Sound Quality Vs. Measurements

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Bog, any input..? we are not here to tear each other down.... :rolleyes:

Input:

As you can see there`s every "get better performance" copy/paste trick you can see in the schematic, you could think of (or find on the web:)

1. The triple OPS as shown in schematic will probably oscillate.
2. Used Vbe multiplier will NOT track temperature well on a triple OPS.
3. 10 Ohm base stoppers?
4. 40A fuse for a speaker!!
5. SOA for driver bjt`s ?

You want more?
 
Only if the experts are really expert.

The claim that I sort of grafted the amp from many schematics found on the Internet is not something anyone knowledgeable would ever say, because that simpy does not work that way. I did use a well known topology, to be sure, as most of us do, at best adding a twist here and there. A new topology surfaces about once in 10 years, and even so, it is usually an evolution of another one.

I never claimed to be an expert, and I don't see myself as one. To me, "expert" is a tall order few can deliver on. At the risk of forgetting someone, the real experts here as I see things would be John Curl, Damien (1audio), Thorsten (not quite here, but lurking) and a few others, people with impressive credentials as long as my arm, and certainly people I look up to to learn from. I can PERHAPS show them a trick or two, but they show me entire principles, a different league altogether.

For confirmation, just look at Damien's list of suggestions and think about it; not just finer points here and there, it's capital stuff. And you haven't seen Thorsten's list, that alone will keep me busy for literally months to chew it up. But rest assured, all of it would make for a better device, one which I dare say would push the amp somewhere into the lower tiers of the true High End at a still very reasonable price.

Besides, I'll still be struggling with it for a few days, and I feel confident that if they were doing it, it would have been finished by now, at least to the point of testing an actual prototype. A man has to know his limitations: on the other hand, being self-taught, I have come a long way, while they are trained engineers.
 
This is my last stand. This is where I call it a day.

1audio and Thorsten have provided good lists of suggestions. The main benefit from them for me is that I now see just how much further this basic design can be evolved to.

Some of their suggestions I adopted "as is", or basically but in a modified form. For example, the input stage CCS is still a 3 transistor job, but they have been completely turned about. Basically, it's a cascode now. But the most interesting part of it is the one lone FET. If I use a plain resistor to terminate the circuit, the amp has a DC offset of -52 mV. With the FET as shown, the offset is reduced to 0.7 mV.

Thorsten pointed out that the BF trannies I use could be replaced by something else. That's true enough, instead of BF 471/472, Wayne could use 2SC3503/2SA1382, for example. The last performance stand is in fact set by one of them, and they do seem better in Data Sheets. It also turned out that they are almost exactly what the BF 720/721 are, especially regardin the C factor, so I checked using them as well. Nothing drastic happened, except that the overall distortion figures dropped a bit - otherwise, the circuit stayed stable in all respects.

Excuse my tendency to throw in bigger caps than really necessary, but there seems to be something wrong with my DNA, so I throw in larger caps. I would NOT advise making them larger still because larger still will also be slower, and that is not really wanted. You hear that, Wayne?

ML means multilayer, and silver mica will do just fine too, TA means tantalum, MKT and MKS are Wima series denominations, do as I wrote and you will be doing what some good names do (e.g. Kensonic Accuphase) and what the Germans have been doing forever.

So, this is is the last variant. Over to Alex now, but I would advise Wayne to consult regardig mechanical outlay, because Wayne is the one who has to decide what it will externally look like. I can help, but I can't decide, I don't want to irritate the lady of the house, and I want even less to answer the doorbell and find a SWAT team looking for me. :D
 

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Nice design.... will you be starting a CFA design soon? What about Current-drive?

Thx-RNMarsh

Thank you, you are very kind, but now that "I've seen the light". meaning now that I have a view of just how far this can be evolved to, I am simply pleased I did the job, but not elated. I gues it's that proverbial bug which pushed us onwards.

