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Old 15th February 2003, 08:02 AM   #1
stigla is offline stigla  Norway
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Default Simple Class A design. Interesting?

Hello guys!

I was scrolling through some old (local) Hi-Fi magazines the other day, and I stumbeled over this circuit. I is states as giving 12W Class A in 2 ohms (2,45A bias current!). What instantly cought my attention was the fact that no NFB is used! I have never seen a transistor amp without it! The article stated that NFB could be used, but the sound was so much better without.

I really like the symmetry and the simplicity of this thing, so I think I want to try it out some time.

This is a pretty old article, so I am really out for some comments on this design, will it do a propper job? Possible improvements (bypassing the Vbe transistor from collector to emitter perhaps?)
Some of the resistor values seems a little strange to me also. (4.7ohms collector resistor of the driver transistor...)

Well, so I just kindly say thanks in advance!

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Old 15th February 2003, 08:30 AM   #2
halojoy is offline halojoy  Sweden
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Default okay

How much voltage gain do you get
without global feedback?
Gv_openloop=20 ?
I would still think that over say 2-4 watt
you will get distortion like 0.5-1%.

So I would use a global feedback.
Because of the low open loop gain,
the feedback would be very low, so it would be harmless,
maybe even ineffective?

Anybody can do some simulation of this circuit?

/halo - likes this amp - no unnecessary junk
and low (controlled) overall gain.
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Old 15th February 2003, 08:33 AM   #3
stigla is offline stigla  Norway
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Yeah, I don't know the numbers, but the article really pointed out the fact that the first stage ie. the FET's where run at 1-time amplification, and thus not needing a cascoded stage to minimize the miller-capacitance....
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Old 15th February 2003, 09:40 AM   #4
SvErD is offline SvErD  Norway
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That's my amp!
It's designed by Knut Řystein Lauvland(DOXA) and published in the norwegian magazine Audio in the early 90's.
A very good sounding amp indeed if. You will not regret it if you build it.
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Old 15th February 2003, 09:53 AM   #5
stigla is offline stigla  Norway
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Holy Cow!

Well, that's something! Yeah, I've got that Audio article. It's really nice, and I got a copy of it, had it for years, but forgot about it until I found it reacently! I hope it's OK that I re-drew and posted the schematic

Do you know where to get those output transistors? (i norge?)

So, this is all kind of funny Reading the article once agian sort of sparkled my mind, in that same way when I first read the Lynn Olsons Aurora amps article at Aloha Audio. They said lots of the same things.


Stigla!
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Old 15th February 2003, 10:21 AM   #6
Hamish is offline Hamish  Australia
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looks very interesting....
i trying to decide which class a will be my first, and i definitely like the look of this one. so now it's between this and a JLH.

SvErD,
anything else you could tell us about the sound? what size (roughly) for the heatsinks?

Thanks
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Old 15th February 2003, 10:28 AM   #7
stigla is offline stigla  Norway
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Quote:
what size (roughly) for the heatsinks?
...article-wise it is stated at 0,4 C/W as a maximum, preferably less. Each channel will give around 90W of heat.

I can draw a schematic over the suggested PSU if anyone would be interested, or maybe SvErD has some tips here.


SvErD:

What's the power in 4 and 8 ohms? Guess it's PSU related though...
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Old 15th February 2003, 10:39 AM   #8
Hamish is offline Hamish  Australia
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i'll probably go with a capacitance multiplier PSU. 3/4 of the money on class A seems to go on caps and heatsinks . until i can get them as samples, i'll cut corners wherever i can!
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Old 15th February 2003, 11:02 AM   #9
SvErD is offline SvErD  Norway
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I don't know if the transistors are available anymore, but aparently you can use 2SA1216 og 2sc2922. http://www.brukthifi.no/cgi-bin/yabb...num=1043658997

The sound is exceptionaly good. I'm no good in describing sound, but some keywords: very pure sound, deep and rock solid bass. One of the first impressions I had when I first heard it was how quiet it was when the music stopped!

My heatsinks are 0.3 C/W
trafos are 2 x 14 300va
caps 230000uF
Power is ~38w/4.5ohm
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Old 15th February 2003, 11:03 AM   #10
SvErD is offline SvErD  Norway
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Inside
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