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previous: Andy Fuchs The average guitar amp input stage ... -- 9/30/1999 12:14 PM view thread

Re: you don't need all that.....

9/30/1999 7:16 PM
kgRe: you don't need all that.....
Andy said:  
 
quote:
"The average guitar amp input stage offers an adequate amount of gain at a reasonable level of noise."
 
 
And this is undoubtedly true. And it is also precisely why I want to try to optimise it. I'm not really in for the gain--the only thing I need gain for is to give the signal enough gumption to be a little more immune to stray noises. The input stage is the single greatest contributor of noise in the amp... it's contribution gets amplified by all subsequent stages. Thus, attention paid here to minimise noise will be most beneficial.  
 
I want an UNREASONABLY low level of noise. Hell, I already have an UNREASONABLY high power output capacity--600W. Now I'm seeking the space in between the notes; I want it black as the cloak of the grim reaper. I don't want it with noise gates or downward expansion or bandwidth limiting. I want it with whacked out, well-engineered design and quality components.  
 
Why not use the right tool for the job--a high gm stage with all WW resistors, operated low and hot?  
 
quote:
"To gain a true benefit of balanced inputs, you would really need a balanced source. Okay, so center tap your pickups...."
 
 
Umm, what do you mean by balanced source? I don't need to center tap the pickup at all. The differential input has no care what the reference of the signal input is--that's the beauty of it. It doesn't matter whether the source is balanced or not. It's not about absolutes at all, but rather the simple, relative difference in voltage between the two input signal leads (+ and -). Ground reference doesn't enter the equation one bit. The only ground I need is for shielding purposes, and nothing more.  
 
quote:
"But seriously, switch the first tube to a 2SK30 ultra hi mu fet, and ditch the tube maybe."
 
 
Methinks you threaten to offend the tube gods... "Ditch the tube???"  
 
quote:
"Perhaps parallel the 12AX7 halfs, and reduce the gain back to where one tube was (and buy some noise db's)."
 
 
This is an option of course. Double the gm, double the Pa, half the rp, half the Rk and Rl, mu stays the same. Resistor thermal noise drops substantially.  
 
quote:
"But cascodes are super sensitive to outside world noises (I've tried them),"
 
 
Hence the balanced input with as high a CMRR as I can muster. I am not sure how a cascode can be any more succeptible to "outside world noises" than another circuit with comparable gain. Could you elaborate a bit on this?  
 
quote:
"...and they amplify the capacitance of the input source, so your cables 'sound' will become an issue."
 
 
I'm not quite sure I follow you here. When you say amplify the capacitance, what do you mean? The voltage output from the pickup remains unchanged. The loading of the input stage can be largely ELIMINATED, using a cascoded FET--the gate is a VERY high impedance, and when cascoded there is NO Miller capacitance multiplication going on since the voltage of the drain does not change. If anything, this extremely high input impedance will LESSEN the effects of the cable construction, not increase them.  
 
Ken Gilbert  
 

 
Replies:
Andy Fuchs Firstly, the noise generated by the... -- 10/1/1999 12:22 AM