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Old 8th September 2005, 08:51 AM   #1
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Default reduce before amplifying?

You may find this a stupid question, but i'm strugling with it....

i'm busy designing an amp with el84 SE, now i found out that i will bias it at -5v, thus needing a max. signal of 10v (14,14v pk-pk)

if i'm correct line-out of a cd-player is 1,7vpk-pk , needing a gain of 8,3.

but all tubes i found will....with an input of 1,7 give out way too much voltage swing (usually 20-50v), if i increase rk they will go into non-lineair regions...

is the trick just to place a resistor behind the input pot which will REDUCE the input before amplifying it???
what current does a cd player produce(max)???

hope someone can enlighten me a bit
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Old 8th September 2005, 08:54 AM   #2
EC8010 is offline EC8010  United Kingdom
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Standard output of a CD player is 2VRMS, or 2.8Vpk, or 5.6Vpk-pk.

Oh Lord, forgive me in advance for saying the words-that-should-not-be-said, but you need the extra gain for negative feedback. There, I've said it, got that off my chest. Now, I'll run away and hide.
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Old 8th September 2005, 09:05 AM   #3
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If it biases at -5V, doesn't that mean that the max drive signal is 10V peak-to-peak, not 14?

EC8010 is in one of his Black moods.
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Old 8th September 2005, 09:18 AM   #4
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sorry, thought the datasheets worked with rms voltages.....
ok, 10v it is pk-pk.....i'd really like to get it running withouth feedback first....feedback will cost some more winternight studying.......

do i have to calculate the input from the cdplayer as if it were coming from another tube

signal -> Rg -> cgk ??? a resistor in series with the signal(rg->cgk) would set a ratio for decreasing the input signal thus risking frequency attenuation through cgk
(see attached picture)

i hope i explained myself a bit......
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Old 8th September 2005, 09:20 AM   #5
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Datasheets use either or both, depending on what's appropriate. A bias voltage is DC, so that sets the peak limits for the AC drive.

In the circuit you show here, you can drop the gain easily by removing the cathode bypass cap.
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Old 8th September 2005, 09:24 AM   #6
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put the multimeter on my cd-player output....seems to give 1v max per channel(rms)......maybe because i'm in europe???
thougt there was a difference between line-levels between us and europe....
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Old 8th September 2005, 09:29 AM   #7
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@sy.... yeah i know, but it was used as an example, it's a different amp....
what does the 68K resistor do?? is this the ' input- reducer' and by placing such a resistor, do i have to take in acount the input impedance of my first tube or can i just use ohm's law to drop some voltage before entering the first tube??
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Old 8th September 2005, 09:30 AM   #8
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The 68K will not reduce anything except very high frequencies. It's a slightly oversized grid-stopper.
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Old 8th September 2005, 09:39 AM   #9
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ah....thanks....

so thats why 1,7v pk-pk dropped only to 1,6983v pk-pk.......

Then rephrasing the question:

With a tube as sensitive at it's screen as an el84.......how do you drive it by a pre-amp tube....???

1. reducing at input (how??? voltage-divider????< practical setup????)
2. using a tube to de-amplify??
3. use a tube with very low mu (6n1p mu=8,5) ???

sorry for being unclear.....it's just so much info working out a schematic and finding all sorts of little problems that i lose oversight sometimes
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Old 8th September 2005, 10:00 AM   #10
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I'd follow EC8010's advice and use feedback to get the gain where you want it. Instead of throwing gain away, you can barter for lower distortion, higher bandwidth, and lower source impedance.
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