Who has tried a current boosted op-amp (with a transistor) as a chip-amp buffer?

[sardonx]

I might try this sometime down the line...anybody dunnit?

[sardonx]

something like this

[theChris]

notes:
1.) without drawing a feedback path, the cirucit is fairly meaningless.
2.) the output stage is classA, which means you will have approximately Vee/R1 amps of current flowing at all times, making for (Vcc + Vee)(Vee/R) watts of dissipation. for anything more then headphones this will be a lot of heat.

there are various designs for this though. most use a classB output stage though. note that the stability of the opamp+transistor is different then just the opamp by itself. you may need a high Gain-Bandwidth-Product opamp to acheive enough bandwidth at the higher gains required for stability.

also, the opamp must be able to handel a load of Beta * Rl. if Rl = 10 and B = 100, this comes to a 1kohm load.


i suggest that you start with a headphone amp and this design, then work up to a loudspeaker amplifer

[DSP_Geek]

You might want to wire Vout to the opamp negative input. I used something very much like this with a 2N3904 to drive Sennheiser 580s, and it works a treat. The only thing you want to watch is the emitter resistor - it has to be low enough resistance compared to the load that the load is adequately driven at all output voltages.


Francois.

[sardonx]

i suggest that you start with a headphone amp and this design, then work up to a loudspeaker amplifer

theChris,

Read the title carefully. I don't want to make a loudspeaker amplifier. "buffer = line amplifier!!"


DSPGeek,

How would you say it improved the sound?

[theChris]

what are you trying to acheive with this? also, what of feedback?

[DSP_Geek]

Originally posted by sardonx


DSPGeek,

How would you say it improved the sound?

Well, it replaced a 4556 headphone driver, so I was starting from a fairly easy place to improve. I used a TL072 with the aforementioned 2n3904 transistors as buffers, and 470 uF bypassed with 0.1uF on each supply line. Given the tests weren't even single-blind, I thought the bass went quite a bit deeper while being more defined at the same time, and the highs seemed a bit more transparent.

This arrangement requires positive bias on the buffer transistor, so before clipping the opamp is always delivering current to its base, which is reminiscent of opamp output biasing proposed by some folks here (although not as high current, obviously).


Francois.

[Stocker]

leaving the question of classA operation of monolithic opamps aside, have you had a look at the "gainclone afterburner" type ideas presented here?

[sardonx]

Hello,


Chris,
I'm not an electrinics wizard but apparently boosting op-amp output currents improves the sound. Go to http://www.zero-distortion.com/start.htm and check out the article.

DSP,
Thanks.. that's what the writer of the aforementioned article says about the sound too.

Stocker,
I just read the thread on the afterburner. Although it seems like an interesting project It doesn't seem like a very proven one as of yet. That's not to say that I won't try it.. but i'd specifically like to experiment with beffering the chip amp first and getting the best sound that way. Then after that i can move on to a different project.

sardon

[sardonx]

Stocker,
On second though.. it might make for a cool amp to have a current boosted pre section and chip amp. Hrmph..