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Old 9th February 2003, 07:53 PM   #1
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Default Large Value Cap For DC Blocking


We have a situation where we need to be able to inject a small dc voltage into a speaker driver while it is being driven by a large power amp. We plan on monitoring the dc current flow through the driver to get an idea of its temperature. We plan on using a large value cap between the driver and the power amp to isolate the power amp from the dc voltage. We will be using an 8 ohm driver and will be testing at levels from 500 to 1500 watts. The test signal is filtered pink noise, 30 hz to 600 Hz. We were planning on using about a 10Kuf cap to be sure we do not filter any of the low frequency information.

My question is, can we use a computer grade electrolytic cap in this situation, or do we need to make a large bipolar cap out of a bank of caps?

The amp is capable of out put voltages of over 120 V.

How should we spec out the cap or caps?



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Old 9th February 2003, 08:00 PM   #2
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It would clearly be best to make a HV bipolar cap. You want *each one* in the back-to-back pair to be rated at something above the peak amp voltage plus the bias that you're injecting to the driver.
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Old 9th February 2003, 08:08 PM   #3
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Um, is your amplifier DC-coupled? If so, why not apply the DC bias voltage you want at the amplifier's input and forget about the output caps?

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Old 9th February 2003, 08:20 PM   #4
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the amp is not dc coupled. It is a QSC Powerlite 9.0PFC. 6000 watts into 8 ohms when bridged. We only use it for high power testing.

We will order caps to make a large bipolar cap bank.


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Old 9th February 2003, 11:30 PM   #5
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Bipolar shouldn't really matter, since ideally, it won't have any voltage across it. You might find some distortion from the caps, but if this is purely for heat in the voice coil, that doesn't matter.
Of course, at 1kW, voltages are getting up there so bipolar might be worthwhile anyway.

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