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Old 21st May 2003, 07:43 AM   #1
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Default simple Amplifier?

Hi,

This is my first posting in here - so be not too strict with me

I just found this schematics on the web.
Can anyone tell me, if it's worth a try?
I just want to build a really simple amplifier. It don´t have to be with shortening protection or anything like that or dc-protection for the speakers.

If you have any other good and simle circuit which runs on +/-45V let me know, please
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Old 21st May 2003, 10:12 AM   #2
djk is offline djk
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A couple of comments:

Sneak another 1N4007 in series with the other two so you end up with a string of three diodes.

Add a 100pF cap from B to C on the TIP41, that may be too big, but better safe than sorry.

Heatsink the outputs real well with either grease and mica washers, or rubber insulator pads.

The 27K on the input would be better if it was 22K, less chance for DC off-set.

Lower the 1K5 on the input transisors to 1K.

And very important: you must add a 0.1F cap in series with 10 ohms and then put this combination directly across the speaker out terminals.

I expect this will do 75W at 8 ohms. I wouldn't run it at 4 ohms without reducing the supply voltages.
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Old 21st May 2003, 10:45 AM   #3
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Quote:
Originally posted by djk
A couple of comments:


And very important: you must add a 0.1F cap in series with 10 ohms and then put this combination directly across the speaker out terminals.

Just for me as a newbie, why is this so important?

What about using BD249/250 for the TIP142/147? Seems they can handle more current and are also Darlingtons...
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Old 21st May 2003, 07:41 PM   #4
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so this is what the amp looks like (after djk's suggestions):

best regards,

HB.
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Old 21st May 2003, 07:59 PM   #5
JDeV is offline JDeV  South Africa
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Default Re: simple Amplifier?

Quote:
Originally posted by technics
If you have any other good and simle circuit which runs on +/-45V let me know, please
Did you look at this forum?

http://www.diyaudio.com/forums/forum...php?forumid=40

Example of very easy/simple circuit which you will find there, and can work from 45-0-45 (with LM3875 for example - which you can get as free sample
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Old 21st May 2003, 07:59 PM   #6
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I would suggest something like this:

any comments welcome,

HB.

EDIT: zener is 12V one
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Old 22nd May 2003, 06:27 PM   #7
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Default Re: Re: simple Amplifier?

Quote:
Originally posted by JDeV


Example of very easy/simple circuit which you will find there, and can work from 45-0-45 (with LM3875 for example - which you can get as free sample
Thanks for this circuit - maybe Ill give it a try. But occuring to the datasheet it will "just" have 56 Watts on 8 Ohm which isnt very much on +/-45V...
Id like minimum of about double this value (say 100Watts continous power on 8 Ohm), but capable of powering 4 Ohm with approx. 200Watts. So that bridging on 8 Ohm is possible.

I got here 4 power supplies with each delivers continous 350VA by trafo and enough caps to get stable power.
So I could build 4 identical amps and bridge each two of them to get 2 bridged channels.
Or I could bridge each two of the trafos and build 2 amps with each running at +/-90V...
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Old 22nd May 2003, 06:30 PM   #8
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I think a forward-biased LED would be better than a 12V zener, or you'll lose lot of drive in the positive direction, the output would probably clip at already 30V or something like that... the resistor values will have to be changed in that case too. I also think a LED is less noisy than a zener.
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Old 22nd May 2003, 06:39 PM   #9
elizard is offline elizard  Canada
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Default Re: Re: simple Amplifier?

Quote:
Originally posted by JDeV


Did you look at this forum?

http://www.diyaudio.com/forums/forum...php?forumid=40

Example of very easy/simple circuit which you will find there, and can work from 45-0-45 (with LM3875 for example - which you can get as free sample

lm3875 cannot run off of 45-0-45 ..
that's 90V, its max being 84V, that's a little too much
i wouldn't recommend more than 40-0-40 ..
just thought i'd clear that up
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Old 23rd May 2003, 10:06 AM   #10
djk is offline djk
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"Just for me as a newbie, why is this so important?(0.1F+10R)"

If the amp doesn't 'see' a load at high frequency it can oscillate and blow up.

"What about using BD249/250 for the TIP142/147? Seems they can handle more current and are also Darlingtons..."

The TIP142/147 are plastic and rated at 125W at 25*C, in a real amplifier I wouldn't expect to see them run harder than 60W for the pair.

If you could find some 200W plastic darlingtons I wouldn't run the pair harder than 100W, and be sure and look at the secondary breakdown curves too. The TIPs start to fold back at 30V, even though they are a 100V rated part.

And when I say 60W or 100W I mean into the lowest impedance the amp will be expected to drive. If you make the voltage high enough to do 100W/8R you will not be able to drive 4R with only one pair of outputs. If you set it up to do 100W/4R, expect to see about 60W/8R

" but capable of powering 4 Ohm with approx. 200Watts. So that bridging on 8 Ohm is possible."

"Or I could bridge each two of the trafos and build 2 amps with each running at +/-90V..."

I would pick the 90V option, less hassle than bridging. If you have a good heatsink and will only be diving 400W/8 you can get away with only three pair of MJ21193/21194 plus driver transistors. If you expect to drive 600W/4R off the same 90V you will need five pair, 1000W/2R you will need eight pair.

If you want to use plastic, the MJL21193/21194 have the same die in a plastic case, but you will need to use TWICE as many to get the same reliability. Or you could do a tiered power supply with 45V and 90V rails and fewer outputs.
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