Direct coupled Amp causing me grief... - diyAudio
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Old 14th March 2007, 03:41 PM   #1
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Default Direct coupled Amp causing me grief...

First thing's first...I'm no EE so be gentle

I've got a HK Citation 5.1 amp, that appears to lack DC decoupling on it's inputs...as far as I know it has DC protection on the outputs. Any time I attach this amp to me AVR via the pre-outs, it's causing the speakers to pop when the when switching inputs or switching the audio signal...

Strange thing is, it will make the speakers pop, even if the speakers are not connected to the HK amp...what I mean is, if I leave the speakers attached to the AVR high level outputs and connect the HK Amp to the pre-outs and then change signals, the speakers whose pre-outs are connected to the amp will pop even though the speakers are not physically attached to the outputs of the HK. Hopefully that makes sense...I had this problem with an adcom amp as well...

I thought it could be that the AVR was discharging some DC across the pre-outs, but the HK has dc protection on the outputs..and since the speakers are not attached to the outputs of the HK, that doesn't seem to make sense. yet when I place a cap between the pioneer and the HK amp, this popping goes away.

So I have been told that the amp is probably directly coupled...well, the pop is driving me nuts on layer changes of dvds so i need to decouple this amp somehow without detrimentally affecting the FR of the mains...any insight is much appreciated.

It would be ideal if this fix was internal to the amp. I have access to scopes and such.
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Old 14th March 2007, 04:00 PM   #2
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You could wire that capacitor into the input of your pioneer amp. This would solve the problem it sounds like from what you have tried. Can you find the input impedance (resistance) of the pioneer amp? You could then calculate a capacitor value that will work well, or just wire in the capacitor you used if it works good. This can be done internal to the amp. You may have to cut traces to get it in or if the inputs have wires going from them to the circuit board, then you can put the capacitors inline with the wire.
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Old 14th March 2007, 04:30 PM   #3
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Just to clarify..the Pioneer is my AVR and I'd rather not crack it open as it's a little more complex. The HK is the amp and I assume that is what you were talking about.

So in theory I could open the amp and insert a cap between the back of the RCA input and the PCB?

The input Impedance of the HK amp is 22kohm. The FR is 3hz to 22khz at full rated output. Ideally I would like to retain as much of that response as possible. The output impedance on the Pioneer AVR is 2.2kohm

which leg of the input would i place the cap, the neg or pos? This amp has the ability to be bridged...would adding the cap to the input lines inside the amp have any affect on its capability to be bridged?

thanks
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Old 14th March 2007, 04:48 PM   #4
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So you have many sources going into your HK amp and you are using this HK amp as a source selector. Then from the pre-outs of your HK amp, you are connecting it to an input on your Pioneer AVR. The speakers are then connected to the Pioneer AVR. Is this correct?

If so put the capacitor in the positive line of the pre-out of the HK amp. It should not effect the ability to be bridged.

What size of capacitor did you use when you hooked it between the two amps?
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Old 14th March 2007, 04:56 PM   #5
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Sorry for the confusion...

Here is how it's setup:

Pioneer VSX-1014 AVR -> HK Citation AMP -> Speakers

The Pioneer is the source selection device ie essentially the Pre/Pro. The HK is connected to the Pioneer's pre-outs.

So the way I see it, I would open up the HK Citation and insert a 1.5uF cap in series with the positive lead before it goes to the PCB.

I arrived at the 1.5uF cap using the calculator found here:

http://www.the12volt.com/caraudio/crosscalc.asp#ccc

my values were 22kohm for impedance and 5hz for Frequency of the filter.

I don't recall the cap value of what I used last nite, I will have to check tonight...they may actually have been 1.5uF that I had for another project.
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Old 14th March 2007, 05:10 PM   #6
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Sounds like you got er figured. Do it up and report back on if it works or not.
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Old 14th March 2007, 05:12 PM   #7
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Will do. If those caps I have are 1.5uF than I can at least do 2 of the 4 channels while I order some more.

thanks for help.

-patrick
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Old 14th March 2007, 05:38 PM   #8
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Your welcome.
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Old 14th March 2007, 10:14 PM   #9
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I know I'm gonna get flogged for even asking...but here it goes...

It's pretty cramped inside the HK:

http://cyberfrogs.net/photo/index.ph.../HK%20Citation

I've got some Vishay caps laying around...220uF 10V. Are these sufficient for DC decoupling:

http://cyberfrogs.net/photo/index.ph..._0464.jpg&id=6

or will I need to get some more Solen caps? The small size of the Vishay's is appealing because as you can see there is not much room to put these caps...unless I extend the input leads and place a board with the caps right underneath the AC input...not desirable either I assume.
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Old 15th March 2007, 02:00 PM   #10
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Those caps should work but definetly are probably not the optimum. The Fast Solen caps or something from them would probably be a lot better, but you could always put the Vishays in and swap in better ones if you think they are effecting the sound in a negative way.

Where it says CH1 CH2 and so on, are those the inputs? Is the thing coming through the PCB the positive lead? You could cut the little wire in half and solder the cap into that break.
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