AMT - Legend Amp Series 2  [traced]

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J0K3RX
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Post by J0K3RX »

Lookie what I found..... :twisted:
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AMT S2.gif
AMT K2.gif
AMT E2.gif

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Post by The G »

Nice :applause: !

Hmm, but 1k input resistance?

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Post by deltafred »

ggedamed wrote:Nice :applause: !

Hmm, but 1k input resistance?
It does seem a bit low, unless it is a typo.
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Post by soulsonic »

It doesn't say which transistors are what, does it? I see where it lists the transistors used, but I can't tell which corresponds to which position.
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Post by DrNomis »

Looks interesting..... :thumbsup
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Post by Ice-9 »

soulsonic wrote:It doesn't say which transistors are what, does it? I see where it lists the transistors used, but I can't tell which corresponds to which position.
nice post jok3rx

Very likely 2SK208GR as most of the other AMT stuff use these Jfets.
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Post by Ice-9 »

after having a look at the S2 diagram and using this as an example it looks like T5-7-9 are being used as the replacements for the BAt diodes in the bias arrangement which now take feedback from the output of its jfet amp stage into the gate. I would like to further look into this arrangement.

Any thoughts on this.
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Post by bajaman »

Thanks for posting these schematics - but I suspect many mistakes in them - the cabsim output stage is impossible to bias correctly and function as drawn - 150 ?
I will try a few elements in my simulator and see what can be done to get working versions although these pedals are priced very cheap on Ebay ;-)
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Post by bajaman »

so many mistakes - how are the input jfets T1 and T2 biased ? There is no gate bias resistor - the 1k input resistor is obviously wrong too - I suspect R2 and R3 should be 10k not 1k etc
Upon checking the S1 schematic - the S2 bears very little resemblance :roll:
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Post by The G »

soulsonic wrote:It doesn't say which transistors are what, does it? I see where it lists the transistors used, but I can't tell which corresponds to which position.
MMBFJ177 and MMBFJ270 are P-channel JFETs specialized in switching, and that's what they're used for. The N-channel JFETs are, as Ice-9 says, the same 2SK208 AMT are using in all pedals. The info is not in the schematics, but I remember they used 2SK208-R, which would be consistent with the use of Schottky ddiodes in the JFET gates.

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Post by bajaman »

spent the best part of a cold winter Sunday simulating the S2 circuit as per the schematic - result = it does not work as drawn :secret:
So many errors and wrong component values makes these schematics next to useless - the only part that appears to work correctly is the drive out filter network :lol:
the cabsim section will not work as drawn even if the two 2k1 resistors which are shown not connected to anything are connected to the 9 volt rail :roll:
I would suggest that someone opens one of these LA2 pedals and takes some hi res photos - at least we will then be able to establish the correct resistor values :slap:
Not wasting any more time on this
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Post by Ice-9 »

bajaman wrote:spent the best part of a cold winter Sunday simulating the S2 circuit as per the schematic - result = it does not work as drawn :secret:
So many errors and wrong component values makes these schematics next to useless - the only part that appears to work correctly is the drive out filter network :lol:
the cabsim section will not work as drawn even if the two 2k1 resistors which are shown not connected to anything are connected to the 9 volt rail :roll:
I would suggest that someone opens one of these LA2 pedals and takes some hi res photos - at least we will then be able to establish the correct resistor values :slap:
Not wasting any more time on this
bajaman
It happens that I bought the S2 last week , but still waiting for delivery, I will post a few pics when it arrives. I like the stuff AMT do and they are a decent price also. I still own my P1
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Post by bajaman »

Hi Ice-9
That is excellent news - I like the sound demos of the B2 and was considering purchasing one for comparison ;-)
Maybe next month :!:
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Post by Ice-9 »

My Legend 2 pedal arrived today, and once again the people at AMT have made a great sounding pedal. I'm pretty impressed with the sound although haven't tried the cab sim yet but if it is like my P1 pedal it will be great to DI into the desk.

Anyway I have taken a couple of pictures of the boards and had a quick look at the circuit. This is the S2 and in reference to the diagram at the start of this thread I have looked at the 2 input buffers. R1 is a 2M resistor and both jfets are biased with a single 2M resistor from the gates to Vref. Both buffers have a source resistor which is 22k. I started with looking at the PSU section which is pretty standard with a voltage divider network for Vref.



