AMT - Legend Amp Series 2 [traced]

[rutgerv]

Great thread! I'm working out a DIY recording box with variable input impedance (pickup loading), high quality buffering, a line level output (to interface to my sound card) and an insert-point for bringing more pedals into the recording chain. I'm planning to use it to record guitars on the computer, mostly with amp modelling, though I'm considering to take most of the high-gain overdrive into the analog domain as well, as I'm not too fond of most amp modellers in that respect. That leaves only power amp and cabinet modelling duties to the computer.

I was considering to try some variation to the FET pre-amp modelling theme, and highly impressed with the VHT clone that AMT made. From youtube I grabbed the attached stills from the internals of the VT2 (which I believe we're still missing in this thread). I'll attempt to trace it myself, in order to learn from the circuit design, but if someone else beats me to it, I won't mind. I have not too much time these days for DIY stuff, but if I get it fully traced, I'll post the resulting schematic.

Best,

Rutger

[Dave37LP]

R2
https://www.dropbox.com/s/64o486qjrf3tr40/R2.zip?dl=0

Please RockKenny, reupload the images of the AMT R2, that link is broken now. I´m interested in that model of the series. Maybe someone else has the file?
Thanks in advance.

Dave

[RockKenny]

https://www.dropbox.com/s/e7pw5wq1ifn40n6/R2.rar?dl=0

[mensur]

Here is assambled R2 schematic from the pictures that are posted
Enjoy

[rutgerv]

I all, I've traced the Vt2 and am very close to testing the circuit on a PCB. In the mean time the AMT V1 also caught my interest, and I saw a schematic floating around. It seems like a Spice simulation of the circuit, and I can't tell if it uses the same components originally selected by AMT. Does anyone know what's inside the V1? What are the diodes and JFEt types? Pictures of the PCB would be even better!

[Gila_Crisis]

I just got 2 these pedals: the R2 and the K2.
Both sound stellar but the clean channel on the K2 to my surprise is more gainy and brighter, like there's a high knob fully open. R2's clean sounds better, more balanced and fuller.
I was thinking in a different tolerance in the transistors around clean channel, but then I decided to look inside the pedal and found out one resistor is different:
R43P (I follow Bajaman schematic) is 130k in the R2, while the K2 has a 100k resisistor instead.
Now I'm away for the next few days, but asap I'm back I would like to check also the other side of the pcb, to see if the caps are all the same or there are some differences.
In the meantime: Do you think this resistor R43P can have such a big impact on the sound?

[Paul Ruby]

Since I own several M2 pedals, here's my contribution of a traced M2 drive channel. There are also a few minor differences in the clean channel as well: R22P 200R; C36P not installed; R37B 2k7; R16B 68k. I have hi-res images as well if desired.

[Paul Ruby]

Have drawn the generic schematic into KiCad and used P2 component values. Here's the screenshot from KiCad of the P2 drive channel including all components. The links are "0R" and the non-installed are "XXX".

EDIT: Changed the naming convention of the components to allow KiCad to understand the annotations. The numbers still match the real PCBs but the number is at the end of each reference. For example, R10P is now labeled RP10. Also found a missing wire and a mislabeled component.

2nd Edit: One connection missing.

[Paul Ruby]

And the C2...
EDIT: found a few mistakes, dang it. Replaced the file.
And again...
Mixed up CP31 and CP35. Another edit...
The new values after editing are...
CP2=22nf, CP33=2n2, CP31=68nf, CP35=100nf

[Paul Ruby]

Since I own several M2 pedals, here's my contribution of a traced M2 drive channel. There are also a few minor differences in the clean channel as well: R22P 200R; C36P not installed; R37B 2k7; R16B 68k. I have hi-res images as well if desired.

I can no longer edit the post but the Volume POT is 220kA, not 100kB.
EDIT... But I can still edit this one, dang it. R27P is 200k. Incorrectly non-existent in previous schematic.

[Paul Ruby]

I just got 2 these pedals: the R2 and the K2.
Both sound stellar but the clean channel on the K2 to my surprise is more gainy and brighter, like there's a high knob fully open. R2's clean sounds better, more balanced and fuller.
I was thinking in a different tolerance in the transistors around clean channel, but then I decided to look inside the pedal and found out one resistor is different:
R43P (I follow Bajaman schematic) is 130k in the R2, while the K2 has a 100k resisistor instead.
Now I'm away for the next few days, but asap I'm back I would like to check also the other side of the pcb, to see if the caps are all the same or there are some differences.
In the meantime: Do you think this resistor R43P can have such a big impact on the sound?

R43P is probably not a key value change by itself. But, I have a few of these pedals and find the following are likely much more important... In some pedals C36P is excluded entirely and in some it is only 470pf. R43P, R44P and C36P are doing some filtering of the high end, so check C36P. The other change I see in some is C5B is 15nF instead of 100nf. This might be taking out some low end.

[Paul Ruby]

Found a mistake in the cab sim output schematics posted here. The transistor types of VT6B and VT8B are reversed. Here's a picture... VT6B is the 5458. VT8B is the J177. As drawn in the schematic, the two transistors act like two forward biased diodes in series to ground, shorting it out. The incorrect snippet of the schematic is also shown here.

