AMT Slap Bass (Provisional Schematic)  [schematic]

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AMT Slap Bass (Provisional Schematic)

Postby bobbass4k » 02 Jul 2017, 21:57

I acquired one of these in a trade many, many moons ago, and it's one of the only "branded" pedals i still have, it's always been so unique I could never let it go. I've wanted to trace it for ages and for some reason, after 6 years, I decided this weekend was finally the time.

First off, this is incomplete - the board is SMT and I couldn't borrow the cap meter from work this weekend. There's a few through hole caps, but the majority of the values will remain a mystery until i can measure them. As it's incomplete, I haven't fine-tooth combed it yet, so there may be mistakes, either in the drawing or the trace.

The reason I wanted to post it now is the circuit looks pretty weird to me. I definitely don't recognize it (though my knowledge of drive circuits beyond the "big" ones is pretty limited). So, does anyone recognize this? Anyone know what it might be based on? The volume section looks pretty odd to me, as well as the filtering (if it even is filtering?) between the second 2262 and the second 062, and I can't say I recognize the tone stack. They market it as a drive/compressor/amp simulator, but again, there aren't any compressor elements I really recognize.
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Re: AMT Slap Bass (Provisional Schematic)  [schematic]

Postby bobbass4k » 17 Jul 2017, 01:02

New schematic with provisional cap values below. I measured everything in circuit, any reading that didn't make sense I took the cap off to confirm, but there's still a few values that seem a bit odd. I was out of a few values so I'll to have wait for my parts order to arrive but it should get breadboarded this week.
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Re: AMT Slap Bass (Provisional Schematic)

Postby bool » 17 Jul 2017, 12:57

Imho you have the R3 placed wrong. It should go right from U1A pin1 to U1A pin2. And consequently the C13 should connect directly to U1A pin2 (now junction pin2, R3 and C13).

It looks like a fixed souped-up mid-boost section. (perhaps as you mention this is intended for some "other" type of pedal).

And the circuit around the U4A looks "interesting". Are you sure this isn't a lowpass section similar to the one at U4B?
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Re: AMT Slap Bass (Provisional Schematic)

Postby bobbass4k » 17 Jul 2017, 23:26

bool wrote:Imho you have the R3 placed wrong. It should go right from U1A pin1 to U1A pin2. And consequently the C13 should connect directly to U1A pin2 (now junction pin2, R3 and C13).

It looks like a fixed souped-up mid-boost section. (perhaps as you mention this is intended for some "other" type of pedal).

And the circuit around the U4A looks "interesting". Are you sure this isn't a lowpass section similar to the one at U4B?


See, that's what I assumed too, but pins 1 and 2 of the IC are shorted by a track and I've double checked the rest of the circuit, it's definitley as drawn. The U4A circuit and the tonestack is the bit I was least sure of, I've had a quick look now and it seems to be as I've drawn it but there are some traces that go under the IC itself that I've made educated guesses at with continuity testing, I might take the IC off to be 100%

Just to reiterate - this schematic is still a WIP, I will be breadboarding it this week and I'll report back.
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Re: AMT Slap Bass (Provisional Schematic)

Postby bool » 18 Jul 2017, 12:57

If U1A pin2-pin1 are shorted, then it's a buffer - and the only logical conclusion in this case would be that they use a similar "deep notch" as SansAmps do in some of their "other" pedals (the misterious caps and resistors hanging off the U1A).
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Re: AMT Slap Bass (Provisional Schematic)

Postby bobbass4k » 04 Aug 2017, 01:02

Finally got some quality time with the breadboard this week. With apologies to bool, the revised schematic is attached. The more I looked at it the more that 100k didn't make sense, there's a track from it that actually disappears under the input jack and comes back out, which I hadn't noticed, and puts it in series with the output of the buffer, so with it as a pot it acts as pre-gain.

I haven't had much chance to A/B it directly but it sounds pretty close, though missing some of the treble bite of the original, so I'm still not convinced the tonestack caps are right, I'll have a tinker this weekend, but the schematic is correct and workable.
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Re: AMT Slap Bass (Provisional Schematic)

Postby phatt » 04 Aug 2017, 09:23

bobbass4k wrote:Finally got some quality time with the breadboard this week. With apologies to bool, the revised schematic is attached. The more I looked at it the more that 100k didn't make sense, there's a track from it that actually disappears under the input jack and comes back out, which I hadn't noticed, and puts it in series with the output of the buffer, so with it as a pot it acts as pre-gain.

I haven't had much chance to A/B it directly but it sounds pretty close, though missing some of the treble bite of the original, so I'm still not convinced the tonestack caps are right, I'll have a tinker this weekend, but the schematic is correct and workable.


Not sure the circuit as drawn is right ??
The output needs to be *Decoupled* with a capacitor,,try anywhere from 2u2 to 10uF. 10uF will keep all the low end intact.
Also add some series resistance on the output or you could blow the last opamp,, somewhere around 500R would save the chip from a dead short output.
Other wise some interesting ideas in that circuit so thanks for posting, I'll have to fire up the simulations to see how well it works. :thumbsup
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Re: AMT Slap Bass (Provisional Schematic)

Postby Dirk_Hendrik » 04 Aug 2017, 13:11

What Phatt says.

As well, for readibility, move R6 to pin 6. It's part of the input filtering and,, although electrically ity connects to the output, it no suggests being a pullup resistor on the output.
Sorry. Plain out of planes.

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