect.
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Buy Chinese. Bury freedom.
;
I remember contentious discussions on this exact topic years ago having to do with the ultrasonic response of piezo tweeters. I'm hoping for several reasons that no one is still using those. :)
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Buy Chinese. Bury freedom.
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