Pioneer SA400 -- Headphones hum, speakers don't. Why?

[mfaughn]

I have an Allied 935 (i.e. a rebadged Pioneer SA-400). The speakers are really quiet even at full volume with no signal. Headphones in the jack have all kinds of hum -- at least two different hums too. One constant and one that fluctuates with the volume pot. The volume hum doesn't increase with volume though, it is just slightly louder in some positions of the pot than others. They aren't huge fluctuations but they are significant. Other than the hum, my headphones sound quite nice through the amp. The low end is much more present than it is from the headphone out on my Emu 0404. Any suggestions?

Thanks,
Michael

 

[Alastair E]

The hum is probably present on speakers too....

Its just that phones are MUCH more sensitive and therefore, yu can hear it through them clearly.

Suggest checking power-supply and filter-caps....

Are any functions more 'hummy' than others...?

 

[mfaughn]

All electrolytics are new. There is a hum present on the headphones when volume is all the way down. There is no audible hum on speakers when volume is maxxed out and ear is 8 inches away. It does sound like line hum but I think there is something that needs to be addressed in the headphone part of the circuit if that is possible. I just don't know what.

 

[regal]

It is actually much more difficult to design a good tube headamp than speaker amp. I know for a fact that headphones are more sensitive than most scopes. I doubt you could do anything cost effective, you are just hearing the PS ripple and AC heaters. Might want to buy a pair of old AKG 240-DF's for this sort of amp.

 

[mfaughn]

Yea, that isn't surprising that it is harder to build a good headphone amp. This thing sure makes me want to build something though. I am using AT-AD2000 cans. This little old integrated brings the low end in them to life far better than any of the headphone amps I've used with them. It is really a huge difference in the listening pleasure.

I've had a couple pairs of sextets in the past and really, really thought highly of them. Not the most detailed or balanced cans but still some of the most enjoyable. Downsized my can collection and now I really only use the AD2000. Why would 240DF be a good choice with this amp?

 

[regal]

240dfs have low low sensitivity, I imagine you have the volume knob way down with the AD2000's?

. I like sextettes too, they would probably be a match as well but no garantees (I am currently struggling trying to design a good sextett tube design that can power 12V's across the 600 ohm voice coil at low distortion.)

For the AD2000's you are probably best to build something like the SOHA II or Millet Max for a nice budget upgrade over the pioneer hp out.

 

[Pano]

Hum that is different at different volume position (not just louder as you turn it up) usually points me to a grounding problem. That's where I'd look first.

Is the headphone jack taken from the speaker out with some resistors?

 

[mfaughn]

Yes, headphone jack is taken through 100 ohm resistors from 8 ohm taps. I will see if I can find any grounding issues. I'll also try and find my most sensitive speakers and see if I can't hear any hum through those. Might increasing the value of the 100R make the output less sensitive and therefore decrease the proportion of hum in the headphones?

I've had a Head-Direct EF-1, Zhaolu D2.5 with upgraded discrete headphone amp, the EMU 0404 and none have matched this old Pioneer for low end. They might have all beat it on top end and detail. I've always found the little SA400 to be colored but very listenable and musical though.

FWIW, schematic attached.