apheared 47 question
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puck

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i just put together an apheared 47 amp on my breadboard. i'm using slightly different cap values because that's what i happen to have on hand. anyway, when i listen to it, it is horribly sibilant and seems to lack midrange in a serious way. i built up a cmoy on the same breadboard and it sounded fine. anyway, does anyone have any ideas what might be causing this?

cheers,
puck
 
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Nezer

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I think you can eliminate the breadboard... :wink:

How far off were the caps you used? What op-amp are you using, it's possible that its oscillating. you might try cramming a 47uf cap in the feedback loop to see if that helps.
 
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tangent

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It depends on how "slightly" different the cap values are, Puck. Also, changing the input impedance resistor on this amp can cause problems like this. I guess you just need to list all your component values....

Quote:

try cramming a 47uf cap in the feedback loop


47pF, surely.
 
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Nezer

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Thats like the 5th time today I have said somthing totally stupid like that. I really shouldn't have got out of bed this morning!
 
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puck

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the resistors all are the same. they're just regular resistors but they seem to all be ok. i used .22 uF caps (specs call for .47uF caps) for the input filters-same as i used in my cmoys. i thought that that was just a basic dc filter though. i thought that at worst, this would roll of the low bass a little. the bass seems to be fine however. i'm using opa2132 opamps.
 
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Nezer

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How much voltage you feeding the opamp? What is your gain?
 
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puck

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a split 9 volt. i cant remeber what the gain is but it is set at whatever it was in apheared's article. i think that its around 3 or 4. does this amp require a duel 9 volt power supply? i don't have any high impedence headphones so i don't need all that much power for them.
 
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tangent

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The input caps along with the input impedance resistor form a high-pass filter. This does indeed roll off the bass. This is why I asked about the resistors. Let's say you reduced the input impedance resistor to 4.7K -- your filter's corner frequency would then be about 145 Hz. You'd be missing all the bass, and you might then turn the volume up to get some bass, which brings the treble up to earsplitting levels.

Touch your op-amp chips, or use a milliammeter and measure the power draw of the circuit. If the chips are hot, or power draw is over about 20 mA, the amp is oscillating. The simplest solution is what Nezer said: don't build critical circuits on breadboards. Use protoboard.
 
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puck

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i'll put an am meter on it and check but the chips aren't getting hot. thats always one fo the first things i check on any circuit-if any components are getting hot. i have to order some more parts soon so i'll get some bigger caps for the input filter then. also, i only built things up on the breadboard first to check and see if they work. i then rebuild on protoboard. thanks for the help though.
 
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