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Old 25th December 2003, 05:19 AM   #1
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Default Help with amp/schematic(newbie)

Hi guys,

I'm pretty new to this forum, and would like to make the class a car audio amp below. Could anyone tell me if the circuit is worth a try?

I was hoping i could get some help with some issues i'm still unclear about.

Firstly, can someone suggest a good value for the cap that is on the inputs, but not labelled? Or is that just a switch hehe

Secondly, at the bottom left there is an arrow that points to the 10K resistor. I assume this to be part of the circuit? If thats the case, should i connect the arrow before or after the resistor itself?

Sigh, another newbie question(LOL)....do bear with me....the 10mF cap is a polar capacitor right? If so, how do you tell which way is + and which way is -?

Finally, i have read through the data sheets on the mosfets in the circuits but am still unclear about how the schematic refers to the source/drain/gate terminals. I think that the source refers to the terminal connected to the voltage source, drain refers to the voltage outgoing terminal and gate is the one in between. Am i correct?

One last thing(thank you so much for reading this far). In car audio, amps usually have lots of capacitance in the power line because with a car, when its running, you get plenty of voltage swing(anywhere from 12-14.4V). I believe this causes ripple(not sure if thats the right term). If this is correct, could someone suggest adequate capacitance to ensure that the power supply is smoothed out?

I've got all the FETs and have planned out how i want to layout the circuit already. Just need to pick up the resistors and lugs etc and hopefully i can get started soon. Its normal to use a piece of wood as a circuit board instead right(breadboard or something?)

Oh, just a last thought, are there any FETs that i can substutute for(without making the circuit any more complex than it already is) to provide more power per channel? I was actually hoping for about 30W per channel Class A but its already really difficult to come across car audio amp schematics so i'm not going to be too choosy about it

Thanks for the help guys, really appreciate it
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Old 26th December 2003, 11:45 AM   #2
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surely someone must be able to help? Please...?
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Old 26th December 2003, 02:04 PM   #3
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Hello,

1. For the input capacitor I would use 10 microfarad also but use a metalized polyester film capacitor.

2. That is the schematic symbol for a potentiometer (variable resistor).

3. The fatter bar on the capacitor symbol denotes the positive lead.

4. The capacitor is used to store lots of energy. The capacitor is charged by your alternator as your amplifier depleats its charge.

5. You better heatsink those transistors or poof........
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Old 26th December 2003, 02:19 PM   #4
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Thanks for the reply!

I'm confused as to what the purpose of a pot is where its denoted. Is it meant to adjust input sensitivity, and if so, why not just put one on the inputs? Also, the arrow denoting the pot appears to be connected to another line leading to ground(where the 220uf cap is). Should i therefore connect them?

Could you suggest a value for a capacitor on the power input line to smoothen the power supply?

Thanks again!
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Old 26th December 2003, 03:20 PM   #5
skaara is offline skaara  Slovenia
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Could you suggest a value for a capacitor on the power input line to smoothen the power supply?


U dont need cap because u have pure dc from car battery.
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Old 26th December 2003, 06:50 PM   #6
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Slut,

The potentiometer wiper arm should be connected to the junction of the 10K and 220 microfarad capacitor. The potentiometer provides bias adjustment for the FET.
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Old 26th December 2003, 08:22 PM   #7
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Thanks a lot for the help so far guys i really appreciate it!

Just a last couple of doubts though.

Is there a FET i can substitute for to provide more power?

If not, can i bridge 2 similar FETs together? If so, how? Just off the top of my head, all i can come up with is

1) FET 1 drain to FET 2 source &
2) FET 1 & 2 gate to ground

I came up with this from studying some schematics where FETs were bridged(at least, i think they were bridged )

Not sure if this is the right way to go about it.

I'm actually hoping for a 30W per channel amp(will be making a 4 channel)

I'm also concerned about whether the amp will still be able to drive a 4 ohm load when the FETs are bridged(i have the pdf fact sheet on the FETs in this amp but i have no idea how to tell whether they are thermally capable of handling the lower impedance)

Regards
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Old 27th December 2003, 09:02 PM   #8
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You will never get 30 watts out of this amplifier. You need to switch to a different topology if you want 30 watts. I also highly recommend you get a book from your local library to start studying up on solid state amplification circuitry..
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Old 27th December 2003, 10:20 PM   #9
osey is offline osey  England
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Cant quite put my finger on it but Im not in love with this design. I think a lot of it might be the inductor which is going to be nothing short of rather large.

I do agree that you should do some more research into solid state and mosfets in general however, they are in principle quite simple devices.

Ima fraid I have little experince with them as such for amplification so ic ant give any great pointers - but you might want to consider using the student evaluation version of Pspice to simulate the circuit. It might help you understand whats going on that little bit better.
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Old 28th December 2003, 10:48 AM   #10
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Guys, thanks for the help. I dont have any kind of electonics background but do love audio and was hoping this could be a little hobby on the side. I have tried to read up on amplification circuits but the books i have come across so far are way to complex for someone with no prior knowledge, but i will keep trying.

I'm kinda going with this design because its the only car audio amplifier schematic i have been able to come across in this forum which looks relatively easy to build and is Class A.

I'm hoping that once i get into building the amp itself i'll have a better understanding of how it works and maybe then i might be able to understand those books in my uni library a little better

Thanks again, if anyone else has anything to add or some tips, do post.
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