Class A from scrap

For the three and more legged things
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Mike H
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#16

Post by Mike H »

Appendix ~ Cricklewood Electronics have a range of audio polyester, if anyone is interested:

http://www2.cricklewoodelectronics.com/ ... hp?cat=151

Haven't tried them tho.
 
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Nick
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#17

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Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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Mike H
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#18

Post by Mike H »

Sorry no. :D
 
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brig001
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#19

Post by brig001 »

Interesting that this page http://greygum.net/sbench/sbench102/caps1.html shows capacitor characteristics against bias voltage. This suggests that I shouldn't go too high with rated voltage in the quest for low ESR.

Brian.
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#20

Post by Mike H »

That is interesting, and kind of bears out my fairly typical approach (latterly, comparatively speaking) which to use a higher Voltage one but the actual DC I put on it is confined to around 1 tenth up to about a quarter of the rated wkg V. Or a half, depending what's possible.

The non-linearity part of it is not so much of an issue for me, my pet bee-in-the-bonnet is what the dielectric does to the signal.

You'd think it's just two plates with an insulator in between, but of course the insulator still has electrons in it which are pulled and pushed around by the AC. That it is difficult (but not impossible of course) to make them leave the dielectric is what makes it an insulator. However if these electrons don't move uniformly at all frequencies is what adds garbage to the signal, to my way of thinking.

By adding a DC bias puts them "under tension" in one direction, so seems logical you can use that to "tune" the optimum operating region.

Fortunately I find most elecs OK as far as I can tell, albeit they're using a kind of semi-conductive jelly to boost the capacitance value (not just an insulator layer).


PS there may be a very remote possibility that page I mentioned might be on backups from the old PC, IF I saved the page and didn't "tidy up" afterwards.
 
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#21

Post by Nick »

You'd think it's just two plates with an insulator in between, but of course the insulator still has electrons in it which are pulled and pushed around by the AC. That it is difficult (but not impossible of course) to make them leave the dielectric is what makes it an insulator. However if these electrons don't move uniformly at all frequencies is what adds garbage to the signal, to my way of thinking.
I think you have to avoid thinking of the electron as a small ball, and consider the effect on its wave function. It won't leave the outer orbits, as if it did it would become a conductor and the cap become a resistor. I think what is more important (and I think what you mean) is the polar effects in the insulator.

http://en.wikipedia.org/wiki/Dielectric

The same effect happens in transformers.
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Mike H
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#22

Post by Mike H »

OK, I was close :lol:
 
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#23

Post by brig001 »

OK, found some time over the weekend and added a CCS made from the output transistors from an old class AB amp PCB, and changed the output capacitors for these http://cpc.farnell.com/panasonic/eca1ca ... dp/CA05734

Must say, the sound is better, but I think the capacitors are the biggest improvement - did that first.

I'm going to look for a case now with plenty of cooling as total dissipation is 15W, but on a brighter note, I now get 1W per channel out of it. Also going to see if one power supply per side improves things at all - I have a second one somewhere.

Brian.
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#24

Post by brig001 »

New schematic attached. T2 is actually a BUZ10 and T3 is a 2SB688. I must figure out how to add components, or try some other spice program.

The main question: is the arrangement of T1 and T2 a good one? It is either a Sziklai pair, or more likely a long tail pair with a tail chopped off, either way, I haven't seen it used like this before.

Any thoughts?
Thanks, Brian.
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Class A CCS.png
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colin.hepburn
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#25

Post by colin.hepburn »

That CCS looks very simmler to the well documented John Lindsey hood Class a Amp
http://www.tcaas.btinternet.co.uk/jlhupdate.htm
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#26

Post by Mike H »

brig001 wrote:The main question: is the arrangement of T1 and T2 a good one?
Perfectly valid, actually, is exactly how early p-p amps were done, before the long-tailed input pair became fashionable.

Also, spookily, is how the input & driver of my germs p-p amp has turned out in its latest iteration.
 
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#27

Post by brig001 »

Thanks Colin and Mike. Good to know it shouldn't be too far off. I'm interested in the JLH amp - it should be quite efficient for class A, unlike my room heater. Actually it's not to bad now with "just" 7.5W dissipated per channel when idle.

I'm going to leave it as it is for now until I get the speakers sorted out and have another listen then, but in the meantime it is sounding good on an old pair of missions.

Brian.
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#28

Post by colin.hepburn »

brig001 wrote:I'm going to leave it as it is for now until I get the speakers sorted out and have another listen then, but in the meantime it is sounding good on an old pair of missions.

Brian.
What 4/5 watt is driving missions what missions are they I thought most were about 89db so there's hope for an 8 watt EL34 single ended valve job driving my 89dB Wharfdales 8.3s perhaps then ?? :shock:
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#29

Post by pre65 »

colin.hepburn wrote:
What 4/5 watt is driving missions what missions are they I thought most were about 89db so there's hope for an 8 watt EL34 single ended valve job driving my 89dB Wharfdales 8.3s perhaps then ?? :shock:
Just get them started Colin, you WILL be amazed how they sound. :wink: :lol:
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G-Popz THE easy listening connoisseur. (Philip)
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#30

Post by chris661 »

Ummm...

It's a watt. Just one. The Missions are 760i, and go cheap on ebay pretty often. They have exceptional performance for their (often ~£20) price tag. SPL's around 88dB@1w/1m, so put a watt each into 2 of them, then there's a theoretical 94dB out.

The first watt is the loudest.
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