Tonestacks and impedance

[Branimir]

I've been looking into oh-so-ever popular FET emulations of the tube amp preamps, and I noticed that most designs include typical bass, middle and treble controls.

Usually the BMT tonestack is preceeded by a cathode follower (in tube amp circuitry) and I that it relates to source follower in the discreet circuitry.
I know the cathode/source/emmiter follower usually has gain of aproximately 1 (no amplification), and that it serves only to transform impedance,

Since I'm trying to omit the usual BMT tonestack from a typical 3 or 4 gain stage FET preamp, and use something else, like AMZ's tone control, do I need to have the last gain stage set up as a source follower or not?

This is my current work in progress:


Think I borrowed the tonestack from the OCD pedal, but I might try the AMZ one too... I put the switch so I can easily take the high roll off if not needed, I have no clue how it will sound through a tube amplifier, first thing I wanted to do is to simplify the circuitry and get rid of too many knobs, if this thing works okay, I'll dig into getting rid of the trimmers for bias, maybe to try mu-amp stages, or even mosfets...

Anyways, do I need the source follower here or should I make an another gain stage like before (common source), I know the BMT tonestack brings a lot of signal loss, this simple highs roll of should not introduce overall signal loss, and the AMZ's tone control should introduce no signal loss whatsoever?

Thanks!

ps: I think the pot wiring is backwards on the schem

[GibsonGM]

IMO, there isn't sufficient insertion loss from the ol' one-knobbers to merit dedicating a source follower/cathode follower to drive them.   A tad, but hardly worth worrying about.   Nothing more than a fixed RC network between stages.  Of course, the absolute values of the R's will have the final say.   Try before you buy - don't have to make it permanent!

Your circuit should provide PLENTY of drive to make the small amt. of loss negligible.   Remember, you're feeding an amp input, so just a few volts will overdrive it, not like pushing a tube amp's power stage...

In the future, if you use software like LT Spice or TINA, etc., you can figure out how much loss will actually occur!  You can also model what happens when you scale the R's and C's, which sometimes will allow you to omit the buffer and use a gain stage (altho bigger R's = more noise). 

Good question.

[Branimir]

Thanks for the swift reply, I only installed this software today, last time I used schematic software was almost 10 years ago, and before that I had some software on an 1,44" floppy that I used to draw pcb's and layouts, so I'm a bit outdated, and mostly did stuff by hand and breadboarded it later...

Will revise my schematic and try it, I'm currently doing this on perfboard so it's always prone to changes.

[Kipper4]

I'm with Gibson.
Not much need for Q4. It might be better behind the stack if at all.
A neat trick is to put a small cap (try less than 10nf) between your R4 and lug 2 of Rv1(gain)
So as gain increases so does the treble. It stops it from getting muddy.

As drawn your pots will work in an opposite way to normal.

I also found some small caps across (lug 1 to 3) of your Tr biasing pots.

[GibsonGM]

Thanks for the swift reply, I only installed this software today, last time I used schematic software was almost 10 years ago, and before that I had some software on an 1,44" floppy that I used to draw pcb's and layouts, so I'm a bit outdated, and mostly did stuff by hand and breadboarded it later...

Will revise my schematic and try it, I'm currently doing this on perfboard so it's always prone to changes.

Everything changes, that is the beauty of DIY!     Often I will assemble the bulk of a circuit, on breadboard or perf, and 'stop' when I get to a point like the tone stack.   Then I'll just 'line out' from it, to a breadboard and try different things. 

Esp. on tube circuits, where you have TONS of level!

LT has a bit of a learning curve, but there are many many tutorials online, and once you get the hang it's 2nd nature.   What are you using for that schematic you posted?

[Branimir]

  What are you using for that schematic you posted?

It's called KiCad, I think it was the first thing google showed up when I searched for some kind of schematic drawing cad for linux.

I'm with Gibson.
Not much need for Q4. It might be better behind the stack if at all.
A neat trick is to put a small cap (try less than 10nf) between your R4 and lug 2 of Rv1(gain)
So as gain increases so does the treble. It stops it from getting muddy.

As drawn your pots will work in an opposite way to normal.

I also found some small caps across (lug 1 to 3) of your Tr biasing pots.

The small cap trick - treble bleed cap as in various Marshall circuits? I am aware of it, but I was following actually the topology of my (now sold ) Laney VH100R amp that I modded a bit, and now I miss it's drive channel, thought I could use a distortion pedal to get a similar vibe on other amps too... Speaking of fet sims, just came across the Thor project from ROG, dunno why am I bothering with my design, seems they already did a similar thing with two gain stages + mu amp Heh, will give that one a try first, also a nice read on their website.

[GibsonGM]

Oh, I've seen KiCad but didn't install it, back when I had Fedora.

Everything under the sun has been invented, but that design is "yours", so don't feel like you wasted anything...