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Thanks for the responses to my inquiry regarding true class A receivers below.
I should'nt have limited the question to receivers and integrated amps, so now, what are the vintage true Class A amps and preamps? And which were the ones that sounded as good as class A is reputed to be?
TIA
Follow Ups:
Read Nelson Pass's article on power supplies. I linked to it below. In it, he describes how a class A amp should weigh one pound for every two watts. Of course, if you add more stuff, as in a receiver, it will need to weigh more per watt. I just saw a post about a JVC 40W "class A" receiver. Bet it doesn't weigh 80+ lbs. There is more class A talk in some of Pass's other articles, so check them out. Also look at pics of Pass's products. They'll give you some idea of the heatsinking required for high power class A (though some manufacturers have assumed clients won't use much continuous power, so they provide sub-par heat dissipation. See the old Musical Fidelity integrated). I keep meaning to try some class A solid state. I've got a couple of tube amps to get through first (one of them I need to rebias into AB to be a dedicated woofer amp), but I'm thinking of building the De-lite when I've got the chance.
Paul
Whenever I read the word 'true' before the term 'Class A' these days, alarm bells start ringing in my head.
The "true class A" hype is a gimmick whipped up by the unscrupulous marketing departments of guitar amp builders and has spread like a virus through the industry.
Class A is a definition, devised by human beings, to describe a state where the active devices, be they tubes or transistors, are always conducting some bias current. They are never "off." I've had to think about Class A over the last few days as I've been trying to fix the vibrato circuit on my Vox Continental Organ; it's making a popping sound because it's emitter follower output doesn't seem to be properly biased. It goes into shutoff on the negative swing of the ~6.8 Hz vibrato signal, and the transition from off to on creates a noise that gets coupled into the signal chain. It needs to be operating in Class A, but it isn't.
On the other hand, a guitar player friend of mine told me he had a "true Class A" guitar amp that gets 18W from a pair of EL84s. It's a nice sounding guitar amp, but it's not Class A. The fact that it's a nice sounding amp has little or nothing to do with the buzz words used to market it.
I listen to vintage Class AB biased tube amps all the time. They sound fine to me and most everyone else who hears them.
Snake oil salesmen love to muddy the waters. Beware.
David
Not to argue, But an awful lot of solid state preamps have class AB outputs, or Op Amps that aren't running pure Class A. It would sure be nice if they did but there are many.. mostly middle of the road including The Dynaco Pat series. Vacuum Tubes on the other Hand are almost ALL Class A.
DagW00d
As was mentioned in reply to my Yamaha post on receivers they (Yamaha)do make Integrated Class A Amps.
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
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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