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Discussion in 'Amp Tech Center' started by Bickert Fan, Mar 3, 2013.
I recently purchased an amplifier built in the late 1970s. see post below for more...
Also should mention it was custom built. Ill get the brand name soon, but the builder made amps from early 70s until mid 80s. All by hand. No schematic or manual. Not sure if this will be a fun project or a complete waste of time and money, but its sure a cool looking head.
If you know brand and model, use Google to search for schematics or specs.
There's no way to measure the intended output impedance of a solid-state amp. Given sufficient knowledge, you might be able to reverse-engineer the impedance that the amp was designed to drive.
As a last resort, it'd be safe to err on the conservative side and use a 16-ohm speaker. It's never unsafe to run a SS amp into a higher impedance load; the only side effect will be a slight reduction in output power.
Thanks for the 16-ohm advice.
I am at work, I'll get the make when I get home. It's some guy's name. There is no model number per se. Just a small 100-Watt head. But it's very well built from what I can see.
I googled the maker a week ago when I was considering the purchase. There is no mention of this amp anywhere except for the fact that the builder did an interview in 2001. He talks about the amp quite a bit. The interviewer, a musician himself, took this model amp all over the world on tour. The builder only made one design that I can tell. That he built a number of them. But there was no schematic anywhere. I cannot get in touch with the builder as he lives/lived in Australia and for all I know he's deceased. Hopefully I can open up the head and figure a few things out. I'm an amateur at best, and this project will be a learning experience. If I get something other than a cool paperweight/conversation piece I'll be thrilled. lol I do know it turns on. So it's a start. More info later.
The amp is made by "Don Mori"...I added a couple of pictures. Below is a link to the builder...this is literally all the information I have on the amplifier. Based on the article it seems as though this amp may be set up for 110 as well as 220. Is the swtchcraft jack on the bottom left of the back possibly a 110 input?
The pictures (amp is about 3'x4"x8")
I'm Don Mori, a professional musician and once a manufacturer of electronic equipment. A long time ago I set out to be the centre of Australian micro-electronics.
And you succeeded, you made some amazing quality amplifiers, we have one here now, it's a compressed piece of electronics which is actually the size of a drainage pipe. The amp was housed in a drainage pipe.
It's actually the size of a brick, 3 by 4 by 8 inches, it was set up to be a small amplifier that musicians could carry around with them, it had two input channels, one could be used with a microphone, the other with an instrument, all the design concepts on it were considering the demands of the professional musician, the live gig, one of the design features was that it was modular and it could be pulled to pieces, it was built in three units which were replaceable, but I never stockpiled enough replacement units however.
"it was inspired when I was teaching at Balmain High School, some of the rat bags were skateboarding through the water drains, it was probably quite dangerous but it was really exciting stuff" JR
I remember it was quite dangerous to bring one back to be repaired, it could end up as parts going out in someone else's amp!
Some of these units I rebuilt three of four times. Sometimes there was a shortage of materials.
My pair of Mori Amps went on at least three world tours, were used in the States and Europe and Japan where they completely freaked out over these things because they thought they were the masters of the micro. It rated 100 watts complete with custom graphic equilizer. What got you going with the idea?
The inspiration was the availability of small components, the invention of the transistor which I witnessed because I'm that old, instead of putting these parts into an enormous package it seemed to me to be common sense to put it into a small package, it's a conventional amplifier, there are innovations now that I didn't have access to when I designed this thing, the first one was 1972, this one we have here is about 1980.
I remember there was a bit of an upsetting moment when the piping changed from imperial to metric dimensions so you had to re-design the bits inside.
Yes, I made jigs for the transformer, finally I had to melt the transformers in the oven so I could re-shape them to fit into the drain pipe.
Another thing, a prototype of this was taken to America and we nearly had the American market, I had made them from the start so they could use the 110 volts they use in America, which was an innovation, the problem with the Americans is that they were so cautious about litigation from the big multi-nationals that they made me register the design, I couldn't ever patent it.
So you might have been a multi millionaire by now if the cookie had crumbled the right way. Also from that period, you built an exponential speaker in your brother's gallery, a very significant thing. We could use this speaker built into the wall for concerts, it was powered by one of your little speaker units. Well, you are also a professional musician as well - talk to me about your music, your opera, for example.
That is a very nifty looking little amp. I'm quite intrigued by the designer having managed to stuff 100 watts into that small a package four decades ago. He was way, way ahead of his time!
Today with switching power supplies and ultra-efficient class D technology it would be easier, but to have managed this in the 1970's is very, very impressive.
