reduce before amplifying?

[kathodyne]

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

 

[EC8010]

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.

 

[SY]

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.

 

[kathodyne]

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......

 

[SY]

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.

 

[kathodyne]

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....

 

[kathodyne]

@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??

 

[SY]

The 68K will not reduce anything except very high frequencies. It's a slightly oversized grid-stopper.

 

[kathodyne]

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

 

[SY]

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.

 

[morfeas]

3. use a tube with very low mu (6n1p mu=8,5) ???

6N1P mu=33 like ECC88, 6DJ8

Consider ECC82/12AU7 mu=17

 

[kathodyne]

Hitting myself

i was so fixed on this one problem that i overlooked something as simple as the input pot and it's value.....

simulated it with a circuitprogram and it turns out you can set ratios to the input voltage!! how could i've been so stupid to completely overlook this????

well i got my answer now!!

.....reducing gain with feedback looks interesting but really i should be able to get it working without before i will be able to use nfb..i just don't know how to use/implement it....