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Old 9th October 2004, 02:38 PM   #31
johnnyx is offline johnnyx  United Kingdom
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I think it's brilliant. Only 9 components - that puts 47Labs claim to shame

I think people forget that some speakers have transformers, Quad electrostatics and Audax gold dome piezo tweeter to name but two.

The design is the polar opposite to some, like Hawksford et al. with their ever increasing complexity.

Yet our hearing is very sensitive to location, and not so sensitive to distortion, so your design is probably more attuned to our hearing abilities than most, something that you have hinted at in your description. The use of line array speakers probably emphasises its strengths in this respect.

Should I dump all my current projects for this minimal approach? what a dilemma
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Old 9th October 2004, 02:42 PM   #32
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Mmmm... I like this amp - not a P-channel fet in sight!
You can do fun things with it by adding a second identical centre-tapped winding to the output transformer and feed the drains with it, the centre tap going to + volts of course. Then add a coupling cap from each drain to the opposite source. All is revealed in the diagram here:
http://www.diyaudio.com/forums/attac...&postid=122402
from the thread here:
Tetrahedron output stage topology.
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Old 9th October 2004, 05:28 PM   #33
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Default Re: You mange to get me totally lost

Quote:
Originally posted by EUVL
Hi Susan,

Congradulations for the reception you got with the new thread. I am also glad that it turned out to be sound advice.
Thank you and thank you, as I am new to this type of forum.

Also I have put in a lot of design effort into this amplifier and then been constantly pooh pooed by "those that know" so I must admit to have been a bit anxious about airing it publicaly on the forum.

Most people tell me to ditch the transformers - totally missing the point - and use conventional circuits. What is the point of that sort of comment? It then just becomes another "me too" design and I wouldn't have started on this path in the first place if what was available worked properly.

Quote:
BUT

you managed to get me totally lost with your tx design. And being a native speaker (yes, I live in Germany; no I am not German) with sufficient IQ , I guess I might not be the only one -- whether they admit it or not. So maybe you would be kins enough to do some detailed explanation. A few pictures or sketches would of course help a long way.
Dont worry, it took me two years and lots of research to figure it out and understand what I was doing.

>> Single chamber bobbin, EI 96 x 45.7mm
>> Quad Filar wound,

> meaning 2 x primary and 2 x secondary windings all side by side ?

Yes. See, not so hard to understand.

This is the key design feature of the output transformer, and why it is NOT the same as a valve transformer.

>> 0.80mm insulated copper wire

> Is this empirical, or do you design on internal resistance or current density ? How do you scale this with output power

Mainly empirical although I did do some maths - long forgotton.

I have a transformer for a certain power over the bandwidth I want to attain.

For 25 Hz to 250 kHz small signal bandwidth I have determined a rough rule of thumb that I needed four times the VA mains rating for the peek wattage I could have (for a 1:1 output that's supply rail x 2 across the load impedance - nominally 8 ohms). So for 50 watts peak I need 200 VA core section.

To scale with a bigger core I would check the bobbin cross section and see how large a diameter of wire I could fit whilst maintaining the same number of turns - i.e. keeping the same(ish) inductance.

For a 75W tranformer I was going to use 1.0 mm wire and for a biggie to push 120 watts plus I was going to use 1.2 mm wire.

If I didn't want the bass response - say I was filtering from 200 Hz - then I could use a smaller core section.

>> No insulation between layers.

> Why is this important (or unimportant) ?

Because mains transformer manufacturers are used to puting the tape in and look puzzled when there isn't any between the primaries and secondaries. So I explicity stated the fact rather then assume it would be understood.

>> Terminations; to tags on bobbin:

> I take it take you are not really centre tapping from one winding (or that the 2 primary windings are not ends on.

Nope, you are correct in the first instance - all four windings done simutaniously. hence quad-filar.


>> 2 x Primary:
>> 12 full layers ( approx. 144 turns each).
>> Start and finish at same side of bobbin

>> 2 x Secondary:
>> 6 + 6 full layers ( approx. 72 + 72 turns each).
>> Start and finish at same side of bobbin ( opposite side from
>> primary).

> What do you mean by 6+6 ?
> In your circuit, you showed a secondary with a centre tap (in dotted lines). Are you using a step down ratio of 4 (i.e. no of turns in primary = 4 x no of turns in secondary) ?

The schematic is the simple version. For the amp in the pic (and this winding specification) I have made provision for configuring the secondary for different impedance loudspeakers. So I split the secondaries in half to get four seperate sections which can then be configured as required.

>> Primary: Start Finish
>> Winding 1: 22 24
>> Winding 2: 25 27

> What do the numbers 22, 24, 25,27 refer to ?

>> Secondary: Start Finish Start Finish
>> Winding 3: 13 15 17 19
>> Winding 4: 14 16 18 20

> Same question as above.

These are the tag numbers on the bobbins that I was using and therefor specified to the transformer manufacturer so I knew which winding went where.

>> Starting from same bobbin end, wind all four for 6 layers,

> All 4 meaning 2 x primary and 2 x secondary, each 0.8mm dia. ?

Not this time.

All four is 2 x primary and 2 x secondary.

Start with four wires from four spools stacked together on a broom handle wedged into a filing cabinate draw (this last bit is optional).

Wind six layers.

Pull out two of the four wires in a loop which will connect to the appropriate tags.

Return the loop so one still has four seperate wires.

Wind six layers.

Wrap in tape and then cut the ends of the wires to length and solder to the tags.

(I would varnish impregnate but I don't have the facilities to do that muself.)

Assemble laminate and attach frame.

Finished.

I hand wind the output transformers, holding the bobbin in one hand and feeding the wires with the other. It takes about 20 minuits to half an hour.

> Thank you in advance for enlightening.

> Cheers,
> Patrick

My pleasure. I hope it now makes some sense.

Best wishes,
Susan.
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Old 9th October 2004, 07:24 PM   #34
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Susan,

Your posts are a refreshing, filled with content, in a forum that was in danger to become a marketplace of people shouting 'hey, I KNOW this should be so-and-so, trust me' or 'I have spoken in 1985 to so-and-so and he agreed I am right'.

Thanks very much.

Jan Didden
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Old 9th October 2004, 08:09 PM   #35
mikeks is offline mikeks  United Kingdom
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Hi Susan.....Welcome to the forum
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Old 9th October 2004, 08:32 PM   #36
jam is offline jam  United States
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Oh ! Oh! Mr.(Excessive) Feedback just popped in.............................
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Old 9th October 2004, 08:42 PM   #37
mikeks is offline mikeks  United Kingdom
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Hi Jam..
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Old 9th October 2004, 08:49 PM   #38
jam is offline jam  United States
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Hi Mike,

I hope you are not going to put Miss Parker to task because of the lack of feedback in her design.......................
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Old 9th October 2004, 08:51 PM   #39
mikeks is offline mikeks  United Kingdom
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Old 9th October 2004, 08:52 PM   #40
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Very interesting - I have a few good-sized silicon steel toroids I may use to wind output transformers to try this out. The only down side is possibly dying of boredom while winding. I'm no stranger to layering and multifilar techniques for reducing leakage/extending bandwidth in transformers, as I design switching power supplies for a living and wind all the prototype transformers myself. This project goes in the pile with the 3-4 other projects that haven't been boxed/listened to just yet, so it may be quite a while before I report resuts. What thickness laminations were used for the output transformer?
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