MOSFET follower headphone amplifier

[EUVL]

OK, I stick my neck out and fire the first shot.




Patrick

[rjm]

The circuit posted by Grey is just a Szekeres amp with a honkin' big unregulated split power supply. Nothin' wrong with that. The split supply allows you to get rid of the gate bias network and input coupling cap, and offers better PSRR.

What I found (following up Greg's suggestion) was that if you use three channels rather than two you can get rid of the output coupling capacitors as well, which I feel are the sonic Achilles heel of the circuit. For the details see my Szekeres VE page.

There is also a thread over at headwize.

/R

[steenoe]

OK, I stick my neck out and fire the first shot.

Thanks for that, Patrick Looks nice. Did you build or simulate the circuit? Good idea feeding the zener with a ccs. Might be just the thing for the shunt-reg supply I am working on (again)

Steen

BTW Maybe you should start a new thread? Its often difficult to find things burried in other threads.

[mullardel34]

Grey,

Given your extensive background with the XA project (Nelson Pass-based SuperSymmetry/Aleph) and other PassLabs-based efforts, I thought that I'd note that Marcello Pellerano posted the following Aleph-based headphone-amp design a few years ago on the HeadWize project page:

http://headwize.com/projects/showfil...lerano_prj.htm

Like the project author, Marcello, I also enjoy the use of the low-impedance (~32 ohm) Grado headphones, so the added control provided by the Aleph current-source really brings these cans to life. The purity and the sense of ease in the presentation of the music delivered by the Aleph headphone amplifier is truly amazing. Even my entry-level in-the-ear Shure headphones deliver an astounding musical experience when driven by this amplifier.

I built this headphone-amp with just a few modifications to the published circuit. The primary modification that I made to the presented design was the use a dual-rail power-supply instead of the single-ended power-supply of the posted project; this design change eliminates the need for the output coupling capacitors. I used a 12-0-12VAC dual-secondary Plitron power transformer, resulting in +/-16VDC power-supply rails. Of course, as with any Aleph-based DC-coupled output stage, all of the output-stage components need to be suitably matched to avoid any DC offset issues...

The Aleph-based headphone amplifier avoids one of the major deficiencies of most high-end headphone-amp designs; over-damping of the output stage. When driven with an excessively low output impedance, most high-quality headphones present a very 'dry' and 'sterile' sonic character. However, the same headphones can really sing when driven with a 'critically-damped' output stage. The observed sonic characteristic of matching the output impedance to the load is quite similar to the very nice effect of driving a ribbon tweeter with a current-source amplifier. My personal preference is a damping-factor somewhere in the range of 4-15. A low damping-factor is really shines with a benign load impedance, while the higher damping-factor is potentially necessary to control a more wide-ranging load impedance. The Aleph-based headphone amplifier allows for the simple reconfiguration of the standing Class-A bias current, which sets the effective output impedance of the output stage. Just be certain to provide adequate heatsinking on the MOSFET's; with 300ma bias (and a 12-0-12VAC dual-secondary power transformer), the individual output devices will be handling about 5-6 watts of heat.

Be certain to use a high-quality input-coupling capacitor in order to avoid degrading the intrinsic performance of the Aleph-based headphone amplifier. The Solen FastCap capacitors are very neutral at a low cost, but the Mundorf capacitors offer a very audible sonic improvement (even the 'entry-level' Mundorf capacitors, which are anything but 'entry-level' from a cost standpoint).

BTW, I also took a pass on the EQ circuit included in the project by the author. I prefer to keep the signal-path as simple and uncorrupted as possible.

[steenoe]

Hi Mullard. Marcello posted about his project here:
ZEN-like headphones amp Username "tortello".
Many of us, use that headphone amp, since digi01 made several GB's on pcb's for it (his own layout, also without the Equalizer) Search for Mini Zen, or something like that.
Thanks for posting your interesting thoughts. Component choice is pretty important in a simple circuit like this one, indeed.

Steen

[rjm]

When driven with an excessively low output impedance, most high-quality headphones present a very 'dry' and 'sterile' sonic character.

If by "excessively low output impedance" you mean opamps, I'm temped to agree, though this is the first time I've seen it attributed to damping factor. Surely if it was that simple, inserting a resistance in series with headphone would solve the issue? Although I haven't tried it, from what I've read this doesn't work.

Be certain to use a high-quality input-coupling capacitor in order to avoid degrading the intrinsic performance of the Aleph-based headphone amplifier

Certainly well-intentioned advice, but can you tell me why a 4.7u film coupling cap on the input is 'degrading the intrinsic performance' when you have two 470uF electrolytics for the output coupling capacitance?

/R

[EUVL]

I cannot quite understand why people would still want to build capacitor coupled circuits when they can in many cases get round it quite easily.

Patrick

[mullardel34]

Originally posted by rjm


If by "excessively low output impedance" you mean opamps, I'm temped to agree, though this is the first time I've seen it attributed to damping factor. Surely if it was that simple, inserting a resistance in series with headphone would solve the issue? Although I haven't tried it, from what I've read this doesn't work.
/R

I've noted an audible correlation between damping-factor and audio quality. In my experience, whether we're discussing headphones or loudspeakers, damping factors greater than about '15' result in the suppression of a number of desirable sonic characteristics. One of great advantages of an Aleph output stage is its ability to dynamically maintain a balance between output impedance and the exertion of control of the load; a static series resistor does not duplicate the adaptive behavior of the Aleph design. As this is a DIY forum, I would suggest that you conduct your own experiments along these lines.

Originally posted by rjm

Certainly well-intentioned advice, but can you tell me why a 4.7u film coupling cap on the input is 'degrading the intrinsic performance' when you have two 470uF electrolytics for the output coupling capacitance?

/R

Please re-read my post; I used a dual-rail power-supply so that I eliminated the output-coupling capacitors in my implementation of the Zen headphone-amplifier.

[mullardel34]

Originally posted by steenoe
Hi Mullard. Marcello posted about his project here:
ZEN-like headphones amp Username "tortello".
Many of us, use that headphone amp, since digi01 made several GB's on pcb's for it (his own layout, also without the Equalizer) Search for Mini Zen, or something like that.
Thanks for posting your interesting thoughts. Component choice is pretty important in a simple circuit like this one, indeed.

Steen

Steen,

Thanks for the welcome and the information. I'm glad to hear that others enjoy Marcello's Aleph-based headphone-amp design. Now, if I could only find a design for a point-singularity power-supply so that I run this Class-A circuit as a mobile headphone-amp without the need to carry a car-battery...

[steenoe]

Hi again Mullard, you are welcome Just for the record; I have to be a little bit against your saying that Marcello's amp is Aleph based. It is not
It is Zen based, with an Aleph based CCS The input mosfet pair in the Aleph design is the main difference from a Zen Which doesnt have a thing like that.

Steen