A rare find.

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Nigel,
So, are you going to build it? I hope you do, that's the purpose of that thing to begin with.

A few years ago I found a Heathkit IT-121 transistor checker. The case had flaws from being shifted around for years, but I built it. I use the thing - just because. It was great to build a Heathkit, maybe for the last time. It was very satisfying.

-Chris
 
Yes I will build it.
I already have a couple of these amps that I bought as faulty.
It was fun getting them going again.
It took ages to fix one of them as the fault just didn't make sense.
I tested every component yet it still wouldn't work.
Turned out in the end someone had replaced a transistor with a pnp instead of an npn.
I really should have picked that up when I tested the transistor.
 
The first amp I ever built was one of these 50 watt Maplin amps.
As usual I ordered the parts from Maplin and of course the pcb was out of stock.
So me being impatient decided to rats nest it.
It just fried when I turned it on.
Magic smoke. So I had to reorder parts and wait for pcb patiently.


Here are some of my Maplin amps.
225WRMS, 50 watt and 75 watt mosfet.
[IMGDEAD]http://www.ckpr.talktalk.net/3amps.jpg[/IMGDEAD]
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Nigel,
That's a pretty decent little amp for the time. I've seen much worse in "high end" product. I'll bet it doesn't sound that bad, and knowing what you know now, you could really improve the sound quality by simply choosing better components (new capacitors are better) and matching transistors. In fact, the processes are better these days, so even replacing same with same could deliver better noise performance. Consider metal film for the feedback and front end resistors and increase the power rating for that 10K feedback resistor.

Sorry, I can't help myself. When building an old unit over, I often will make these changes, and the sound quality is well worthwhile.

-Chris
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Nigel,
Build it improved. The original circuit remains and you can judge it against the other Maplin amps you have. You don't need to go crazy, just use the better industrial parts without delving into audiophile approved bits that won't fit anyway.
or ...
why not? You've done the original builds, do something a little different. The original parts are good spares for the earlier builds. :)

-Chris
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I don't know about niceties. Spartan designs and doodles continue to pour in here from newbies with simulation software and those with a conviction that elementary or easily understood designs are intrinsically better.

I find it it interesting that the design is more than one pair of output devices in an inverted CFP arrangement, such that the emitter resistors couple to the power rails, not the output rail. I haven't seen that in a medium power BJT amplifier since Randy Slone used it as an example of CFP design in his "High power audio amplifier construction Manual" High-Power Audio Amplifier Construction Manual: G. Randy Slone: 9780071341196: Amazon.com: Books Any comments on that aspect?
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi rayma,
Thank you, but ...

I'm not that much into RF. Just precision time references and GPS monitors and 10 MHz oscillators. The rest is solid baseband for me.

It's really great that the Heath name is going to be out there. If you're into RF, that looks like a neat place to watch.

-Chris