Realistic MPA-20 Solid state 40W P.A amp used for guitar

[momo]

I thought I would start a new thread with this one.
I went and bought the thing and it sounds good!
It can work on 12v and thats what I wanted. I will have to change the 12 plug to something better, but besides that its all good for a portable busking setup.
2 inputs so I can put a mic in there too and even aux in for playback audio.
It sounds clean and as you can see I put a tubescreamer and valvecaster with it and its great.

Now some guy did a mod with the feedback circuit going to the output transformer, sais it makes it breakup and distort earlier.
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The best solid state harp amp I have currently is one I constructed from an old radioshack PA amp I bought for $8 at the Goodwill. It's a "Realistic" brand, model number MPA-20. Very importantly, it's an older one, and the circuitry is 100% discreet (ie. it uses power transistors in the poweramp section, not a chipamp). The newer MPA-20's use a chipamp, and don't sound nearly as good. The only modification I did was to remove the "phono" section (I'm not feeding it input from a record player!), and to change how the negative feedback loop get's tapped to the output transformer. Originally, the NF loop was connected to the 8ohm tap of the OT. I removed this tap, and added two new leads, one to the 16ohm and one to the 4ohm tap of the OT. I made it switchable. When you go to the 4ohm position, it get's crunchy and breaks up early. It's also a bit louder. When you go to the 16ohm tap, it's a little quieter, but MUCH cleaner, even cleaner than it originally was. Both settings have their purposes, and I think both sound really good. This amp also has two input channels (with two separate pre's and two separate volume controls), so it's a really versatile amp. I can plug both harp and guitar into it, or harp and a footdrum (porchboard). BTW, this amp also sounds killer with my cigar box guitar (it's an acoustic with a piezo pickup).
I built this PA into a custom cabinet I threw together with singe 25 watt 8" paper-cone speaker with a smallish magnet. It's one of a pair that came from an old organ. The BEST harp speakers that I've ever found.
Anyway, the point of all this is to say that PA amps are a really great place to look for nice solid state Harp amps. PA amps have been used for years for harp (eg. look how everyone loves those old Bogen PA heads), and a nice solid state one will work as well or better than a tube one. That amp you are buying (the Lapai) is basically a PA amp. I'm not very familiar with those tripath chips, but everyone seems to think they sound good for instrument amps. You are probably on the right track. If you don't get THE sound you want out of it at first, then you'll need to start experimenting with Preamp circuits until you find one that works well with that particular chip. I suggest starting with something like the Fetzer Valve or the Tillman pre (both very simple one-transistor designs), and then move on to something like the Peppermill or Blue Magic (relatively simple two-transistor designs). Any of these preamps should let you dial in a relatively clean tone, or get a nice mild overdrive.
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I tried to make it breakup and could not.
Mind you I only have a 15w speaker and did not want to push too much, input was full on and volume at 2.....still it was plenty loud, what I have been looking for.
Its just a bummer that I did not build it myself...

Here is a link to the guy mentioning the mod, I would be interested to see if this is a good mod.
I recommend this amp if you want a 12v portable amp......30w for 12v!
I did not try the 12v yet, just used ac power, I hope it does not change the sound quality..

http://www.ssguitar.com/index.php?topic=1326.15
http://www.youtube.com/watch?v=v3SEULc6eCQ

[PRR]

Note that the power amp topology is identical to a Darcy (Deasy??) which is a cheap half-watt amp hacked into a pedal used by some Famous Name. Also the original PigNose.

Get a 4x10" Half-Stack speaker and beat that thing. The NFB mods may do something but not-much until you bang on the limits.

With battery (and away from all wall-wire) you will lose the hum/buzz. Even if you don't hear it, there's always some in any wall-power gear. One test on a classic tube amp suggests that a perfectly hum-free amp sounds "different", and perhaps less exciting. Or it may have been a bad test.

I think this amp is an honest 20 Watts clean sine power. As such it can suck up to 2.5 Amps from a 12V supply. You won't get full roar from any flashlight battery. (Two 6V lantern batteries will run it, but not for real long before stressing-out, and this is expensive power.) A 50AH car-battery computes as 20 hours, though you should not play hard more than 5 hours per full charge because deep-drain hurts the batt. For shorter or less-loud gigs, motorcycle and lawn-tractor batts may be fine.

[momo]

It sais its a 40w amp with mains power and 30w on 12v...that thing is loud!
Its also one of the cleanest amps I have played as far as buzz and 60Hz.
I play a strat in it so of coarse I get the usual single coil noise, but thats about it and some minute amount of hiss.

