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Old 3rd June 2012, 09:24 PM   #1
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Default Inverting stage not working...help!

Help!

I am modding an old Hartke HA 5000 bass amp. It has a crossover, but these never had an inverting stage to bridge the 2 power amps inside it. Silly!

So, I pulled out the crossover in the head..and am trying to invert the signal going to one of the power amps so I can bridge the out put.

My issue...I believe I have the inverting op-amp wiried correctly..but it's not inverting the signal.

Click the image to open in full size.

So we see IC-202B not really being used. I can take a signal off the wiper of the pot...and put in series a resistor 1k For now...to pin 6. And then copy the caps and resistors from the out put of the non-inverted stage...for the inverted output.

I do that, but the signal is not inverted....

So just remember..we're using IC202B to invert. not A. I asked this question on other forums, and they're telling me I cant invert A....which is NOT what I want to do!
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Old 3rd June 2012, 09:54 PM   #2
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hi mike.
added the inverting components.

R
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File Type: gif needinvert.gif (16.5 KB, 79 views)

Last edited by robo7; 3rd June 2012 at 10:02 PM.
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Old 3rd June 2012, 09:54 PM   #3
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Your idea is basically correct about using IC202B as an inverter for bridging. But, the 1K will present a pretty low impedance to the wiper of the pot. Instead, I would connect one end of the 1K resistor to pin 1 of IC202A (or any component electrically connected to the output) and connect the other end to pin 6 of IC202B. That way, IC202A works as a signal buffer and IC202B will give you an an exact, but inverted signal at pin 7. Then use the same R & C network that's on the output of pin 1 that feeds the rest of the amp.

That should work. The reason it may not be working now is that pin 6 will always be at 0 volts adding a 1K load to the wiper of the pot and to the preceding preamp. It may be enough to swamp out the signal so you have no output.

Does this make sense?
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Old 5th June 2012, 07:34 PM   #4
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Perfect sence! Thanks!
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Old 5th June 2012, 07:45 PM   #5
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robo7 said the exact same thing I did, only with a picture instead. Same solution that should do exactly what you want.
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Old 5th June 2012, 08:33 PM   #6
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Yep! Thanks guys!
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Old 5th June 2012, 08:41 PM   #7
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I changed Resistor 218 to a 3K

And I am using 10K for the inverting stage input and feedback. Should I drop those to 1K? I raised 218 to 3 K because I took out the crossover circuit..which added gain as well as the crossover.
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Old 9th June 2012, 07:35 AM   #8
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Be wary when you bridge a pair of amplifiers these see half the rated impedance of the speaker you intend to use them with - they should be safe with 8 ohm ones, but they may not like 4 ohm loads.

If you use the same values of resistor in the input and feedback circuit of an inverting stage the gain of that stage is -1 times. 10k values in those positions will work but in theory 1k ones will give less noise. If you are using for a sub you don't need to worry.

mjona
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Old 10th June 2012, 04:47 AM   #9
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Yeah, I'd keep the resistors at 1K. Sure it's down to theoretical current noise, but it's best-practices to keep values (and Iib (input bias current)) for both channels the same.

Regarding altering R218 to keep the gain the same, as long as the overall system gain (from input jack to speaker terminal) is the same, you should be fine.

I second what mjona said about bridging amps. Be very cautious of your amp's ability to drive low impedance making sure to only bridge it into a load it can handle. So if the amp is rated as being stable into 4 ohms, it can be bridged into 8 ohms. If it can handle a 2 ohm load, you can bridge it into 4 ohms. If you exceed the ratings, best case is current limiting and a hot amp; worst case is a dead amp and more repair and rework needed.
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