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Old 19th January 2007, 10:37 PM   #1111
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Hi unclejed613

Thanks for your post.

Quote:
Originally posted by unclejed613
... SLOWLY use the drill to twist the wires evenly until you have the desired twist pitch...
This technique is great for keeping the multi-filar windings together but one needs to watch out (particularly if one is making bobbins for an EI transformer) as the twisting will decrease the number of turns one can get in a given space.

For hand winding a toroid this is a good way of keeping the wires together and one doesn't need that many twists to keep everything together. Be careful of pressure points where the wires cross over each other on a corner.

Interwinding capacitance will be in the nF range, however with the small number of turns, the low driving impedance, and the fact that all windings are seeing the same voltage swing this isn't a problem as the output bandwidth is still a couple of MHz (with my EI120 transformer).

Looking at the transformer spec page (3/4ths way down)...

http://www.audiophonics.com/audiopho...ut-tx-75w.html

... you will see that in 2:1 mode there is about 10nF between primary and secondary.

Many thanks.

Best wishes,
Susan.


[M1009 + RT-524 + R-442 + PRC-77]
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Old 21st January 2007, 01:42 PM   #1112
Tyimo is offline Tyimo  Hungary
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Hi Susan!

I would like to ask something about choke loaded amps power calculations, maybe you can help me!

I know, if I ignore stuff like saturation voltage and losses in the choke, just divide the rail voltage by 1,414, square the result and divide by the load impedance.

(+U/1.414)2/Rload

But, what about the current??? How will I know that my amp is voltage limited or current limited into any given load?????
I need to know this, because I would like to bias the amp for the correct level.

I know this formula for current limit calculation:

Prms = Irms*Irms*R.
Irms = 0,707*Ipeak
Ipeak is my bias current.

for example with 2A bias:
Irms=2*0,707=1,414Arms
Prms(I)=1,414*1,414*8=16Wrms

What would be the correct supply voltage with 2A bias and 8 ohm load? ca.16V??

Greets:

Tyimo
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Old 21st January 2007, 02:44 PM   #1113
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Hi Susan, and everyone else!

I'm only on page 90 yet, but thought I'd chime in and say that this thread was extremely interesting, and I will be looking to try to implement Susan's design as soon as I can. Time to start looking for old transformers that can be used for experimentation, I suppose!

Regards
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Old 21st January 2007, 04:58 PM   #1114
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Quote:
Originally posted by Patrik Floding
I'm only on page 90 yet...
Only on page 90!?!?

You need to go to the user cp and set your options for 50 posts per page. Then you'll only have 23 pages to read.

se
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Old 21st January 2007, 08:10 PM   #1115
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Hi Tyimo,

Quote:
Originally posted by Tyimo
...
What would be the correct supply voltage with 2A bias and 8 ohm load? ca.16V??
Is this SE or PP?

8 ohms is nominal, and most speakers will dip at some points a lot lower than this.

For SE inductance determines the LF response as well as the current one is using for the bias.

For PP the current sets the transition point for Class A to AB.

Also allow for the necessity to have some headroom (which varies with mosfet type) so you may want to allow for a couple of extra volts than the calc says.

Best wishes,
Susan.
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Old 21st January 2007, 08:33 PM   #1116
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Hi,

I've been following this thread since its inception and have been (slowly) constructing a pair of Zeus 75s. These have strictly followed Susan's circuit and use all her recommended parts.

Configuration
Input transformer: Sowter 8160 with 50% mumetal laminations in 5:1 configuration (primaries in series)
Output transformer: Sowter 9840 in 4:1 configuration
Mosfets: STW34NB20s Vgs matched to < 1mV at 500mA in test jig
Bias: 750mA per Mosfet supplied by L200 tracking current source
Heatsink: 0.2/W SK158 300x200x80 from Dau

I've encountered the difficulties mentioned frequently by Susan in driving this power amplifier. A variety of DACs and preamps objected to its the low input impedance (stated to be 600 ohm). I'm currently constructing some line drivers, AD845+THAT1646 and AD8610+THAT1646 based. The THAT chip, a recently introduced line driver, has an output impedance of around 60 ohms maximum. Before finishing these off I decided to run an Audio Precision across the design to determine if this line driver would function well.

