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Old 7th April 2009, 08:10 PM   #1
vuohi is offline vuohi  Finland
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Default Troubleshooting hum issues with batteries?

Hello everyone! I just thought I'd ask about troubleshooting an amp with the help of some batteries. OK, so here's the scenario: I put together an experimental 300B amp based on the JE Labs 300B SE schematic. Basically this thing is just a pile of test leads on my desk and of course it had some serious hum issues because of this rather interesting construction. Since I wasn't quite sure how much of the noise/hum was because of the PSU, I decided to heat the valves with lead acid batteries, thus eliminating(?) hum caused by AC heaters. The hum and hiss went away almost completely, but there's still some left. The question is, is it now safe to assume this residual hum/hiss is originating from the perhaps not quite well enough filtered PSU?
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Old 7th April 2009, 09:03 PM   #2
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Default Re: Troubleshooting hum issues with batteries?

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Originally posted by vuohi
The question is, is it now safe to assume this residual hum/hiss is originating from the perhaps not quite well enough filtered PSU?
That's possible, however I would be more suspect of all the exposed clip leads gathering stray pickup as well as, perhaps, not having a proper central (star) ground. Go ahead and build it as it should be (DC filaments) and what little hum may be left should be easy to remedy.

Victor
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Old 8th April 2009, 01:38 AM   #3
mwiebe is offline mwiebe  United States
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You dont mention what you are measuring hum wise on your speaker jacks so Im not sure what you are finding is too much.

AC on the filaments of DHT power tubes may cause a bit of hum though sensitive speakers, but using a hum pot you can pretty well null things to less then 2mV hum on the speaker jacks, and with a bit of experience with wire dress and grounding issues you can get a bit lower--that is with AC heaters on an indirectly heated driver tube.

No it is not safe to assume all residual hum is coming from your power supply. There are many other issues that come into play, a schematic would help with suggesting areas to check, as would mentioning if the power supply is on a separate chassis.

Assuming you have grounded the input to your amp, if the gain of your driver tube it too high for your system then that can cause hum/hiss. If the voltage potential between the driver tube and heater is too great that can cause hum, I assume you have AC on the driver tube. Tube switching driver and power tubes can make a big difference chasing hum/hiss, some tubes are just more hummy/hissy then others.

And stepping into the unknown, you could have one or a host of grounding issues, bad solder joints, bad ground dress, signal carrying wires that need to be screened, signal wires picking up AC, the list goes on. If you are in an old house your ground pin could have DC offset or AC on it. Your power bar, if you are using one, could have a ground issue so plug straight into the wall. If you are using a directly heated driver, the tube itself could be picking up and amplifying an electromagnetic field. And the list goes on.

And you could have a hummy power supply.
Matt
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Old 8th April 2009, 08:54 AM   #4
vuohi is offline vuohi  Finland
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Default Re: Re: Troubleshooting hum issues with batteries?

Quote:
Originally posted by HollowState

Go ahead and build it as it should be (DC filaments) and what little hum may be left should be easy to remedy.

Victor
Oh yes, that's what I'm going to do. My previous construction of the same amp was AC heated, and it was pretty much on par with the "standard" 2mV output hum. I know the question is a bit silly. I mean, how on earth could one suppose anyone can really give a definitive answer based on such a small amount of data? Ideas is what I'm fishing for, and I've gotten plenty already. Thanks for that, gentlemen.

Now, in reality the hum is not the issue here. The hissing tweeter is my main concern. The hiss could be described to be pretty much the same kind you get when you hook a "dirty" switching wallwart to a headphone amp.

Oh well, I'll keep experimenting and probably the best thing to do is to put the amp together in some sort of a chassis instead of just having the parts speard on my desk and hooked together with a bunch of alligator clips.
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Old 8th April 2009, 12:48 PM   #5
pointy is offline pointy  United Kingdom
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in construction of an amp the metal chassis is used to screen all connecting leads within it. and so with the leads all over the place you will have at the moment a big hash antenna (sounds good but it does not work the way we would like).

i also think when you have put the amp together chassis and all it will be fine (as you have said its only the hum that is a problem).

short leads and screening
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Old 8th April 2009, 09:49 PM   #6
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I had quite a few problems with hum on a 12AX7 pre amp I built.
In the end I went to a DC heater feed and that reduced the problem.

Some of the hum can be down to the power supply smoothing and decoupling. I also found running the heater wires near signal wires was a mistake.

Another mistake was mounting the valve close to a transformer as that induced hum into the high impedance inputs.
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