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Jonathan Bright 12th October 2004 01:08 AM

Congratulations Susan. All the excitement in this thread has reminded me of a small sign I saw once. It was on a wall behind a secretary's desk and it said something like this.
"When it came to dancing Ginger Rogers was just the same as Fred Astair............except she did it backwards in high heels."

Allexx 12th October 2004 01:12 AM

I am somewhat confused about contradictory information about life expectancy of Bass Loudspeaker (Life expectancy: 100 years plus for the cabinet (the drive units may need refurbishing before this) and Spheroidal Marble Speakers (Life expectancy: 100 years plus for the cabinet (the drive units may need refurbishing before this).
About electrolytical capacitors in power supply nobody has no any doubts...

Graham Maynard 12th October 2004 07:18 AM

Hi Susan,

I guess the question ' What's inside your 'speaker ?' would also have been as relevent, because the amplifier-loudspeaker combination is more important than either the amp' or 'speaker when viewed alone in isolation.

You quote 2.78 ohms output impedance. That would not satisfy me for use with 'conventional' loudspeakers.

However your Marble enclosures appear to house what looks like a Jordan JX92S, which can be 'bright' when driven by a low output impedance amplifier, this to the extent that many users prefer to use them with a series L//R 'baffle step' corrector. Now wouldn't that make for a super A-B comparison; your system, versus a good conventional amp plus baffle step, using the same source and loudspeakers!!

You suggest small drivers like the Jordan are 'bright', but when you get 9 or 16 of them in a line, the brightness disappears due to physical spacing, which is why a tweeter becomes necessary at hf for non-'far-field' listening. But then who could afford 9 or 16 Jordans per channel ? I'm not so sure that parallel connection down to 0.5 ohms is going to sound any better than evenly or 'field' distributed series/parallel chains; as you say, even the Quad has distributive response tapering.

Your amplifier does have its own internal voltage NFB - at the sources wrt input. Indeed, when there would be any transformer/driver resonance leading to momentarily high impedances at the sources there would be a potential for waveform rectification, though your amplifier controls this via its push-pull arrangement. Did you ever try any resistor damping at each source wrt ground, say 22 ohms ? I don't think your circuit would work so cleanly if it were single ended.

You mention
1 pre-amp cables............ not a problem if the pre-amp designer knows to buffer properly with resistor feed and loading.
2 NFB .............. not a problem if the amp designer examines for amplifier response to predictable crossover/loudspeaker induced back EMF.
3 cables .............. not a problem if the crossover is before the amplifiers or driven at genuinely low impedance then with inexpensive cables going to the loudspeaker drivers.
You have covered these aspects with your own design in a way that most commercially available systems have yet to do.
It is not signal path capacitors or NFB that are the problem, indeed there are those of us who could equally complain that transformers are a problem, but then you know your own design in a way that we cannot.

I thank you for your offer for remote listening, but alas I have my own as well as family health probs to cope with. You would not believe the difficulties I had just completing the text for my article; my own hands on work has become almost non-existant. Hows about comparing your amp at home with a class-A like the JLH, for Jordan loudspeakers can be well driven by any of the JLH class-A's ?

Never let us wear you down,

Cheers ............ Graham.

PMA 12th October 2004 08:19 AM

Graham - thanks.
We should not participate in building the new superstitions.

dimitri 12th October 2004 08:30 AM

As it was mentioned the amp is push-pull source follower with transformer as a load. I would like to ask what is the quiescent current of each FET? Is it class A or AB? During one half-cycle of the input signal one follower will feed not only the load but the other follower too. What will be if the FETs are slightly different? What measures are taken to avoid dc component in the output transformer due to imbalance? Having experience with high-current triodes like 6C33C, 6C18C I know that the output transformer core jumps to saturation even with small imbalance.

Susan_Parker 12th October 2004 09:20 AM

Hi,

Thank you for your question and interest in my amplifier :)

Quote:

Originally posted by Circlomanen
Could you make a much more powerful version without sacrificing
the wonderful simplicity???
400 watts????

I assume you mean 400 W of full bandwidth hi quality audio rather than an FM RF amp or ultrasonic driver for side scan sonar?

