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Old 22nd June 2008, 11:45 PM   #1
amo is offline amo  United States
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Default Please explain what I did

I got inside an inexpensive amp, which sounded great to begin with, and began making some obvious mods. I wanted to bypass all the bad pots, cheap op-amps and needless switches. The entire input section, along with all volume/balance circuits and needless input selection circuits, is connected to the rest of the board via a ribbon cable. I disconnected the unneeded circuits and connected the line signal directly into the board. (The speaker is connected directly into the board as well) It sounds great! But I am concerned that I killed the input impedance or allowed too much voltage to go to output devices or something else which I have no idea about. Please take a look at the schematics and explain what it is that I did exactly. BTW I plan to replace the input section with Rod E's active XO... Thanks!

Input imedance?????
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Old 22nd June 2008, 11:47 PM   #2
amo is offline amo  United States
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schematics..
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Old 22nd June 2008, 11:48 PM   #3
amo is offline amo  United States
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Sorry dont know how to attach all in one... This is the most important page i suppose...
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Old 23rd June 2008, 12:26 AM   #4
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Guys, please don't ignore my post this time around. My understanding of this issue is paramount to a large project I have undertaken. If it is ok to do what i did, I will replicate this many times, build many power supplies and active XOs to drive my almost rebuilt B&Ws 683. I have been diligently collecting photos of this "sort of" interesting project. By the way, what do you think of the amp circuit it self? I am redoing much of the mechanical work, but don't feel like messing with the circuit much more. Should I abandon these amps (they are cheap!) for something a little better? I liked this design because I would be able to build Rod's Xo with a balanced input and single ended output, connect output directly into the board, and have a balanced preamp drive the xo from some distance. THANK YOU!!!!!!!!!!
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Old 23rd June 2008, 01:40 AM   #5
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Okay well its pretty hard to make sense of those really complicated schematics. I gave up but I guess you basically bypassed the volume control PCB and switches. Theres no real reason to go through this trouble. Those circuits are there for a reason and just because they're opamps doesn't mean its bad. There's nothing wrong with them despite what you might here otherwise. No need to get more expensive amps, any amp should perform more or less before clipping.
Perhaps someone can help out more I really can't see what happens after the switches. Ah well, I guess all I'm saying is there's no need to do what you are doing with bypassing all that stuff it just doesn't matter.
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Old 23rd June 2008, 03:01 AM   #6
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I agree, those schematics are massive and I can't fit enough on my screen to look at it all.

I think the easiest way to see if the impedance is messed up would be to just try and measure it.

Also, if it sounds different in a bad way, this could be a sign.

Not to mention, that if the input impedance is good enough, the circuit should be able to handle full input level.

I'm afraid I can't really help you beyond this, though I hope that someone can.

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Old 23rd June 2008, 03:13 AM   #7
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Sounds like amo just connect the signal directly to the power amp.
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Old 23rd June 2008, 03:42 AM   #8
amo is offline amo  United States
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I think i need to take a picture. First of all, the sound is much better! I am very happy with the result.

That is correct. All the input circutry is bypassed and it goes straight into the amp... is this bad? will it still be bad when there is an actrive xo between this input and the pre?

These amps can be had for pennies right now, so they are perfect for active set ups. The only thing that bothers me is the bad crosstalk measurement since its stereo.. its like 65... But at these prices I can get away with using only one side. thanks!
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Old 23rd June 2008, 03:58 AM   #9
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I think I see now. In this case I think it might be best if you connect a 22k resistor in series with your input. As it is it looks like the impedance is not much higher than 6.8k.

Whatever you're driving it with might be able to handle a 6.8k load but something cheap like from Walmart may not. I'm not quite sure what the standards are.

It might be best if you feed your input into a preamp with proper input impedance and then feed the output of the preamp into the amplifier.

If it sounds good, though, that is a good sign!

Also, if you're worried about crosstalk you might be able to just bridge the left and right amplifiers and get a single channel amplifier with 2X the power! Two of these units and you'll have two channels, but in different cases. I believe this is what they call a monobloc system.

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Old 23rd June 2008, 04:20 AM   #10
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Thank you soo much Keantoken! This is what i was looking for. I simply dont know enough about electronics to come up with that answer, but I knew somehow that I bypassed a needed component. Unfortunately I can not bridge as I am driving a single driver with each channel. Each driver is 4 ohms, so its too much for a bridged system. That is why I need so many chanels, because there are a total of 8 drivers, plus 2 powered... I recently took out passive crossovers from my towers and now building some actives. Very exciting. The Preamp will drive the active crossovers because they have the right impedence ratio, and the active xo will feed the amp through a resistor, like you said. That will take care of everything I believe. And thank you for your help!
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