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Old 27th January 2003, 10:52 PM   #21
7N7 is offline 7N7  United Kingdom
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The amplifier now plays music.

Actually it sounds very good - especially considering the appalling output transformer fitted.

Earlier I measured the distortion. At 15W THD is around 4%, so I tested at 12W (it's always worth remembering that the 'scope screen is not the place to look for distortion).

So I measured at 12W which is, in dB terms only a gnat's below 15W anyway (-3dB = 7.5W).

THD at 12W is -32dB, 2.5%. This is almost entirely made up of 2nd and 3rd Harmonics (3rd is -6dB i.e. -38dB = 1.25%) fourth and fifth were at -25 and -26dB respectively, i.e. about 0.125%.

This is with a 4k a-a load.

I have the option with the transformer to use an 8k load, so I shall try this tomorrow. I expect lower distortion, but I shall be inetrested to see the effect on the power output. As I am operating it, each section of the 6528A has ra of around 400 ohms.

The amplifier is playing-in very nicely, and now that it is later, the mains has improved of course, so it's sounding very well.

After I have tested at 8k tomorrow, I'll measure all the voltages and start to draw the thing up.

7N7
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Old 27th January 2003, 11:02 PM   #22
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Default GOOD NEWS.

Hi,

Quote:
After I have tested at 8k tomorrow, I'll measure all the voltages and start to draw the thing up.
At 4K a-a,you're already at a 1:10 ratio.

I wonder what the 1:20 would bring.

Don't you expect some loss of power due to the higher ratio and longer windings?

Just a thought,
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Old 27th January 2003, 11:40 PM   #23
7N7 is offline 7N7  United Kingdom
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This is one area where I constantly have a problem: it is said that if one has a valve with ra = 1k, then in a single-ended stage one uses a 2k transformer for best power (slightly larger for best distortion). Now apparently for p-p this equates to an 8k component! I had always assumed that since the output valves in a p-p stage are in series, then one simply doubled the load.

I imagine that there is a square function in the horrid mathematics associated with transformers that perhaps accounts for this.

With 8k I do expect a small loss in power output, but I suspect that it will not be much. What I am looking for is a reduction in distortion and the 8k load may give me this. Incidentally, the transformer offers a 6k winding too, so I can organise a classic compromise

All the best

7N7
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Old 27th January 2003, 11:56 PM   #24
dhaen is offline dhaen  Europe
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Default The art of compromise...

One of the Forums posters, (name escapes me), has a signature to the effect of:
Engineering is the art of compromise.

I cannot disagree...

Cheers,

BTW, I've spoken to your "Boss" and he says you can post the schematic now.
(Only joking)
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Old 28th January 2003, 12:20 AM   #25
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Default A LA BELGE

Hi,

Quote:
Engineering is the art of compromise.
Yes...and some compromises are over-engineered.

Cheers,


P.S.My *Boss* can't wait either.
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Old 28th January 2003, 12:39 AM   #26
dhaen is offline dhaen  Europe
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Default The art of compromise...

OK Frank,

You lot, (and my roots), have an art. But is it so difficult to find a midway point between 2 variables? Surely, more difficult is to find a position between several...

Read into this what you will.

Cheers,
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Old 28th January 2003, 12:49 PM   #27
7N7 is offline 7N7  United Kingdom
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Default Re: The art of compromise...

Quote:
Originally posted by dhaen
BTW, I've spoken to your "Boss" and he says you can post the schematic now.
(Only joking)
Harrumph!

7N7
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Old 28th January 2003, 01:55 PM   #28
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Default ..art for arts sake...

John,

One definition of an art is a 'science with more than seven variables'...

By that definition engineering isn't art since the whole basis of engineering is to freeze most of the variables to allow the maths to be solveable and then construct the real world solution according to rules that ensure the math works in practice.

I guess this could be argued as 'second order art' since the art is hidden now in the choice of variables to freeze so that the solution works in practice...

The choice of mid point between two variables is itself a variable ... but I guess its a dependent variable...does that count?

ciao

James
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Old 28th January 2003, 02:02 PM   #29
dhaen is offline dhaen  Europe
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Default Delta force..

James,

I concur with the 7 variables break point.
For what it does, is roughly separate the "repeatable" from the "unique".
Thus Science from Art.

Cheers,
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Old 28th January 2003, 02:03 PM   #30
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Default Re:Delta force..

Hi,

Jeeez....and I thought we were talking politics.

Ciao,
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