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Old 8th June 2006, 02:06 PM   #1
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Default 100-150watt amp

Hi there
I found a 100-150w circuit on the web and decided to build it.
It comprises two darlingtons tip 142 and 147. The other transistors are tip 41, and two Bc 558.
After two attempts the BC 558 that is linked to the + voltage bursts.
I am also measuring 45 volts across the speaker leads. Is this normal?

Has anyone built this circuit? Did it work?
It a pretty simple circuit and I would really be pleased if it works.

If anyone has a simular simple circuit with matching power that works well, can u please let me have a look.


By the way I could locate the Tip 147 so I used the replacement NTE 271.:confused:
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Old 8th June 2006, 02:11 PM   #2
Netlist is offline Netlist  Belgium
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Welcome ignatius,

I've seen this schematic pop up on a regular basis.
You might want to look at this thread.

/Hugo
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Old 8th June 2006, 03:27 PM   #3
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Default what about the power supply

Is it possible the voltage is too large, because I folowed some not all i will admit but the circuit still smokes; and to think I am using 38+38 Volts
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Old 8th June 2006, 03:53 PM   #4
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Where did you get the schematic from? There is a warning on the website where it originates from about the BC558...
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Old 8th June 2006, 07:57 PM   #5
traw is offline traw  United States
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Still in midst of reading other thread provided (thanks, didn't come up in searches for me before)

I built it... had some hiss and vulnerable. Had decent sound and punch . Gain seemed too high. I was running of 39-40 volt rails. Also has some problems where it didn't seem to settle in and sounded very distorted once in this funky state (bias?). It was like I needed to turn it on with no input, let it sit for a bit, then ran good. Then tried changing my input caps as I had small ones in, 1uf rather than 4.7uf electrolytic in schematic, to some 5.0uf crossover caps to test and smoked the amp. Haven't gotten around to trying again as I have more 142 and 147s on hand as I know it has some inherant problems.

Will study the other thread as will be a good learning exercise
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Old 8th June 2006, 09:01 PM   #6
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A few years ago I reconstructed the following schematic from the drawing shown in the start of this topic.

Remarks:
1) Note that the output transistors are darlington's
2) This is real Class B design (bias current in output stage is 0), so you can expect quite some crossover distortion, especially at low volume levels and high audio frequencies.
3) Do not expect hifi quality from this design. I do not recommend to build this schematic other than for educational purposes.
4) Like mentioned by richie00boy below, it is not unlikely that this amplifier will act as oscillator.
5) It is very well possible to improve performance by means of a few simple design changes.
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Old 8th June 2006, 09:04 PM   #7
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The design is also unstable with no pole compensation.
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Old 11th June 2006, 09:15 PM   #8
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I also built this circuit as my first amp and indeed it is a 100 watt oscilator. I'm by no means an expert or even remotely qualified to give analysis on circuits but I did design and build a circuit based on that one that I have been using for about a month. It performs well and has decent sound quality but is great for learning your way around. Hope it works for you and remember, no matter how tempting it may seem NEVER stick your tongue accross the big black nichicons
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Old 11th June 2006, 09:45 PM   #9
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Sorry for the double post but my attachment appears to not have gone through.
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Old 12th June 2006, 03:04 AM   #10
CBS240 is offline CBS240  United States
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In looking at the schematic in post #6, I sketched out a few ideas to improve that circuit but still keep the basic idea, just a few peripherals such as the mirror to balance the current in the diff-amp stage and a current source to set and limit it. Might keep the BC556's from exploding. A CCS to load the VAS with BD139/140's. Vbe multiplier is a must for these darlingtons if they are to be biased in class AB to compensate the temperature coefficient and maintain thermally stable bias. 10-50pf cap for miller feedback may help with stability. Finally a Zobel network may also help quell oscillations.

Wouldn't expect get more than around 50-60WRMS from one pair of these.


Any other quick improvement ideas from the actual 'experts' out here? and to point out where I went wrong.


I might be willing to try this circuit if the TIP141's I have in my drawer weren't fakes...and bad ones at that! getting ripped off is quite irksome.

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