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In Reply to: RE: the idea is borrowed from BH Foreplay ver.1 ... posted by deathtube 667 on June 30, 2017 at 06:46:37
I have two outputs per channel on my DIY preamp.
One feeds the input of my SE 300b amp and the other feeds the input to a Opamp based low pass filter that feeds the SS amp driving my woofers.
I don't have isolating resistors and see no need for them.
I use a cap at the input of the 300b amp to create a 6db per octave high pass filter but even if I wasn't using a cap and the signal was just seeing the grid resistor of the input tube I see no need to isolate.
The Opamp low pass filter input is a Opamp followed by the filter parts followed by an opamp. The first opamp isolates the input from the filter so there is no need to further isolate the preamp output.
I asked the question about your application thinking that the isolation resistors might really be needed.
If you are just "doing it because BH did it" without knowing why you might want to re-think.
If there is a reason that those resistors need to be there then by all means use them, if there is no reason then I say get rid of them as they are serving no useful purpose.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
The 4.7k isolation resistors seem to be a VoltSecond Foreplay mod idea, but he said that if both output are being run at the same time, the 4.7k value can be dropped down to 100 ohms.
The basic idea is to provide a bit of isolation between the two power amp inputs.
My powered sub has a 3rd order low pass filter built in, so no extra filtering is needed.
I'll probably try lower ohmic value isolation resistors and hear if anything changes.
Try it without the isolation resistors. They should not be needed.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
For LTSpice to simulate both channels, there needs to be some resistance.
I didn't notice any subjective difference using 100 ohm vs. 4.7k iso resistors.
dt 667
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