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Old 12th October 2012, 08:27 PM   #7421
dvv is offline dvv  Serbia
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Originally Posted by AndrewT View Post
I call it passband adjusting.

I firmly believe that the passband of a power amplifier should be determined by a pair of passive filters at the input to the power amplifier.One decides either by listening, or by measurement, or maybe a combination of both, what the required passband should be to let the audio signal pass through without corruption. I set my filters quite a bit wider than F(-1dB) from 20Hz to 20kHz, but that is my choice. Each builder/designer/manufacturer can choose what they want as the passband.

I do not use active filtering inside the amplifier's global loop to determine the passband of the power amplifier.
THIS is what I've been looking for, a clear personal opinion - thank you, Andrew.
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Old 12th October 2012, 08:39 PM   #7422
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DVV . I find as I am sure you have that playing with the input capacitors gives you plenty of options ( and LTP cap ) . My instinct tells me that 3 Hz - 3db and 200 kHz - 3dB should be aimed for . It is then a simple task to add filtering if required . I think you yourself said it should be inside the pre-amp if anywhere ? I think you will find what I have said contradicts what you might perceive my preferences to be . I have always preferred wide bandwidth amps ( makes piano sound real , don't know why ) . Sometimes the typically British way suits very well . That is perhaps 20 Hz to 20 kHz - 1 dB ref 1kHz . I have a very early Quad 33 / 303 which is my everyday amp . Mostly I listen to MP3 style stuff via TV . It never is less than pleasing . I have learned to like it very much . At first I used it because it was the only bit of hi fi I didn't sell when feeling the pinch . Now it is my honest friend . It lacks detail is my only criticism ( much more detailed than any would guess ) . I have adjusted it very carefully so as to use it's capabilities well . The 33 is not the best , even so mostly that is about matching and not disaster of performance by bad design ( my original assumption ) .

You might have detectors that kick in filters if excessive ULF or HF detected . These could be reset and have a warning light . You might also have PA mode .
Nige, I don't believe in special modes of operation, I believe in getting it right in the first go. Sit down, think what would be a nightmare for the amp and make sure that even it comes his way, he survives and does not become unstable, does not overheat, does not need to push out unholy quantities of current and/or voltage (meaning choose wisely according to your needs), and so forth.

Where the industry gets away with as little as one pair of power trannies, or in better sample two pairs, or in very good samples three pairs, I use 4 pairs. Sure, it's more complicated, more matching to do, and it surely costs money, but it pays with dividends in the end. If I can get away with two toroids rated at say 300 VA, I will likely use two 500 or 600 VA toroids.

All the while KNOWING full well that because of the easy load my speakers present to the amp, and their relative effciency, I will never get even close to its limits. It's not needing the resources, it's the pride of owning them. Of KNOWING that it can do a hell of a lot more if ever required.

So I try to keep it as simple as I possibly can and know how to do. As few switches, detectors and sensors as possible, but if required, don't think about it, just do it.
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Old 12th October 2012, 08:41 PM   #7423
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My general feeling is, all depends. If I design an amp for consumers I will make it as well "fool proved" as possible. If I design it for myself I would make it to add as less changes as possible, including phase shifts on infra low frequencies.
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Old 12th October 2012, 09:09 PM   #7424
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My general feeling is, all depends. If I design an amp for consumers I will make it as well "fool proved" as possible. If I design it for myself I would make it to add as less changes as possible, including phase shifts on infra low frequencies.
In other words, if you know EXACTLY the conditions it will work under, AND you trust the user (you do trust yourself, don't you? ), you will let it ride for all of the money.

But if you're unsure of either, or both, you will add some bandwidth limiting?

Good logic - I agree.
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Old 13th October 2012, 06:49 AM   #7425
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Right. If I trust the user I mean that he will not blame on me when uses the gear improperly. Like his precious vinyl player without rumble filter damages his precious subwoofer because my amp honestly amplified the signal. Or his boutique DAC fried his super-tweeter.
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Old 13th October 2012, 07:23 AM   #7426
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Right. If I trust the user I mean that he will not blame on me when uses the gear improperly. Like his precious vinyl player without rumble filter damages his precious subwoofer because my amp honestly amplified the signal. Or his boutique DAC fried his super-tweeter.
As Albert Einstein once remarked - there are only two things I know which have no limits - space and human stupidity. I'm not sure about space.

