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Old 18th February 2006, 10:43 PM   #21
Johan Potgieter is offline Johan Potgieter  South Africa
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Sith and Beppe61,

I suppose one can criticise just about every circuit available, even the good ones, even if in the latter case alternative suggestions become with greater frequency opinions or beliefs, the origin of which the commentator is often not too certain of.

Your given circuit is one of many so-called trouble-free/easy/simple options. But quite an improvement can be had with no real increase in cost. To keep this brief I would state a few general pointers to look for in future, rather than another circuit.

But firstly, you mentioned the very valid point of whether 0,01% of distortion is audible in the presence of loudspeaker distortion orders higher. Very low total harmonic distortion (thd) figures in amplifiers are very often of promotional value only. The point to grasp (and this is a whole subject on its own) is that it is the composition of the distortion that is important. High order odd harmonics, especially the 7th, 9th, 11th and 13th can generate such discordant artifacts in the human hearing mechanism, that it can be detected even at levels right on the threshold of hearing. It is thus totally a matter of what that 0,01% consist of. E.g. 0,1% distortion can not be objectionable when consisting mainly of 2nd, 3rd and no higher products. On the other hand amplifiers with thd of 0,01% can generate what is called listener fatigue if containing high order products. This as a summary, but it is this very matter that mostly singles out good semiconductor designs from the rest.

In summary, just the rest:
1. The best output configuration is a full complimentary pair. Dual emitter followers are 2nd, but especially Darlington types as per your circuit are, I fear, low on the list because of limited frequency ability.

2. Somewhat more difficult to see just from a circuit, is the basic requirement that the response of the amp before feedback should cover about the whole audio spectrum, so that feedback stability requirements [compensating capacitor between collector-base of the driver (also called the dominant capacitor)], operates as far up the audio spectrum as possible. Too severe compensation is one of the main causes of high order harmonic generation, etc. The circuit you posted originally does not fulfil this requirement.

3. A driver fed from a current source, an element of which is normally an improved way of temperature sensing over the transistor between the output transistors' bases, is again a better option than doing it with "bootstrapping" as in your posted circuit (that is another harmonic multiplier).

4. Being something of a rebel, I leave maximum currnet limiting out. They do have an effect on distortion, even if marginal, and there are other reasons. Fuses are simple but they can be non-linear if of low enough rating to be effective. So my simple philosophy: Do not short the output terminals! There is no need for that to happen in a domestic amplifier, anyway.

And so on. I may not have done you a favour because I cannot begin to go into any of these details without totally exceeding my welcome. Much of this can be found on web-sites; I am also not unwilling to post basic details to you privately in a very simple document I have prepared. (Only I will have to restore that on my PC; my previous one with a lot of stuff on it was stolen just before Christmas!)

The bottom line is then that your better circuits will contain the above topologies, without necessarily costing more.

Good hunting!
Regards.
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Old 19th February 2006, 12:06 PM   #22
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by consort_ee_um
Hi Beppe
1) The output devices, because they are slower than the rest of the electronics tend to dominate the frequency response and higher frequency distortion characteristics.
The other electronics decides the low frequency distortion and power supply rejection etc.
2) I think the easiest and best route for you is to try removing the output protection circuits of your Samson amp.
This could be as simple as snipping the lead at one end of a resistor.
If you email me the circuit I will identify where to make the cuts.
I will try to do it so you could restore it easily in the unlikely event that it does not do the business.
I will also check if the Samson design is robust enough to allow this.
Dear Sir,
thank you sincerely for your very kind and much valuable support.
I hoped very much in a your positive reply because before starting any search for a new power amp solution I would very much like to check if with a little mod on the amp I have at hand I could get some nice improvements, as I said.
1) Very very interesting indeed.
The fact that I could get a sound as nice as the one of the Albarrys from a very simple DIY kit is even amazing for me.
Now I understand that kits can be so good to be on a par with very fine commercial products.
2) Thank you very much again for your very kind support in my experiments.
I will scan the circuit as soon as possible and I will send it to you for comments.
As I said apart from the weak bass I found the amp quite good sounding after all.
But the bass response is so weak that it seems that a low filter is engaged.

Thank you so much again for your kindness.
Kind regards,

beppe
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Old 19th February 2006, 03:40 PM   #23
richie00boy is offline richie00boy  United Kingdom
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is this any good?
Beppe, I thought you were currently using an Adcom amp, not a Samson one?
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Old 19th February 2006, 05:15 PM   #24
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by richie00boy
Beppe,
I thought you were currently using an Adcom amp, not a Samson one?
Dear Friend,

you are right, I am using a old GFA 545, but I have also a Samson Servo 260 that I bought on the basis of very favourable reviews.
After that, looking for a stronger bass, I bought the Adcom.
A little better but still not up to a enjoyable performance.
The music is without the necessary body.
"I still haven't found what I am looking for".
Well I have (the Albarrys) but they are so difficult to find used.
I would like a similar performance.

Thank you.
Kind regards,

beppe
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Old 6th October 2006, 09:49 PM   #25
mikeks is offline mikeks  United Kingdom
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Default Re: is this any good?

Quote:
Originally posted by sith
found it over the internet is it worth to try?
It's exceeding awful!
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Old 7th October 2006, 07:57 AM   #26
beppe61 is offline beppe61  Italy
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Default Re: Re: is this any good?

Quote:
Originally posted by mikeks

It's exceeding awful!
Excuse me Sir, have you actually built it?
How would you rate its sound?
Someone here reported about a not so awful performance.
Please report your findings.

Thanks and kind regards,
beppe
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Old 15th November 2006, 10:20 AM   #27
DanDini is offline DanDini  United Kingdom
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I've built this kit to use as a sub amp and have had nothing but trouble with it blowing up. It's fine up to a certain point but once I crank up the volume it suddenly makes a very loud hum from the speaker and then the output pair die. I'm using the exact components from the schematic. I'm interested in the suggested removal of the current limiting components, but am concerned that will make it even more keen to blow.

Any advice welcomed.

If you couldn't already tell, I'm a complete novice to this whole subject.

Many thanks,

Dan
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Old 15th November 2006, 11:51 AM   #28
Workhorse is offline Workhorse
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Default Re: Re: is this any good?

Quote:
Originally posted by mikeks


It's exceeding awful!

Mikeks nothing more than an artist of wonderful explanations
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Old 15th November 2006, 12:31 PM   #29
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by DanDini
I've built this kit to use as a sub amp and have had nothing but trouble with it blowing up.
....
Dan
Dear Mr Dan,
thank you very much indeed.
I hoped that this simplicity were a good basis also for a nice sound.
I understand it is not.

Kind regards,
beppe
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Old 15th November 2006, 12:34 PM   #30
DanDini is offline DanDini  United Kingdom
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Probably should have said, it's going into a 4 ohm load...
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