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Old 26th May 2015, 04:37 PM   #1
Hamman is offline Hamman  United States
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Location: So. Calif.
Default Bass blocking cap at amp input

I need to block frequencies of about 150hz and down from going through the input of my amp (Ashly SRA 120) This is for an organ set up whereas I "lifted" the audio signals of the organ to feed a separate reverb unit which in turn feeds a 2 channel amp ( mentioned) which in turn feeds smaller full range speakers for the reverb effect. The pedal bass proves to be too much yet cannot be separated out prior to mix down.
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Old 26th May 2015, 07:08 PM   #2
djoffe is offline djoffe  United States
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The manual says the input impedance is 20K Ohm balanced. Thus with a differential drive, each conductor sees 10K to ground. Assume that the driving source impedance is low, perhaps 600 Ohms differential, or 300 Ohms each side to gnd (or an imagined ground if transformer isolated).

With all those assumptions stated, you can make first order high pass filter at 150 Hz if you place a 0.1 uF cap in series with each of the signal input leads.


1/(10300*2*pi*150)=1.03e-7, or quite close to 0.1 uF.

To maintain best hum rejection, the two caps should be well matched.

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Old 26th May 2015, 07:16 PM   #3
Hamman is offline Hamman  United States
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Thanks djoffe!
Your suggestion is exactly what Leslie did in their model 720 speaker to keep bass out of the rotary channel (.1 mfd)
However, I'm feeding my (reverb) amp from my reverb unit with mono 1/4" to 1/4" cables. Would your suggestion still hold true? BTW its an Alesis Nanoverb unit.
Again, thanks for your help!
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Old 27th May 2015, 01:25 PM   #4
djoffe is offline djoffe  United States
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So, if I read your reply correctly, you are using single ended driver and receiver.
Ideally, we'd just need to know single ended Zout of the Driver and single-ended Zin of the Rcvr.
Then we could calculate the capacitor.

Can you dig those numbers out of your docs?
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