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Old 19th December 2003, 11:08 AM   #1
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Unhappy ODL amp bias-how to help

OK so I got suckered into an ODL HT2 amp on e-bay before learning ALL the facts-knowing what I know now I would not do it again

but lacking a time machine I want to make lemonade from my lemon-and to tell the truth it does sound somewhat better than my adcom 555II

here's the problem-a moderate hum/hiss in both channels-I tracked it to the amp and it still happens even without any interconnects connected(just power and speakers) so I measured the output at the binding post and get 15.4 mV(the adcom had 0.1)

so could this be from the bias being improperly set?(I've read some accounts of that being one of the major probs with these amps) and if so I think I could adjust it(I can read a meter and turn a screwdriver) there are 2 pots per channel-one at the front near where the signal comes in and one near the middle where the heatsinks attach--which one would be the more likely culprit?or both? would I have to measure somewhere besides the output posts for either?

any help would be lovely-including any warnings or procedures
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Old 20th December 2003, 09:31 AM   #2
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No one elase has chimed in...so I will.

Of course, you just measured DC offset. 15mV is a perfectly acceptable figure...not optimum, but decent. If the bias is off, it certainly can affect offset, and vice-versa. But I doubt that bias or offset are the root of your problem.

(The Adcom is so low because it uses a DC servo opamp to do the job).

I'd start looking for wiring problems. Measure voltages between respective ground points and look for something funny. Check all internal connectors to make sure they are all seated and whatnot.
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Old 20th December 2003, 09:49 AM   #3
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I have been looking at it some more and it seems to be the transformer itself that is humming-quite loudly once I took the lid off-and it seems to go away occasionally-but usually it's there
the transofrmer is a bare toriod(you can see the wraps)-the adcom has the trans encased-I guess for just that reason

maybe I could wrap it or encase it somehow?

it seems to be getting better though(this amp supposedly has about 50 hrs on it) and I adjusted the DC offset down to 10 mV also

I've ordered an aragon 2004 and if it sounds as good as this ODL I'll just sell the ODL and keep the aragon--if not then I might send the ODL to klaus and have him upgrade and fix it-he says for $400 he can make it 90% of what a current version stratos is
interestingly the aragon I bought cost $400 so I'd rather it work out than end up with $1000 in an ODL amp that will never have that kind of resale value-but I will if it's better than the 2004-otherwise it might be best to get out now and recoup what I can

only time will tell

thanks for the help-people probably don't want to mess with me-they'd probably rather see me suffer
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Old 20th December 2003, 10:02 PM   #4
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Something I have observed not with any commercial units but with ones I construct the higher quiescent current the more noise I can hear at the speaker, hum and hiss.

I don't know if this is just because most of my amps have been on proto boards or because it's just the nature of my designs or a common issue among all amps. I must add; when the quiescent current is correct like 7~12mv across output resistor there is almost no noise out at all.

Maybe you could check your quiescent current?
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Old 20th December 2003, 10:04 PM   #5
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Looks like took to long to post, ohwell glad you figured it out.
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Old 20th December 2003, 10:08 PM   #6
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I don't know if I figured it out but at least I can't hear it humming from my listening position anymore

thanks
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Old 20th December 2003, 10:36 PM   #7
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I was just thinking about your problem and thought although a bit onorthodox maybe you could try putting a little dynamat or the like around or under the transformer mount.
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Old 20th December 2003, 10:54 PM   #8
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actually it already has something like that under it

maybe I could put more or something else?maybe something more effective-or some damping applied to the PCB also?
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Old 21st December 2003, 01:02 AM   #9
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I guess you could unscrew the transformer from the chassis and separate the two by holding it in your hand or whatever to see if it's just transferring hum to the chassis or if it itself is creating the hum then you'd know which to isolate.
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