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GRollins 6th February 2007 06:22 PM

MOSFET follower headphone amplifier
 
2 Attachment(s)
If a picture's worth a thousand words, surely a schematic is worth at least five hundred.
I tried recently to describe a headphone amplifier, using only words. While it is possible, in principle, to unambiguously define all the parts of a circuit and how they fit together, it's pretty cumbersome. In an attempt to clear some of the fog, I dredged up the schematic.
It's a trivial circuit, something you can build using only junk box parts, but it performs very well, indeed.

Grey

sek 6th February 2007 06:30 PM

It is easy to, e.g., draw a 1000uF bipolar cap (and claim to have it in the junk box, for that matter). :D

*duck* I like it! ;)

EUVL 6th February 2007 06:53 PM

Grey,

Shame on you !!
You should have posted a schematic with Loveltech's !!!

:)


Patrick

PMA 6th February 2007 08:16 PM

Hi Grey,

what do you think about current source instead of source resistor, like here:

http://www.pha.inecnet.cz/macura/follower_e.html

and here:

http://sound.westhost.com/project83.htm

GRollins 6th February 2007 09:54 PM

sek,
Guilty as charged. I always draw caps with both sides as straight lines; the ones with one or both lines curved look funny to me. It's an esthetic thing, nothing more complicated than that. Yes, I could put a + on the silly thing, but I'm lazy.
It's how you know I'm not an anal-retentive EE.
For the record, the MOSFET shown is a (now discontinued) Motorola part very much like the IRF610. It's an enhancement mode device, so the Source is more negative than the Gate. Given that the Gate is at ground potential, that puts the Source negative relative to ground. Attach the DC blocking cap accordingly.
And what do you mean you don't have 1000uF film caps in your junk box? You need an upgraded junk box, man!
EUVL,
I'll be doing a Lovoltech power JFET headphone circuit, by and by. Note the date down in the corner of this one. I'm just posting something I threw together after treating myself to a new set of headphones a few years ago. I got frustrated trying to describe the circuit using only words a couple of days ago and have been trying to remember to post the schematic ever since.
PMA,
Now we're entering deeper waters...
Take any or all of the following thoughts and throw them in a pot. Stir over medium heat. Serves one.
--I have no problem with the idea of using a current source instead of a resistor to bias the circuit.
--A Darlington? Why not use another MOSFET so the characteristics match? (This may be answered in the project's writeup, but I'm short on time. I'll try to read it later tonight.)
--Given that the intended load is a pair of headphones, there's not much energy storage to be dealt with. The CCS doesn't really have to absorb a lot of backlash. Also, 99.999% of the headphones on the market are one way, hence no crossover. Again, a simple load, very easy to drive. There's no mass to speak of, so there's not much need for a stratospherically high damping factor, either. In short, simple is sufficient, and quite possibly better, which leads us to...
--It adds complexity. Whether this is good or not depends on who you're talking to. In this case, I've gotten rid of the input cap and all the biasing stuff, partially due to using a bipolar power supply.
--I have run into cases where a simple resistor actually sounded better than a current source. Why? Ugh. Ask me on the second Tuesday of next week and I may have an answer for you. My suspicion is that it's related to high frequency compliance difficulties, but I've never been able to think of a good way to test my hypothesis. It's offered only as a basis for discussion and/or ridicule. Given the relatively modest voltage swings required in this case, I honestly doubt that it's really much of a factor. Don't misquote me, as others have. I'm not saying that resistors always sound better, just sometimes. Have I built two versions of the circuit, one with a resistor, one with a CCS? No. It's possible that this is a case where a CCS might sound better. Or maybe the two would be indistinguishable. Who knows? As it sounds now, the thing sounds damned near perfect. I'm reluctant to mess with it.
In the DIY progress report thread, where I originally attempted to describe this circuit, paulb compared this circuit to the Szekeres headphone amp. There are similarities, but there are differences, too. Again, my circuit uses a bipolar power supply, which allows me to dispense with the input cap and the biasing circuit. I use caps sometimes if they will allow me to delete a bunch of other stuff. Do caps sound perfect? No, of course not. On the other hand, their imperfections are relatively predictable compared to gain devices. Drive them with enough current and they can sound pretty good (ask any tube guy). That said, if I can remove a cap from the circuit simply by putting in another rail...to me it's a no-brainer. Getting rid of the bias circuit just sort of falls in your lap when you add the negative rail. You've got the opportunity--might as well take it.
Two more quick notes about the circuit.
1) If you want more bias, decrease the value of R4. Have good heatsinks handy, as the circuit runs pretty warm. Conversely, if you want to decrease the bias or heat, increase R4. The bias will be:
I= (Rail voltage-Vgs)/R4
Multiply that current by the rail voltage to see how much heat you'll be needing to get rid of. Warning: It's more than you think.
2) You are in no way required to build the power supply as drawn. That was just some junk box stuff I wanted to use up. The transformers are literally two separate transformers. They were too puny to do much else with. Yes, I could have tied the secondaries together and made a 24-0-24VAC out of it, but I have a bunch of bridge rectifiers to use up and this seemed like a good chance to do so.

Grey

moe29 6th February 2007 11:48 PM

Grey,

I'll just say thanks for posting the schematic :devilr:

I wanted to try something else besides an opamp-based amp.

GRollins 7th February 2007 12:35 AM

Opamp headphone circuits? Been there, done that. Gave the T-shirt to a homeless fellow living under the Gervais St. bridge.
Besides, it's cold down in the Dungeon. Anything that produces heat is welcome. I suppose it goes without saying that this circuit is pure, red-blooded class A. And, yes, it is capable of driving a speaker, providing your volume requirements are modest.
Or you can start increasing the bias, perhaps even paralleling output devices. After all, it's capable of something like 45V, peak-to-peak, albeit with asymmetrical clipping; call it 40Vp-p symmetrical, or about 14Vrms. That's nearly 25W/8 Ohms if you provide sufficient current.
Remember that this circuit can only swing the voltage that you put into it (actually a whisker less). In my case, I don't need much. First off, I don't listen as loud as I used to. Second, the various sources I use all have sufficient output to drive the thing directly. Finally, even if I were to want more voltage swing, I have a tube preamp, and if there's one thing tubes can do, it's voltage...though I can't remember ever needing it.

Grey

adason 7th February 2007 01:44 AM

i built this one quite a few years ago, it worked fine, but in comparison to tube headphone amp it was nothing special
http://headwize.com/projects/showfil...szeke1_prj.htm

adason 7th February 2007 01:47 AM

here is improved version, i did not try it
http://headwize.com/projects/showfil...murdey_prj.htm


but i modified the circuit to eliminate the output cap by adjustable power supply

paulb 7th February 2007 03:28 AM

Quote:

Originally posted by GRollins
...That said, if I can remove a cap from the circuit simply by putting in another rail...to me it's a no-brainer. Getting rid of the bias circuit just sort of falls in your lap when you add the negative rail. You've got the opportunity--might as well take it. ...

Hi Grey, and thanks for posting the schematic.

I built a Sezerkes a while back, but needed some gain so added (gasp) an op-amp in front of it. Oh, and I used a P-channel FET. Or maybe it was one of each, N- and P- so I could draw equal currents from a dual rail supply. In any case, I removed it from the preamp...it's been dismantled.

Looking forward to the Lovoltech version...I have high impedance phones so need a bit of gain.


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