Class A gainclone/chipamp idea
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Steven
diyAudio Member

Join Date: Oct 2003
Location: The Netherlands
Re: Re: Re: Re: Class A gainclone/chipamp idea

Quote:
 Originally posted by millwood Create a big DC offset right on each amp, so each amp will be working on its positive half only (you can do negative half but for discussion purposes just stay with the positive half for now). One amp will have just the DC offset, the other the DC offset + the signal. The load will bridge the two outputs so you get nothing but the signal. Think of each amp as a transistor in the differential pair. One has the input signal on its base and the other doesn't. And the load is cross the collectors of two such transistors. Or one of those Zen amps with unbalanced input. The only purpose to have the 2nd amp is to get rid of the DC offset on the load.
Millwood,

I'm sorry but I think you misunderstand how the amplifiers are put into class A. It is not the voltage offset at the output of each amplifier that causes class A operation, it's the current offset that is important. In the case of the proposed circuit the voltage offset is only there to create the current offset in a simple way.

Your proposal to have one ouput at a certain DC voltage level and the other amplifier at the same DC voltage level plus signal applies the signal part to the load if the load is connected between both the amplifiers. That is correct. But none of the amplifiers will be in class A for that matter. The output currents will still be positive and negative, in both amplifiers.

Steven

 15th February 2004, 11:27 AM #12 Pjotr diyAudio Member     Join Date: Nov 2002 Location: Netherlands Hi Steven, Don’t see the offset problem either, the outputs of the two amps are at the same level as their inputs at no signal.. But even then, the output swing is much larger than the input swing. If this would cause problems any op-amp would have a problem. Think the whole thing works. Only disadvantage is you have a 0.11 ohm resistor in series with the load, reducing damping factor. But will that be a real disadvantage? Curious about the sonic results. Are you intending to build the amp Steven? Cheers
 15th February 2004, 12:18 PM #13 Steven   diyAudio Member     Join Date: Oct 2003 Location: The Netherlands Pjotr, This is still a damping factor of 80, more than many other good sounding amplifiers. I don't think it is a problem. Anyway, it is possible to lower the voltage offset and use an even smaller resistor, but then the circuit becomes more sensitive to tolerances. Or use even more amplifiers in parallel, the output impedance caused by all these summing resistors becomes smaller too. At the moment no intention to build, but who knows. Steven
 15th February 2004, 02:30 PM #14 Ed LaFontaine   diyAudio Member     Join Date: Feb 2004 Location: where the Appalachians rise from the Blue Grass similar to ? Steven, I'm fascinated by your proposal. I'm wondering if this approach has similarities to using tubes in class A applications and in push-pull for greater output. I've not much to offer as I have little experience. I'm reaching for something I have seen before in order to grasp the concept. Interesting. Please continue. Sincerely, Ed
 18th February 2004, 08:07 PM #15 mikelm   diyAudio Member   Join Date: May 2003 Location: England Hi Steven, It's interesting that we should come up with a similar idea at about the same time albeit with different implementations. I will not try mine for a while. I'm focusing on my class A amps my idea is here: http://www.diyaudio.com/forums/showt...129#post327129 I wonder who's cct will sound best ? mike
 18th February 2004, 10:37 PM #16 Steven   diyAudio Member     Join Date: Oct 2003 Location: The Netherlands Hi Mike, Yep, both circuits will run in class A, but the circuits are completely different. You use an external CCS, I use the other amp to force a DC output current. Your circuit uses mainly the upper transistor in both bridging amps, mine use the upper in one amp and the lower in the other and both amplifiers are in parallel, not bridged. Difference in sound? Who knows? Steven
 19th February 2004, 07:32 AM #17 Pjotr diyAudio Member     Join Date: Nov 2002 Location: Netherlands They are fundamentally different. Mike’s circuit basically works in SE class-A, Steven’s circuit works in PP class-A.
KlausB
diyAudio Member

Join Date: Dec 2003
Location: Dresden
Class A inverted Gainclone

Hi there,

I implemented a class A gainclone based on the idea by Steven. Each channel comprises of two modules of an inverted gainclone which are paralleled.

One module receives a positive biasing voltage at the non-inverting input, the other module receives a negative biasing voltage. These voltages induce a current across the two LM3875. The two 0.22 Ohm resistors set the idle current to 0.9 A. The arrangement biases the “upper” output transistors in one chip and the “lower” output transistors in the other chip into class A. Since the idle current is 0.9A, class A drive limited to 13W at 8 Ohm and to 6.5W at 4 Ohm.

Instead of a simple feedback resistor I used a T-network as recommended by Joe Rassmussen. The T-network with relatively low value resistors simply sounds better than one resistor of 600k I the feedback loop.

Instead of an input buffer I use an audio transformer. The winding arrangement and the 1k pot at the secondaries result in an input impedance of 4kOhm, which is suitable for most solid state preamplifiers IMHO.

The development of the amp followed three steps:

One module of an inverted gainclone per channel: The simple inverted gainclone delivered a rich and detailed sonic image in combination with my Mission DAD 7 CD player and my SL 600 speakers.

Paralleling the second module without class A biasing: Noticable differences were small. A little more control in the bass and may be a little more depth in the soundstage.

Paralleled modules biased to class A: The chane was clearly audible. Class A biasing added “body and soul” to the sonic image. It sounded less as a transistor amp but without loosing detail.

Listening tests were done blinded by two independent persons. In conclusion it can be resumed that Stevens idea of class A biasing is definitely worth a try.

Cheers
KlausB
Attached Images
 schematic.jpg (48.0 KB, 1969 views)

KlausB
diyAudio Member

Join Date: Dec 2003
Location: Dresden
I forgot to add an image of my amp.
KlausB
Attached Images
 chipamp.jpg (45.2 KB, 1868 views)

 26th February 2005, 02:01 PM #20 Steven   diyAudio Member     Join Date: Oct 2003 Location: The Netherlands Hi Klaus, I'm happy that it appears to be a useful approach. Congratulations. Steven __________________ The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973

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