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jaycee 21st April 2007 12:43 PM

Headphone amp

I got bored and started noodling around in LTSpice. At first I was simulating the usual buffered opamp, using 2 diodes for bias, something like ESP's Project 113. Then I decided to try out diamond buffering. I came up with this:

Pretty basic I think and seems to sim well, but I'm not a great expert on designing amplifiers. Any experts out there care to tell me what they think?

I was even thinking about trying it out using BC850/860 instead of BC546/556, but i'd have to get some of those...

Juergen Knoop 21st April 2007 02:35 PM

I would reconsider the gain setting.
A gain of 22 seems far too much. :confused:

jaycee 21st April 2007 02:44 PM

True, ideally I want full swing with a 1V RMS input signal. It's easy enough to change R6 though.

edit: I set R6 to 3K and that does the trick. It still seems stable but I dont think the model of the OPA(2)134 is particularly good in that regard.

peranders 21st April 2007 02:57 PM

The output stage will run in almost class B.

Forget about the cascodes in my design. Check which values the emitter resistors have.

jaycee 21st April 2007 03:43 PM

Hi Per-Anders... I must say I've seen your boards and they are works of art :) Sadly I'm just a home hack, doing my boards in a tray of sodium persulfate being warmed in a bath of heated water.. I haven't tried anything double sided yet as I havent worked out a good (cheap) way of doing vias.

I am guessing the cascode stages are T3/T7, and T14/T4/T8. What purpose do T1 and T2 serve? I am guessing they act as references in the current mirror.

I had thought about emitter degeneration and will probably add it as it makes the PCB easier to construct into the bargain.

The output is class AB but only just, there is about 18mA of bias according to the sim. Class A would be better though, especially at this low power.

Conrad Hoffman 21st April 2007 08:00 PM

The design looks nice, and I'd be curious if it performs as well as the sim. No reason it shouldn't. Long ago I did a class A headphone amp I really liked, though the design was a bit unorthodox. It was basically an op-amp driving a power FET, but with a separate op-amp/FET current source underneath. The interesting thing about that is the output of the op-amp runs at the gate voltage of the FET. Thus, until you reach fairly high levels, no zero crossing occurs in the op-amp- it runs pseudo class A as well. The thing sounded great, and I can probably dredge up a schematic if any body's interested, though you'll likely be a bit skeptical at first :whazzat:

peranders 21st April 2007 08:21 PM


Originally posted by jaycee
The output is class AB but only just, there is about 18mA of bias according to the sim. Class A would be better though, especially at this low power.
Remember that you must consider temperature stability also. The simulations give you nothing about this.

jaycee 21st April 2007 09:26 PM

Yeah, I was concerned about thermal stability. Not sure how to go about that one yet. I might try and put it together with some parts I have laying about, on a piece of veroboard or so.

As you said, the sim doesnt tell the whole story.

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