In the attached graph the cyan curve shows the input impedance. The input signal level was 500 mV. At high frequencies ~8kHz the input impedance is approximately 900 ohms - low but can be driven by a source with a low output impedance. By 600Hz it has fallen to 600 ohms - a standard reference value. As the frequency decreases to 20Hz the Zeus75 input impedance drops to 120 ohms. This is a very difficult load for most preamps and perhaps explains Susan's like of 'chunky' preamps

James
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Old 21st January 2007, 08:34 PM   #1117
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In view of this difficult load I decided to investigate the THD+N characteristics. The input signal level was 500 mV and the measurement bandwidth was 22Hz to >500kHz. The curves in order:

red - source output impedance = 600 ohms
black - source output impedance = 150 ohms
cyan - source output impedance = 50ohms
green - source output impedance = 25 ohms

I was tempted to drag in a conventional power amplifier to provide a <0.1 ohm source impedance but the trend is already clear. As can be seen, not unexpectedly, the distortion decreases as the source impedance is reduced. I can now see that my line driver with its 40-60 ohm output impedance will not be a good match for this.

James
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Old 21st January 2007, 08:38 PM   #1118
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There are five solutions that I see:

(1) Take the low impedance preamp route that Susan has pursued with transformer, headphone and AD815 designs.

(2) Modify the Zeus 75 to have a higher input impedance. I see several ways of doing this. In the Zeus designs voltage gain is provided by input transformer. Running of 34V rails the output Mosfets will run out of steam about a volt below this (being conservative, allowing for loading). Most line drivers running off +/18V can be persuaded to supply 16V peaks.

An input transformer step up ratio of 2:1 would therefore suffice. This necessarily locks the Zeus to sources which can supply higher outputs than normal. This is fine as the Zeus is such a unique design that being coupled to a specific (or small range) of preamps is unavoidable.

Transformer impedance is proportional to the square of the turns ratio. Thus the impedance reflected from the secondary input is divided by 25 with the current 1:5 step up transformer. With the substitution of a 1:2 step-up the impedance reflection is 4:1 which should increase the input impedance across the audio bandwidth signficantly and make the Zeus considerably easier to drive.


The next three ideas are based around creating a two stage design.

(3) Change the Zeus75 to have two stages, input/interstage and interstage/output. In a sense this is what Susan's transformer based preamp and the current Zeus75 already are. The two box solution requires four transformers. In effect the preamp output transformer and the power amp input transformer are merged into an interstage tranformer, saving costs.

This question has already been asked back in post #1076

http://www.diyaudio.com/forums/showt...64#post1043464

Susan's comments re. bandwidth are relevant and yet to be tackled.

(4) Change the Zeus75 to have two stages but allow (non transformer) voltage amplification in the first stage. This is what

Rozak has done in his design where the cascoded BF245C provides some voltage gain.

http://www.diyaudio.com/forums/showt...682#post636682

However, philosophically this is difficult as it breaks Susan's idea of using wire based amplification only.

(5) Use a combination of ideas from (2) and (3). Again use a source which can provide a higher than normal swing. The problem of the b/w of the interstage transformer can be solved by making it a 1:1 ratio or 1:2 step-up. The voltage follower on the input can use lower power and lower capacitance FETs and thus the input impedance should be higher since the reflected Xc^2 will be much lower for FETs with Crss of a few pF.

None of the above should be viewed as critism of the existing design - Susan has produced and supported one of the more unusual and interesting designs seen here. I've had a lot of fun building them. With a sunk cost of 500 UKP in transformers and 200 UKP in heatsinks in addition to batches of MOSFETs I'm not about to let this one go. I'm simply offering some ideas on how the main quirk of the Zeus75 - the low input impedance could be tamed. I'm still as enthusiastic about this design as when I first saw it...

James
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Old 21st January 2007, 10:50 PM   #1119
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Quote:
Originally posted by nemestra
Transformer impedance is proportional to the square of the turns ratio. Thus the impedance reflected from the secondary input is divided by 25 with the current 1:5 step up transformer. With the substitution of a 1:2 step-up the impedance reflection is 4:1 which should increase the input impedance across the audio bandwidth signficantly and make the Zeus considerably easier to drive.
I think the input impedance issue with the current transformer isn't so much the turns ratio as it is the relatively miniscule primary inductance. Keep in mind that the primary inductance is in parallel with the reflected impedance and constitutes the limiting factor at low frequencies.

Anyway, I think the best approach would be moving the bulk of the voltage gain upstream using some good quality micrphone input trannies.

se
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Old 21st January 2007, 11:40 PM   #1120
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by nemestra
[B Susan has produced and supported one of the more unusual and interesting designs seen here. I'm still as enthusiastic about this design as when I first saw it...
[/B]
I agree. Great discussion. Here's where I'm headed:

Sheldon
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