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

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something like this
 

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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
 
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 said:


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.
 
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
 
Just remember you also want to keep an eye out for instability. The buffer transistor itself has a finite bandwidth. That'll throw a zero into the system, possibly someplace you really don't want to see it, so you want to compensate for that in the feedback network. I didn't bother doing that since the 2n3904 was way faster than the TL072, but that might become an issue with faster opamps and bigger transistors.


Francois.
 
sardonx said:
I might try this sometime down the line...anybody dunnit?


Old thread but I built a current boosted TL072 amp back in '88.
It sat front and center in my livingroom and it was used DAILY
till Dec 2004 as my main amp, finally being replaced with a 5.1
Yamaha!

Fed it originally with a little JVC ghetto blaster. Then later I
bought a remote controlled kenwood mini system to drive
the amp. (Mistake BTW. The sound of the JVC was noticably
cleaner than the overpriced Kenwood.)

So yes I done it. 2N3055 output transistors. Fed with some
sort of darlingtons, driven by the TL072.
 
I had a commercial amp that was I believe an opa2604 boosted to 25 watts per channel with 'zero feedback' and Class A. To make the long story short, saying that i was totallly unimpressed with the sound is an understatement. Especially because almost every single review 'zine' out there raved about it like it was the only amp ever. In reality though, it sounded no different then a receiver. Typical solid state sound. Couldn't tell the difference between it and a 1980's bryston. Maybe a little but that was probably due to power output differences. That should tell you something. Same goes for the bryston stuff... so no offence to any bryston or monarchy fans.
 
Cool. This thread lives.
My current boosted amp was 3 channel originally.
One 100 watt RMS for Sub.
Two 50 watt RMS for stereo.


to Upupa Epops:

I'll assume the sr-buf634_00.pdf sound refiner is to make my
Kenwood sound better.

What does it do technically speaking? Sell me cause I'm not
sold.
 
If you use similar circuit beside your CD player, output circuit will very low loaded ( input impedance of buffer is 1 MOhm ), so distortion of output circuit will be much lower than by normal condition. If you connect at the end of cabel ( on the input of amp ) the same resistor like is at output of buffer ( 50 Ohm ), summary gain will be unity ( the same like by normal interconnect ), but signal transmission will be run through on much lower impedance ( 25 Ohm ), so connection get bigger immunity against hf disturbing. By this solution are results shocking, belive or not. :cool:
 
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