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Old 27th January 2013, 11:33 PM   #21
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Thanks a million. I do have a quality multimeter just never knew it would be accurate with high frequency ac voltages.
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Old 27th January 2013, 11:51 PM   #22
JMFahey is offline JMFahey  Argentina
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Well, nobody mentioned "high frequency "

Anyway, don't worry; most are guaranteed full precision up to 400 or 440Hz , check the manual, and at 1KHz they are reasonably close.
Of course, at 10 or 20 KHz most can't be trusted at all, but there are other methods, an "RF Probe", built with 1 diode, 1 capacitor and 1 resistor is usable and flat, as the name says, up to many Mega Hertz, how's that? .
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Old 28th January 2013, 07:21 AM   #23
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but, more to the point, they would at any frequency indicate the same on both channels, at least they should with a mono signal.
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Old 28th January 2013, 12:31 PM   #24
JMFahey is offline JMFahey  Argentina
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Yes, of course.
And some can reach quite high, it depends on what the designer used as a full wave rectifier.

But anyway I suggest you build the RF probe.
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Old 28th January 2013, 08:29 PM   #25
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problem is solved... unfortunately, because it can't be solved completely. i reversed the input from the soundcard when the sound appeared to be panned awkwardly and IT is the main problem (beside the volume pot on the amp). i "pimped" it by removing some smoothing / frequency limiting caps and the effect (also tilting the balance by software) seems to be more limited.
Still happy about solving the mistery i played the amp with the volume at 5% for about 1 hour then had to stop because the now sharper highs felt like pressing really hard on my head to where it got painful

Oh, also replaced all paper caps with polypropilene, they seem slightly nicer.
Thanks for all the help!
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Old 4th February 2013, 02:13 PM   #26
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since it's still related to the same amp i'm gonna keep posting here, at least until an admin gets tired of this and locks the topic.
i eliminated the previous power supply as it seemed to sag when pushing the amp close to 1/3 of it's power. now using one double U-core transformer 2x14Vac for output stage (LM3886) and a "classic" E+I core for the preamp 6,3Vac+300Vac+30Vac.
There is a slight hum apparently 50Hz the network frequency that wasn't there before independent of the volume potentiometer. It is only audible with my ear stuck to the test speaker. What i did:
1. rearranged rectifier bridge for the output stage thinking the doubler scheme i was using had insufficient filtering. now with classic rectifier bridge + 2x4700uF per rail the hum is very much the same and frequency has not changed (so it's not bad filtering).
2. removed tubes from sockets and disconnected the heater wires (thinking they would bring in AC somehow into the signal). still the same.
3. i disconnected the preamp transformer completely as it was also audibly humming in an annoying fashion. now the amp casing no longer hums but the slight hum in the speaker stays the same.
4. moved the remaining transformer around as it's not fixed yet to the case (just sits on rubber legs) still there is absolutely no change in the hum level.
There no ground loops i can see but... i must be doing something wrong
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Old 4th February 2013, 02:40 PM   #27
djoffe is offline djoffe  United States
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Can you provide measurements on the hum at the speaker leads? A sensitive AC VM is fine...In my experience, 1 mV is clearly audible, 0.1 mV is typically inaudible. It does depend, also, on the spectrum of the hum, the quietness of the room, the sensitivity of the speakers, and the acuity of your hearing!

It seems like you're doing the right thing...a divide and conquer approach to find the source of the hum. Schematics and pictures might help the remote troubleshooting procedure.

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Old 4th February 2013, 03:56 PM   #28
gootee is offline gootee  United States
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Is your AC input wire pair tightly twisted, everywhere? What about the pairs from the transformer secondaries? And they should all be far away from any low-level or signal conductors and components, or at a right angle to them.

Check your input signal and input signal ground conductors again. They are either twisted or shielded, and signal ground is not connected to anything until it's on the amp board. Input jacks are isolated/insulated from chassis. There is a separate connection to star ground from signal ground on amp board. Signal and signal ground traces stay very close to each other everywhere, on amp board.

There should be sufficiently-large electrolytic decoupling capacitors near amp power pins.

Last edited by gootee; 4th February 2013 at 04:00 PM.
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Old 4th February 2013, 05:21 PM   #29
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Yes and no... the lowest scale for AC on this meter is 2V, not very precise. The reading fluctuates sometimes even though the speaker hum does not change. So much for accuracy. It says between 1-3mV with speaker connected, 5mV straight if there's no speaker. Won't that include the error because of the LM's offset? (keep in mind these are not original ic's). It's barely audible in quiet conditions if i keep speaker 10cm from ear however with my regular use speakers that have higher sensitivity and most importantly they have enclosures, it just may be audible from 1-2 meter distance.
Yes gootee there is a main star ground in the middle of 2 of the large capacitors in the power source. The preamp having different power supply has it's own star and since there has to be the same potential of the signal and power ground, there is a wire uniting them.
Solved part of the problem removing small resistor that connected signal gnd with main gnd in the output stage. Was not needed anyway since they come together elsewhere. Also on twin shielded cables carying audio only one shield connected at both ends, also helped diminish hum a little.
No... eager to test the thing i did not twist anything except the wires for tube heaters. I'll do that this evening to see if it kills the hum. Makes sense as there's AC going from back to front whre the switch is then back to the transformers... a long path.

** hum apparently doesn't register on the frequency counter perhaps it's too weak or in spite what i'm hearing it may not be a single clean frequency

Last edited by Bloodmist; 4th February 2013 at 05:27 PM.
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Old 4th February 2013, 11:12 PM   #30
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No, the wires now all arranged and twisted and no change whatsoever. I measured the outlet and there seems to be a problem. Between the hot and earth i got 127v and between cold and earth there is 107. That can't be right, earth and the cold wire must be at 0 right?
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