As everyone who has ever developed an electronic product for the general market knows, half of the development time goes to idiot proofing the product.

Got you Wave, thank you.
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Old 13th October 2012, 11:25 AM   #7427
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back to the advantage of single pole passive filtering vs two pole filtering.

Adding a single pole passive filter to the input of every receiver is a real pass band adjuster.
But it is not the sole filter in the feed to the input.

Look at any Source to Power Amp connection.
The Source is likely to have an output impedance.
The cable will have some capacitance and some inductance.
The power amp has some input impedance.

All of these add up to at least two pole filtering and more likely a staggered three pole filter before the power amp.

Our job is the try to get the stagger about right that the wanted audio signal gets through and as much of the unwanted signal gets attenuated. That's where the advantage of multipole filtering comes in:- more attenuation.

I suspect we all already have at least two pole passive filtering at or before the input to our Power Amplifiers.
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Old 13th October 2012, 11:53 AM   #7428
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Fool Proof .

A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. Douglas Adams ...
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Old 15th October 2012, 07:32 AM   #7429
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Originally Posted by AndrewT View Post
back to the advantage of single pole passive filtering vs two pole filtering.

Adding a single pole passive filter to the input of every receiver is a real pass band adjuster.
But it is not the sole filter in the feed to the input.

Look at any Source to Power Amp connection.
The Source is likely to have an output impedance.
The cable will have some capacitance and some inductance.
The power amp has some input impedance.

All of these add up to at least two pole filtering and more likely a staggered three pole filter before the power amp.

Our job is the try to get the stagger about right that the wanted audio signal gets through and as much of the unwanted signal gets attenuated. That's where the advantage of multipole filtering comes in:- more attenuation.

I suspect we all already have at least two pole passive filtering at or before the input to our Power Amplifiers.
I believe this was not so much a problem in the past, when the transistors we had tended to roll off rather early anyway, but these days, with very high transistor Ft, I agree your above scenario is more than likely.

On the other hand, we actualy do do some subsequent filtering, like it or not. Eventually, we need to curtail the response of one or more stages, and in the process, we also do some filtering. The typical comensation capacitors ultimately reduce the transistor gain and their in situ response.

Stability issues make us do that.

Completely agreed with you that we need to investigate the entire signal path from the preamp output to the amp input. A simple example - the output impedance of preamps varies wildly from say 47 to say 600 Ohms, a 13:1 ratio. When deciding what your input filter should look like, you have to be resigned to the fact that if you work it out for say 120 Ohms (as per the IEC recommendations) and place your -3 dB cutoff point at say 200 kHz, with 47 Ohms it will be higher, and with 600 Ohms it will be lower than that.

I remember many years ago, the Norwegian company Tandberg had an interesting approach to this. In their 2075 top of the line receiver (cca 1977), they had a classic input filter at some relatively wildly high point, say like 300 kHz. Then, between the input differential pair and the VAS differential setup, they installed an RC filter with a lower cutoff point, say 200 kHz, both in inverting and noninverting lines. The ultimate effect would be a second order filter at say something like 150 kHz, but the input pair was given more leeway for better slewing purposes, I'm guessing.

However, in their prime time 3003 power amp (cca 1980), they avoided this setup and did some VERY serious input differential stage compensation, with a rather complex RLC setup instead. I'm still trying to understand the logic and purpose of it all. Unfortunately, they used some long gone BC transistor (447), which I do not have in my simulator, and frankly which I have never even heard of, so it's not easy.
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Old 15th October 2012, 09:54 AM   #7430
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This amp isn't as wrong as it looks .The simplicity is so close to disaster as to invite it . Many things here look badly wrong or you have read many times are wrong . Forgive any errors as it is to show an idea and might be weak on detail or things the wrong way round etc .

What is right with it ?

The input pair is reasonably low noise .

DC offset is reasonable , better than 25 mV if a little care is taken .

Input degeneration is not bad and is half resistor and half re of the transistor ( about 50R per side ) .

The input pair is reasonably balanced . Any imbalance will cause second harmonic distortion which might be nice to have in small quantities .