I won't get much time to look at this pedal for a couple of weeks as I'm off on holidays this sat.
Attachments
PCB 2 Top.JPG
PCB 2 Bottom.JPG
Main PCB Top.jpg
Main PCB Bottom.JPG
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

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Post by bajaman »

Well - that certainly makes a lot more sense - I suspected that the vref line was missing on all stages - so many mistaken component values (and that is just the input buffers :roll: ). It does look like the general layout is correct - just completely wrong values for most of the components. I have an E2 on it's way to me, so I will open it up on arrival and post pictures etc. accoering to the AMT blurb, all the cabsim and clean / boost sections are identical. The clean channel is supposedly modelled on the Fender clean - the boost just gives it a bit more gain in the mid frequencies I believe.
interesting to note that there is NO true bypass function available on any of these pedals - you either get the Fender clean or the legend amplifier high gain channel :!:
Cheers
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Post by Ice-9 »

bajaman wrote:Well - that certainly makes a lot more sense - I suspected that the vref line was missing on all stages - so many mistaken component values (and that is just the input buffers :roll: ). It does look like the general layout is correct - just completely wrong values for most of the components. I have an E2 on it's way to me, so I will open it up on arrival and post pictures etc. accoering to the AMT blurb, all the cabsim and clean / boost sections are identical. The clean channel is supposedly modelled on the Fender clean - the boost just gives it a bit more gain in the mid frequencies I believe.
interesting to note that there is NO true bypass function available on any of these pedals - you either get the Fender clean or the legend amplifier high gain channel :!:
Cheers
bajaman
Yes all that is correct, no true bypass, having a quick look over the two pcb's, the main pcb has the buffers and cab sim and other filters for the different outputs to the jacks and I expect will be identical in every version pedal.
The upper pcb with all the pots on will also be identical in all models but with different component selections between the different pedals, there is a selection at the side which has a 0 ohm resistor placed which denotes the model of pedal. (the resistor is not part of the circuit it is just a tick box).

I haven't had the chance to look up some of the transistors but they are marked

JR = these I know are the 2sk208R
1GW = can't remember off hand what these were
3GW

I will try and do some tracing before going away on Saturday (isle of man TT , I go every year ) but I do hope to get a bit done before going.

Happy you have one ordered as well Bajaman, It will make it easier to compare details. :applause: saying that you will probs have yours finished before
I even get back :thumbsup
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

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Post by bajaman »

1GW = BC847C (BC547C in through hole)
3GW = BC857C (BC557C in through hole)

Enjoy the TT ice-9 :D
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Post by bajaman »

I have worked out most of the cabsim and drive out circuits (nothing like the earlier schematics in this post :roll: ) from ice-9's pictures. one very interesting feature is the two transistor (BC547 /BC557) +9v dc auto switch which shuts down the 9v power rail to the cabsim section to extend battery life when the cabsim output socket is not being used. :thumbsup
The circuit consists of two transistors and two resistors (VT7B, VT10B, R318 and R 328) directly under the cabsim jack output socket :secret:

Another interesting feature: when using the drive output socket to connect directly to the input of a standard guitar amplifier, only the high gain amplifier simulator is used and the footswitch gives a true buffered bypass signal to this socket when the amp sim is turned off. Both the cabsim and preamp output sockets give the choice of high gain or Fender style clean when the footswitch is engaged - nice design feature :hug:
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Post by Ice-9 »

bajaman wrote:I have worked out most of the cabsim and drive out circuits (nothing like the earlier schematics in this post :roll: ) from ice-9's pictures. one very interesting feature is the two transistor (BC547 /BC557) +9v dc auto switch which shuts down the 9v power rail to the cabsim section to extend battery life when the cabsim output socket is not being used. :thumbsup
The circuit consists of two transistors and two resistors (VT7B, VT10B, R318 and R 328) directly under the cabsim jack output socket :secret:

Another interesting feature: when using the drive output socket to connect directly to the input of a standard guitar amplifier, only the high gain amplifier simulator is used and the footswitch gives a true buffered bypass signal to this socket when the amp sim is turned off. Both the cabsim and preamp output sockets give the choice of high gain or Fender style clean when the footswitch is engaged - nice design feature :hug:
I have just about got the same as you bajaman, the cabsim is quite close from the original drawing with some errors and missing bits and most of the component values are wrong, the drive output is as you say a direct output just going through a buffer.

I only have the BC857 B847 parts left to trace out and was then going to draw the schematic so far. If you already have this part drawn out then I can confirm the component values.
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?

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Post by Ice-9 »

A little update of schematic so far, unfinished cabsim as I need to look at the last section, just posting this to see if I am the right tracks.
Attachments
image-amt-S2-part.jpg
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?

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