ADA MP1 into a pedal

[caspercody]

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

[Scruffie]

It's more of a limiter, I think you really want Q6 & 7.

[caspercody]

Thanks for the reply!

Is Q6 and Q7 more like a boost to hit the front end of the tube circuit?

[teemuk]

By quick glance gain control seems to be the LDR in opamp input. I see a digital pot driving a "servo" circuit, which keeps track of actual resistive value of the LDR (they are good switches but terrible variable resistors). There's a similar control for resistive divider establishing "OD2" gain.

AFAIK, the opamp gain stage, driven by the input stage, does some preliminary clipping and you need both that and the discrete input stage to accurately replicate true signal path of MP-1. The tube part is just a little fraction of the circuitry.

[caspercody]

Teemuk

Thanks for your input on this!! 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]

Here is what I came up with from the ADA schematics. I cannot make out some values around Q6 and Q7, and not sure if need the 1M to ground right before the 1M gain pot. Please let me know what you think?

[caspercody]

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

[PRR]

... But I still think R92 is 510, and not 910.. ...

My eyes tend to agree with you, but....   Why do we care, 1k or half-k??

[caspercody]

Thanks PRR for your reply, I do not know if it would make a difference so I thought I would ask.

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.

[caspercody]

The circuit at the beginning (Q6/Q7) appears to be what is called a JFET/BJT feedback amp (or Sziklai pair, compound transistor), this boosts the signal a lot. Just the U4 circuit by itself barely gets the signal to unity gain. Right now I have a circuit of the Q6/Q7, and U4 in a separate pedal which then connects to the tube circuit (in a separate box) and it works, but it is very trebly sounding.

I want to add a pot between the (2) circuits, because I think this will be a better gain control, since the Q6,/Q7 is hitting the U4 circuit wide open. Maybe this is causing the trebly sound.  When I add it there is no volume, until you turn to 9 then there is sound. From 9 to 10. Anyone have any ideas how to add a pot to adjust the output from Q6/Q7, and into U4?

Another reason I added this into a pedal, is if I use my VFE Ice Scream in front of the tube circuit, it sounds really nice. And with all the controls on the Ice Scream you can really change up the tone.

But I have worked too much on this project not to get the whole circuit working. Just want to figure out how to adjust output of Q6/Q7 into U4, I think this might work?

Please let me know if you have any ideas?

[caspercody]

Okay, I grabbed another pot, just to make sure I was not grabbing the same one every time I try this, and for some dumb luck that pot was bad. It was not bad.

What I did different this time was turn the amp up! The 1M volume pot is connected connection 3 of a 1M pot. Connection 2 will go to U4 and connection 3 will to ground. When I turn the pot from zero to 1 I hear no signal, but a rubbing dull noise, then at 1, I get a low signal. As I turn up the pot I get a clean signal, that does not really go up in volume. When I get to say 8, then volume goes up and slight distortion. There is a very thin line as I turn it up from 8-9 where the volume jumps way up. The distortion is not good. When I switch on the 100K at end of circuit, have a big volume jump, and a lot of bass added. But the pot works the same way as I described.

The only way I have gotten something to work like a gain pot, is to remove the 20K resistor in the U4 stage loop, and connect the end of the 20K pot to Pin2. Then I have a pot that works great, but the sound is very trebly, thin sounding. When I connect it this way, I am not putting in the 1M pot

[Willypp]

Have you had a look at the layout here?

http://guitar-fx-layouts.42897.x6.nabble.com/ADA-MP-1-Preamp-td43638.html

[caspercody]

Yes, but that will not sound as good with that layout. The tube circuit has a -15vdc coming into the bottom stages of the first two tube stages. I connected the circuit just like the veroboard layout in that thread, and it did not sound great. When I connected the circuit per the schematic (adding the -15vdc) it sounds so much better. I put SPST switches in place of Q3, and Q4, and when they switches are shorted, this adds a lot more gain and bass.

I finished what I can with this project. I made a separate pedal of the Q6/Q7, U4, and U9 portion (see attachments). So I can just use patch cords to put this pedal before the tube circuit, or use a different pedal before the tube circuit. It sounds okay, but it sounds so much better with my clone VFE Ice Scream in front of the tube circuit.