All Discrete Class-A Headphone Amp

[geraldfryjr]

Very cool I have been contemplating a design like that and I am glad to see that some one has tried it!

More details on how it preforms would be great!

Cheers! jer

[Chris Daly]

Hi
Schematic shows input + op amp, also serving as signal input as grounding !

I suspect you meant to have an inverting op amp arrangement where signal input is to neg of op amp, then op amp + grounding and feedback returning to inverting input via a pole.

Cheers/ Chris

[Conrad Hoffman]

Mnturner, we seem to have tread the same ground. Ignoring the errors in the schematic, I've built this same circuit for headphone use, but with a power MOSFET device and better opamp. I've posted it here previously to mostly laughs and derision. Some interesting things happen in that circuit, especially if you use a MOSFET. The output of the opamp operates at the gate turn-on voltage of the MOSFET, so it doesn't pass through zero until the level gets up higher, so for small signals there's no crossover in the opamp. I used another MOSFET with fixed gate voltage for the current sink, rather than an IC, but either works. A large heat sink is essential because there's no temperature compensation and below a certain current MOSFETS are positive just like a BJT (depending on type). I ran a hefty enough bias that I could also drive speakers at moderate levels. I'm not a big believer in "amplifier sound" but I think this design sounded incredibly good compared to anything else I've ever plugged phones into.

[mnturner]

I actually did build it in non-inverting configuration (for no particular reason), there is no pole in the feedback loop; gain compensation comes from the cap from opamp output to -ve input (without which it oscillates). There's a little ringing on square wave which will be minimized i think by adding a small lead cap across the feedback resistor.
Yeah in this build with bipolar outputs the opamp's outputs sit at about 1v dc; the output transistor's Vbe + the voltage drop across the 1000 ohm base resistors, so the opamp is in class A up to that point.
I like the principle of this design, it would beat the he'll out of those single MOSFET buffer amps people seem crazy about at the moment. Needs no output cap and the output stage (which is already quite linear) is enclosed within the opamp's feedback loop, so essentially you end up with only the distortion of the opamp.
I have some irf switching fets, I might try them out at some point.

[mnturner]

Also, any suggestions as to better op amps? I know it's been done to death but I've never really lOoked into high end op amps, just stuck with the good ol' 07x's.

[Conrad Hoffman]

I built mine with a 5534, but today I'd try a National LME opamp. The fets I used were IRF530 because those were what I had a few of. This was at least 15 years ago, so there must be better parts. I'll look for the schematic this weekend and post it if I can find it. Maybe my blameless design too. It would be interesting if somebody wanted to simulate it, since I know what it actually does in real life if properly constructed.

[EUVL]

For the AK701 with 62 ohm impedance, the DAO follower is hard to beat.

http://www.diyaudio.com/forums/headp...ml#post2133565

http://www.diyaudio.com/forums/headp...ml#post1130805
http://www.diyaudio.com/forums/headp...ml#post1131708
http://www.diyaudio.com/forums/headp...ml#post2133200

Some other Source Follower COnfigurations
Some other Source Follower COnfigurations

You can still get matched LU1014's cheap enough.
And you can even build the circuit by hard wiring.

I would encourage you to try it out for yourself.


Patrick

[EUVL]

This is what I use myself with the AK701 :

Downscaling Zen v.9 for use as headamp
Some other Source Follower COnfigurations


Patrick

.

[mnturner]

Hi Patrick,
I'll Probably try this circuit out with MOSFETs, either some irf something's or exicon audio MOSFETs I have lying around. the DAO follower you linked has no voltage gain, which I need as none of my sources drive the phones as loud as I'd like. This necessitates some voltage gain stage of some sort, and if an opamp is used, as in this case, I don't see any point in not including it in the feedback loop which reduces the significant THD of a single FET follower and reduces output impedance to very low levels.
I'm thinking about building a tube based version at some stage which might use a FET follower similar to the one you linked for the output stage though.
Thanks for the reply

[EUVL]

If you need gain, you could build a blowtorch as pre.

Another easy solution is to use a LM3875 or LM3886 at +/-24V rails to drive the AK701.
Won't be pure Class A, but plenty of drive.
(Yes, I tried that myself. Not bad at all.)


Patrick