RDF is correct.
The reason for the big change in load seen by the CD is that the power amp input impedance is too low for a 10k passive pot.
I would suggest that the amp input impedance be at least 10times the source impedance. 10k pot has max output impedance of 2k5 (+Rs/4). The amp sees this as source impedance and it's minimum input impedance must be 25k.
I prefer 20times source impedance telling us that a 10k pot needs to see a 50k Zin.
A 20k pot needs 100k Rin, 50k pot needs 250k input impedance.
All of these pot options will need fairly short, fairly low capacitance cables. The 20k and 50k pots will need modifications inside the power amp to adjust the RF filter to above the audio band.
BTW,
the CD needs an output impedance of <500r to drive that 10k+10k combination and preferably <250 if one adopts the 20times rule of thumb. But can the CD drive [ 5k // cable capacitance ] ?
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Andrew,
What you say and rdf is correct but this is not the question I previously answered. I was referring to the source impedance of the amp instead of the load impedance of the source (CD). So, the question here, what is the real question (quiz) brought up by Micheal?
Also, as you indicate, the amp RF input cap value must be adjusted to take into account the high impedance of the pot otherwise the upper frequency limit will be reduced considerably compared to an active preamp with a low output impedance - which represents a low source impedance for the amp. It also reduces the "resistor" noise entering the amp compared to a passive pot.
Thanks
Fabien
I'ld be leery of going much past a 10K pot for a passive. Tests I did with a 10K driving six feet of highly capacitive Canare quad-wound interconnects showed the tiniest worst case measurable roll off starting around 50kHz. For most users with equipment in a single rack or closely grouped it shouldn't be an issue. Of greater concern is the higher susceptibility to hum pickup for those keen on unshielded interconnects.
For best voltage transfer, we want a source with low output impedance and a load with high input impedance.
Source is CD Out. Load is Power Amplifier's Input Impedance.
The drawings below are the equivalents of A,B,C.
With a Passive Preamp, the Source Output Z and Power amp Input Z varies as the pot is adjusted.
Source is CD Out. Load is Power Amplifier's Input Impedance.
The drawings below are the equivalents of A,B,C.
With a Passive Preamp, the Source Output Z and Power amp Input Z varies as the pot is adjusted.
Attachments
Michael,
You're confusing what is a very simple concept.
The "source output Z" and "power amp input Z" never change. (They are constants.)
The passive preamp creates 'its own' variable output Z and variable input Z based on setting position and resistance of the potentiometer.
Cheers,
Dave.
You're confusing what is a very simple concept.
The "source output Z" and "power amp input Z" never change. (They are constants.)
The passive preamp creates 'its own' variable output Z and variable input Z based on setting position and resistance of the potentiometer.
Cheers,
Dave.
One thing I don't understand is how to properly size the "series" resistor in a shunt-mode potentiometer/preamp. I've seen a lot differing views on this...so it's become confusing.
In and attempt to better understand, I've built several using a 100k log taper pot and series resistors from 13k - 15k ohm to 91k ohm and they all seem to work pretty well. However the 91k doesn't sound as good as the lower values. My amplifier has an input impedance of 100k and I am using a cdp as my source.
In and attempt to better understand, I've built several using a 100k log taper pot and series resistors from 13k - 15k ohm to 91k ohm and they all seem to work pretty well. However the 91k doesn't sound as good as the lower values. My amplifier has an input impedance of 100k and I am using a cdp as my source.
An externally hosted image should be here but it was not working when we last tested it.
Hi,
your Zin=100k power should have a source resistance of <=10k preferably <5k.
The passive pot you have varies in output impedance with both the series resistor and the position of the wiper in the shunt pot.
The maximum pot value I would recommend for a 100k amplifier is 20k since it has a maximum output impedance of 5k.
To mimic that worst case you need a 10k pot as the shunt resistor and a 10k series resistor feeding it. Compare that to what you listed in your post.
your Zin=100k power should have a source resistance of <=10k preferably <5k.
The passive pot you have varies in output impedance with both the series resistor and the position of the wiper in the shunt pot.
The maximum pot value I would recommend for a 100k amplifier is 20k since it has a maximum output impedance of 5k.
To mimic that worst case you need a 10k pot as the shunt resistor and a 10k series resistor feeding it. Compare that to what you listed in your post.
Thanks Andrew...Not having had experience with passives, I used the 100k because that was the size used in the infamous "Harry Haller" passive....located somewhere in diyaudio.com. It certainly seems to work well though (sounds good)...the only real problem I have with it is that the volume doesn't completely mute at full ccw rotation.
