I'll start a new thread, but any chance anyone has the schematic for the mpa-25? I have one of them as well.

Oh man, thanks so much for the schematic. This is so helpful.

J M Fahey, great to know about about the output transformer, I'll heed your words.

I haven't had a ton of time, but without the schematic I thought I'd at least do what I could, so I removed the one mic transformer and the inputs for the aux and phono inputs and installed a 1/4" speaker out jack. It has those spring clip speaker terminals, one of which was for the 70v that you mentioned. I couldn't see needing to use it so I unsoldered it from the connector, used it as my speaker out. Now all I have to do is jumper from that terminal to the impedance I need. So far so good.

I tried it out into a 2x12 at 4 ohms, and man, it's loud. It sounds quite good really. The tone control doesn't do much, probably not designed for guitar which I expected. I put a preamp pedal in front of it and it helped shape the tone quite a bit. It gets pretty fizzy at high volume. I'm guessing the sound of the chipamp clipping isn't the greatest. An assumption as I hadn't seen the schematic.

I also wonder if the mic inputs are just too sensitive, and maybe I should be tying into where the phono/aux does. Might tame it a bit? I'll have time to go over the schematic tomorrow.

Thanks everyone for the help!

Kaz Kylheku,

All of that makes sense to me. Really good point on a DC blocking capacitor in the cf loop. And I'm sure you're right about wanting the cf on the primary side. You'd want it directly off the output I would imagine.

Do you feel like there's value in having current feedback as well as an output transformer? I'm sure there's no harm in trying it, just wondering about theory.

It is really interesting to see how older "traditional" technologies influenced the ones that came after them in ways that might not actually make much sense, but also using what you have to overcome the limitations of the new technology.

Hey phatt, thanks for the reply.

I'll definitely read the links you posted. I ran across that site in my searching but I wasn't entirely following it. Part of my issue is that I don't know exactly what I'm working with so I can't follow along, but I am keen to learn. I recently built a noisy cricket and really want to get more into solid state amps.

Your explanation of damping speakers and why in a guitar amp you would want more movement and back emf makes sense to me, and I get how you would get that with current feedback and also with an output transformer so that clears some stuff up for me.

I don't disagree about leaving the pa amp stock. The amp has four inputs, two microphone, an aux and a phono. My plan was to remove the phono, aux and one of the mic inputs, the one with the microphone transformer, and leave the second mic input. I want to build a preamp as a learning tool and also to take advantage of the already built power supply. I haven't decided on a preamp yet, but I like the look of the dirt road special.

My question about bridging amps is more to do with the fact that this amp is already doing that, so I'm interested in learning what's going on in there and if there's ways to modify/tweak. I totally agree that if I was to build from the ground up I'd just pick something else, or if I was to rob this for the parts maybe make two 5w amps rather than one 20w.

Thanks again for the information, it really helped me.

I just reread my last post and clearly I was trying to get out of work on time. The two chips are bridged and apparently outputting 20 watts. I'm not sure what that looks like because I don't have a schematic and haven't had time to trace it out.

Is the output transformer useful to me or would I be better off without it? It has taps for 4 ohm to 16 if I recall. My original plan was to have one output and then use a jumper for the output impedance based on the transformer. I'm very open to ideas.

Also, if anyone has any info about how one would bridge an opamp like the one I mentioned above I'd love to read it. I've done some searching but haven't been super successful yet.

Thanks!

That's interesting. I'm still learning about this stuff, so correct me if I'm wrong here. Current feedback would mean taking the output, through a resistor to pin 7.

I've heard of this before but I'm still not clear as to why it's beneficial in guitar amps. Do you know of any good explainers on this? I'll keep searching.

The MPA-20 uses two of these chips and then goes through an output transformer. Is this necessary or beneficial? If I'm reading the datasheet correctly it is based on a 4 ohm load. Would it be fine at 8 ohm as well?

Thanks!

Hey gang, I'm looking for documentation for a
Realistic mpa-20 PA amplifier. It's not the discrete version, it's the one with two kia7217ap chips. I'm looking to drastically modify it into a guitar amp. I'm hoping to leave the power amp section mostly the same, possibly tweak a few things and add a new preamp section.

The board is labelled P-200902.

I've only found a snippet that someone traced of the preamp section. Hoping I can find something a bit more comprehensive.

Thanks for your help!

Thanks for the input. Yes, I want to be able to have a mono input that would go to both power amps if possible.

Phatt, you're correct, the in/out between board 1 and 2 is exactly like that in the amp, very easy spot to access, very reversible. I think I'll try it there when I get a chance. Free time is hard to come by these days, but thinking at work I can manage!

Thanks for the confirmation. I'll update when I get around to doing it.

I'm guessing I would put a jack in between where the two boards connect just after the volume, and just pick one of the two? Either use switching jacks or just run an in and out and jumper with a patch cable?

I'm assuming between the output of board 1and the input of board 2. Is there a better way to do this?

Hello all, I'm looking for some help adding an effect loop to a jc120. I mostly want this so that I can use it as a power amp in for other preamps, not so much for adding stereo effects like delays. I have attached the schematic for the amp in question. I'm just not clear exactly where to split off.

I wasn't going to do aby because I'm making them into heads.  I should have clarified that. What I was thinking was more like having my clean effects y off to the jc120, then all dirt pedals to the peavey. I just want one 212 cabinet for the whole affair though.

It's probably unnecessary, it's true, but I have most of the stuff on hand. So I have it set up to switch the positive and the negative, but I probably have to keep the rings separate from each amp, do I not? I would imagine you need to keep each power supply isolated.

Hello all.  Hopefully this is the right place to ask this.  I have two solid state stereo chorus amps, a peavey stereo chorus and a jc120, and I had a thought to use them both with the same speaker cabinet and make a foot switch so I could do it on the fly.  I came up with a schematic that involves six relays. It works very much like a reversing starter.  You push the button for the peavey, it seals in a relay that closes the contacts for the speaker output for that amp and disables the jc120 relay.  To change you hit a stop button, then hit the jc120 button.

I want to do all that to prevent the outputs of the amps from connecting.  I am wondering if this is safe, and more importantly is there anything I haven't thought of.  I expect I'll need three separate plates to mount the jacks, one for each amp and one for the output so that the negatives won't be connected?

If there's a reason I should abandon this please let me know. It's one of those things that I couldn't stop thinking about.

I'll definitely double check.  It looks correct so far.  The amp was modified, so I'm going to have to go through it pretty extensively.  I'm not sure what the guy was up to.  He added two inputs to the back, and three relays.  He also put a small circuit board in there, and a transformer.  The end result is that none of the effects work, so I'm going to try and put it back to stock.  I don't think he changed much on the boards, mostly the connections between the boards.  The schematic I posted matches the serial number of my amp.