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Old 26th November 2008, 01:00 PM   #1
Dan2 is offline Dan2  South Africa
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Default volume limiter please help

hey guys, i have been busy on Rod Elliott's project 67 - a fast audio peak limiter. it can be found here:

http://sound.westhost.com/project67.htm

i have wired it up on a breadboard - hooked up the headphone output from my computer speakers to the input, and hooked up my headphones to the output. i have cranked everything to full volume and it doesn't do any limiting at all.

i think the problem is the FET - 'cos i have double checked everything. the problem with the FET is i cannot find a datasheet for it. it is an F2458PH75. i have tried connecting it up with all the possible pin configuration and the only time anything happens the volume is soft.......all the time
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Old 26th November 2008, 01:49 PM   #2
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I am not familiar with that JFET number.. not does google provide any info.. maybe you could buy a few of the correct JFETs that are indicated in the diagram: 2N5459...

This is no way telling what is wrong without knowing the parts used...
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Old 26th November 2008, 01:57 PM   #3
Dan2 is offline Dan2  South Africa
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haha i tried to get the right JFET - no stock. tried to get an equivalent JFET - no stock. so i just got these thinkin i would b able to find the datasheet.

as for the rest of the parts they are the same as on Rod's site. C5 is tantalum.

as for the circuit - is the JFET the part that does the attenuating??
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Old 27th November 2008, 10:39 AM   #4
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yep, the JFET is the attenuating part.... you could ask Rod Elliot for equivalent JFET suggestions, but start my trying out the 2n5458 or... or 2n3819, maybe even j310 .. the are all very diffenrent parts, but they share the same property that they are N-channel, and thefeore need a negative bias volatge..

grt, T
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Old 28th November 2008, 10:49 AM   #5
Dan2 is offline Dan2  South Africa
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thanks for the help - i will have to get some others maybe.

do JFETs have current gain like BJTs do???
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Old 28th November 2008, 11:18 AM   #6
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FET means field effect transistor, i.e. controlled by a voltage field.
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Old 30th November 2008, 08:26 AM   #7
Dan2 is offline Dan2  South Africa
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ok so the difference between FET and BJT is that BJT needs some current on the base while a FET does not??

but there must be some kind of gain or something right? i mean - i can't just slap any 'ol FET in the circuit and expect it to work....... can I?

BTW sorry for the noob questions, my electronics course covered BJT's not FETs.
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Old 30th November 2008, 02:36 PM   #8
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There are several types of FET and they work in different ways. Too much to cover here. Try and get down to a library and dig out. The Art of Electronics by Horowitz and Hill.

However for this project you need a J-FET. 2N3819 is very popular so have a look for one of those, just keep the supply limited to 12V rails though as this part is only rated to 25V.
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Old 1st December 2008, 10:50 AM   #9
Dan2 is offline Dan2  South Africa
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thanks guys.

so thats the problem with my circuit, the rest should be fine (im sure it is 'cos its not exactly a complex circuit)

one more question, is it possible i blew the FET i have?? 'cos afta not finding the datasheet i just tried connecting the legs in any order to the circuit.
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Old 2nd December 2008, 09:28 AM   #10
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Yes it's highly likely. Also you need to take static precautions as FETs are much more easy to damage by touching with your hands.
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