I'll try the 20k/10k combo. I don't happen to have a 10k pot in hand. Maybe RS has a cheapo I can play with.
Hi Dave
This is only true if the 3 components are unconnected. In use, the "variable output Z of the passive pot" is in parallel with the input impedance of the power amp and the "variable input Z" is in series with the CD output.
In the picture below, do you agree the 3 drawings are the same?
Regards
Mike
Attachments
no, I don't agree.
You have 3 pieces of equipment. The music source, the passive pot, the receiver.
Consider each pair only, i.e. source and pot. Consider what it is doing and what it needs to do. It's fairly simple.
Now consider the next pair, i.e. pot and receiver. Analyse again. Simple.
Don't try to combine all 3 into a complex analysis.
You have 3 pieces of equipment. The music source, the passive pot, the receiver.
Consider each pair only, i.e. source and pot. Consider what it is doing and what it needs to do. It's fairly simple.
Now consider the next pair, i.e. pot and receiver. Analyse again. Simple.
Don't try to combine all 3 into a complex analysis.
AndrewT....Yesterday I "cobbled" together a dual-mono shunt-mode passive configured as per your recommendations (10k (audio taper) pots + 10k series resistors) using very inexpensive Radio Shack parts. This works well into my amplifier (100k input Z) in that it gives much better volume control than any of my previous efforts, providing comfortable listening levels at about 10 -11 o'clock; plenty of dynamics and spacious soundstage. The only criticism at this point (a slight lack of high frequency detail retreival) is probably related to the low quality parts.
Do you know where to find the RF filter in your power amp?
Go and see what components are fitted.
Go and see what components are fitted.
We can agree to disagree.
A Passive Preamp is nothing more than a potentiometer.
Potentiometer Input is in series with the source. Potentiometer Output in shunt with power amplifier.
Don't try to separate them. There are no buffers.
Davey disagrees with your complex methods of analysis, of what can be a very simple pair of analyses.
all the above is pointless if you put next to them a ladder type step atennator ( that can also be constructed to any impendance almost ) then again the construction may as well preserve impedance to where its supposed to be but if not constructed properly might pick up noises ...
I will agree though to :
higher imp plays better
wippers are noisy
and average 100K pot is very cr**ppy
got my shelf a blue alps 100 k pay for 35 euros still in very low levels diference betwen R-L is actually terrible
I will agree though to :
higher imp plays better
wippers are noisy
and average 100K pot is very cr**ppy
got my shelf a blue alps 100 k pay for 35 euros still in very low levels diference betwen R-L is actually terrible
why?
It's still a load for the source and a source resistance/impedance for the receiver.
ok then Andrew ....
let us skip the all thing ...
let us not use potentiometers cause they have wippers that are noisy
also effect the impedance of both
let us also skip ateunetors of all types cause they imitate the potentiometers
let us through away all amplifiers that have input impendance lower than 50K
and close the subject by listening direclty from CD to amplifier at full power
let us skip the all thing ...
let us not use potentiometers cause they have wippers that are noisy
also effect the impedance of both
let us also skip ateunetors of all types cause they imitate the potentiometers
let us through away all amplifiers that have input impendance lower than 50K
and close the subject by listening direclty from CD to amplifier at full power
passive
hi guys sorry to jump in here
i have a question maybe one of you guys could answer
i have a really high quality custom p&g 10k log pot (from a now defunct audio firm i worked at) which i want to use a passive volume control
it would be connected as a passive volume control into a 15K power amp input
it is being driven by a cd player with a tube output stage
unsure of the output impedance of the cd player
would this be ok do u think?
thanks a lot
hi guys sorry to jump in here
i have a question maybe one of you guys could answer
i have a really high quality custom p&g 10k log pot (from a now defunct audio firm i worked at) which i want to use a passive volume control
it would be connected as a passive volume control into a 15K power amp input
it is being driven by a cd player with a tube output stage
unsure of the output impedance of the cd player
would this be ok do u think?
thanks a lot
not enough info.
I need cable capacitance before and after the passive pot & I also need the RF filtering in the receiver.
What about the LF filtering in the CD output?
I need cable capacitance before and after the passive pot & I also need the RF filtering in the receiver.
What about the LF filtering in the CD output?
thanks for the response
i will open them both tonight and have a look and post back
thanks
cable is 0.5m from cd to pot and 0.5 from pot to p/amp i/put.
its a cut down branded audio cable, cannot remember which type , will look
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