Yes I really have to try this thing on a decent cab, for now its my 10 inch 15w and it sounds great.
I'm tempted to crank it a bit but I would not want to blow my vintage speaker..

Also, if I had no valvecaster in there my reference would be just the amp and I would say it has nice tone, but with the tube preamp
it really comes alive in tone and a bit of finger control for dynamics. I have a mullard in the valvecaster and it sounds real nice at 12v.

Note that the 2 Hitachi transistors are PNP germanium......awsome!
That should help with nice tone when pushed to the limit....
Apart from having a classic tube amp, I think this is a great setup for portable kit with nice tone and power.
Again, I would of loved to build it, but I will take it and run as it does the job for me!

Now about that mod, if I understand, he took the wire off of the 8ohm that feeds the FBL and put it on a switch, he then put wires from the 4 and 16ohm respectively on that switch as to decide which output feeds the original wire that was on the 8ohm post?
Simple as that?
I would like to try that if its that simple..

[momo]

I'm sort of converting this amp a bit, beboxing it and upgrading the connectors.
Now one of the input pots is split between an aux input and a phono input.
The pot has the phono positive on lug 1 and aux positive on lug 3 while lug 2 givs the output of those.
So you can pan fully to one side for one input only or mix both I guess if the pot is not fully rotated.
The mid point seems to be the null point.
Now I just want to use this for aux input and dump the phono....
So
whats the best way to do this?
obviously if I just cut the phono wire, only one half of the pot will work, but how can I convert it to use the pot fully for the aux input?

??
I want to mess as little as possible with this, never encountered this pot situation.....
sorry if this is obvious....
I'm sure its simple!
but
I don't want to mess up....

[PRR]

The aux/phono pot has a ground tap in the middle. (It may be "hidden", grounded to the bushing.) So the center is "silence", not "mix".

You want to un-ground the center, then wire the pot like any Volume pot. It may be better to just replace it with a "normal" pot, possibly Audio Taper (depending on if you need a wide range of inputs or all your inputs are the same level).

To take your WalkMan or iPod *stereo* output, keep both RCA jacks as L and R. Mix the L and R together with two 1K resistors (can be soldered right to the jacks). Then that mono signal goes to the top of the pot, and the wiper to the AUX input in the preamp.

[momo]

I am now into the last details concerning wiring, my question would be to you Paul, here are detailed pics of the aux input split between phono and aux. As mentioned before I will convert this to just aux input on a stereo 1/4 inch jack. As you see there are caps on there at both ends. on the input side, its a .002 ceramic cap from signal ground to chassis ground. Then on the pot side, the pot on the right is the "blender between both the phono and aux input on lug 1 and 3. The wiper goes to lug 3 of the volume pot that is on the left accompanied with a 68k resistor as well as a 330PF ceramic cap.
The cap at the input, whats that for?, phono sensitivity to radio interference?, do I keep it?
Do I keep the resistor and cap on the aux pot going to volume as it is ? and just combine the left and right wires on lug 1 with 1K resistor as you suggested?

Sorry for all these questions which might be obvious to some, I thought of posting these questions here so that everyone can benefit with the answers!

[PRR]

Keep both caps, or omit, whatever is easy.

The one at the jacks is radio bypass (sends radio-crap the short way to the box instead of the long way through the audio ground) and probably not needed 99% of the time. Obviously RS had to cover the 1%.

The 68K+330pFd seems to lift (probably level) the response above 7KHz, probably balances some stray capacitance elsewhere. As 7KHz is the very top of the guitar band, it isn't critical.

[momo]

I removed the input cap and kept the rest. Not much volume coming out. So I tried a 10k pot and there was not much more volume but I then realized the originl pot had an extra tab at the bottom and was indeed connected to ground....
So I will experiment and see what pot and configuration to use to have more volume with the aux input....
Here is the schematic of the amp.....
A few things I noticed, first DC voltage indicates 18v instead of 12v......I noticed the led indicating power is low on 12v compared to when using ac power. The transistors indicate NPN while the datasheet for the ones I have indicate PNP...
Also, I see a 40K pot for the aux input and I have a 100K pot......

[momo]

OK this project is done,....well almost.
There is a slight hum using the AC power, I would say a bit more than before, but its still not much. The wireing is clean, I would probably have to probe with a wooden stick to fine tune that. With the DC power, boy, its like it was never on..
There is another part to tthis metal box that goes underneath it if I need more space.
I think I will put the battery in the other copmpartment and keep this space for pedals and stuff...
I still have to ID the stuff but it works for me.

