Headphone amp - low Z load and ... "echo" - diyAudio
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Old 27th March 2007, 09:46 PM   #1
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Unhappy Headphone amp - low Z load and ... "echo"

My amp made with 6c4pi-ev and 6c45pi-ev works well with Sennheiser headphones. When loaded with 32 ohm headphones one can hear an "echo" effect - vocals are distant. Any guess why? Phase distortion?
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Old 28th March 2007, 04:18 AM   #2
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Hi,

Your headphones are either:

- Fed out of phase
- An incorrect contact is making them be fed in series (are they pushed all the way in?)

Cheers!
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Old 28th March 2007, 07:49 AM   #3
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They are fed in-phase for sure. A bad contact is not an explanation. Those effects do not exist if I connect 300ohm Senns. I had had similar "echo/chorus" effect with Senns before I changed a coupling cap value to 1uF. I will attach schematics to help trace the bug.
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Old 29th March 2007, 10:23 PM   #4
kevinkr is offline kevinkr  United States
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How do you know there is not a contact problem with the 32 ohm phones - have you measured for continuity to all three contacts on the plug??

You provide almost no information about the headphone amplifier and yet you expect a reasonable answer, and when you don't get one you dismiss what under other circumstances would be a very plausible explanation. (I've had phones with a broken ground lead that caused a problem quite similar to your possibly inaccurate description.)

OTOH If this is an OTL design 1uF is way too small a coupling cap for 32 ohm phones - I would recommend a minimum of 100uF and preferably 220uF or more, but I can't be sure without knowing more about the design. The -3dB point with a 1uF cap and 32 ohm load is almost 5kHz which might be why vocals sound so distant, and there would also be no bass. 220uF would give you a -3dB point of about 22Hz.. For acceptable performance with a 300 ohm headphone you would need at least 20uF!

Please post a schematic, or at least tell us whether this amplifier uses output transformers or is an otl of some sort.
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Old 29th March 2007, 10:52 PM   #5
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It would be great to get echo/chiorus effect such a way, without complex digital /noisy analog time delays, etc...
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Old 30th March 2007, 09:42 AM   #6
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Quote:
Originally posted by kevinkr
How do you know there is not a contact problem with the 32 ohm phones - have you measured for continuity to all three contacts on the plug??


Yes, I have measured it. They are ok - I have tested it also with other headphones (sennheiser).
The Phillips HP1000 are also ok - tested with other amp.


You provide almost no information about the headphone amplifier and yet you expect a reasonable answer, and when you don't get one you dismiss what under other circumstances would be a very plausible explanation. (I've had phones with a broken ground lead that caused a problem quite similar to your possibly inaccurate description.)


I did not mean to be rude and in fact, after I had fired the amp the sound was strange - I had forgotten to connect gnd of the socket! That is why I said so. Sorry for my English.


OTOH If this is an OTL design 1uF is way too small a coupling cap for 32 ohm phones - I would recommend a minimum of 100uF and preferably 220uF or more, but I can't be sure without knowing more about the design. The -3dB point with a 1uF cap and 32 ohm load is almost 5kHz which might be why vocals sound so distant, and there would also be no bass. 220uF would give you a -3dB point of about 22Hz.. For acceptable performance with a 300 ohm headphone you would need at least 20uF!

Please post a schematic, or at least tell us whether this amplifier uses output transformers or is an otl of some sort.


Find the schematic attached. TIA
Attached Images
File Type: jpg headphone_amp_6c4pi_6c45pi_sch.jpg (23.5 KB, 245 views)
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Old 30th March 2007, 10:08 AM   #7
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The effect described above is exactly what happens when there is no ground connected on the phones. Then everything that is in the center (like solo voice for instance !!!!!) is missing and only the parts that are different between left and right is heard (like stereophonic reverb for instance).

Regards

Charles
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Old 30th March 2007, 10:15 AM   #8
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May it imply different cable connection in HP 1000 than in Senns? Hm, must re-check it with right-left channel tests.

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Old 30th March 2007, 01:06 PM   #9
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Have you taken the obvious step of testing your 32 Ohm phones (and their lead and plug) with a different amp, to eliminate the most likely fault area?
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Old 30th March 2007, 01:09 PM   #10
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Quote:
Originally posted by dnsey
Have you taken the obvious step of testing your 32 Ohm phones (and their lead and plug) with a different amp, to eliminate the most likely fault area?
Yes. Already written it above. Yet there might be wiring fault of the socket of MY amp which result in such a behaviour of HP1000.
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