Yet another OPA2134-based headphoneamp

[Mikael Abdellah]

I have a pair of AKG 240Monitor that could use a good amplifier. I would like to create my own amp using BB OPA2134, for a couple of reasons. They sound good (I think), they are capable of driving 600Ohm, but most important... I have NO experience with OP-amps. I'm fairly new to analogue-electronics and the only thing I have built so far is a sweet sounding tube-amp. I think it's time I got the hang of OP-amp now as well. I've been looking a lot at this amp: http://raymondaudio.nl/projects/project7/project7.html but in order to learn more about the basics, I would like to build my own from scratch.

I've studied the pdf-documentation for the OP-amp. The maximun powersupplyvoltage seems to be +-18VDC, and to get the most out of it that's pretty much the voltage I'm going to run it from. There is a current-limit of +-40mA but in order to get 40mA at 600Ohm I seem to need 24V so the current-limit shouldn't be any problem as far as I can see. But there are a couple of things that puzzle me. In the spec-sheet it says that the maximun output voltage at 600Ohms is +-2.5V. Is that correct? How can I get more output-voltage? The other thing is the gain, but I'm going to read more about it so see if I acn figure that one out myself. A few pointers in the right direction would be appreciated though.

[hjelm]

The output voltage is V+ -2.5 positive and
v- +2.5 negative so for the positive it is 18(if you feed it with that)-2.5 =15.5 Vout peak at 600ohm. 31V swing.

Gain is 104dB which is 158489 times voltage gain.

For more output current check the link you supplied where he used two opamps to drive the circuit, i would have gone full parallel but nevermind.

About headphones the link indicated that the output resistor was inside the feedback loop which would make the output impedance very low. Doesn't headphones require an output impedance of 50-120ohm to perform as expected?

[Mikael Abdellah]

Ahhh.. thank you for clearing that out for me. hmm.. 31V P-P seems to give a maximum power of 1.6W. That's a heavy load, right? If I want .775 input signal to be clip-level, can I just find the right gain?

[Pan]

Go with a BUF634 right away... full bias/class A.

As for gain, about 12dB should be enough I guess.

I could probably get by with no gain since I only listen at low/normal volumes with cans. I have used HD580/600 and AKG-K240 with a unity gain buffer and it was definately ok with the Senns.

The difference in sound quality going from OPA2132/34 to BUF634 was not a small one IMO.

/Peter

[Mikael Abdellah]

HOLY MACARONI!!!!! Is that for real??? 2kV/us slewrate?!??! But is it possible to use it as an audio-amplifier?

[hjelm]

Yes 31/0.775 = 40

The 1.6W is peak RMS is ~1.6/1.4 W

Saw a project recently where the whole 2134 based headphone amp was built in a 10pin IC socket, think it was 10pin could have been more.

[hjelm]

Maybe i am thread stealing here but peter did you include the output resistor and if so did you put it inside or outside the feedback loop?

[Mikael Abdellah]

Should I use the first half of the 2134 as a buffer first and then use the second as "amplifier" or should I amplify right away without a buffer-stage?

[wim]

Mikael,

I build the same headphone-amp and PSU as you are talking about.

i drive a Sennheizer HD600 with it and it sounds great. It's a very fine amp that i can recommand.

[Pan]

BUF634 is often used in audiocircuits and it´s really good. It´s often used in highend DAC as output buffer after I/V stage consituting of OP627/134.

For a mini amp a lá Cmoy it is maybe not so wise since it will draw plenty current compared to an opamp, but if you want really good sound you owe it to yourself... use a BUF634 that is.


I never used opamp + buffer, I used both solutions separate, since I didn´t need gain I got by with only BUF634. I think I have a 50 ohm resistor at the output. One can try different values to tailor the FR of the cans. 50-150ohms or so, could try lower still if short cables are used... maybe, dunno.

If consumed power is no problem, use BUF634 with full bias and also use one as virtual ground driver for a very fast powersupply.

As for dual opamps, I think you should use one amp as gain stage and just use the other half as buffer. But of course it´s possible to run both in symmetric paralell mode as well.

/Peter