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What can cause Headphone Amp's output resistance to increase over time?

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May 28, 2020
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#1
I have a really old Aune T1 DAC/Amp. And I recall that when I first got it, it could drive very low resistance IEMs just fine. But after all these years, all at sudden, it can't anymore. It can drive planars such as my Sundara.

The specific problems it has with IEMs now are:
1) Overall, volume is tiny.
2) No base and mid is thin. Everything sounds like far away.
3) If I really increase the volume, a lot of distortions and pops kick in.

So it feels like the output resistance is just too big now. And planars are less affected, but IEMs are just not able to get decent power anymore. I am just curious if this is common and if so, can I salvage it?
 

egellings

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#2
Is this headphone amp by chance a vacuum tube model? If it is, aged-out tubes could explain your problem. I don't know what in a semiconductor based amp would cause this slow decline outside of some sort of a fault that might need repair. Possibly dried-out electrolytic caps, maybe? One odd problem I saw once was an amplifier in which a transistor's B-E junction was temporarily reverse biased into Zener breakdown upon power off of the amp. This is a no-no, as breaking over this junction with reverse bias eventually clobbers the hfe, or amplifying ability of the transistor. I fixed that with a reverse biased diode across the junction to clamp the reverse voltage to a safe amount. This would amount to a design flaw in the product, and all copies would show such a problem over time. Another possibility is a bad batch of transistors, and some sort of migration problem might be going on. I would find that to be unusual, though. After that, I'm grasping at straws.
 

Pluto

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#4
But after all these years, all at sudden, it can't anymore.
Check the power supply. It might have gone low voltage, asymmetric (if split rails) or unable to supply sufficient current (?reservoir capacitors?)

If power supply OK, next thing to look at is output devices and work backwards from there.
 
OP
S
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Thread Starter #5
Thank you all for the responses!

1) Yeah, it is a tune dac and an ss amp. And I tried directly feeding the RCA input and bypass the tube circuit, and the sound is still tiny. :(
2) I gently twiggled my headphone jack and it felt solid.
3) I think most likely it is some busted cap. I probably have no time to open it up and fix it all right. I will use it with my sundara as is. Is it going to damage the headphone in anyway?
 

Vini darko

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#6
Thank you all for the responses!

1) Yeah, it is a tune dac and an ss amp. And I tried directly feeding the RCA input and bypass the tube circuit, and the sound is still tiny. :(
2) I gently twiggled my headphone jack and it felt solid.
3) I think most likely it is some busted cap. I probably have no time to open it up and fix it all right. I will use it with my sundara as is. Is it going to damage the headphone in anyway?
If you have a multi meter you can check for DC offset coming out of the amp. Use the cable from your sundara and measure both L+R sides with the amp at idle. The number should be low like a millivolt or two.
That would at least let you know wether the amp is safe.
 

twsecrest

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Nov 27, 2018
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#7
I have a really old Aune T1 DAC/Amp. And I recall that when I first got it, it could drive very low resistance IEMs just fine. But after all these years, all at sudden, it can't anymore. It can drive planars such as my Sundara.
The specific problems it has with IEMs now are:
1) Overall, volume is tiny.
2) No base and mid is thin. Everything sounds like far away.
3) If I really increase the volume, a lot of distortions and pops kick in.
So it feels like the output resistance is just too big now. And planers are less affected, but IEMs are just not able to get decent power anymore. I am just curious if this is common and if so, can I salvage it?
That is so weird, something old does not work well any more.
So maybe you should replace it?
Schiit Fulla 3.
 

Bob-23

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#8
I'd open the case and look if there's any obvious cracked solder joint; look if there's (yellow) glue on the board (which not rarely gets conductive and corrosive over time: remove it if you find any); and I'd clean the pot, if it is an open one. (Don't forget to discharge big capacitors completely before you touch them!)
 
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