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Need help with passive preamp and cables impedance and capacitance calculations

aikofan

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I‘m inserting a passive stepped attenuator between my miniDSP Flex and my power amp after I experience a couple of nasty and potentially destructive volume surge/noise incidents recently that I described in another thread. I need help determining whether I need to buy a pair of low-capacitance cables.

The attenuator has an impedance of 10k ohms, so I think I should be fine on the source side (Flex output impedance is 200 ohms). However, my power amp has an input impedance of 28k ohms, only 2.8x higher, so what is the max capacitance for 1-meter RCA interconnects that’s acceptable to avoid audible frequency response changes?
 

RayDunzl

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These days, I'd measure a sweep, with and without the attenuator, and eyeball the result.

Back when, I ran twin 10k pots, using coax, one per channel for ability to keep the balance
balanced, for years between a CD player and power amp without being annoyed.
 

RayDunzl

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My present DAC manual claims 1360 feet of unspecified unbalanced cable and load impedance for a 0.1dB loss at 20kHz

1678503970659.png
 
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kchap

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I‘m inserting a passive stepped attenuator between my miniDSP Flex and my power amp after I experience a couple of nasty and potentially destructive volume surge/noise incidents recently that I described in another thread. I need help determining whether I need to buy a pair of low-capacitance cables.

The attenuator has an impedance of 10k ohms, so I think I should be fine on the source side (Flex output impedance is 200 ohms). However, my power amp has an input impedance of 28k ohms, only 2.8x higher, so what is the max capacitance for 1-meter RCA interconnects that’s acceptable to avoid audible frequency response changes?
I do not think it will be an issue. Assuming it's the attenuator is a simple L pad design the power amp will see a max source impedance of 2.5 kΩ when the attenuator is set to -6dB, 2.55 kΩ if you include the Flex source impedance. For all other settings the amp will see a source impedance less than 2.55 kΩ. For example if the attenuator is set for 120db -20dB the amp will see a source impedance of approximately 900 Ω.

Of course I'm making simple assumptions by ignoring the affects of L and C but, Good enough for Jazz as my father, a part time musician would say.

Edit: -20dB instead of 120db
 
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Blumlein 88

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1 meter is not likely a problem. If it were 3 meters or more it might become one. If it physically isn't a restriction on placement, you could use a half meter between amp and attenuator and 1.5 meter from source to attenuator. It is tehcnically better even if one meter isn't a problem.
 

DonH56

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aikofan

aikofan

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Update: I ran a sweep to see if I lost any high frequencies. I don’t notice a huge difference, although the 2k-7k region looks a little worse. I guess I can live with it.

Before adding attenuator (green curve)
Graph 20220529.jpg


After adding attenuator

20230312 Attenuator LRS.jpg
 
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