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Old 20th December 2012, 12:19 PM   #7871
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Join Date: Mar 2007
Location: Oxfordshire
Just was sent this ( Thanks Chris ) .


Pythagoras' theorem - 24 words.
Lord's Prayer - 66 words.
Archimedes' Principle - 67 words.
10 Commandments - 179 words.
Gettysburg address - 286 words.
US Declaration of Independence - 1,300 words.
US Constitution with all 27 Amendments - 7,818 words.
EU regulations on the sale of cabbage - 26,911 words
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Old 20th December 2012, 07:00 PM   #7872
a.wayne is offline a.wayne  United States
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LOL....

Guys, i need to drive two amplifiers with one pre , do i have to do any impedance matching or can i just parallel the damn inputs .....
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Old 20th December 2012, 07:27 PM   #7873
dvv is offline dvv  Serbia
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Quote:
Originally Posted by a.wayne View Post
LOL....

Guys, i need to drive two amplifiers with one pre , do i have to do any impedance matching or can i just parallel the damn inputs .....
I think that depends mainly on the preamp's drive capability.

There's a reason why I used 50W (bipolar, 75W for MOSFET) devices for my preamp output. You could stack 3 Otala amps, with its ridiculous 6K8 input impedance, and it wouldn't notice. Stack 'em up, Wayne, it's all right, so long as the ultimate impedance the preamp sees is 8 Ohms. It can do even lower, but distortion starts to rise.

I was anticipating it having to drive 3 separate power amps.

But if your preamp has a weedy op amp output, well then you're in trouble.
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Old 20th December 2012, 07:29 PM   #7874
dvv is offline dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
Just was sent this ( Thanks Chris ) .


Pythagoras' theorem - 24 words.
Lord's Prayer - 66 words.
Archimedes' Principle - 67 words.
10 Commandments - 179 words.
Gettysburg address - 286 words.
US Declaration of Independence - 1,300 words.
US Constitution with all 27 Amendments - 7,818 words.
EU regulations on the sale of cabbage - 26,911 words
Friendly greeting - 2 words, second word "off!".
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Old 20th December 2012, 07:33 PM   #7875
dvv is offline dvv  Serbia
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Originally Posted by nigel pearson View Post
I feel with a clean supply it works very well . My beloved Hitachi design has a 47 K tail resistor . 7 active devices which could be reduced to 5 if the double VAS was made single VAS and bootstrap CCS if so . Some will tell you bootstraps need nearly 100 % positive feedback to work . 70 % still is usable . 97% as in complimentary feedback pair is a bit high and sometimes needs a series resistor . 94 % is ideal .

The Hitachi I sometimes build is 8 devices + 3 diodes as I use a tail CCS .
Nige is one of those people who first design an amp, and then start taking bits and pieces out from it until it stops working - he then quickly inserts back the last piece taken out, and calls it a day.
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Old 21st December 2012, 07:07 AM   #7876
dvv is offline dvv  Serbia
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It is 8 AM locally as I write this, and the very fact that I am writing it proves that the world is still here.

Seems we beat the rap this time.

For the time being, at least unto 12:12 PM 21.12.2012. Just in case, I'll postpone paying the credit card bill, I really don't want to go out poor.
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Old 21st December 2012, 10:43 AM   #7877
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Originally Posted by dvv View Post
Nige is one of those people who first design an amp, and then start taking bits and pieces out from it until it stops working - he then quickly inserts back the last piece taken out, and calls it a day.
Correct
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Old 21st December 2012, 11:58 AM   #7878
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Join Date: Mar 2007
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Quote:
Originally Posted by a.wayne View Post
LOL....

Guys, i need to drive two amplifiers with one pre , do i have to do any impedance matching or can i just parallel the damn inputs .....
If the output impedance of the preamp is at least 5 times lower than the paralell input impedance ( resistance ) of the two amplifiers it probably will work . One problem that often occurs is if one amplifier is switched off the diode of the input stage be it transistor or valve will cause distortion . Use some cheap speakers to be sure there are no ground loops or worse . Parallel resistance is Rt = A x B / ( B + A ) input A and B that is . I was taught to say it that way because it is derived from 1/Rt = 1/A + 1/B and B + A is from transposition . + being commutative it can be A + B .
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Old 21st December 2012, 12:11 PM   #7879
dvv is offline dvv  Serbia
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It appears the End of the World was arriving by Serbian railways, and is consequently slightly late. However, it will get here, just give it a few million years ...

Or, it may have arrived, walked out of the Belgrade railway station, looked around and thought: jeez, wrong address, I've been here already.
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Last edited by dvv; 21st December 2012 at 12:14 PM.
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Old 21st December 2012, 12:15 PM   #7880
dvv is offline dvv  Serbia
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Join Date: Sep 2004
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Quote:
Originally Posted by nigel pearson View Post
If the output impedance of the preamp is at least 5 times lower than the paralell input impedance ( resistance ) of the two amplifiers it probably will work . One problem that often occurs is if one amplifier is switched off the diode of the input stage be it transistor or valve will cause distortion . Use some cheap speakers to be sure there are no ground loops or worse . Parallel resistance is Rt = A x B / ( B + A ) input A and B that is . I was taught to say it that way because it is derived from 1/Rt = 1/A + 1/B and B + A is from transposition . + being commutative it can be A + B .
Beggin' yer pardon, good sir, wasn't that supposed to be 10 times, rather than 5?
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