60 Hz hum in a PL2000 preamp.

J!m

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#81
Or, you use both sets of contacts (in parallel) so you are running half the amperage per contact set.
 

George S.

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#82
Now that is a interesting idea! Wonder how that would affect turn on thump? I use power strips to turn the system on and off so thump isn't a issue, but would still like to get it as low as possible.
The unmodded original unit I use as a control drives the 2 400 WOPLs into DC protect mode if I turn it's power on while it's in the system (bad caps I think). Thanks for the idea. Going to do some testing.
 

J!m

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#83
I think capacitors are used to limit the thump; the smart people on here know how to implement that properly... I don't, but I've seen them many times before.

Doubling up on the contacts is not a new idea. It is used often for industrial applications. I rebuilt a lathe here a while ago, and I ran the e-stop that way, so as to greatly increase the life of the contacts. One set was adequately rated, but running two in parallel, you are running them at half their rating. Sonic properties were not a concern, and pulling the e-stop does not incur full-load in-rush amperage (because the motor is stopped), but it COULD handle the full in-rush, if some jackass left the motor lever in the "on" position...
 

George S.

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#84
The accessory power plugs need to be fused separately from the preamp if they're to be kept active. This would be easy enough to do, but I really don't see the point of doing it, given the plethora of power strips that are now available. Add the fact that the volume pot/power switches are no longer available.
My understanding of "thump" in a preamp is it's the inrush of current as the capacitors charge and how that affects the opamps.
Switching the line across two sets of contacts may be worthwhile in this case. Easy enough to do with small jumpers. Be interesting to see if it changes "thump".
 

J!m

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#86
Cool.

I have no idea if it will effect the "thump", but the switch will last beyond our lifetimes I would guess, after doing this "mod".

And, if it is the capacitor charging that causes the thump, perhaps a "stand-by" mode could be created, whereby the main caps remain powered, and therefore charged, and the front switch brings the regulated power on-line. It would require a rear-mounted "main power" switch (which could be over-rated, and maybe even be one of those integrated noise-filters with attached fuse, used on computers).

I have some components set up like this- the phono preamp has exactly zero switches (always under power, if plugged in), and my CD transport has a rear main power switch, to keep some components "warm" and ready to use. I think my preamp is the same as well.

My "ultimate" design would be to have the transformers and rectification external, bringing only DC up to the main cabinet. Further capacitor reserve and voltage regulation can happen there, to reduce loss over the cable length from the A/C external power supply. Then use the rear main/front sub-system power switching scheme.

But again, I don't know anything...
 

mlucitt

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#87
The accessory power plugs need to be fused separately from the preamp if they're to be kept active.
Yes, the accessory power jacks should be used to connect to equipment that is already fused internally. Otherwise, you have a fuse protecting a fuse! If the PL2000 1/8th Amp fuse blows, only the preamplifier shuts down. If you wanted to fuse the accessory jacks with a single fuse, it would have to be rated at 7 Amps, which the PL2000 would never blow!
I think Phase Linear had the wiring right, they just used too much of it. This is my wiring before:
Power Wiring Before.jpg

I'll post a picture of the after, when I clip the red wires and re-route the power wires (black - AC Supply, yellow - Preamp Power, and white - Accessory Power).

Turn on thump:
Not caused by any switch, but seems to be because it occurs as the switch is used.
It is related to power supply voltage ramping up at turn on. The transformer and rectifiers turn on immediately. But it takes a few AC cycles for the filter caps to get charged up to full voltage.

Look at how transistors and the really small transistors in the OPAMPs work. They are like diodes. So they don't start working until the forward voltage or bias voltage gets above 0.7 volts. In normal quiescent state they are biased on. But as the power supply comes up to full voltage, they suddenly come alive. That may be part of the thump.
Of course at shut down, it takes a while for the caps to discharge. This is why you turn things off and there is a slow fade of sound.


Mark
 
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George S.

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#89
Mark, I have to respectfully disagree. I really do not like that switch being in line between the cord and fuse.
Additionally, I think the accessory outlets should have their own separate fuse before the switch as well. Consider someone plugging in a lava lamp or something else that's not fused.
 

George S.

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#91
I say use Jim's idea with parallel switch contacts. Put fuse before switch, no accessory AC plug wiring, really not needed. May the switch last another 40 years.
 

mlucitt

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#92
If someone plugs a lava lamp into my PL gear, the shotgun is coming out!
Is it necessary to protect the power switch with a fuse, perhaps. But the low current consumed in the PL2000 (.125 Amp) means it is not really an issue.
However, I concede that most of the PL gear fuses the AC line as it comes into the unit.
I use the switched accessory jacks for a PL1000 and a soon to be acquired SAE 5000 noise reduction unit. They are less than 1 amp and present no threat to the PL2000 power switch. I would never use the PL2000 to switch a PL400 or PL700, especially a WOPL amplifier.

