An overdrive/distortion.... A little different, this one.

[WGTP]

I just figured ou the drive control.  Cool.   

[brett]

That non-linear amplifier is sooooo cool  (http://www5b.biglobe.ne.jp/~houshu/synth/VcaMos0210.GIF). It has the biasing control and everything.

The other link (to the OTA schematic) also has interesting piece of information in it, despite it looking so simple.  Hmmm....the lower the supply voltage and Gm of the devices, the less linear the inversion.  I didn't know that, but in hindsight it seems likely to be a reason why many people run their tube-sound fuzzes at low voltage.  That sort of distortion seems analogous to what you get lots of in a class A tube amp.  (?)  My TSF-based circuit + shorted inverter is set to run at about 6V, but there's no reason why it wouldn't work ok at 4 to 5V, which might well make it less linear, and more compressed and tubey.

cheers

[Barcode80]

do we have a working layout for this? i'd love to give the build a shot and compare notes, but have no extra time to create a layout at the present.

[Ben N]

As long as this thread is still around, can we get an update on the links for Brett's schematic and layout? They have gone 404 on me.
TIA,
Ben

[brett]

Hi
the jpgs should be in the gallery.
My area is called something like "Brett's bin of bogus bits".
I'd be very pleased to hear anyone's thoughts.
cheers

[puretube]

Brett, Gez: Hsu, and g**gle: "Nauta" (Bram)... 

[gez]

Interesting.  So the extra stage with short from output to input makes transconductance independent of voltage supply.  Or, to put it another way, it loads the preceding amp to such an extent that gain is pitiful across the whole voltage-supply range! 

[brett]

Good work puretube.
And thanks for digesting that stuff Gez.  My brain is a bit dead tonight.

I hadn't thought of what it looked like in terms of a load -  "CMOS" and "low Zin" are stored in different parts of my brain.

So is the full-feedback inverter a "gain soak" for the previous stage because it is an active load (in the wrong direction) ?
thanks

[gez]

So is the full-feedback inverter a "gain soak" for the previous stage because it is an active load (in the wrong direction) ?
thanks

That's my understanding, Brett.  The author describes the preceding stage as the 'driver transistors' and the 'gain soak' stage as the "load transistors".  Adding more driver stages increases transconductance, and adding more load stages decreases it.  This is how he suggest controlling/setting gain.  IMO, then, your term "gain soak" is an excellent description. 

[WGTP]

So, is the effect the same as reducing the size of the resistors in the feedback loops?  I have noticed that it dropped the output some when connected.   

[gez]

So, is the effect the same as reducing the size of the resistors in the feedback loops?  I have noticed that it dropped the output some when connected.   

Not really, although if you don't include an input resistor and the feedback resistor is of low value, then you are going to load the previous stage as input impedance will be low.  It will still invert, though, whereas an inverter with a connection from out to in won't (it just loads).

[earthtonesaudio]

So, is the effect the same as reducing the size of the resistors in the feedback loops?  I have noticed that it dropped the output some when connected.   

Not really, although if you don't include an input resistor and the feedback resistor is of low value, then you are going to load the previous stage as input impedance will be low.  It will still invert, though, whereas an inverter with a connection from out to in won't (it just loads).

This still hurts my brain... 
So what if the feedback resistor is super small like 2 ohms?  Does it still invert? 1 ohm?  0.5 ohms?  Clearly these would give gains of less than negative one, but what does the transition between inverting and non-inverting look like?

Maybe I'll shed some light on it for myself by putting a small value variable resistor in the feedback loop, and then turn the resistor down to zero while watching the output on a scope or something.  Yeah, I think that would do it.

[gez]

So what if the feedback resistor is super small like 2 ohms?  Does it still invert? 1 ohm?  0.5 ohms?  Clearly these would give gains of less than negative one, but what does the transition between inverting and non-inverting look like?

With feedback resistance that small, and if no input resistor is included, then I'm pretty sure it would act the same as if there was just a wire from output to input (non-inverting, 'active load' as Brett put it).  With an input resistor, though, you'd have nothing/a minuscule signal at the output.

[puretube]

moretofollowsoon...

[David]

Hi
the jpgs should be in the gallery.
My area is called something like "Brett's bin of bogus bits".
I'd be very pleased to hear anyone's thoughts.
cheers

I think it's great your stuff is online again.  I don't see your Hall-effect wah or the Aussie compressor, though...

[TELEFUNKON]

sorry, folks,
but I don`t see an inverting buffer in U1f:
just a dead short between in & out;

I see two (shunting) FET-diodes going from the (center-biased) output of U1a to the respective rails. ("clipping"...)

maybe not "diodes", but "resistive loads" ? "constant current"?, "biasing devices"?
in Fig.2, here (N2/P2) an inverter is shown with in-/output shorted.

more theory, here...

Make that voltage dependant resistor workalikes, and you`re there where even the most ignorant
reworse-engineer can understand it.