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Old 1st November 2007, 01:44 PM   #1
Dan2 is offline Dan2  South Africa
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Default VERY old school amp

i have a mosfet amp that looks like it came from the stone age ( well you can see all the parts have been hand soldered in) but it still works well and can drive a pretty decent speaker.

it has the preamp built in - so all u have to do is hook up a cd player, speaker and it plays music.

the only problem is that the amp distorts before it reaches its power limits (i think), because the supply voltage doesn't drop too much (maybe 1 volt) and the bass doesn't thin out, and the amp doesn't even get warm

is this just how mosfet amps work or does it sound like i could get some more power out of this amp?
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Old 1st November 2007, 02:25 PM   #2
poynton is offline poynton  United Kingdom
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Could the CDplayer be overloading the input ??

A photo and schematic would help.


Andy
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Old 1st November 2007, 02:55 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
Mosfet amps generally perform better with a higher bias current than a BJT amp.
This usually means that it runs warmer. Odd that you say it does not warm up.

I wonder if the distortion is a crossover problem?


Can you identify how to measure the output stage bias current?
Can you estimate the heatsink capacity and temperature rise?
Can you tell us what the output stage consists of? Driver and output device type numbers and number off.
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Old 2nd November 2007, 01:11 PM   #4
Dan2 is offline Dan2  South Africa
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i don't have a schematic of this amp, sorry. i can tell u what is written on the mosfets : SGS 9805 DB116 (i think they are mosfets)

i tried looking for the datasheets of the DB116's but no luck.

here are some pics
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Old 2nd November 2007, 01:13 PM   #5
Dan2 is offline Dan2  South Africa
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from the side
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Old 2nd November 2007, 01:14 PM   #6
Dan2 is offline Dan2  South Africa
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Old 2nd November 2007, 01:27 PM   #7
Elvee is offline Elvee  Belgium
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Hi,

Looking at the pictures, it seems pretty sure it is not a mosfet amplifier. In fact, the output devices might even be germanium.
The first thing I'd do is to check the electrolytics : there is a number of them, and after 40y or so, there is a good chance one or two have dried out.
LV
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Old 2nd November 2007, 02:15 PM   #8
poynton is offline poynton  United Kingdom
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What are the numbers on the other transistors ?



Andy
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Old 3rd November 2007, 11:33 AM   #9
Dan2 is offline Dan2  South Africa
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how do you check the electrolytics??? and what happens to the amp when the caps dry out? 'cos it still works well, it just doesn't seem to run at its full potential.

i don't suppose you can guess the power output, maybe??
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Old 3rd November 2007, 01:42 PM   #10
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by poynton
What are the numbers on the other transistors ?

Andy
Any answer ?


Also what volume control arrangement are you using?


Andy
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