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Old 26th June 2011, 12:17 PM   #41
AndrewT is offline AndrewT  Scotland
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The supply voltage and the current ability determine the Power output.

The undistorted volume we hear is solely dependant on the undistorted voltage into our chosen loading. We don't need any more information, particularly if we use the recommended 8ohm speaker.
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Old 26th June 2011, 12:18 PM   #42
col_s is offline col_s  Australia
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hi Hugh,

You can put me down for a pair. I'm in NSW.

Colin
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Old 26th June 2011, 01:02 PM   #43
AKSA is offline AKSA  Australia
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Default FetZilla Power Output

Paul, Greg and I have done extensive Spice modelling and now have a real indication of power output.

This is a single ended voltage gain block and as such will always have an asymmetrical clip. This slightly lengthens the usual SS PP distortion profile, and tends to produce more even than odd at least at clip onset. Using 36V rails (25-0-25Vac trafo) we think it's around +28.3Vp on the positive half cycle at 1KHz into 8R. Negative half cycle clips at just under 2V more.

This means that undistorted power will be exactly 20Vrms (peak x 0.7071). Square this, divide by 8R for power output, and it comes to exactly 50W. This is a very reasonable power output for a medium sized room with 87dB/watt/metre speakers, and more than enough for a moderate sized apartment in a city. Of course there will be headbangers who want more, but for most, and particularly for middle aged people and older, this is more than sufficient for most music, even Hotel California.

The most linear region of operation for the lateral output fets chosen is 1A to 5A. With 28Vp, we would achieve nominally 3.5A into 8R speakers, and max 5A into a reactive load, so this accords very well with strong, linear power output. FETs have a much broader SOAR, so with a 200V rating you would never blow them with a difficult load unless of course it was a short.

Hope this explains things,

Hugh
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Old 27th June 2011, 02:50 AM   #44
AKSA is offline AKSA  Australia
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Default FETzilla PCB Group Buy

Keep 'em rolling, now at 32 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand
2 Colin S. (?) (city?) NSW Australia

Acting on advice from Paul, I have added a 1K5 resistor to the drain circuit of the input stage to prevent damage to the VAS gate during switch-on transients. It does not affect loop gain, but does reduce the dissipation in the input device, no bad thing. Now down to just under 80mW with 36V rails, about right for a 400mW package.

One of the two LEDs (the green one) used as voltage reference for the input stage bias generator is now off the board and doubles as the front panel 'on' indicator. It can also be mounted on the pcb if desired.

There is now ground plane separation between 1st and 2nd stages and the high excursion output stage. This should improve intermodulation performance.

Board attached,

Hugh
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File Type: gif FetzillaPCB.gif (47.0 KB, 1113 views)
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Last edited by AKSA; 28th June 2011 at 07:50 AM.
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Old 27th June 2011, 09:30 AM   #45
AKSA is offline AKSA  Australia
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Andrej,

Yes, one 300x75 mm heatsink per channel, each rated to 0.4C/W. Each channel (one board) will dissipate around 25W, so that would raise the temperature 10C above ambient at idle, more when pushed hard. This is the same heatsinking I use for the NAKSA 100, you are correct.

I would think 4R loads would push this amp. It would work OK, but strain it at high volume. I would think 6R should be the minimum at highish volume. In this connection it's not dissimilar to the AKSA/Lifeforce 55.

At 350mA idle current with an 8R load, the first half watt would be pure Class A. At 750mA idle current (54W dissipation into the heatsink, 22C temp rise above ambient at idle), we get a full 2.25 watts of Class A. This is surprisingly loud in fact and is essentially a Class A power amplifier.

Thank you for your interest,

Hugh
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Last edited by AKSA; 28th June 2011 at 04:03 AM.
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Old 28th June 2011, 08:15 AM   #46
crispow is offline crispow  Hong Kong
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I will also take a pair Hugh
Chris p
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Old 28th June 2011, 08:40 AM   #47
AKSA is offline AKSA  Australia
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Done, Chris, thanks for the interest!

Do we have a mutual acquaintance, Ian McP?

Keep 'em rolling, now up to 34 boards. Check the graphic from #44, just been updated.

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand
2 Colin S. (?) (city?) NSW Australia
2 Chris Powell (?) Hong Kong China

Cheers,

Hugh
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Old 29th June 2011, 06:59 PM   #48
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Hello Hugh,
I'm also in for a pair.
I need something cooler than my F5 now that it is summer :-)
Regards,
Danny
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Old 1st July 2011, 12:28 PM   #49
AKSA is offline AKSA  Australia
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HI Danny,

You're in!! Now at 36 boards. Check the graphic from #44 for the PCB update.

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand
2 Colin S. (?) (city?) NSW Australia
2 Chris Powell (?) Hong Kong China
2 Danny (?) Lierde Belgium

Cheers,

Hugh
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Old 1st July 2011, 06:56 PM   #50
sonnya is offline sonnya  Denmark
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Add me as well. - 1 pair.
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