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Old 26th March 2011, 07:33 PM   #1
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Default Basic 50W hifi amp -for beginners?

This amp is designed to provide high quality sound yet be fairly simple to set up and build. Unfortunately, perhaps, it uses quite a few transistors, so isn't the simplest possible amp, but does have some good features.

The overall architecture is based on the one which Doug Self expounds. It differs in a number of respects. First, the input stage is degenerated with quite high value resistors - 1k. This reduces the gain to a measly 1 mA/V (actually half this but is multiplied by two in the current mirror). The benefit is that the input stage won't "clip" or cut-off for any input signal up to 1V input, so should keep transient distortion to a minimum.

The low gain makes it essential to use a Darlington VAS pair, in this case two PNPs, driven by a PNP mirror. The reason for the mirror is that the input impedance of the Darlington VAS is so high even a 10k resistor degrades the gain significantly, but with a mirror the dynamic impedance is around 1M.

The VAS transistor, a BD140, operates at a fairly high current- 15 mA compared with the usual 6 mA or so- for the same reason as the input degen resistors - so that the VAS does not cut off in a transient. The emitter resistor is split into two so that the base resistor is connected to the tap point. This allows us to keep the base-to-emitter resistor ratio at 1k to 10 ohms, cutting the gain by about half, but reducing Early voltage distortion by raising the effective Early voltage, otherwise a ratio of 1k:20 would have given an unnecessarily large gain reduction. The additional 10 ohms, however, is included in the short circuit protection with a simple current limiting transistor on the VAS.

The compensation capacitor need only be a small value at 33 pF because of the large input degen. To aid stability, 100pF sprog stopper capacitors are used across the driver collector-base junctions. The compensation capacitor is connected between the output rail and the feedback compensation point (Darlington VAS base) which reduces crossover distortion remarkably. However, it is not quite as stable as connecting the capacitor to the collector of the VAS, but several of these circuits have been built and at worst may need 150 or 220 pF stopper capacitors. AS a result the THD figures are around 0.02% at 20 kHz and .005% at 1 kHz. This is not the headline THD performance which the so-called "blameless" amplifier might achieve, but it offers this performance with transient intermodulation distortion effects minimised while using a fairly safe compensation approach.

Short circuit protection is passive current limiting to about 5A in the output transistors. The amplifier is intended to operate with 8 ohm loads and a 2A fuse in each rail is a good idea.

Quiescent current should be about 50-100 mA in the output transistors. It could be set with just two fixed resistors in the bias stabiliser, but if the current is significantly outside these limits a parallel resistor can be used to adjust the current as needed. Alternatively the base to emitter resistor in the bias stabiliser could be a 680 ohm plus 470 pot to allow the current to be tweaked.

The output transistors are the very popular 2N3055/MJ2955 pair. These should be the epi types (all PNPs will be; the NPNs should be) as these have a higher frequency response than the original hometaxial RCA devices. The latest from ON semi also have an SOA almost as good as the original.

John
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Old 26th March 2011, 07:55 PM   #2
lineup is offline lineup  Sweden
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Good project.
We can come a long way with this model.
And with suprisingly low THD.
Thanks for schematic
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Old 3rd April 2011, 10:57 AM   #3
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Hi Lineup.
Thanks for your comments. Indeed the performance of such a circuit is to be heard rather than seen. I have found that its quality is surprisingly good. I would go further and say that it still sounds OK even if the sprog-stopper capacitors are eliminated and the Miller capacitor moved to the normal position (collector from the VAS) but the THD at 20 kHz rises to about 0.2%. It still has a low THD at 1 kHz though. This version would be recommended for anyone needing an unconditionally stable amplifier - such as when using transformers for line matching, other loads (motors) etc for non-hifi use.

I would also ask contributors to diyaudio for their listening impressions of dual-slope Miller capacitors (the ones with the split miller and resistor to ground). I have found these to be highly strange. It's hard to describe but a sort of "artifically crisp". However, evaluating simulations reveal that the leading edge transients can be completely wrong: the overall amplifier response is delayed, but when the edge "cuts in" it over-compensates and goes faster than the actual signal. Ears were not lying. My version of the blameless sounds better!