Have you tried asking for information on the Aussie Guitar Gearheads (AGGH) forum? Not only is that site based in Australia, but there are also active forum members who've been involved with Australian-made music electronics for decades. You might luck out and find someone who knows more about this intriguing little amp.
Here's where you can find the AGGH: http://www.guitargear.net.au/discussion/index.php
Do let us know what if anything you turn up!
Gnobuddy...wow what a lead! I guess I'm a bit naive in thinking the only place to find out information on electronics is here. lol I'll log onto there in a bit. The amp is en route to me as I type. Once I get it I'll take some detailed pictures of the insides.
I'm hoping someone over in the other forum knows a bit more about this thing. From the sounds of the above interview this Don Mori had made a number of amps for possibly 10 years. My hope is that someone there has at least seen one. I'm still at a loss as to what imedance I need to use, but I guess 16 ohm is the safe bet.
Thanks again. I have a few more pictures...
The world is full of smart and knowledgeable people, and thanks to the Internet we're lucky enough to be able to contact them all over the world with almost no effort.
I thought about this, and I believe I've figured out an easy way for anyone with an oscilloscope, an audio signal generator, and a basic understanding of basic electricity to work out the proper impedance for you.
Here's how: use the signal generator and 'scope to find out how much voltage the amp puts out before it starts to clip, indicating that it has reached its maximum. Then, knowing that it is rated to put out 100 watts, you can easily calculate the proper impedance.
For example: suppose you measure and find the amp puts out a maximum of 20 volts RMS just before it clips. (Note that the oscilloscope will show the peak-to-peak voltage, which has to be divided by 2 and then divided again by the square root of 2 to give you the RMS voltage.)
The equation for power is (voltage x voltage)/(speaker impedance). So now we know that (20 x 20)/impedance must equal the 100 watt rating of the amp.
In other words, we know that (400/impedance) = 100
Cross-multiply that and you get: impedance = 400/100 or 4 ohms.
That's just a simplified example. In real life the numbers may not work out so prettily, and the amp's output voltage will fall a bit when you connect a load to it. But I think things will work out close enough so you can tell if it's designed for 4 ohms, 8 ohms, or 16 ohms. And that's all you need to do!
So now all you need is to find someone with that test equipment and the ability to follow the basic arithmetic I outlined above. Maybe a local amp tech, or a local community college with an electronics program, or a local university with an EE student who actually knows how to measure real electronics (most of them just fiddle with software these days).
Even better, maybe you'll find someone on the AGGH forum who knows all about these amps - that would be the perfect solution!
Cool little amp, and it seems to have been the original "lunch box head" that all the amp companies think is such a brand-new concept.
The person I bought it from actually has the cabinet, also built by Don Mori. But they are keeping the cab. I asked the person I bought it from to search the cab for any kind of numbers, or even the speaker magnet. However, I don't think they know anything and wouldn't feel comfortable taking anything apart.
I can't do what you mentioned as far as detmining the impedance, but I know someone that can. Hopefully I can get a hold of him. I'm sure he has the equipment needed to test this stuff as he is a long time commercial electrician. He has fixed some old italian built machines in our building. Loves the challenge.
Looking at the pictures, is there any chance at all that the jack on the bottom left is 110V? I wouldn't dare hook anything up to it without knowing more. I am excited to just open this up and see what's inside. it's like one of those little easter eggs..just cant wait to open it and see whats inside. Hopefully it's not bad candy.
Did the amp come with the ac cord shown in one of the pics? If it did, see if that red connector fits one of the sockets on the back.
No need to take anything apart - all the owner has to do is measure the resistance between the two speaker terminals on the cab. This can be done with a cheap $5 Harbor Freight digital meter ( http://www.harborfreight.com/7-function-multimeter-98025.html ) if the cab owner doesn't have a meter already. Heck, you could buy one and mail it to him for the measurement, at that price it would be worth it considering the hassles it will save you!
If the guy can be persuaded to take a resistance measurement, you can interpret it this way: if he comes up with rougly 3 ohms, it's a 4 ohm speaker; if he comes up with roughly 6 ohms, it's an 8 ohm speaker; if he comes up with roughly 12 ohms, it's a 16 ohm speaker.
Same here...I wouldn't dare either. Hooking up AC line power to the wrong thing is a guaranteed way to fry this invaluable one-of-a-kind amp, so that's definitely a thing Not To Do!
Once you join the AGGH, if you don't get a response to your question, try contacting forum members Darryl or Roly. Both of them have been actively involved in Australian audio electronics for decades, and both of them seem to be walking encyclopaedias of electronics knowledge, so there's a very good chance at least one of them will know something about Don Mori and his amps.
