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In Reply to: RE: All preamps are Class A -nt- posted by Ruinthere@sbcglobal.net on February 27, 2010 at 14:20:38
I am afarid you are incorrect. The vast majority of preamps are single ended, even if they use opamps. Single ended has to be class A. The only preamps that could pontentally be AB are differential ones, and no one is going to go to the trouble and expense of designing a differental preamp, and then skimp on the power supply to the point were they need to go class AB. There is no class AB preamp that I am aware of, and if there was, it would be a sonic and marketing flop.
Dave
Follow Ups:
No really Dave too many of the mid priced used op amps that weren't running Class A. Sometimes you can bias into Class A by tying a resistor or FET current source to the usually negative rail around 2ma. The LT1010 app note mentions that you can bias into Class A. This is the driver used in the preamp stage of the Adcom 565 I believe it is. Unfortunately it's not uncommon to use them. Again in mid level. Now when you get into higher priced discrete amps it's different. Op Amps not so much.
DagW00d
It is true that I am not that well versed in op amps. The LT1010 is a buffer, desinged to work in a feedback loop. I would think that that would sound terrible. I have heard the Adcom (I used to sell it). It did not sound bad to me. Also, the idea of a push-pull buffer operating is class AB, sounds quite bizarre to me. There is also nothing mentioned about this it the chip information. Can you point me to some information on this? I am very curious.
Dave
Dave,
From the LT1010 Application Information found here;
http://cds.linear.com/docs/Datasheet/1010fd.pdf
From page 10
"This shows the buffer being used with a wideband amplifier
that is not unity-gain stable. In this case, C1 cannot be used to
isolate large capacitive loads. Instead, it has an optimum value
for a limited range of load capacitances.
The buffer can cause stability problems in circuits like this.
With the TO-220 packages, behavior can be improved by
raising the quiescent current with a 20Ω resistor from the
bias terminal to V+. Alternately, devices in the miniDIP can
be operated in parallel.
It is possible to improve capacitive load stability by operating
the buffer ....class A.... at high frequencies. This is done
by using quiescent current boost and bypassing the bias
terminal to V– with more than 0.02μF."
So it doesn't operate normally in Class A it would appear it needs to be rebiased to do so. This is a very common practice with op amps. You tie the output to one of the supply rails usually negative I believe.
I'm pretty sure the LT1010 was better than the BUF 634, also National has a new series of Buffers LME49600 They rave about them in their app notes and Joe Curcio has an app note from National where he shows some high end phono preamp designs.
App note for Preamps: http://www.national.com/an/AN/AN-1651.pdf
These Buffers are Diamond Buffers I read it somewhere in the App note but my eyes are so coss right now I've been dong my 1040 online . The LME49600 is supposed to be a very capable driver of 32 Ohm phones. I have some I just have to build one. I wish I could get the app board form National but they evidentally want abou $150 for it.
DagW00d
I was completly unaware that op amps could operate this way. It makes me want to shy away for op amp preamps! (But I know some have good reputitatons.) I am fairly sure that my Yamaha C-60 preamp had op amps in it and it did sound good. My Yamaha C-2x does not and it sounds quite a bit better. The Adcom you had mentioned did sound good when I used to sell Adcom, but not good enough to make me want to own it.
Thank you for the information.
Dave
Dave,
There is only one Op Amp I'm aware of that has a Class A output. Thats an AD744. Walt Jung uses it in his "Op Amp Sandwich" which has a 744 feeding a AD811. There's lots of info on it. You take the output from pin 5 of the 744 (which is the comp pin) and feed it into the input of the AD811 which is a video op amp. This way you have the current drive also. Read this article by Jung on the Composite Op Amp.
http://waltjung.org/PDFs/ADI_1992_Seminar_Audio.pdf
There is an 4 part article in Audio Express back in 2004 or 2005 where they did a rebuild of the Adcom 565 by Gary Galo 11/03, 12/03.. 01/04, 02/04 gee that way you need to buy the 03 and 04 series. Pretty comprehensive rebuild effort.
I have a Yamaha C50, that is in bad shape relay problems, and a C4 which has a slight problem also but should be easily repaired just have to make the time to do it. I haven't even repaired my Sansui CA-3000 which I think is a killer preamp.
DagW00d
I have a stack of preamps on the workbench. My Yamaha C-4 has some distorion problem I have not tracked down but My C-60 is working fine. I don't have a 565, but I do have a GFP-1. I tracked down it's problem to a push button swich. I just need to drill it and spray some deoxit in it.
That Sansui CA-3000 is a sweet piece. I remember seeing one at Dixi Hifi when I was a young teenager. Your going to use a lot of Deoxit on that!
I read the article and it was very interesting. I like the idea of DIY with opamps. I have thought about building a Gainclone. That looks even more interesting.
Dave
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