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Old 27th June 2002, 12:31 PM   #1
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Default much less crazy idea

Okay, I took some advice here and there and came up with a design that I think is much more "feasible"... I took a look at esp project 3 http://sound.westhost.com/project03.htm and decided to make my own version with more power output and to be more capable of driving a 4 ohm load. I decided to use complimentary pairs of a1492/c3856 for the OT and BD139/140 for the VAS and drivers, even though I have tip40/41 in my diagram... Does anybody think this is a good idea to make this amp I modified? Please mention if you see something wron in the pic, as in a reversed biased diode,(that sort of thing) Thanks...


P.S. I somehow calculated that this amp sould put out 3000 watts
into a 4 ohm load, which I figure is quite improbable, can I have some pointers for calculating OT power for a class AB please?
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Old 27th June 2002, 12:59 PM   #2
e96mlo is offline e96mlo  Sweden
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You are on the right track but you should probably read Mr Elliots articles on how to design amps first.

Some pointers:

You don't need both diodes and Vbe multiplicator. Choose one.

The collector resistors should be one common for both transistors and it should be in the order of 0.1 Ohm. This goes for both npn and pnp side.

You might need to balance your diff amp with resistors. A current mirror is also a good idea.

There should be a DC blocking cap on the input.

Maximum dynamic power into 4 Ohm: Vcc^2/4 minus losses
RMS power into 4 Ohm: Vcc^2/4/2 minus losses

So a value in the right direction is reached with Vcc^2/4/2. This will show a too high figure but it is in the right order of magnitude.

Hope this helps.

/Marcus
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Old 27th June 2002, 01:22 PM   #3
Duo is offline Duo  Canada
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Thanks for the pointers, there is a reason I left the double diodes in with the bias servo, if the the wiper on the pot ever opened, the OT transistors would go way way off bias, the idea of leaving the diodes in was to prevent that from ever happening. Next, I did mean to put a cap in the input, just tired from all the reading I've been doing and didn't notice that I forgot it. Next, the collector resistors thing was an accident, I meant to have .5 ohm
common collector resistors, but I accidentally drew two instead, so I just made up their values to "act like" a .5 ohm resistor, but another accident occured when I forgot to put the . in 1.0, ending up with 5 ohms per set of collectors... oops! I will do like you said and put in .22ohm matched resistors, since I have some
nice dual resistor blocks designed specifically for this purpose that came out of an amp with the same transistors... Next, I'm not sure what to do about balancing the diff amp.. Also when you
say vcc^2/4/2 do you mean vcc squared times (2/4/2) or something else?
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Old 27th June 2002, 01:34 PM   #4
e96mlo is offline e96mlo  Sweden
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Read more about the diff amp on ESP and on Douglas Self's site. They explain it better than I do.

The calculation:

Prms = Vcc*Vcc/Z/2, where Z = 4 Ohm in your case.

/Marcus
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Old 27th June 2002, 01:41 PM   #5
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So with your calculation, my amp would put out 1800 wrms (dependant that I could attain full rail to rail swing.) So If I compensate for voltage loss in components, I could get around 1300 watts rms, honestly? I know that the OT devices can take that much current, but it seems kinda fishy... I guess it must be 1300 watts, wow a lot of power!
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Old 27th June 2002, 01:47 PM   #6
e96mlo is offline e96mlo  Sweden
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On your drawing (which is rather unclear) it looks like you have +/-60 V rails. This gives:

Prms = 60*60/4/2 = 450 Wrms minus losses in the O/P stage. Perhaps around 400 Wrms in reality.

Since the speaker is connected between the O/P and ground there can only appear one railvoltage over the speaker at a time.

/Marcus
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Old 27th June 2002, 01:53 PM   #7
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Yes, I just figured that out... At first I thought I had the full rail to rail into the speaker, then I though, no wonder it's fishy! So I calculated 450watts for an ideal world and about 400 for a real world... So , I did get on the right track for that one!
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Old 27th June 2002, 01:55 PM   #8
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BTW: Thans for all the info, you've been a great help!
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Old 27th June 2002, 01:58 PM   #9
e96mlo is offline e96mlo  Sweden
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That's what the forum's for. I'm glad I could help.

Good luck!

/Marcus
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Old 27th June 2002, 02:07 PM   #10
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One last question, do you have any ideas on free software I could download to draw proper diagrams so they are easier to read? I've heard of something called pspice, is it good?
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