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Has anyone built the MJamp?
Has anyone built the MJamp?
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Old 7th November 2000, 07:32 AM   #1
Jon T is offline Jon T  United States
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I was wondering if anyone has built this little amp:


There is a certain simplicity -- an almost tube-like topology -- about the circuit that really appeals to me. I'm curious how it sounds.

At any rate, it should be cheap, so maybe I'll throw it together when I have a moment.

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Old 16th November 2000, 06:38 AM   #2
Leo Chan is offline Leo Chan
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Default It's not bad

One of my friend builded it b4, he told me its very good, but i didn't prove about it....

If u want to build this amp, u need to match the Hfe and the Vbe of the input stage Tr...
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Old 10th January 2001, 11:18 PM   #3
promitheus is offline promitheus  Europe
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Has anyone built the MJamp?
Default not so cheap output mosfets

It looks simple and might have a lot of watts I dont know the rating but the output transistors are not so cheap and easy to find I think.
Anyway it is very simple so good luck if you decide to make it.
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Old 14th July 2001, 08:10 PM   #4
tschrama is offline tschrama  Netherlands
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Have you build it? I have been simulating the schematic
extensively now... it performs very well.. see my post-reply
on the Hiraga amp which is similar in topology .. I'm rely
thinking about building it.. it has such nice features..
any experience?

email my for any quwstions?

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Old 15th July 2001, 12:34 AM   #5
dshortt9 is offline dshortt9  United States
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I've been wanting to build this one also, as usual with more power. 40 volt rails would give about 100 watts with the only drawback being that the gain depends on the feedback ratio. As you have designed it with a gain of 5 one would need 8v. peak or about 5v. rms for 100 watts out. The higher the gain, the less feedback available to correct for crossover distortion (assuming one is running the amp class AB).
There is an interesting 150 watt amp using this style of output at: http://www.ne.jp/asahi/evo/amp/J49K134/zu3.pdf
This person uses the output stage with no gain and uses small mosfets instead of transistors to drive the output beasts. He uses a very complicated front end to drive it all.

I am thinking about using a tube for the first stage and capacitor coupling it to the MJamp in a similar fashion to the one here:

What does anyone think?

There is a good substitute for the 2SK135/J50, the BUZ900 series MOSFETS from http://www.semelab.co.uk

The American distributor is:
Martinez & Associates,Inc. is pleased to announce that we have been designated the sole US distributors for Magnatec's Lateral MOSFET product Line.

We can be reached at:

Martinez & Associates, Inc.
234 Boston Post Road
Wayland, MA 01778
Tel: (508) 358-0131
Fax: (508) 358-2361
email: john@martinezandassoc.cnchost.com

We accept payment by check or VISA, Mastercard, Discover and American
Express credit cards.

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Old 15th July 2001, 09:06 AM   #6
djk is offline djk
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The amp at http://www.ne.jp/asahi/evo/amp/J49K134/zu3.pdf is missing a connection dot in the schematic.There is no connection from the drivers to the outputs.Without a feedback connection the gain will be unbelievably high.The front end is not that complicated.It is the same circuit.It just has cascodes and current sources added.It is also missing gate zeners for all the fets.
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Old 3rd September 2001, 09:36 PM   #7
tschrama is offline tschrama  Netherlands
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this circiut can be easely biased in class A push-pull..
it's a little trikky .. but the itterative adjustments of the collector risistirs in my spice sim's it simple.. and very good spec's!!! really unbeliveble!!....

any results yet anyone?

greetings Thijs
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Old 3rd September 2001, 10:14 PM   #8
P.Lacombe is offline P.Lacombe  France
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I have built several amplifiers with this scheme. It's difficult to obtain the same operation in all them. It's necessary to insert adjustable resistors in collectors of the driver transistors, and to carefully sort and match all transistors. Therefore, it's necessary to buy many transistors. This may be a problem with the expensive output devices.

But whith this care, operation is reliable, and sound very pleasant, fluid and clear. Power supply unit must be as big as possible, big transformer, many capacitors, and very short wiring. Power supply unit is 50% of an amplifier !

Regards, P.Lacombe.
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Old 12th February 2004, 02:24 PM   #9
Teo is offline Teo  Italy
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I've built a version of the mjamp for guitar amp use.

Topology is the same, only resistor values are changed to have an open-loop gain and feedback loop similar to tube amps.

I can't tell you how it will perform in an hifi situation but I like it a lot!
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