Switchable Hi-Z input impedance, how ?

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made some changes to my other preamp project

ordered better att pots
 

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Ex-Moderator R.I.P.
Joined 2005
Again, why all the un-needed and pointless resistors round the pots?


hello Nigel, good to hear from you again

yeah, flawed curcuit, I know

it only works this way
controls are wrong values, old, worn, and noisy

no, not very good,.... but sound is...playable :joker:
and, while fooling around, Im getting closer to get a feeling of what I want

will pick up the BJT buffer again later, and try to mount pots on output
but through all of this, its essential I always have to have a functioning preamp
 
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no, tried without resistors, ofcourse
too much gain for that

I started without the resistors
not good because of curcuit
attenuation works better with resistors
tried several other values
these gave the best result so far

but it doesnt matter
will have new att pot in a few days
onboard pot might be converted to a baxandall bass tone control

but until then, this one will do nicely

but thanks for your concern
 
FWIW I *think" that our friend Tinnitus is "designing" the way many do, by "borrowing" designs from tube amps (where they may have a reason to exist) and adapting them to FETs (where they in principle don't).

Many very successful Guitar amps do exactly that: instead of setting stage gain to the proper value, drive them always full throttle and pad output to the desired level/gain.

There's 2 reasons for it:

a) tubes are old technology, can't easily be set to any convenient gain as, say, Op Amps; all they can do is choose 1 out of a couple available tubes (today's Menu: 12AX7 *or* 12AT7 :( ) , modify plate resistors within a narrow range or choose to bypass or not the cathode resistor. Pretty crude.

2) besides gain, in this case tubes are meant to overdrive.

Being unsymmetrical, sound changes *a lot* whether you choose to clip first the top peak, or the bottom, or both more or less at the same time.

So if you found a nice sounding bias/load combination, you can't modify gain, you are stuck, so you add otherwise unneded extra resistors.

Look at this Mesa Boogie Preamp.

An externally hosted image should be here but it was not working when we last tested it.


You will spot at once many "unneeded" resistors, such as the ones in series with V1a/V2a/V3a grids (which in theory have "infinite input impedance") ; in this case they are used as lowpass filters coupled to the unseen but very real Miller capacitance shown by tubes; the 22K resistor in parallel with the 22K "presence" pot (why not use a 10K one? ) ; the other 22K in series with said control which *severely* limits its range , etc.

Ok, all those look like post-design tweaks to get tiny little sound changes, absolutely by ear, which probably were liked by certain Famous Musicians. Fine.

But copying values and applying them to fresh and *very* different designs, not because they were calculated or arrived at after long tiring live tests, but simply because "Mesa/Marshall/Soldano/etc. does it this way" is , as you point , pointless. (pun intended ;) )

For a nightmare of factory tweaks, and a zillion switchable mods, the Peavey 5150 (designed specially for Eddie Van Halen) :

An externally hosted image should be here but it was not working when we last tested it.


Later Mesas are known to have up to 24 relays and a dozen Vactrol (Led/Ldr pairs) to get different "modes".
Oh well :)

By the way, there is a very popular Forum where all they do is take classic Tube schematics and reproduce them with FETs .... with the original Tube resistor values , without realising they those were chosen to provide *specific* load, gain, and above all proper *bias* for those tubes.
Absolutely ridiculous when applied to Fets without much thinking (what :confused:) and even worse because now we have a 9V supply insted of original 160 to 300V.
So each and every one of those FETs requires an individual trimmer :eek: , which to boot does not control bias (it should be placed in series with source) but load (they are in series with drain) which makes each stage vary wildly and at random, both in gain and impedance.
Oh well [2].
 
On JMFahey's comments, I agree with all that he said. I still scratch my head when looking at the EVH 5150 amp! For a lot of reasons, maybe subjective but ''meltdown'' overdrive in the preamps on many later model tube amps is a bit tiring on my ears.

Tinitus, I would suggest again that for a bass pre, that a simple fet gain stage with attenuator then emitter follower out (to drive loads/passive filters etc) should be more than enough.
Building simpler, well constructed circuits IMHO are going to sound quite good, an ideal probably lost at Mesa engineering (and many many others) but an ideal of which many are followers of here on this forum. Simpler doesn't mean ''easier'', as JM pointed out with fets, you need to get the source biasing/drain current and gain right. Fets aren't plug and play.
 
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now working on a very nice distortion 'machine' :D

hey, Im not going to use any of these curcuits for very long...
but man, not long ago I didnt even know the words emitter, drain...or whatever
that is if its of any use to me at all
I guess only if its possible to learn from mistakes :clown:

but will soon leave this crap, and build other properly designed curcuits ...and thats a promise ;)
 
That's the point Tinitus, keep trying stuff, and ''mistakes'' are lessons learned. If you take some valuable knowledge away from it, great. Have fun! Better to try this stuff on low voltages when you are starting, If you blow a fet or 2 no big deal. But mistakes with high voltage tubes can be dangerous and harmful. An underrated cap or resistor can not only damage the circuit..but the builder as well. YOu need to know theory and foremost follow safe practices before starting on that.
 
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