Switchable Hi-Z input impedance, how ? - Page 53 - diyAudio
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Old 24th April 2013, 09:20 AM   #521
tinitus is offline tinitus  Europe
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made some changes to my other preamp project

ordered better att pots
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File Type: png bass pre jfet new att.png (321.9 KB, 35 views)
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Old 24th April 2013, 11:36 AM   #522
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Again, why all the un-needed and pointless resistors round the pots? - at least 5, and 7 if you count the 1M grid resistors on the second FET's (which aren't needed either).
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Old 24th April 2013, 11:44 AM   #523
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Nigel Goodwin View Post
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
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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Old 24th April 2013, 11:57 AM   #524
tinitus is offline tinitus  Europe
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oh, btw, new main att pot coming is a dual concentric tandem thing...two controls in one
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Old 24th April 2013, 04:59 PM   #525
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Quote:
Originally Posted by tinitus View Post

it only works this way
controls are wrong values, old, worn, and noisy
It will work better if you connect it properly, on the attached image the red circles want removing, and the blue ones aren't really needed.
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Old 24th April 2013, 09:53 PM   #526
tinitus is offline tinitus  Europe
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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
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Old 25th April 2013, 05:15 PM   #527
AndrewT is offline AndrewT  Scotland
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then change one or all of them to buffers.
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Old 26th April 2013, 01:50 PM   #528
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Quote:
Originally Posted by tinitus View Post
no, tried without resistors, ofcourse
too much gain for that
Adding random resistors in the wrong places doesn't help - if you have too much gain, reduce the gain! - it's not a difficult concept. As a poorer solution, add a proper attenuator - not just random and incorrect connections.
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Old 26th April 2013, 03:15 PM   #529
tinitus is offline tinitus  Europe
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still good enough to reveal any adjustments on my 3way speaker
Im ok with that....considering its only a bass guitar

anyway, I found a small trafo with dual 15V secondaries...next try ...maybe Nelson's B1 buffered LDR volume control
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Old 26th April 2013, 03:41 PM   #530
JMFahey is offline JMFahey  Argentina
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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.

Click the image to open in full size.

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) :

Click the image to open in full size.

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 ) 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 , 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].
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