John
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Old 3rd April 2011, 01:09 PM   #4
Bigun is offline Bigun  Canada
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Am I reading this right, C4 and C5 are the compensation caps - loading the VAS collector ?
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Old 3rd April 2011, 01:43 PM   #5
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And your point is?
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Old 3rd April 2011, 03:41 PM   #6
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by john_ellis View Post
And your point is?
.. to understand your design approach and where it differs from what I've heard or seen before. In my designs I've found the amount of compensation to have a noticeable impact on the sound of the amplifier quite unconnected to stability concerns. I've only ever 'played' with the usual Cdom type compensation. Everyone seems to hate Cdom but they always end up using it, you've gone a different route. I'd be particularly interested to hear what your impressions are on any changes in the sound of the amplifier as you adjust compensation with the scheme you have employed.
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Old 3rd April 2011, 04:14 PM   #7
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Hi Bigun
I first came across this scheme in an article by Cherry. I have used it ever since.
While the overall capacitive burden on the VAS is about the same as a conventional Miller (150-200 pF) this is not translated into base current needed by the input stage.
However, the high degeneration in the input stage means that in practice (as Self pointed out) the overall effects between a 330 pF Miller (say) and 100 ohm degen is the same as a 33pF and 1k.
What the scheme does have though is reduced crossover distortion, because the current needed in the input stage is only that needed by the VAS darlington pair (capacitor current divided by the gain) rather than the input stage having to provide the current as well.
And because the transistor stages remain conducting there should be no transient distortion.
I have not tried different capacitors. I generally use the smallest sprog stoppers necessary for stability.
The scheme seems to work best with the slower transistors (2N3055/MJ21194 etc) - the newer ones (MJL3802/1893 etc) show oscillations in the crossover region (internally) unless really large capacitors are used, in which case something else has to be done.
John
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Old 3rd April 2011, 09:39 PM   #8
Bigun is offline Bigun  Canada
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I'm trying to understand what you mean by the way that crossover distortion is reduced. Are you referring to the non-linear input impedance of the output stage as seen by the VAS and how this is transferred back to the input via Cdom in a Lin topology ??? I have a feeling I'm being a bit dense here...
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Old 3rd April 2011, 10:29 PM   #9
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Default There is some mistake Ellis, in your tittle, the amplifier

is basic, but it is not really for beginners..more for seniors or lucky beginners.....it is the end design i have made and have listened too..anything beat these Blameless style design...no chance for others.

Not a try for beginners...they will find the ONE in their first try....lucky guys!

If beginners decide to build..well...they are going the the best possible sonics and they will feel very deceptive in further tries.

As you know, John, despite i respect your knowledge a lot, i have build a lot of units and no chance to anyone of them in these last fifty years of activity..absolutelly no chance..it is my final design, my final amplifier..i can put power, or increase bass, or different sound stage..but globally it is unbeatable, and unbeatable design.

My own reaches 0.018 of THD in 20 Kilohertz just tuning the miller..anything more is needed...and yours had that because low speed output transistors...give a try in others more modern to see what will happens.

The harmonic distribution is lovely, less than 0.002% is very easy with these circuits.

Lucky your beginners...i am in the near the end and i found that after half century.... beginners believing in you, in me, in Doctor Self, Ostripper and others that supports this kind of schematic, will be very lucky and will jump to the happy ending...starting with the best possible unit they would ever listen too.

All these Blameless...the sons of blameless, are awsome...do not matter how much mistakes you make, bad decisions taken...the one will sound great.

regards,

Carlos
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Last edited by destroyer X; 3rd April 2011 at 10:32 PM.
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Old 3rd April 2011, 11:06 PM   #10
wahab is offline wahab  Algeria
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There s some things that don t please me in this schematic,
but if it works, why not ?.....
Has this amp been already built ?....
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