ADA MP1 into a pedal

caspercody · Post by **caspercody** » 12 Nov 2020, 22:23

Hello

I created a pedal size project of the tube section of the ADA MP1. I am attaching a copy of the tube schematic, and input for the ADA MP1, a picture of my ADA MP1 tube build.

So far it sounds okay, but I want to add the circuit shown on the input schematic page the op amp diode clipping stage ( U4) that feeds into the tube section. I drew it up and attached my drawing. This op amp stage looks to have the gain control to the tube circuit. For my tube build I removed R19, and changed R18 to a 1M pot for gain control. I want to change R18, and R19 back to the way the tube schematic shows.

Does the op amp diagram look like that is what it does, feed the tube stage and act as a gain control? What would be best way to design this up? I do have a charge pump circuit on my tube circuit that can provide +12, and -12 vdc. I am using this to provide the -15 shown at junction J on the tube schematic.

The (2) switches are to replace Q3 and Q4 on the tube schematic.

Thanks
Rob

Optical · Post by **Optical** » 13 Nov 2020, 00:37

Cool project
Q6 and Q7 are a high impedance input buffer
U4 is the processor controlled gain control
U5 is a pcb mounted gain trimmer with clipping diodes to avoid opamp clipping (supposedly to allow gain trimming to match the input response across various high/low output pickups)
Q3 is active on gain to short the R73 100k and decrease the bass response - I think, might be remembering backwards

caspercody · Post by **caspercody** » 13 Nov 2020, 00:53

Sorry, but that is over my knowledge. I understand the input buffer, and was thinking of using the other half of the open op amp as a input buffer, on the hand drawing I included.

But, i do not understand what you meant about U4 and U5, or Q3?

Optical · Post by **Optical** » 13 Nov 2020, 04:22

U5 is an opto-resistor controlled by the microprocessor. Essentially a pot and is the main gain control. You can use a pot there instead for a pedal.

T2 in the feedback path of U4 is a gain trimmer for pickup variations, and also probably because U5 had a wide tolerance range between units, so the trimmer meant consistent gain across units.

There is a mp1 mod to break this out to the back panel to allow a physical gain control instead of it being inside the housing. For your pedal, putting this on a pot would be useful. But it could just be a trimmer on the board somewhere instead.

Q3 is important - it changes the input gain and bass response to the tube circuit by shorting that 100k resistor. You'll need this

caspercody · Post by **caspercody** » 13 Nov 2020, 15:21

Is there any way you could please draw up a schematic for me of what you think I should build going into the input of the tube circuit?

caspercody · Post by **caspercody** » 14 Nov 2020, 03:59

Here is what I came up with from the ADA schematics. I cannot make out the C34 input capacitor value. All I know is it is a ceramic capacitor. Not sure if need the 1M to ground right before the 1M gain pot. Please let me know what you think?

caspercody · Post by **caspercody** » 15 Nov 2020, 22:27

Look what I found on the web. But I still think R92 is 510, and not 910.. See picture, what do you think?

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caspercody · Post by **caspercody** » 16 Nov 2020, 02:16

I made this buffer with the U4 op-amp circuit and plugged it (without the tube circuit) and it works. When I turn the gain pot up, not much sound. When get half way then start to hear clean sound all the way up to say 9 out of 10. From 9 to 10 then it gets really loud and distorted.

deltafred · Post by **deltafred** » 16 Nov 2020, 11:13

caspercody wrote: ↑15 Nov 2020, 22:27 Look what I found on the web. But I still think R92 is 510, and not 910.. See picture, what do you think?

Looks like 510 to me but it probably doesn't make any difference as the current into the gate is miniscule so the difference in volts drop across it will be also.

Optical · Post by **Optical** » 16 Nov 2020, 20:15

I have an MP1 in pieces on my desk. R92 is 510, but as said it doesnt matter much.
The input buffer could probably just be an opamp. I cant remember the explanation of that input circuit and why it was used, I believe it was just intended to be a high impedance buffer

caspercody · Post by **caspercody** » 16 Nov 2020, 21:19

Is C34 47nf?

