ZEN-like headphones amp - Page 2

tschrama · 24th February 2002, 11:03 AM

Dear Paulb

Hum problem....:

the original szekeres does not use a decoupling cap for the MOSFETs bias circuit.. that could very well reduce the hum......

also 200mA DC with 2mA AC though a 0.1 Ohm ground lead causes enough hum to be audible so carefull gronding is nesscery..

you could also put the MOSFET inside you opamp feedback loop... an opa134 could be stable at a gain of 3 .. or use a compensation cap around the feedback loop to give it a 100KHz roll all .. should be more then enough... This is what I want to try if I had just a litle more time for things. :-(

greetings,
thijs

paulb · 24th February 2002, 03:07 PM

Thanks, tschrama, I've been pretty darn careful with my grounds but I may have messed something up. The hum is too low to see on my scope, but it's audible at zero volume setting. I haven't had a chance to look more closely at it yet, too busy listening to the power amp section.

tschrama · 24th February 2002, 10:55 PM

When I checkt my amp and found a DC offset I thought I messed up.. then I realized the DC current and the tinny wires I used... anyways I use a 22000uF/16V cap ... at these low voltages these are not expensive (10$ ??) that takes care of the hum ... no regulated supply needed :-)

paulb · 24th February 2002, 11:36 PM

The hum is definitely not from the power supply - I'm using a capacitance multiplier that reduces the ripple from about 100 mV to essentially 0. Works like a charm.

tschrama · 25th February 2002, 09:05 AM

hmmmmmmm... nice!!

I leave my chassis floating, but that shouldn't matter... the only ground connetion is at the audio inout connectors..

Nelson Pass · 25th February 2002, 06:49 PM

Really nice job, Tortello!

tortello · 26th February 2002, 08:20 AM

Thanks Nelson, you're very kind,
... even if all this flour is from Pass' bag!!

anelar · 26th February 2002, 09:56 AM

Very nice work I'd say. I would greatly appriciate if you would send me a better quality pdf file.

Thanks in advance...

//A guy from Sweden

paulb · 26th February 2002, 03:52 PM

Just a quick update on my hum problem.
I found that it was the ripple coming in to the cap multiplier that caused the problem. While the multiplier did a very effective job of removing the ripple, the input to it was close enough to the headphone amp board (2 inches or so) that it induced ripple into the amp.
The power supply is in a separate enclosure, so I added a simple RC (4R7, 1000 uF) there. Voila! The ripple was cut down by about a factor of 10, and it softened the edges a bit. The hum is now inaudible.

tschrama · 26th February 2002, 04:30 PM

It's good to read you solved your hum problem. Can you explain a bit more.. I would very much like to understand what was going on..
Is this inductive indused hum, maybe due to the high impedance of the MOSFET and bias resistor?

Happy listening :-)

thijs

26th February 2002, 09:56 AM	#18
anelar diyAudio Member Join Date: Jan 2002 Location: Luleå (Sweden)	Nice work! Very nice work I'd say. I would greatly appriciate if you would send me a better quality pdf file. Thanks in advance... //A guy from Sweden

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24th February 2002, 11:03 AM	#11
tschrama diyAudio Member Join Date: Jul 2001 Location: The Netherlands	Dear Paulb Hum problem....: the original szekeres does not use a decoupling cap for the MOSFETs bias circuit.. that could very well reduce the hum...... also 200mA DC with 2mA AC though a 0.1 Ohm ground lead causes enough hum to be audible so carefull gronding is nesscery.. you could also put the MOSFET inside you opamp feedback loop... an opa134 could be stable at a gain of 3 .. or use a compensation cap around the feedback loop to give it a 100KHz roll all .. should be more then enough... This is what I want to try if I had just a litle more time for things. :-( greetings, thijs

24th February 2002, 03:07 PM	#12
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	Thanks, tschrama, I've been pretty darn careful with my grounds but I may have messed something up. The hum is too low to see on my scope, but it's audible at zero volume setting. I haven't had a chance to look more closely at it yet, too busy listening to the power amp section.

24th February 2002, 10:55 PM	#13
tschrama diyAudio Member Join Date: Jul 2001 Location: The Netherlands	When I checkt my amp and found a DC offset I thought I messed up.. then I realized the DC current and the tinny wires I used... anyways I use a 22000uF/16V cap ... at these low voltages these are not expensive (10$ ??) that takes care of the hum ... no regulated supply needed :-)

24th February 2002, 11:36 PM	#14
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	The hum is definitely not from the power supply - I'm using a capacitance multiplier that reduces the ripple from about 100 mV to essentially 0. Works like a charm.

25th February 2002, 09:05 AM	#15
tschrama diyAudio Member Join Date: Jul 2001 Location: The Netherlands	hmmmmmmm... nice!! I leave my chassis floating, but that shouldn't matter... the only ground connetion is at the audio inout connectors..

25th February 2002, 06:49 PM	#16
Nelson Pass The one and only Join Date: Mar 2001	Really nice job, Tortello!

26th February 2002, 08:20 AM	#17
tortello diyAudio Member Join Date: Jul 2001 Location: Genoa (ITALY)	Thanks Nelson, you're very kind, ... even if all this flour is from Pass' bag!!

26th February 2002, 03:52 PM	#19
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	Just a quick update on my hum problem. I found that it was the ripple coming in to the cap multiplier that caused the problem. While the multiplier did a very effective job of removing the ripple, the input to it was close enough to the headphone amp board (2 inches or so) that it induced ripple into the amp. The power supply is in a separate enclosure, so I added a simple RC (4R7, 1000 uF) there. Voila! The ripple was cut down by about a factor of 10, and it softened the edges a bit. The hum is now inaudible.

26th February 2002, 04:30 PM	#20
tschrama diyAudio Member Join Date: Jul 2001 Location: The Netherlands	It's good to read you solved your hum problem. Can you explain a bit more.. I would very much like to understand what was going on.. Is this inductive indused hum, maybe due to the high impedance of the MOSFET and bias resistor? Happy listening :-) thijs