Phase Linear has used a variety of AC power themes, at least this is the way they are drawn in the schematics:
The PL400 has the fuse on the Line wire and the thermal switch on the neutral wire.
The PL400 Series II has the fuse and the thermal switch on the Line wire.
The PL700 puts the fuse before the power switch, and two thermal switches on the Line wire with nothing on the neutral wire (except the transformer). The accessory jack is switched.
The Early PL700B puts the fuse on the Line wire and the thermal switch on the neutral wire, no power switch.
The Later PL700B places the fuse before the power switch on the neutral wire and the thermal switches on the Line wire.
The PL700 Series II places the fuse before the power switch and the thermal switches on the Line wire but the accessory jack is not switched.
And...
The PL1000 has a fuse on the Line wire and the switch on the Neutral wire.
Our beloved PL2000 is the oddball, the power switch is a DPST with one pole controlling the switched accessory jacks and a second pole to power the preamplifier via a 1/8 Amp fuse after the switch). One accessory jack is powered on and unfused all the time.
The PL4000 has a fuse on the Line wire and a relay that controls the Neutral wire to three accessory jacks. Three more unswitched and unfused jacks are on all the time as is the transformer primary. The contacts of the power switch control the relay and also the secondary of the transformer.
The PL5000 is just the opposite with a fuse on the Neutral wire and the switch on the Line wire.
I ran out of schematics at this point.
 

George S.

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#93
Years ago, long ago, in NC, I used to plug a vacuum cleaner into the unswitched power plug on the rear of the PL2000 because the small house we lived in had no extra wall sockets unless I unplugged something. And at times I've had various lamps plugged into it.
I see your point, and aknowledge your years of experience. And, damn, nice writeup about the other PL gear! Once you get it wired please post photos. Thanks.
I'm going to wire mine with the fuse in line before the switch, omit the accessory power plug wiring, and probably use use both switch contacts wired parallel like Jim suggested with the appropriate caps across the lugs. I no longer use those accessory power plugs or want to use them. I just leave everything on and use power strips. Trying to make these old PL power switches last. I have seen a few PL2000s on eBay over the years with toggle switches for power switches.
Also, no turn on thump if using power strips. Yeah, I know to turn the preamp on before the amps, but other people in the house don't understand.
 

George S.

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#94
Got the Quasimodo boards partially built, unfortunately short a few parts. I'll get those ordered and try to get both PL2000 dual primary export and regular non export transformers tested and snubbed next couple weekends and post results. PXL_20210912_150039449.jpg
Can power the Quasimodo boards with a 9V battery, but decided to get some old power supplies out of storage. I have one good Power Designs Inc unit and one parts unit. Also a old Graymark kit supply that I really like.
Going to let these sit "power on" for a few hours then check output with the Fluke and scope. Yeah, they're past due a recap.
 

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Gepetto

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#97
I shortened the wire runs about 27", dressed them away from the circuit board, twisted the AC pair, wired the fuse (correctly) and before the switch, and gave myself another (low wattage) switched accessory jack.
View attachment 54203
Not sure why you have 2 hot un-switched wires running forward to the switch?
 

George S.

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#98
Think he's using the lower lugs on the power switch to make and break the accessory power plugs.
I ended up wiring mine differently. No wiring to those accessory plugs and used Jim's idea of doubling up the switch contacts to power the unit on. Would post a photo but I'm enjoying it in the system right now.
 

mlucitt

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#99
Not sure why you have 2 hot un-switched wires running forward to the switch?
I did not change the PL power switch wiring scheme. My color coding is as follows:
Black AC cord "Line" to fuse tip > through fuse to black 22ga wire to both Poles of the DPST power switch
Black AC cord "Neutral" to lower side of accessory jacks > then to transformer primary 1 and 3.
From AC power switch on upper Pole 1 (this Pole is rated 1A at 250VAC) with yellow 22ga wire > then to transformer primary 2 and 4.
From AC power switch on lower Pole 2 (this Pole is rated 3A at 250VAC) with white 22ga wire > then to upper "hot" side of accessory jacks.

Is 3A at 250VAC the same as 6A at 125VAC? Half of that power switch is pretty rugged, eh? It is well marked which terminals are rated higher.

I hope this is sound logic! My goal was to tightly twist the AC wires and run them away from the circuit board and closer to the chassis to reduce hum and noise. While I was in there, I shortened the transformer secondary wires because they were considerably too long (even though the voltage is low). The power switch poles each have a quench arc capacitor across the contacts. And I may never plug anything into the AC accessory jacks so that one side of the power switch will see no current flow.

See what I did there?
PL2000 All Switched Outlets.jpg
 
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George S.

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Got the Quasimodos built and 3 transformers tested. Unfortunately all 3 are dual primary export, however they all test very similar, so that's good. All were tested with the recommended TDK film capacitors .01 uF and.15 uF, and a 1k ohm variable resistor used to arrive at the correct value. Resistor value didn't seem to be highly critical, with a give and take of approximately 20 ohms from the optimum damping. 120 ohms seems to optimum, but good damping was also seen at 100 and 140 ohms. I used 100 ohm because that's what had on hand.


PXL_20210920_154133035.jpg PXL_20210920_143632118.jpg PXL_20210919_153103829.jpg PXL_20210920_155651333.jpg
The PL2000 transformer has two secondary outputs and a center tap. A .01 uF gets soldered between each secondary and center tap, and a .15 uF and 120 ohm carbon film in series also gets soldered between each secondary and center tap.
Can I hear any difference? No, not really. The preamp had no hum or noise before. The spectrum analyzer does show some improvement. After reading Mark Johnsons' paper (he designed the Quasimodo) published in https://linearaudio.net/sites/linearaudio.net/files/volumes/v10 mj abstract.png, I'm going to try some Vishay SBYV27-200 rectifier diodes.
PXL_20210920_170355901.jpg
The old PL2000 circuit board is starting to get really beat. That's OK, It'll remain a test unit.
 
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