[blackboarcult]

Since I own several M2 pedals, here's my contribution of a traced M2 drive channel. There are also a few minor differences in the clean channel as well: R22P 200R; C36P not installed; R37B 2k7; R16B 68k. I have hi-res images as well if desired.

I can no longer edit the post but the Volume POT is 220kA, not 100kB.

Thanks for the trace, mate!
Shouldnt there be a resistor to ground between C18 and R18? I fail to see how VT8 would get its bias at the gate the way it is drawn

[Paul Ruby]

Since I own several M2 pedals, here's my contribution of a traced M2 drive channel. There are also a few minor differences in the clean channel as well: R22P 200R; C36P not installed; R37B 2k7; R16B 68k. I have hi-res images as well if desired.

I can no longer edit the post but the Volume POT is 220kA, not 100kB.

Thanks for the trace, mate!
Shouldnt there be a resistor to ground between C18 and R18? I fail to see how VT8 would get its bias at the gate the way it is drawn

You have a good eye!!! Dang it...
That is R27P that I incorrectly marked as non-existent. But there it is... 200k.

[blackboarcult]

Niice! I will have to breadboard this one of these days. For some reason all jfet marshall emulations I've tried to this day have been a wee lacking. Never 100% satisfied. 10000th time's the charm

[Paul Ruby]

Niice! I will have to breadboard this one of these days. For some reason all jfet marshall emulations I've tried to this day have been a wee lacking. Never 100% satisfied. 10000th time's the charm

M2 is the core of my live pedal board. I'm old and won't carry around a tube head anymore... I'm tracing these out to make some PCB modules for a custom pedal with all the key things in one: Prince-of-Tone pre-boost; M2, both clean and drive and the cabsim for direct to house; Spark Mini post-boost; Zuul noise gate. This will all be in a 1590DD case with three footswitches. I have all the PCBs. Building and debugging now... So far, only the cabsim has been fully built and debugged. M2 is built and ready for debug (with a bodged in R27P).

[Paul Ruby]

Have now built the M2 drive channel, Clean channel and Cabsim as independent modules and they all sound identical to the M2 pedal. Note, in the clean channel, do not install C36P. This adds too much low end (actually cuts too much top end). My real AMT M2 and P2 pedals do not have C36P installed. My C2 uses 470pf. I suggest installing nothing there. 3n3, at least, makes it sound very dull. Since the clean channel has no EQ adjustment of it's own, you will need to select component values you like. I like it exactly as it is in the original schematic with the exception of leaving out C36P entirely.

[Paul Ruby]

It's coming together. At the center is a full M2 pedal with clean, drive and cabsim. On the left is a zuul noise gate and mini spark lead boost. On the right is a prince of tone boost. An AMT C2 drive channel is also there as an alternate drive channel. Holding the channel switching footswitch down for a half second will change between using the M2 or C2 as the drive channel. An external drive can also be used as the alternate using a send/return that is wired to replace the C2 drive module. All the PCBs worked fine despite some mistakes here and there needing a bodge.

[Paul Ruby]

Another build update... In a previous post, I suggested leaving out C36P because it was not installed in my real M2 pedal. But, the result was a shrill clean channel. I found myself adding a 1k resistor in series with C12P, which boosts the high, to reduce the shrill tone. But, in the end, I removed that 1k resistor and installed C36P and that resulted in a perfect clean channel. I'm really impressed with the amount of work AMT put into these to get the frequency response perfected.

My pedal is very modular, despite being hard-wired in an individual build. The Zuul and Spark on the left are always there with the Zuul always on. The full LA2 in the middle can be any LA2. The PCB has all the component options on it, but I really like the M2 as the core of my board. I have several different preboost PCBs that all fit the same three knob holes: Prince of tone, TS-808, OCD, Plexi Drive, Landgraph OD (which is a TS-808 with one capacitor change and different clipping choices). I also have a REVV G2 PCB for pre-boost but that needs two more knob holes. Above the pre boost is an alternate drive channel. I have a PCB that can make B2, C2, E2, K2 or M2 as a standalone drive module to fit that spot. And I have a send/return for a 3rd drive channel as well. The picture below shows this in action to compare the various LA2 drive channels. M2 and K2 are installed in the main pedal and then B2, C2 and E2 in separate modules for comparison. But, any OD pedal can be put in the 3rd option as the drive channel.

The channel select footswitch has an ATtiny84 as it's controller to manipulate 3 latching relays. It always switches between clean and drive but, if held down for a half second, it will switch to the next available drive channel option: M2, then other internal (K2 here), then external (C2 here) and back to M2. Once you see the one you want, release the switch to make it the drive channel. It also senses if there is an external drive channel plugged in and will skip that if nothing is being returned there.

[Paul Ruby]

FYI, I found the reason for my confusion on C36P. The problem is that the original schematic posted here is mislabled. C36P in the schematic is actually C26P on the real boards. And it is installed on all my real pedals.

There is also another actual C36P on the board. This one is correct:

realC36P.png (39.07 KiB) Viewed 225 times