"CFA" - I assume you mean current feedback? No, I have heard a few samples of the ilk and was not blown away. In all honesty, they sounded just like any good amp design should sound, but gave me no incentive to go down the current feedback path. Just like AC and DC amps - theoretically, DC amps should soud better because there are no caps in the direct sugnal path, and hence no phase shifts they rend to introduce, but to this day, I cannot say for sure that this is quite so.

My personal yardstick is Harman/Kardon's Citation 24 power amp; it's an AC design, but is the most neutral sounding amp I have ever heard to date, bar none. While my Karan Acoustics KA-i180 integrated amps is a DC design, costs like 3 Citations, and while truly excellent and still the best INTEGRATED amp I have heard to date, it still lags that little bit behind the 24 in absolute neutrality.

If you go back some, you'll find my Centurion project, which I am about to finish up. The design embodies everything I think and feel about power amps, insomuch that it uses all the principles I swear by (wide open loop bandwidth, low global NFB factor of no more than 26 dB max, curret capability ultimately limited only by the PSU line at least in peaks, high overall peak power, etc), plus uswer flexibility. This means that yu as the user get to choose what you feel is best for you - solid state protection, or fuse protection, or both, with a 3 point power status indicator (green LED for signal present, orange LED for nominal output, red LED for near clipping power), full overheat and excess DC protection, etc. Nominally 100W/8 Ohms, it should be able to peak at just over 180W/8 Ohms before clipping.

Due to oversize caps offered locally, I needed to order new caps from Germany, and I expect then to arrive soon. Then I finish filling up the boards (there are 2) and get down to the nitty gritty of it, but on a live model. After that, I will need to develop an alternative output device board, currently housing 3 pairs of 200W devices, to more pairs for the real freaks out there.

I will put it in public domain, that was the idea from the start. After that, well, I will be needing a preamp, won't I? :cool:
 
more amplifier 101

Continuing with the theme of understanding conventional amplifiers I am thinking about conventional linear op-amp type amplifiers with negative feedback, in particular the non-inverting unity gain buffer with 100% feedback. It seems the voltage gain is 1, that is, 1, to a high degree of accuracy, unlike a typical emitter follower circuit where the gain is slightly less than 1. This accuracy seems somehow intrinsic in the circuit, it is not subject to variation, so far as I know, but rather precise and dependable. I believe that if I were to take say 10 individual op-amps and substitute them in one circuit they would all show gain 1.0000 within the accuracy of my ability to measure. If this is correct it is a remarkable achievement.


The way I understand negative feedback, in this case of the unity gain buffer, the feedback is 100%, equal to 1, and this feedback is "subtracted" from the input signal. However a simple subtraction would leave a sum of zero not 1, so perhaps the opamp input differential amplifier, where the "subtraction" occurs, has a gain of 2, is that right?


I wonder how this is done using a mass-production process where selection and precision trimming are not possible. I found this article on Wikipedia, Differential amplifier - Wikipedia, the free encyclopedia the paragraph about current mirror says this:

differential amplifier collector current mirror

The collector resistors can be replaced by a current mirror, whose output part acts as an active load (Fig. 3). Thus the differential collector current signal is converted to a single ended voltage signal without the intrinsic 50% losses and the gain is extremely increased. This is achieved by copying the input collector current from the right to the left side where the magnitudes of the two input signals add. For this purpose, the input of the current mirror is connected to the right output and the output of the current mirror is connected to the left output of the differential amplifier.

The current mirror inverts the right collector current and tries to pass it through the left transistor that produces the left collector current. In the middle point between the two left transistors, the two signal currents (current changes) are subtracted. In this case (differential input signal), they are equal and opposite. Thus, the difference is twice the individual signal currents (ΔI - (-ΔI) = 2ΔI) and the differential to single ended conversion is completed without gain losses."


Is this current mirror structure perhaps the source of the precision unity voltage gain?
 
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