Slewing should be OK with real music

The VAS transistor has about the right input impedance and is running about 60% it maximum ( 400mW max ) . The input impedance is better than 1k3 ( { 25/5.3} x 280 )

VAS degeneration is minimal ( no emitter resistor ). Linearity is reasonable as the Cob is low using the external capacitance to dominate . Price is 4 devices / $

The bootstrap seems wrong on two counts . 10 uF seems too low , try it you might get a surprise as MOS FET's are odd animals . It can be a polyester 100 V 10 uF if so . Also the gain of the MOSFET's seems wrong to drive a bootstrap , not so . Be careful the other way , above gain 0.95 you might need a series resistor . Doubtless a constant current source ( CCS ) will be preferred . Some still prefer these bootstrap versions . Forget simulations , give it a try sometime . Like savory ice cream it might be OK ? Remove the capacitor and see how life changes .

No tail CCS ? Some poor chap put a tail resistor on a simulator the other day and was shock how good it was . It is only by going to JC's perfected CCS that he got a dramatic difference ( Thanks John , I never suspected it was for that reason the extra resistor added ) . Instead try a clean supply . It will cost peanuts to do . Use CCS and clean supply , why not ?

The filtering of input looks all wrong . As long as your preamp is conventional it should be OK . If using a transformer volume control as I did recently it might be ideal . I was very pleasantly surprised . It was a loan , glad I had a listen .

The 1000 uF is so as to be close to DC . That means only first order input filtering at the experimental stage . You might be surprised . You might keep it this way . As far as my ears can tell this is where being too cleaver and buying expensive capacitors might be a mistake . If you get your spectrum analyses out and measure this area you might find it is not the world you imagine . Mostly 100% DC feedback is the requirement .

The MOS FET's are like a little world of components ( 2 MF = 7 bipolar if including vbe bias , also many resistors ) . They love feedback like it is Christmas every day . Thus what seems so wrong with them is not so bad . They will not measure good open loop . However they love closed loop . Bias is not critical and can be set by ear . What a shame we don't listen to amps . If we did that might be an option ( ? ) . Sorry that was naughty . 100 mA is very correct as it is a design parameter known by manufactures .

Gain of 40 is about right . Could be used with a passive pre-amp and CD .

47 K input is reasonable , can be reduced . The 2K2 might be reduced .

The other by ear parameter is the 47 pF input cap . As John says there might with Vinyl or poorly designed equipment be some slewing issues . If not going to 50V /uS slewing we should adjust this capacitor to taste ( As Andrew said ) . Helps keep radio out . If marketing a product how good your ears are will be judged . Ultimately this can not be taught . I think it can be learnt . Sometimes you have to say I don't care what the simulator says I don't like that . 20 years later you find out why . Good news is 90% of the time if it measures well it will be fine . Excellent is a bit more work than fine .

The power supply will interact with the filtering choices . 2 x 22 000 uF I think is a minimum and 250 VA per channel .

It is assumed MOSFET's need more driving ( 60 V rails to VAS ) . Be careful and listen . The driver clipping first seems a virtue to me . Even my oscilloscope agrees .

From here one can get back to the usual school of design . Some might find usual is not to their liking . The Hitachi amp which this is a reduction of should be tried also .

I think some amps I see are the equivalent of playing 3D chess , fine if you like . I more than admire the ones who do it . Yes we can do much better than this design . I suspect 90% do worse when they play 3D ? I must thank Roy Gandy of Rega for teaching me this . He said make something as simple as you can then build from that . Never change more than one thing at a time . If not you never have truely learnt as you have no real idea what happened in the transition . Roy tested 2 Nylon screws verses 2 steel screws on the Rega turntables . His staff heard in seconds it was OK . It took him weeks they said . These screws were to stop people tightening them which caused problems . He was careful to be sure he liked it . Say what you like about Rega he is no idiot . Roy told me people like to look at the bearing . He then said no idea why . For that reason alone he made it 1 micron finish when 6 is OK . He said it was easy enough to do. I agree with John that when it is easy to do and has no down side we should go further than theory suggests . Sorry John if I misinterpret , It has long been thought less than 0.1 % distortion etc is the limit of human perception ( I have no idea if correct , I think it might be true ) . However if one amp sounds great and is 0.1 and another 0.0000001 % why not have the latter . The customer most likely will not hear a difference . If they do it is more likely bandwidth or stability / power delivery .
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