Would this be suitable replacement Pot for main volume pots on PL700 ii Diagrams mention 100 K ?
PDA241-HRT02-104B0 Bourns | Mouser
PDA241-HRT02-104B0 Bourns | Mouser
PL 700 II REPLACEMENT MAIN VOLUME POTS
- Thread starter Greg
- Start date
I originally used these 100k.
Then went to 50k to match the input impedance of all WOPLs to the original 50k attenuators I kept in the 700 Pro.
I use solid shafts and these set screw knobs, but it isn't to difficult to Dremel a solid shaft to a "D" shaft.
https://www.tubesandmore.com/products/knob-aluminum-set-screw-notched-tip-125-diameter
Then went to 50k to match the input impedance of all WOPLs to the original 50k attenuators I kept in the 700 Pro.
I use solid shafts and these set screw knobs, but it isn't to difficult to Dremel a solid shaft to a "D" shaft.
https://www.tubesandmore.com/products/knob-aluminum-set-screw-notched-tip-125-diameter
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Glenn, the 700 Pro were built with very nice Alps 50K attenuators that easily come apart for cleaning and greasing. I wanted to keep those. Because I'm using it biamped, I wanted to match that input impedance on the other amps.
I have yet to really understand input impedance. I've read about how to check it, and Joe has commented on it and how PL used a high input impedance probably because people were using tube preamps. My understanding which may be faulty, is that total resistance of the attenuator pot affects input impedance. Hopefully Joe will comment on this.
I have yet to really understand input impedance. I've read about how to check it, and Joe has commented on it and how PL used a high input impedance probably because people were using tube preamps. My understanding which may be faulty, is that total resistance of the attenuator pot affects input impedance. Hopefully Joe will comment on this.
It does. WIth the 100K pot all the way up, it puts 100K in parallel with the 49K impedance of the control board input resistors. This pushes the filter pole formed by these resistances and the 220pF input capacitor up in frequency. This also reduces the input impedance below the nominal 49K. Not a huge change but a change.
With the pot at the 50% position, it adds 25K of additional impedance which brings that same pole down in frequency. This also increases the input impedance above the nominal 49K. Not a huge change but a change.
With the pot at the 50% position, it adds 25K of additional impedance which brings that same pole down in frequency. This also increases the input impedance above the nominal 49K. Not a huge change but a change.
Joe,.So, let's say I have a 100K on one channel, and a 50K on the other and both are wired correctly and "turned to the right" for lowest resistance and highest amp output.
Are they electrically equivalent to each other at that position? Seems like they should be, but reading about input impedance awhile back made me question that assumption. Thanks.
Are they electrically equivalent to each other at that position? Seems like they should be, but reading about input impedance awhile back made me question that assumption. Thanks.
No, because channel 1 has 100K in parallel with the 49K input impedance on the control board and channel 2 has 50K in parallel with the 49K input impedance. With a low output impedance preamp of say, 100 ohms, you will not notice a difference but with a tube preamp with 50K output impedance, you will need to turn the preamp up much higher to offset the extra loading.
Joe, thank you! Yes! Now I'm starting to understand this a little better.
Glenn, this is why I matched the 700 Pro 50K attenuators on my other 2 WOPLs.
I noticed the "gain structure" wasn't equal between the 700 and a 400 when they were built exactly the same other then the attenuator values. The 700 was louder at all volumes and this caused a balance issue in the biamped system. Building all the amps exactly the same including the attenuators fixed this issue.
I remember Joe saying he prefers no attenuators at all in circuit, but forget what value he suggested if one must use them.
This started my quest to learn more about preamp output impedance and amp input impedance, but I got hopelessly lost on the other forums because I lack understanding of much of the fundamentals.
There are numerous threads on other sites about this, particularly about preamp output impedance, and "rules" are often stated.
Glenn, this is why I matched the 700 Pro 50K attenuators on my other 2 WOPLs.
I noticed the "gain structure" wasn't equal between the 700 and a 400 when they were built exactly the same other then the attenuator values. The 700 was louder at all volumes and this caused a balance issue in the biamped system. Building all the amps exactly the same including the attenuators fixed this issue.
I remember Joe saying he prefers no attenuators at all in circuit, but forget what value he suggested if one must use them.
This started my quest to learn more about preamp output impedance and amp input impedance, but I got hopelessly lost on the other forums because I lack understanding of much of the fundamentals.
There are numerous threads on other sites about this, particularly about preamp output impedance, and "rules" are often stated.
Glenn, glad you found that spec for the Carver. Someday I'll get around to accurately measuring my PL2000 output impedance as I've been unable to find it published anywhere.
The so called rule:
Output impedance of a preamp is something most people don't think about, but can be influential to the sound. Lefty's rule of thumb of having the preamp output impedance at least 1/10th of the power amp input impedance is sound advice. At this ratio, there will be only 0.83dB of loss in the interconnect.
That's the simple version, then it gets complicated very quickly.
The so called rule:
Output impedance of a preamp is something most people don't think about, but can be influential to the sound. Lefty's rule of thumb of having the preamp output impedance at least 1/10th of the power amp input impedance is sound advice. At this ratio, there will be only 0.83dB of loss in the interconnect.
That's the simple version, then it gets complicated very quickly.