Actually it did not. The person I bought it from assumed it was 220. he got the 220V plug and used a Voltage converter. But the article states that the builder designed these amps to run at 220 or 110 from the get-go. Since this probably predates switching, I wonder if maybe there were simply two sockets on the back and you could use one plug for 110 when you were in the states, and one for 220 for abroad.
Again, I'll post pics of the interior as soon as I get my little hands on it later this week.
Good idea. Next question, what is the mains voltage in the country the seller is from? 230V or 120V?
I'm guessing the other socket on the back of the amp is for the speaker?
Mebbe the designer put a voltage selector switch inside the housing - look for something like that when you open up the amp.
Now you've got me all excited over this easter-egg amp, and it's not even my easter egg!
Gnobuddy...awaiting my registration to be approved. Apparently this builder had a reputation in Australia. The interviewer, "John Rose" seems to have used these amps extensively on his tours. I did my best to find him, but I could only find pictures of him...which leads me to believe he is at the very best, very old.
It's somewhat fristrating since I keep coming up with leads, but none of them go far enough to give me any information, but enough to keep me looking. lol If this Don Mori really made amps for 10 years I would imagine someone over in the aussie forum would have an idea.
220V for sure. However the builder, looking to patent and sell these in the US, did make them from the start with the ability to run 220 or 110. And in the interview the interviewer mentions that he used these amps all over the world. So he either aleays had a voltage converter or somehow, somewhere, it is switchable. Hmmm....
I guess we'll get some better perspective once I open it up and post some pictures.
I opened her up and here are the pictures. The control panel attaches with a simple plug. The whole thing, essentially a massive transformer, is pushed into the amp and held in place with the screws on the bottom. I made the pictures as big as I could without being offensive.
What a very ingenious design! Your gut shots make it even clearer how incredibly tiny this thing is.
I have been wondering all this time where Mr. Mori put his output transistors, and how he was keeping them cool - there's no room for a heatsink inside that tiny enclosure. One of your pictures revealed just enough to find the answer: the power transistors are under the big transformer, and the heatsink is the entire metal case itself!
I put a red circle around the part of the one power transistor you can see in your photo, and attached that image below.
Now for the (possibly mythical) voltage selection switch. If it exists it will be wired to that huge transformer. Most likely it will be between the socket where the AC voltage enters the amp and the transformer primary coil. There won't be a lot of components in between here, usually just a fuse and the main power switch for the amp, so if there is a voltage selection switch it should be relatively easy to spot - though, of course, the tiny size of that amp makes nothing really easy to spot!
It is unlikely, but not completely impossible, that the switch could be on the secondary side of the transformer instead.
Either way, that switch should be wired to one of the transformer wires, possibly with one or at most two components in between, but most likely direct to one of the transformer leads.
I can't help noticing that the photo I'm attaching shows what looks like two mounting screw holes and an elliptical hole in the middle - it looks like it was designed to have a slide switch mounted there. But the switch hasn't actually been installed. (I'm talking about under the transformer, at the end near your fingers in the photo.)
As for the AGGH, yeah, I had to wait quite a while for someone to approve my request to become a forum member. I think I waited weeks, and eventually gave up checking. Even more weeks went by, I tried logging in with the info I'd put in my membership request, and presto! the login worked. (Nobody had notified me that they'd approved my membership request, so I don't even know when that happened!)
Let's hope that was an anomaly, and they approve your request to join quickly, so you don't continue to suffer pangs of unresolved curiosity about your Mori amp!
Hi Gnobuddy. That hole where a slide switch looks possible is indeed odd. That entire unit slides into the outer shell tighter with barely enough room to fit it in there. There wouldn't be room in the physical design of the amp to facilitate a switch there. Even if there was a hole in the outer shell for the switch, it would never make it in/out of the shell as the entire unit literally slides into the case about as tight as a gun round into a barrel. Still, since it's all custom built...that hole was deliberately put there by Mr. Mori for some purpose.
One question (this shows my true lack of knowledge), that connector on the back is in fact an XLR connection. I currently do not have a cabinet at all, but I do have an MBOX. Is it possible to use the amp with only the XLR out and go directly into the MBOX to try to see what kind of sound this generates? I'll build a cab once I determine the impedance.
I keep checking twice a day to see if I can log onto the Aussie forum. It seems it's very sparsely populated with only a handful of posts daily (compared to TDPRI). I'll keep on trying. It would be nice to know how some of these controls work. They look pretty sraightforward. That "tube" toggle is curious. Wonder if it's just an overdrive. Seems odd to see the two trim adjustments on the front panel too.
The quest continues. Hopefully someday I'll post a video of it in operation.