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Old 23rd August 2003, 12:31 AM   #1
diyAudio Member
Join Date: Dec 2001
Location: SIUE, Illinois, USA
Default topology and PSRR

firstly, i'm still basically learning electronics. in any case i'm working on a project that interest me that i want to use as a learning experience. its an overdriven tube pre-amp and a gainclone. why? most people are saying that the SPIKE protection circutry in the national power ICs is sonically bad and should be avoided.

so i was thinking about what would happen if you basically pre-distorted the input so that it is compressed/limited a bit. from more research i see that tubes have some of these abilities, and more so at low plate voltages.

so i was looking at requirements for this combo:
1.) the spike protection must never be activate during even clipped input signal. so output power will be limited, and i accept this for a prototype/fun test.
2.) the PSRR of the tube pre-amp limiter would have to be high because i'd like to keep this thing small. having multiple chokes that can't be within a half foot of anything is bad for size!
3.) there are 3 tubes that come to mind for this application: ecc86. ecc88. 12u7. i've heard the ecc88 is decent for low voltage apps. nothing about the 12u7.
4.) a minimum number of tubes should be used per side i wanted 1 tube per input channel in order to reduce any error between tubes.. i may make amps for bi amping that use a common tube preamp but i'm worried that the effects of the audio crossover will affect the signal and accidently trigger SPIKE in rare cases.

but i was looking into the topolgy for the input, like i've seen some long tailed pair designs, and at low voltage i feel i could use a current source in the tail instead of a resistor, but i forgot if that affected PSRR any. cathode follower is ok, but i'm worried it may be harder to overdrive to get the pre-distortion i am trying to test. and i'm not sure if it has better PSRR then anything else.

so the main question is which topolgy has the best PSRR. the purpose is so i can try pre-distorting a signal in the hopes that the added distorting sounds better than an alternatively added distortion. the PSRR is becasue i hate hum.
if only it could be used for good, not evil...
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Old 23rd August 2003, 12:36 AM   #2
tiroth is offline tiroth  United States
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Join Date: Dec 2001
Location: Pittsburgh, PA, USA
5687 LTP is a natural; other tubes can work well here. CCS much more linear.
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