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Old 18th September 2012, 05:03 PM   #21
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Quote:
Originally Posted by Dave Zan View Post
Bob is not a real fan but does actually write that "many fine amps have been made" with complementary symmetry, so the comment doesn't have much bite as a joke. You haven't read the book I think? But no problem, I've had a little discussion with him about symmetry in circuits and he's not doctrinaire about it. So I will take this as a chance to recommend his book to the OP and anyone else who hasn't read it, to learn some of the subtleties in the subject.

Best wishes
David
Dear David

I have browsed the book and would agree that it could contain useful information for an audio enthusiast.

I am not doctrinaire about any particular "topology, ideology or methodology" either and I commend you for highlighting some advantages and disadvantages to the thread starter.

With my sarcastic remark I lost total sight of the focus of this forum - that of helping our fellow enthusiasts in making informative decisions regarding the amplifier he will eventually invest in.
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Old 19th September 2012, 08:00 PM   #22
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Default Krell Look Alike

This should be a much closer schematic of the amp you propose in the first post and yes it is almost identical to a Krell KSA50 from the 80's. I just chose transistors that came to mind. Resistor values are not optimised but this amp will work. And it is dual complimentary symmetry throughout and this is possibly biased as Class A.
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File Type: pdf KSA50_SIMILAR.pdf (14.9 KB, 28 views)
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Last edited by Nico Ras; 19th September 2012 at 08:04 PM.
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Old 19th September 2012, 08:33 PM   #23
Pano is offline Pano  United States
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OT remarks and bickering removed. Stay on topic.
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Old 19th September 2012, 08:51 PM   #24
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OT remarks and bickering removed. Stay on topic.
Thanks
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Old 27th September 2012, 05:38 AM   #25
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Hi Nico .
I did not propose any amplifier.. I was simply asking questions about its topology. The faulty scematic i posted is supplied by the manufaturer. The amplifier its self works fine . I am in the process of modding it into a dual mono . and i m also intrested in upping the bias . I am aware of the extra heat it will produce and have a choice of much larger heatsinks i can use.. Lower bias = less heat = less heatsink = less cost.. I am trying to remove some of the costing departments compromises.. Also the efficency issues do not matter as im only after 10 to 15 Watts class A.. Ps thankyou for the krell schematic

Last edited by madtecchy; 27th September 2012 at 05:49 AM.
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Old 27th September 2012, 11:44 AM   #26
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Madtecchy, if you are not going to use the amplifier for listening at high levels of pure class A, then upping the bias has very little positive effect.

In my humble opinion, bias it into class A for the average power you expect and do not be surprised that normal listening is less than 5 watts although peaks of +20dB and higher is quite feasible, but for these small duration peaks it can switch to class B operation. I can vouch that you would never hear the difference. Let's hear from other Class A enthusiasts what they think is the optimum class A power or bias.
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Old 27th September 2012, 01:51 PM   #27
AndrewT is offline AndrewT  Scotland
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If you start with the premise that -20dB is your average listening level and that -10dB is reached frequently then ClassA at the -10dB level might be a sensible target and any peaks that call for currents exceeding the -10dB level will be in ClassAB.

That would result in 5W of ClassA in a 50W ClassAB amplifier.
Since 50W of ClassAB is 28.28Vpk that would require ~+-35Vdc from the PSU.
5W of ClassA would require ~1.2Apk of output and the bias required would be ~600mA (total for all output device pairs).
Dissipation would be ~40W,
Certainly doable.
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Old 27th September 2012, 02:48 PM   #28
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Well explained Andrew.
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Old 27th September 2012, 03:20 PM   #29
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by Nico Ras View Post
Madtecchy, if you are not going to use the amplifier for listening at high levels of pure class A, then upping the bias has very little positive effect.

In my humble opinion, bias it into class A for the average power you expect and do not be surprised that normal listening is less than 5 watts although peaks of +20dB and higher is quite feasible, but for these small duration peaks it can switch to class B operation. I can vouch that you would never hear the difference. Let's hear from other Class A enthusiasts what they think is the optimum class A power or bias.
Take for example a 30W class-A amp. Upping the bias will usually improve the sound, even if we listen below 5W, or even if we simulate THD for 0.3 volt input (which usually is below 5W).

With high capacitance HEXFETs, it is easy to listen to the difference (5W signal) even if current has reached 5A. With BJT, my experience was that up to 1A it becomes less significan. With LATFET, often upping the bias has no noticeable sound improvement.

BTW I'm still searching an example of high end amp where LATFET is used and highly biased. I want to know how far they might go.
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Old 27th September 2012, 07:13 PM   #30
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[QUOTE=Nico Ras;3181036]Madtecchy, if you are not going to use the amplifier for listening at high levels of pure class A, then upping the bias has very little positive effect.QUOTE]

Hi Nico
Im starting to think against the Class A idea With the advice you have kindley given and Andrew T.s calculatons are for 8 ohm,s . Thing is im running 4 ohm speakers and at the moment i am very happy with them. At 8 Ohm's andrew calculates disipation at 40w per channel will that not double for 4 ohm . I would have to reduce the supply rails or maybe increase the number of output devices. Maybe this would alter the sound of the amplifier as it stands and maybe also not to my liking. Im a liker of even harmonics. class A i understand is mainly second hormonic distortion. And the topology of this circuit is also that way incllined..

Krell! i am pleasently surprised. so what im listening to at the moment is class A/B krell topology built in china with cheap components . I have changed the res capacitors for elnas i had lying around and im inputting audio directly into the power amp section So bypassing the horrid I.C based input selector and preamp. It sounds very nice . Maybe there are other ways to improve the sound of this amp . Im sure the mod to dual mono will help . And maybe some decent components . ok ill shut up now . By the way anyone that maybe intrested .. the amplifier in question is a Tangent Amp-50 integrated.
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