PL 700 II REPLACEMENT MAIN VOLUME POTS

I have been chasing some distortion and uneven level representation on LED indicator amplitude meter on display between left and right channels most noticeable when DB switch is depressed
 

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Last edited:
In any case ditch the plastic knobs.
 
OK Thanks so much for sharing Experiences its super cool to hear the varieties very much appreciated !!
I appreciate the input
 
grapplesaw said:
George when did we move from 100k to 50k pots. I’ve not seen any documents for that change. Please guide me
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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.
 
George S. said:
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.
Click to expand...

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.
 
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.
 
George S. said:
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.
Click to expand...
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.
 
Sounds good George
still not clear on best choice for pot 100k or 50k
l have bypass switch to eliminate the pots all together but option to use them
The carver has 910 ohm output impedance so I guess not much to worry about if one uses 100k or 50k
 
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.
 
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