AMT Legend R2 pedal or schematic?
- Thread starter Paul.Ruby
- Start date
Paul.Ruby
Well-known member
Found... Now I'm looking for an O2.
Paul.Ruby
Well-known member
Don't know if you've seen my combo build thread. I've made a handful of PCBs for the AMT series (and many other things). I have a generic PCB for AMT LA2 pedals that fits in a 125B but meant to be the center of my combo pedal. I've not shared the fully generic PCB on PCBWay as yet but I'm willing to do so. I have shared a simpler version of it that only supports the ones with the fewest gain stages: B2 C2 E2 K2 M2.I haven't visited this thread in a while, I might have missed it. Strange, because I have the schematics for the E2, K2, S2, B2, D2...
Thanks for posting
Combo build for pedal board core: Pre-boost, Clean/drive channel, cabsim, noise-gate, post boost.
I've spent an inordinate amount of time creating a combo build with 3 footswitches in a 1590DD case. The picture tells most of the story. I'm happy to share info about it if anyone is interested... I'm wondering if anyone else has any interest in such a thing.... To make this all fit, I had to...
forum.pedalpcb.com
Paul.Ruby
Well-known member
Love it after tweaking the frequency response to match the real one. Still baffles me why I needed significantly different component values get a match. Both us guitar players in the band use them and get complements on our tone. But, we still use both use a 2x12 behind us and the the cabsim feeds the board through a balanced XLR driver built into the power amp pedals.
Paul.Ruby
Well-known member
I think I did see it in there... But I wouldn't bet a dollar on it.
temol
Well-known member
So you feed the real cab with a cabsimed signal?
I need to breadboard it. In the combo build thread you posted frequency response of the cabsim - looks like there's a lot of top end.
I've never been able to find the schematic for the O2. If it's been traced, I don't think it's been shared. Not sure how willing AMT would be to provide service manuals with factory schematics, but I've seen some online before
temol
Well-known member
@Paul.Ruby - O2 schematic appeared today on freestompboxes
Paul.Ruby
Well-known member
I believe it is emulating tube saturation. The softer clipping as a tube triode is pushed to higher currents. It's softer because it's not a hard clip like you get with a diode or LED in the feedback of an op-amp. Tubes, of course, will hard clip in the other direction as plate current lowers.
The J177 provides a variable resistance to ground on the gate of the 2SK208R. When the input signal goes up on the 2SK gate, the drain voltage of the 2SK comes down, just like a triode. This reduces Vgs of the J177 and turns it on and will limit the how high the input can go and how low the output can go. So, it appears to me to behave like gradual distortion as the input goes high (and output goes low) to emulate tube triode behavior.
I presume they choose to do this based on what ever real tube amp they are emulating. If the triode stage is biased hot and more susceptible to saturation distortion, they include the J177. If biased cold more toward hard clipping, then no J177 and, instead, the Schottky diode from 2SK gate to source in some way.
This stuff really impresses me about AMT. They went to great lengths to emulate the behavior of the real amp circuits.
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Paul.Ruby
Well-known member
I built two gain stages on a bread board, one with J177 and one without. Both have 15k drain resistor, 820R source resistor and 220k gate resistor. The J177 is actually sharpening the clipping rather than softening. Here's a picture. Yellow is input, red is no J177 and blue is with J177. This is with 9V supply. If the supply is raised, the drain voltage raises way up and keeps the J177 off for much larger input. At 18V, headroom of the stage on the lower end goes way up while on the upper headroom remains the same.
Now add a BAT48 Schottky from gate to ground and, of course, it really adds clipping to the blue curve for the same input and raising supply voltage does not increase headroom on either upper or lower end of the output. The blue curve is unchanged with supply voltage increase.
This seems to debunk my previous thought on the J177 influence on behavior. It can add more clipping at low supply voltage but seems to be nothing at higher supply voltage.
Now add a BAT48 Schottky from gate to ground and, of course, it really adds clipping to the blue curve for the same input and raising supply voltage does not increase headroom on either upper or lower end of the output. The blue curve is unchanged with supply voltage increase.
This seems to debunk my previous thought on the J177 influence on behavior. It can add more clipping at low supply voltage but seems to be nothing at higher supply voltage.