Do the (4) diodes in the op amp U4 look to be 1N4148?

caspercody · Post by **caspercody** » 16 Nov 2020, 22:13

I hooked everything up, and does not sound good. I now it has something to do with the U4 op amp circuit. When I turn the gain pot up it is clean until up to 8 of 10 then starts to distort. Then at 9 of 10 it distorts way to much and feed back is uncontrollable.

I removed the U4 op amp from the path and just had the buffer and tube circuit. It is a very medium gain, there is no gain control at this point. I put my VFE Focus clone in front of the MP1, and then this sounds a lot better. The level control on the Focus works like a gain control and boosts the gain. However the Focus adds some color to the overall sound with its mid boost and controls.

So, does anyone have a good clean boost circuit I could put into this?

Optical · Post by **Optical** » 18 Nov 2020, 00:45

caspercody wrote: ↑16 Nov 2020, 21:19 Is C34 47nf?

Do the (4) diodes in the op amp U4 look to be 1N4148?

Yes and yes

Optical · Post by **Optical** » 18 Nov 2020, 00:51

caspercody wrote: ↑16 Nov 2020, 22:13 I hooked everything up, and does not sound good. I now it has something to do with the U4 op amp circuit. When I turn the gain pot up it is clean until up to 8 of 10 then starts to distort. Then at 9 of 10 it distorts way to much and feed back is uncontrollable.

I removed the U4 op amp from the path and just had the buffer and tube circuit. It is a very medium gain, there is no gain control at this point. I put my VFE Focus clone in front of the MP1, and then this sounds a lot better. The level control on the Focus works like a gain control and boosts the gain. However the Focus adds some color to the overall sound with its mid boost and controls.

So, does anyone have a good clean boost circuit I could put into this?

The U4 opamp and trimmer is intended to allow a small amount of gain trimming. The OD1 clipping light is connected to show when you've run out of headroom there, so dialing it up to max gain is probably assured to clip when it's supposed to be tuned to something less than the clipping threshold. Something like 18V is the max Vpp signal intended to hit the tube stages.

The VFE focus circuit would be a good one, an adjustable high and low pass filter and a variable boost on the input sounds perfect...

caspercody · Post by **caspercody** » 18 Nov 2020, 01:05

The gain pot I was referring to is the 1M pot I added between the buffer and U4 op amp stage. When I turn the trim pot in the U4 op amp stage I hear the volume go up and Dow a little, but no matter where the tr pot is set if I crank the gain pot it sounds terrible. Maybe I did something wrong on my U4 op amp, maybe I will try to breadboard one and see what happens

Optical · Post by **Optical** » 18 Nov 2020, 01:38

what's your power supply for U4? The diodes in the feedback path are designed to clip before the signal hits the rail and clips the opamp. The MP1 has +/-15V on the opamps
Might be clipping the opamp on your setup?

caspercody · Post by **caspercody** » 18 Nov 2020, 01:53

Using 12vdc power supply in a charge pump to give me both (+) and (-) 12vdc.

I took off the buffer and gain pot between buffer and U4 op amp, so it was a 3PDT push button switch to switch in and out just the U4 op amp in to the front of my amp. It was just a loud feed back squeal. Turning the trim pot just reduced the overall volume a little. When I turned the guitar pot down a little the squeal went a way.

Thanks for your replies!!

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Optical · Post by **Optical** » 18 Nov 2020, 02:48

I'd be inclined toward ditching that whole input stage and replacing it with a clean boost into that 33n cap on the U4 output.
There isnt any frequency shaping that i'm aware of within the input stage, so a simple noninverting opamp with variable gain might do the job..

Have you tried shorting that 100k resistor (R73)? - pretty sure this is just what happens for the clean channel but could be wrong

edit - I think this occurs on the gain channel, shorting the 100k makes a 2khz input high pass (cutting a lot of bass before the tube stages)

Optical · Post by **Optical** » 18 Nov 2020, 02:51

btw what are you doing for an EQ?

caspercody · Post by **caspercody** » 18 Nov 2020, 02:54

Could you draw up a non inverting stage?

When I short out the 100k it either deduced bass or added more can not remember

I just added a Marshall tone stack for now. If I get this working I might try to do the MP1 tonestack