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In Z query
In Z query
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Old 9th October 2019, 12:44 PM   #1
diyrob is offline diyrob  United Kingdom
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Default In Z query

Hi - am building a budget 2nd system and have settled on MarkAudios thanks to advice here. However, I am starting with the amp having picked up a 2nd hd pair of BK mosfet modules, which I will re-cap and play with (separate power supply etc) have used in the past - quite good with better caps.

anyway; it would be useful to drive these passively - however due to source limitations(manufacture reccs.) i need to keep the inZ @ 10K

i note the first thing on the input is a 100K across +/-

would it be problimatic to simply replace this with a 100k pot, or a bigger value + a pot to = 100k - the aim being to maintain inZ at 10K (i know linearity of the pot will suffer...)
otherwise i guess its pre amp time (though have a psu i can use)

thanks in advance if you have any input on this basic q.
R
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Old 9th October 2019, 02:57 PM   #2
mjf is offline mjf  Austria
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the 100k res is closing the dc-path of the input cap.......should be no problem to change this res.

but the picture show the 100k res to ground, a 1k res in the signal path and a
47k res again to ground, so your input impedance could be (estimated) aroiund
30k........
if you use a 22k pot at the input (without changeing the input parts) you will be close to 10k........
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Old 9th October 2019, 03:01 PM   #3
Mark Tillotson is offline Mark Tillotson
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The 100k is to discharge the electrolytic. The actual load resistor is the 47k after the cap, but in combination with the input resistor that's about 33k input impedance.


Adding 13k in parallel with the 47k, or replacing the 47k with 10k will give an input impedance of 10k. A pot on the input would compromise noise performance.

Last edited by Mark Tillotson; 9th October 2019 at 03:03 PM.
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Old 9th October 2019, 06:50 PM   #4
sgrossklass is offline sgrossklass  Germany
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It's not usually a good idea to use high-value pots on power amp inputs for distortion reasons... 10k (maybe 20-22k) is about the limit for most.

If you do not have reason to suspect that the preamp output is particularly wimpy (what does it use on the output?), I'd just slap in a 10k pot and call it a day. 8.2k min, 10k max... close enough.
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Old 9th October 2019, 08:56 PM   #5
diyrob is offline diyrob  United Kingdom
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thanks for your replies - i usually leave something out... BK do say the inZ is around 10K..
there is some interesting things to research in your replies - thank you
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Old 10th October 2019, 05:47 AM   #6
diyrob is offline diyrob  United Kingdom
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thanks - the manufacturer says the inZ is around 10k.
how much do you think i could push the values up here - gain reduction is not an issue
R
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Old 10th October 2019, 05:50 AM   #7
diyrob is offline diyrob  United Kingdom
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yes - its an instruction from the CD player manufacturer that 10k is minimum impedance it should drive (rega apollo) and amp manufacturer gives 10k as inZ
R
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