OH and some quirky thing, I need to open the aux in pot a bit for anything to go thru master out..
If thats an easy fix I will do it, otherwise, it does not add noise so its not really a problem.
I would love to understand it though.
I added a switch to change feedback loop source, 4 or 16 ohms and it does make a difference.
Might call it a fat or clean switch.

This will now sit on my worktable plugged in DC and so I can test anything with really clean sound.
Will do a great portable battery amp too, just have to figure out a good battery to stand the full out 20W volume on this !

[PRR]

> first DC voltage indicates 18v instead of 12v......

Wall-power is abundant, battery power is limited. Also battery power "can" be all over the place (15V on some old cars with misadjusted regulator, plus spikes), more so than wall-power. It seems likely that it is 20 Watts on a car-battery, but 40 Watts when using the wall-plug.

> transistors indicate NPN while the datasheet for the ones I have indicate PNP...
Also, I see a 40K pot for the aux input and I have a 100K pot......

I think this catalog-number was sold for decades. In that time there were probably many minor changes, and perhaps some major ones. It may span the era when NPN (Si) power transistors became cheaper than PNP (Ge) power transistors, and Radio Shack would change the design to take advantage.

> I need to open the aux in pot a bit for anything to go thru master out..

Something not wired the way that diagram shows.


TR102 amplifies MIC. VR2 sets MIC volume. VR3 sets AUX volume. R108 R9 mix the two (and isolates interaction). Apparently they had excess gain, R124 knocks it down a bit (and further reduces interaction).

[momo]

Again Paul, thanks for the info. I opened up the box again,
The extra wires I added to switch from 16 to 4 ohms on the feedback loop were not sheilded, so I thought the hum could come from that but its not the case. So now I will audio probe the thing to see where it comes from.
One thing to check first is the new grounding on that aux/phono pot.
This pot had an extra tab at the bottom for chassis ground. I changed that to circuit ground, so maybe thats the problem, will check it out. There is no reason why I should not be able to remove the 60Hz hum which was not there before.

I still have not figured out why I need to open the aux pot to have anything come out of the master volume...

in progress!

[momo]

OK this is done and finished.
I finally dumped the aux input after trying many pots and grounding configurations.
It was either less hum but messes with the master volume or no mess and extra hum.
Its not a problem, the second input can be the aux in and it sounds even better.
Now this 40W amp is super clean on AC power, fat tone with great clean low end,
an added switch for clean or crunch tone , 2 inputs and 4 or 8 Ohm output.

Awsome!

[momo]

Surprise surprise....


I knew and felt that the frequency response of this amp was a bit limited, but now I would like to have a bit more high end.
I think this goes only to 10k. Is this a mere case of swapping a few caps or I am pretty much left to use it as it is?
Its nice and fat in the low end, but the high end is a bit too smooth....I never thought I would say that of an amp.

[PRR]

If you have a 2.2nFd cap on the "mic" input, snip it off.

That's a heavy load for guitar. Might roll-off as low as 1KHz.

Without the cap you could pick up radio stations. 220pFd might tame that for most situations.

Also the input impedance may be as low as 26K, which is awful low for guitar.

You might solve both birds by just adding an input buffer.

[momo]

Yes thats it, if I use a buffered pedal in front it sound fine, ....wait, it sounds great!
So thats the trick, I now have to put this on the list of things to do, but meanwhile I will just use a buffer in front of it.
Since a buffer is not passive, its alittle more work that I thought, just figuring out where to tap in for power for the buffer.
Probably right of the 12 DC feed?
thanks for the answer..

[PRR]

> right of the 12 DC feed?

It's the first stage of the amp, where signal may be weakest. You usually want very clean power to an input stage. While a battery won't hum, a battery feeding 20 Watts of output stage may have distorted signal on the DC voltage.

You have a fine power-point on your present first stage. And signal level is probably low enough to use a simplified JFET bias.

[momo]

Wow Paul!
thanks so much for doing this....
great stuff and it will help me get thru it.
many thanks!

[PRR]

I realized that the left 1Meg and the 0.01u are not strictly needed. Input to the junction of the other 1Meg and the 33K.

There will be a few DC-leaky sources that will upset the FET bias. They shouldn't, because a zillion tube amps also have no input capacitor; but the FET may be slightly easier to upset. If you get trouble with "some" sources, you can add the 1Meg+0.01u, or fix your leaks.

[chemosis]

i know this is old but i bought pretty much the same thing but it says radioshack instead of realistic and im very suprised by its sound quality as a guitar amp. im running an old 8" realistic speaker and am pretty suprised.