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Old 6th May 2003, 02:26 PM   #1
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Default 6C45 PP IT 845SE amp.

Here is the schematic of my latest design. It has a PP driver stage IT coupled into an 845 SE output stage. I had been given a pair of 845 and I wanted to use them in a two stage amp. I needed a voltage gain of about 90 from the driver stage. As the mu of the 6C45 is between 40 and 50, a push pull pair would nicely deliver the 90 times voltage gain.

I was worried by the chance of my son getting a 800Volt shock whilst probing the circuit so I decided on a + & - 400V supply for the output stage taking advantage of the ITs ability to isolate the ground structures throughout the amp.

I have just finished quick sketches of the amp and bipolar supply. This is an evolving breadboard at present so the values of components are changing...

All comments gratefully recieved.

ciao

James
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Old 6th May 2003, 02:28 PM   #2
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Default ...bipolar psu...

Here is the B1+ and B1- psu.
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Old 6th May 2003, 04:23 PM   #3
Aiace is offline Aiace  Italy
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Default Electroshock !!!

50 mA il lethal for a man !!!
The electrical resistance of your (cleaned) body is about 1k, so 50mA x 1K = 50Volt. If you afraid the healt of your son, you have to use under 50 volt in your circuits.

Bye, Aiace.

P.S. Your project is very nice but not very cheap, isn't it.
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Old 6th May 2003, 06:12 PM   #4
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Default Re: Electroshock !!!

Quote:
Originally posted by Aiace
50 mA il lethal for a man !!! The electrical resistance of your (cleaned) body is about 1k, so 50mA x 1K = 50Volt. If you afraid the healt of your son, you have to use under 50 volt in your circuits.
If it's across your heart. You can take much larger hits and live (I have) if you're lucky and it doesn't directly bridge your chest. But don't tempt fate! I even have a photo sequence I used to use in class of a man who grabbed 66kV and lived. But you <i>really</i> don't want to end up like he did. Yuck.

<b>Obvious disclaimer: work safe, one hand in your pocket at all times on anything above 32V, and if you don't know what you're doing, and the correct safety practices, DON'T!.</b>
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Old 7th May 2003, 08:31 AM   #5
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Default ...Burnout...



I was always taught that 100-200mA across the heart was the fatal level. To check I searched the web and the following URL confirms this level and has some interesting safety data. It is worth a read.

http://www.dartmouth.edu/~machshop/safetelec.html



ciao

James
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Old 7th May 2003, 06:02 PM   #6
EC8010 is offline EC8010  United Kingdom
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Default Ker-baaang!

I found myself agreeing with most of what the Dartmouth post said until I came to the bit where it told you to short-circuit capacitors with a screwdriver. Now I know we've probably all done this, but we don't do it anymore because:

(a). The arc sends molten metal everywhere.
(b). It doesn't do the capacitor any good.

Either have a bleeder resistor permanently wired in circuit (best), or one that can be safely dabbed across capacitors.
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Old 7th May 2003, 06:09 PM   #7
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Default Shifting ground

James,

I like the idea of the PP 6C45 to drive the 845, but I'm wondering about the power supply, and particularly about the inductors in the 0V rail. If the signal current from the + rail is different from the - rail, won't it cause a voltage to be superimposed on the 0V?
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Old 7th May 2003, 07:18 PM   #8
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EC8010,

There is that potential but that is the reason for the delta configuration of the psu capacitors. This way they can couple between whichever pair of rails the signal return appears in. I've started looking at 3-phase psu and how to arrange them and the design is based on that but I've only just started looking at the theory so I might be completely wrong!!!

The breadboard version that I have running doesn't have the earth line chokes as I ran out of them. It is working very well without so I might just drop the idea.

How about the 6C45 PP for your tweeters?

I agree about the screwdriver and capacitor! I learned about bleeder resistors the hard way - on the 25kV EHT supply for a Phillips colour monitor Damm bleeder was open circuit and hadn't discharged the smoothing capacitor. THe old saying is true - 'Any lesson that doesn't kill you leaves you wiser' but that one was too close a call.

ciao

James
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Old 7th May 2003, 07:32 PM   #9
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Default CHOKING ON CHOKES?

Hi,

Quote:
If the signal current from the + rail is different from the - rail, won't it cause a voltage to be superimposed on the 0V?
It would.

Quote:
I've started looking at 3-phase psu and how to arrange them and the design is based on that but I've only just started looking at the theory so I might be completely wrong!!!
IMHO, a 3 phase is not what you want...you need a bipolar PSU so putting chokes in the + and - rail should do the trick nicely.

If you make both rails independent from eachother ( the purist approach) than you can move the choke into each earth return, no problem there as far as I can tell.

Cheers,
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Old 7th May 2003, 07:47 PM   #10
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Quote:
Originally posted by James D.
How about the 6C45 PP for your tweeters?
I did wonder, particularly since I've got lots of them, but the Miller capacitance would be horrendous. OK, I could neutralize them, but I think I would end up with an amplifier having a completely different character to the main amplifier.
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