PL 700 Pro Build
- Thread starter George S.
- Start date
- Joined
- Nov 15, 2021
- Messages
- 482
- Location
- near Liverpool, NY
- Tagline
- Lifelong student / listening = bliss
J!m,
As a young freshly-minted USAF tech school certified '3-level' airman reporting to his first Avionics shop back in the summer of '78, I learned pretty quick that 'the elders' with the calm demeanor & 100-yard stare had literally forgotten more than everything that I knew at that point in my life.
For example, I'd try to align the IF stage of a single VHF radio all day long, back & forth, close but never quite to the T.O. mandated specification.
Tears of frustration...and invariably, about 20 minutes before the end of the shift, one of the elders would come over, ask me what the status was...and then, within just a few minutes, have the unit a couple of db beyond the spec. (If we had been in the movie Dune their eyes would have glowed blue...)
They had done this for so long it was like muscle memory for them. I was in awe...and just made me want to go from just being able to accurately recite the theory -- to comprehending it enough to be able to use it in anger to actually diagnose/fix a busted box of electronics. :0)
****
From my perspective, Gepetto has proven by his words & his actions to definitely be one of The Elders.
Cheers --
As a young freshly-minted USAF tech school certified '3-level' airman reporting to his first Avionics shop back in the summer of '78, I learned pretty quick that 'the elders' with the calm demeanor & 100-yard stare had literally forgotten more than everything that I knew at that point in my life.
For example, I'd try to align the IF stage of a single VHF radio all day long, back & forth, close but never quite to the T.O. mandated specification.
Tears of frustration...and invariably, about 20 minutes before the end of the shift, one of the elders would come over, ask me what the status was...and then, within just a few minutes, have the unit a couple of db beyond the spec. (If we had been in the movie Dune their eyes would have glowed blue...)
They had done this for so long it was like muscle memory for them. I was in awe...and just made me want to go from just being able to accurately recite the theory -- to comprehending it enough to be able to use it in anger to actually diagnose/fix a busted box of electronics. :0)
****
From my perspective, Gepetto has proven by his words & his actions to definitely be one of The Elders.
Cheers --
Last edited:
Joe, O.K., so increase capacitance value to improve low frequency performance.
PL had two 2.2 uF electrolytics in series + to + on each output trace to create a bipolar.
Per the photograph of the old test board, a jumper replaced one cap and a 4.7 uF film the other old cap. This is how both working preamps are configured.
What should I study to figure out the optimum value for the output coupling caps. I'm at a loss for terminology and thinking I'm approaching the need of a good spectrum analyzer once again.
PL had two 2.2 uF electrolytics in series + to + on each output trace to create a bipolar.
Per the photograph of the old test board, a jumper replaced one cap and a 4.7 uF film the other old cap. This is how both working preamps are configured.
What should I study to figure out the optimum value for the output coupling caps. I'm at a loss for terminology and thinking I'm approaching the need of a good spectrum analyzer once again.
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Hi George
The good news is that the move to the single 4.7uF has improved the situation ~5 fold from the original (1.1uF to 4.7uF)
You have to consider what you are driving, the PL amp input load with a RevG1. You indicated the measured impedance is 386 ohms at 50Hz which makes total sense given what you measured a decade above that at 500 Hz. Lets assume 25 Hz, your measured impedance should be somewhere around 772 ohms. Lets use that number. This means that your amp output at 25 Hz will be down approximately 1.5% from and equivalent signal at 500 Hz. Not huge but something.
Compare that to the original back to back 2.2uF situation and your 25 Hz signal would have been down ~6%. You have improved the situation significantly.
The good news is that the move to the single 4.7uF has improved the situation ~5 fold from the original (1.1uF to 4.7uF)
You have to consider what you are driving, the PL amp input load with a RevG1. You indicated the measured impedance is 386 ohms at 50Hz which makes total sense given what you measured a decade above that at 500 Hz. Lets assume 25 Hz, your measured impedance should be somewhere around 772 ohms. Lets use that number. This means that your amp output at 25 Hz will be down approximately 1.5% from and equivalent signal at 500 Hz. Not huge but something.
Compare that to the original back to back 2.2uF situation and your 25 Hz signal would have been down ~6%. You have improved the situation significantly.
I am sneaking up on the Curmudgeon stage...
Snow day in NE Ohio, so pulled the 100k attenuators out of the 400 S2 WOPL that powers my biamp system. Installed 50k to match the 700 Pro 50k attenuators.
There's a lot about amp input impedance and it's effect on preamp output that I don't understand, but it makes sense to do this as the 700 seems to be more "sensitive" and have higher gain at the normal volumes I listen to. Need to get these balanced without going into the MiniDsp.
There's a lot about amp input impedance and it's effect on preamp output that I don't understand, but it makes sense to do this as the 700 seems to be more "sensitive" and have higher gain at the normal volumes I listen to. Need to get these balanced without going into the MiniDsp.
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So ran into a problem with the 400 S1 WOPL and 50k attenuators. At 40 VRMS output, the meters peg. With 100k attenuators the meters were perfectly aligned at 100% at 40 VRMS output.
I don't understand why the Cylon meters were still closely calibrated but the mechanical meters aren't. Going to open her back up, find the resistor on the WOAD light board for the meter, and install a decade resistance box to find the proper value. Get those new resistors installed and retest.
I don't understand why the Cylon meters were still closely calibrated but the mechanical meters aren't. Going to open her back up, find the resistor on the WOAD light board for the meter, and install a decade resistance box to find the proper value. Get those new resistors installed and retest.
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27k ohms puts the needle right at 100% at 40 VRMS output, which is 200 watts per Joe's tech documents. The WOAD light board has a 13.14k ohms resistor installed.
So, dropping the attenuator value by 50% requires doubling the resistor value to keep the mechanical meter calibrated.
Now to search for 27k resistors. Parts are not organized.
So, dropping the attenuator value by 50% requires doubling the resistor value to keep the mechanical meter calibrated.
Now to search for 27k resistors. Parts are not organized.
Attachments
Easy answer, the original light board calibration was for 100W at the 0dB setting on the meter. The new light board (LBC) provides 3 jumperable or switchable settings for the 0dB mark, 20W, 100W (to match the stock setting) or 200W. Ask Dave (Phase Linear Phanatic), he has several of the switchable light boards that he has installed with switches on the back to match his mood.
PS: the input pot value has nothing to do with this meter sensitivity setting.
You may now be disappointed that the analog needles do not move off of zero at most normal listening levels. All of my customer requests have been how do I make my PL400 needles move more at the levels I listen at? That led to the birth of the LBC light board.
The LED meter has a log amp in it to give you some sizzle at low listening levels while flattening out at higher volume levels. Analog meters don't do that.