Problem with Denon AU-320 step-up transformer

Jeff449

Jeff449

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PROBLEM:
The unit does not sound right. It makes cymbal crashes and other highs way too bright. Overall, the sound seems "unbalanced." Vocals sometimes get lost behind the music. It just sounds bad.
SOURCE:
So I just bought this unit on ebay for $200. It shipped from Japan. I cleaned the RCA connections, then removed the top and sprayed De-Oxit on the switch contacts, then worked them up and down several times.
I did not try it out before cleaning it.
SETUP:
Here is the signal path: Denon DL103 on EPA-100 arm in a Technics SL-1000 (SP-10 Mk2). [Alignment, VTA, VTF all good.] OEM Tonearm cable (w/ grounds) to the Denon SUT input. Toggle switch set to 40ohms as per the instruction manual. Output cable (incl. ground) to MM input of Cambridge 651P MM/MC phono preamp. Phono preamp output (Blue Jeans RCA cable 6') to AUX input of my Sony audio preamp.
TROUBLESHOOTING:
I tried both inputs on the SUT (1 & 2) with the same results.
I know my phono preamp is fine (along with the rest of my system). I use the MM input for my MM carts and they sound great.
Records sound much better with the DL103 going straight into the MC input of my phono preamp (Either wiring straight in or using the "bypass" setting on the SUT) instead of using the SUT.
QUESTION:
Anything wrong with my setup?
Is this a common problem with SUTs "of a certain age"?
Can it be fixed? I'm pretty handy.
Should I return it?

Please don't suggest other SUTs, preamps, carts, cables, etc. right now. I'll ask for that on a separate thread if I want to. I just want to focus on thie Denon SUT for now.
..and yes, my records and stylus are very clean.

Thanks, guys.
 
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Could it be wired out of phase between the channels? That would cancel out a lot of anything that's panned center, which would be vocals and bass usually. Also stereo image will sound "weird".
 
muovimies said:
Could it be wired out of phase between the channels? That would cancel out a lot of anything that's panned center, which would be vocals and bass usually. Also stereo image will sound "weird".
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I suppose that's possible. Could you explain that further? I assume you mean inside the SUT? You mean, for example, that L+ input is connected to L- output and vice-versa?
 
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Jeff449 said:
I suppose that's possible. Could you explain that further? I assume you mean inside the SUT? You mean, for example, that L+ input is connected to L- output and vice-versa?
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Does the cartridge play ok with other equipment? It would be more likely that the cartridge tags have been wired-up incorrectly than the internal wiring of the SUT be wrong, unless the SUT has been repaired/rewired/modified at some point. It might be worthwhile to look inside to check for signs of any work having been done.
 
I agree that it's more likely someone rewired or modified the SUT, maybe to use with a different cartridge with different specs than a Denon. Seems like all else in your system works fine together with the SUT out of the chain.
 
I agree. The connection of the output cables to the impedance/bypass switch is covered in white RTV. I have not seen that on any other pictures I have found online of the inside of a Denon SUT.
I can't find a wiring diagram anywhere. I am going to return it and buy another one.
I like this particular model because it has two load settings AND a bypass switch. I switch cartridges often, and all my other carts are high output.
 
Update:
I asked for a refund. It was granted, but they did not ask for the unit to be returned.
I used the refund to buy another Denon AU-320 from a different seller. I hooked it up as soon as I got it. It sounds great. It has better clarity, focus, and imaging than the MC stage of my Azur 651P.
What I just discovered is that I may be to blame for the first unit sounding so bad. What I failed to mention was that while I was checking out the first unit, I took a multimeter to it to check the wiring. I just read in the manual today that it should not be done. I might have magnetized the transformer core by running DC voltage through the coils while testing continuity. Tonight, I will try to find out how much voltage my DMM puts out in resistance mode. I did not (and will not) touch my new SUT w/ a DMM.
Anybody have experience with such a bonehead move?
Would it cause the symptoms mentioned above?
 
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The battery in the DMM supplies those killer volts. Usually not much.


I guess the question is can it be demagnetized back to proper function?
 
I just measured my DMM. it puts out one to three volts depending on the setting.
Could a tv degausser demagnetize it?
 
Jeff449 said:
...I took a multimeter to it to check the wiring. I just read in the manual today that it should not be done. I might have magnetized the transformer core by running DC voltage through the coils while testing continuity.
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Jeff449 said:
I just measured my DMM. it puts out one to three volts depending on the setting.
Could a tv degausser demagnetize it?
Click to expand...
Volts isn't really the issue, it's current passing through the windings which magnetizes the core. What happens is you get a kind of "magnetic offset" which is a bit like having a DC offset in a circuit. It causes distortion, but the effect is actually fairly benign and doesn't really explain the poor sound you were getting. Anyway, DMMs usually won't cause much, if any, magnetization of the core. It is quite simple to demagnetize a transformer by applying a high enough AC signal to saturate the core, then reducing the voltage slowly to zero.
 
Thanks for the response, Rothwell.
It looks like I'll have to cut through the RTV to check for proper wiring.
If I wanted to demagnetize the core, how much AC voltage would I need? Can you walk me through the setup or send me a link?
 
Jeff449 said:
It looks like I'll have to cut through the RTV to check for proper wiring.
Click to expand...
I don't know what RTV is.
Jeff449 said:
If I wanted to demagnetize the core, how much AC voltage would I need? Can you walk me through the setup or send me a link?
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Connect a signal generator to the transformer's primary and observe the waveform on the secondary with an oscilloscope. Increase the signal until you see the core saturate, ie a horribly distorted waveform. Then slowly reduce the input voltage back down to zero. A few volts (single figures) is usually enough to cause heavy saturation.
 
Should I do both channels at once, or one at a time? Any specific frequency?
 
Because of the apparent mods done to the wiring of this SUT, I would want to check that out before trying the demagetization. If the wiring looks good, then I would do as @rothwellaudio suggests. But some photos of the wiring with the RTV or whatever covering removed may provide some clues before you go sending volts through the transformers.
 
I intend to compare the wiring to the new one I just bought, which seems to be wired correctly.
 
Jeff449 said:
Should I do both channels at once, or one at a time? Any specific frequency?
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Doing both channels together or one at a time makes no difference. Saturation is frequency dependent, so it's difficult to saturate the core at high frequencies but easy at low frequencies. 50Hz should be fine.
 
Thank you Rothwell, bb, and BS. I'm not sure when I will have the time to work on this unit, but I will post any further developments
 
hello; I should demagnetize my sut but i don't have an oscilloscope to check distortion/saturation; can i connect my portable dac/headphone amplifier (chord mojo) at high volume to the primary of the sut to have saturation and then turn slowly down the volume?
is there a "too loud" volume?
thank you!!
 
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