Quote:

Originally posted by john curl
No, it would be impractical, BECAUSE the distortion will rise directly as the power out, (presuming 3rd harmonic is dominant). How much distortion can you tolerate? 1%, 10%? High power amps require a re-think of the approach. Sorry, Susan, didn't mean to break-in like this, but I stated this principle on another thread, recently.
No problems John, supportful comments always welcome.

===

I see the practical limit at around the 120 watts point, not just because of the rising distortion but the more pragmatic one that I simply wouldn't be able to lift an amp of any greater power due to the weight.

However I would posit that you would not need 400 watts in a normal domestic listening situation (for which my amplifier is designed) unless you were after a bass guitar amp or such like?

As previously mentioned I am of the opinion (rightly or wrongly) that one should split out the low bass where speakers are operating in piston mode (and therefore require lots of power) from the rest of the audio range - where one should use single wide range drivers (perhaps in line arrays). The discussion then becomes one of whether crossover phase distortion is more or less problematic than Doppler distortion.

Low bass - which could benefit from several hundred watts if you are really going for the sub 20 Hz stomach rattler - can be driven directly by a conventional solid state amp - perhaps with motion feedback BUT one needs a big room to be able to hear this as most domestic living rooms (here in the UK anyway) as sufficiently small that true low bass can't be reproduced because the (half?) wavelength of the sound is bigger than the room dimensions and it cancels out.

Crossover point at say 120 Hz.

The rest of the audio band with decent sensitivity speakers shouldn't need more than 30 to 50 watts max even in a large room.

Hope this helps.

Best wishes,
Susan.

Susan_Parker 12th October 2004 09:32 AM

Re: Hey Susan!!!
 
Hi,

Quote:

Originally posted by darkmoebius
Talk about a grand entrance - sheesh!! 5,600+ views in 2 days.

Congratulations on an exciting debut post. In case you don't know, for just about every response there have been 50 other people reading this thread.

I can't wait to hear the reports from others trying out your design.

BTW, thanks for finding a US manufacturer to wind the trannis, there are plenty of us across the pond who will be interested in this project.

A couple of trannis, 3 resistors, 2 zeners, and a pair of Mosfets - brilliant!!!

Ah! I hadn't realized that so many people were watching - now I am feeling a little self conscious. I had better try to keep my spelling in order - not one of my stronger points being dyslexic!

Thank you, and everyone else, for the encouragement.

A quick point, the US manufacture is a source for the laminations, which is the heavy part of the transformer. The transformers will still need to be assembled (not a tricky thing to do for someone who is up to bolting power devices onto heat sinks).

I can supply wound bobbins etc. although hand winding the output transformers is not difficult due to the large wire size and relatively small number of turns.

The input transformer is a different matter which is why I was suggesting to buy them from Sowter ready made.

Best wishes,
Susan.

Susan_Parker 12th October 2004 09:46 AM

Hi Allexx,

Quote:

Originally posted by Allexx
I am somewhat confused about contradictory information about life expectancy of Bass Loudspeaker (Life expectancy: 100 years plus for the cabinet (the drive units may need refurbishing before this) and Spheroidal Marble Speakers (Life expectancy: 100 years plus for the cabinet (the drive units may need refurbishing before this).
About electrolytical capacitors in power supply nobody has no any doubts...

The Bass speaker is made from hardwood (beeswax polished) with proper carpentry and therefor should last at least a hundred years if not bashed around.

The Sphere's shells and their stands are hand carved from single pieces of solid marble, and therefor should last a minimum of a thousand years - with a possible life of five thousand years plus is well looked after.

The drive units however incorporate materials like thin aluminum foil cones and rubber surrounds and these are susceptible to corrosion and deterioration in shorter timescales. Hence the likelihood of the drive units requiring refurbishment in a shorter time scale.

I hope this answers your question.

Best wishes,
Susan.

TonyF 12th October 2004 09:56 AM

Quote:

The Bass speaker is made from hardwood (beeswax polished) with proper carpentry and therefor should last at least a hundred years if not bashed around.

The Sphere's shells and their stands are hand carved from single pieces of solid marble, and therefor should last a minimum of a thousand years - with a possible life of five thousand years plus is well looked after.
Just the stuff for future Antique Road Show ...?

traderbam 12th October 2004 10:05 AM

Might even qualify for the "Time Team". Oo arr. :clown:


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