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lndm 29th December 2006 12:20 AM

6J5 input swing
 
What is the best way to have a 6J5 accept a 7Vrms input cleanly whilst retaining a good operating point. I want to avoid an unbypassed Rk as I want a low rp for low noise.

Geek 29th December 2006 01:43 AM

LED's? Mu-follower?

SY 29th December 2006 02:08 AM

Nope and nope. You can't exceed the bias voltage at peak in order to avoid grid current. The requirement is odd- lndm, care to elaborate?

lndm 29th December 2006 08:07 AM

Sure. I am designing a phono/pre and have a cascode on paper. With EQ it puts out 7Vrms below 1kHz with my MM cart giving occasional peaks of 70mV. I simply want the second stage to endure these peaks and I have had trouble with another amp that couldn't handle that level. The cascode seems to be happy with this level.

Miles Prower 29th December 2006 08:24 AM

Re: 6J5 input swing
 
2 Attachment(s)
Quote:

Originally posted by lndm
What is the best way to have a 6J5 accept a 7Vrms input cleanly whilst retaining a good operating point. I want to avoid an unbypassed Rk as I want a low rp for low noise.
If you don't want to use an unbypassed Rk, the only way to do what you want is to run at a very high Vpk. Since 7.0Vrms is a bit less than 10Vp, that's what you're looking at: Vgk= -10Vdc. From the plate characteristics, that'll be right around 250Vdc. Depending on how large the passive load is, the DC rail will go a lot higher than that (or use an active load).

Other than that, unbypassed Rk is the only other option.

SY 29th December 2006 11:28 AM

Do you have passive EQ between the stages?

lndm 29th December 2006 06:08 PM

Yes, the 318us which allows 20kHz to be up by 20dB over the input.

SY 29th December 2006 07:12 PM

Other way around- that knocks down the level at high frequencies. With passive EQ, it's very doubtful that you need 7V of overload. But let's go through the exercise: what's your cartridge output spec and the first stage gain?

lndm 29th December 2006 07:38 PM

Below 1kHz the gain is 43dB. I am using an Ortofon OMB. I'm not aware of any specs for it, I just loaded the cartridge, played a few records and watched the screen. Most program material seems to be within 5-10mV with regular peaks at 30mV, some up to 50mV with some recordings, and occasionally 70mV.

Wavebourn 29th December 2006 07:49 PM

If you don't want a local feedback put a global one on the same cathode, no other way except attenuating input signal, i.e. loosing precious signal power.


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