DIY Audio Projects Forum

MX50 SE Amplifier, Transistor Subs
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Author:  BowToEd [ 21 Jun 2017, 00:14 ]
Post subject:  MX50 SE Amplifier, Transistor Subs

I ordered an MX50 SE power amplifier kit from the eBay. I won't actually have it in my hands for quite some time. However, I'm cautious about the possible invalidity of the transistors coming with the kit. Personally, I've never had an issue with transistors ordered from China, although I also don't buy the expensive ones from there. Example - if I need a handful of BD139s for RF amplifiers, I can get them for a dollar. Never had to order a matched pair of 2SC1969s or RDHHF16s, so I can't say whether or not eBay ones are any good.

The listing for the kit specified all the transistors...2SA1941 and 2SC5198 for the final transistors, and 2SB649 and 2SD669 for the drivers. It has crossed my mind to shelve these transistors and pop in some of my own, which I obtained from more trustworthy sources. For example, I have some transistors I pulled from a blown home theater receiver which I planned to reuse some day. I also have two pairs of TIP35/36Cs that I bought from Jameco a long time ago. I could also find good replacements for the driver transistors. I used BD139/140 once as low power finals and got an annoying hiss in the output. I'm not entirely certain that's because of the transistors.

I'm also thinking of ditching the film caps that come with the kit and ordering some better ones. Tube depot has a good variety of film caps.

My power supply will center on a large transformer I got from an AV receiver (see above) and whatever large caps I can find. I feel like I should use the large can caps I have, but plan for replacement as those tend to wear out eventually. Instead of running the thing at maximum power and getting 100 watts, I'm planning on adjusting the supply voltage to set the maximum power closer to 50w or so, allowing the transistors to enjoy a much longer service life. 50 watts of audio is more than enough if you're not powering a small venue.

The only other issue will be finding a suitable heat sink. Guess where I salvaged some of those from?


Author:  Peter W. [ 27 Jun 2017, 06:13 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs


Purchase a good transistor tester, one capable of matching. Test anything you install prior actual installation. There are some very nice, inexpensive devices out there for this purpose.

Similarly for the capacitors. Caps are commodities these days, the technology for making them is simple and well understood - to the point where making poor examples is probably more costly than making good ones. Further, no reputable kit-maker would dare use marginal parts. Given how little the average consumer understands about electronics, and how little skill they have typically, marginal parts, poor instructions or other issues would be fatal.

If you really do want to upgrade, wait until the kit is in your hands. Then make component-by-component decisions.

Author:  BowToEd [ 27 Jun 2017, 06:47 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs

What you're saying is that the matching of the transistors to each other is more important than what type of transistor?

Author:  Peter W. [ 28 Jun 2017, 07:47 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs

BowToEd wrote:
What you're saying is that the matching of the transistors to each other is more important than what type of transistor?

That is exactly what I am stating - with the further caveat that the 'type' of transistor used is - at least - suitable for the application. Example: A TIP31C from ON Semiconductor will be as suitable as one from Signetics, if a 31C is what is wanted in the first place. And if there is a range of options, those used should be from the same specific option rather than across the range.

Author:  Soundbrigade [ 31 Jul 2017, 10:19 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs

The answer you have gotten is "don't repair what isn't broken". I would like to add, putting a RR-emblem on your VW doesn't make it a luxury car.
I bought that kind of kit too and even assembled it and then the project cane to a halt. Anyway, som 10-20 bucks for a 50W transistor amp is so cheap, that I cannot see any point in trying to squeeze more "quality" from it. Build it and enjoy it. If you find any flaws, try to figure out where and how to fix them.
Enjoy your kit!

PS. Being a tubaholic, actually, I have been experimenting with cheap switching amps. There's a 60W mono amp, TPA3118, that sells at a few dollars.

Author:  wdecho [ 17 Aug 2017, 07:26 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs

I received my boards yesterday and installed them with an Antek 300VA 28V AC transformer with 45,000 uf per bank of capacitance. Sounds much better than it should considering the cost. Ideal first project for a diy'er. My understanding is this design is based on Douglas Self's blameless amplifier as are many retail amplifiers being sold. I question the longevity of the parts used but I cannot really fault the sound. Great bass with my horns and very good soundstage. When pushing less efficient speakers the results might be different.

Author:  wdecho [ 21 Aug 2017, 05:57 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs

I took the time to A/B the MX50 build against one of my Firstwatt clones and it was not surprising that it fell short against a good class A build. But it is a decent sounding cheap to build 100 watt amplifier. I ordered a set of the LJM L12-2 boards to give them a try. From what I found on the net the L12-2 boards are supposedly a Chinese clone of the Destroyer X amplifier by Carlos Eugênio Bezerra Mergulhão which is also based on the Self blameless amplifier. I can use the same PS section with a different toroid so it will be easy to change out the boards.

Author:  Soundbrigade [ 11 Sep 2017, 04:44 ]
Post subject:  Re: MX50 SE Amplifier, Transistor Subs

I must admit that I wasn'¨t that observant when I first saw this thread. I had to google the MX50 amp and found out that I have built exactly that one myself, but still haven't tested it.
After gathering components for the SymAsymand (original) Leach amps and even having PCBs made for the SymAsym, decided to put those projects on hold.
Instead, when I ran into the MX50 cheaply on AliExpress I bought it, hoping to get it up and running FAST! But that was a few years ago.
Maybe try to fix the last few details, like hooking up the PSU. Haven't gotten a chassis, but with two power transformers, monoblocks could be an idea.

So honestly, I am thankful that you shared your amp ith us, giving me a push forward. :up:

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