Favorite Coupling Cap Brands and Types

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tubegroove

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Re: Favorite Coupling Cap Brands and Types
« Reply #80 on: 17 Jul 2014, 07:09 am »
The high pass is the easiest to do. Cable capacitance is no more an issue with or without the high pass. You need to know the output impedance of your preamp and input input impedance of your power amp, the latter will probably dominate when they are added together.

Here is a good calculator I have recommended before. http://www.sengpielaudio.com/calculator-XLC.htm

I worked out a typical example for you. If the preamp output impedance is 1 Kohm and the power amp input is 47 K add those together and put 48K into the calculator. For 80 Hz crossover the series cap is 0.414 uF. As you can see the output impedance doesn't matter much adding 1 k to 47 K wont change the answer appreciably. If you don't know the output impedance of your preamp you can likely ignore it as it should be small compared to the input impedance of your power amp.
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Once one has the data on hand of the pre amp and amp respective impedances and the freq response of the main speaker, how does one practically decide the high pass crossover point (80Hz in your example)?  Is it typically the-3dB point of the main speaker's low end of the freq response?  Should it be somewhat higher than the -3dB point thereby creating some overlap between the main speaker's bottom end of the freq response and where the intended sub woofer kicks in?  Whats good practice to come up with the crossover point? Or does it not matter so long as its within the sub woofers crossover range?
Thanks

sfox7076

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Re: Favorite Coupling Cap Brands and Types
« Reply #81 on: 17 Jul 2014, 02:38 pm »
Someone should get Dennis Murphy in here.  He is the crossover guy for Salk and his own speaker line.

Roger A. Modjeski

Re: Favorite Coupling Cap Brands and Types
« Reply #82 on: 17 Jul 2014, 04:09 pm »
I just looked, and found my receipt for the caps Tom installed in my amp, one on each channels input jack; 00.0033 MFD +/-10% 200V Siderealkap (matched). With an input impedance of 50K Ohms, does the frequency end up around 80-100Hz? I think that's what I was shooting for.

Someone missed a zero there. 0.0033 and 50K form a corner at 965 Hz. However if the cap is actually 0.033 then the pole is at 96 Hz which is fine.

numbers, numbers, numbers.... :(  This is why I teach electronic numbers in $$$. People pay attention to $$ because they have as real a value as electronic numbers have to  me.

bdp24

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Re: Favorite Coupling Cap Brands and Types
« Reply #83 on: 18 Jul 2014, 01:52 pm »
Yeah, I assumed that number had the accidental zero. I'm going to open the amp (I still have it, but it's been in a box for years. Toobs rule!) and look at the cap to make sure.

Roger A. Modjeski

Re: Favorite Coupling Cap Brands and Types
« Reply #84 on: 20 Jul 2014, 08:51 pm »
Once one has the data on hand of the pre amp and amp respective impedances and the freq response of the main speaker, how does one practically decide the high pass crossover point (80Hz in your example)?  Is it typically the-3dB point of the main speaker's low end of the freq response?  Should it be somewhat higher than the -3dB point thereby creating some overlap between the main speaker's bottom end of the freq response and where the intended sub woofer kicks in?  Whats good practice to come up with the crossover point? Or does it not matter so long as its within the sub woofers crossover range?
Thanks

Since I don't want male vocals coming out of the sub I chose 100 Hz and 24 dB/octave. If your slope is not that steep on the sub you have to choose a lower crossover point.

I would not go to the -3 dB of the main speaker, try to stay half an octave above that especially if there is a peak before the fall as there often is. There are lots of trade-offs to explore. Experiment if you can. If your satellites don't go low then you have to go higher with the Sub. If they go low and you have amp power go lower in the crossover. Listen to each speaker separately.

In my ESL system and with QUADS I recommend 100 Hz, 24 dB octave in both directions.