No more 60 Hz hum in a PL2000 preamp.

Calculated a maximum transformer VA for the 2000. At idle it pulls 18 mA across the fuse contacts. Uses a 125 mA fuse, and the most I saw the mute board pull was 200 mA. So I'll go with .125+.200 times 36( the secondary voltage), equals a VA of 12. Pretty damn sure the transformer they used is way smaller than this as my numbers are way high.
Did some searching. Found a Hammond 166J33, VA of 33. Yup, it's big, and over a pound heavy, 33 VAC secondaries with a center tap should get me around 16-17 VDC on the rails. Way overkill and probably won't see much of any voltage sag, ever.
Made some paper templates and looks like it will just fit. I'm going for it!
 

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George S. said:
If you don't have the stepped washers and thin bolts, try Don's idea of insulating the bolts with clear packing tape. That should work.
Steve, no need to move the fuse holder, with care it fits.
Click to expand...
Good catch - I forgot about insulating the bolts. Easily enough to pull them back out. No stepped washers on this assembly

Plenty of room between the IEC and fuse holder, Steve; there is no contact.
 
These are a very good fit. Going to calculate the new snubber values and get those soldered in. Then use jumpers to set up the primary configuration for lowest 60 Hz noise using the QuantAsylum. For those of you not having a Real Time Analyzer, Joe documented another way to do it on the following page, posts 147 and 150.
https://forums.phxaudiotape.com/threads/pl-700-pro-build.9965/page-8
 

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Ringing the transformer secondary with the Quasimodo test jig in order to calculate the CRC snubber values for the resistor. Two film caps are in the jig, a .01 uF, and a .15 uF in series with a precision trimmer. The trimmer is adjusted to eliminate ringing, then it's resistance is checked for resistor value to use.
The ringing simulates 60 Hz rectification noise
 

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Added a 100 ohm dropper resistor to each rail supply wire to bring them down to +&- 15.2 VDC. Going to order some 75, 80 and 85 ohm Dale 1% metal film and get it up to 16 VDC.
The mute board works best with 500 ohm droppers. I'm pulling 37 VAC off the secondary before the rail dropper resistors. Was using some 500 ohm 1/2 watt, but they were heating. Remembered I had some very old Dale 500 ohm, 10 watt, 1%. Perfectly good overkill, work perfect, but I want chassis mount, so going to order some.
It's been a interesting experience first time really working with a unregulated supply. It's like a fine balancing act.
 

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Going to double the output caps and see If I like the result. These are Panasonic ECW 4.75 uF polypropylene. They're the largest value Digikey or Mouser carry in this series and form factor.
I already have one installed at each output. Think I'll just temporarily tack them on the foil side and give it a listen.
 

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No good, treble is slightly distorted on all sources. Pulled them out. This preamp seems to work best with a maximum of 4.7 uF on the outputs.
The mute board is a no go. Need a regulated power supply to run it and the preamp. Not a issue, I'll deal with turn on thump the usual way.
I do like the larger transformer. Bass is noticably tighter. I'll be upgrading the other 2000 with the same model transformer.
Also upgraded the rail voltage dropping resistors to 1 watt, 0.5%, metal film, 100 ohm. Rails are at +- 16.25 VDC. I've read that equal rail voltage is essential for the opamps, it's equal.
I'll look at adding output caps to the rear channels later. Going to have to cut the trace and drill the board. I'll probably add eyelets in the through holes.
 
George S. said:
No good, treble is slightly distorted on all sources. Pulled them out. This preamp seems to work best with a maximum of 4.7 uF on the outputs.
The mute board is a no go. Need a regulated power supply to run it and the preamp. Not a issue, I'll deal with turn on thump the usual way.
I do like the larger transformer. Bass is noticably tighter. I'll be upgrading the other 2000 with the same model transformer.
Also upgraded the rail voltage dropping resistors to 1 watt, 0.5%, metal film, 100 ohm. Rails are at +- 16.25 VDC. I've read that equal rail voltage is essential for the opamps, it's equal.
I'll look at adding output caps to the rear channels later. Going to have to cut the trace and drill the board. I'll probably add eyelets in the through holes.
Click to expand...
It is not essential George...what source said that. It is essential for equal positive and negative output swing but not much else. Op amps will run off +30V and Ground for example.
 
Joe, read that a good while back on the DIYAudio Forum. They were matching a power supply to some board that used opamps.
If it doesn't matter, then that's alright. I'm glad I have my rail voltage exactly equal though. I was rather surprised as I thought the transformer wouldn't be that close on both sides of the center tap. I find it rather astonishing that it is.
So what's the result of raising and lowering rail voltage. The original opamps were rated for a max of 18 V. I supposed higher rail voltage increases output. My thinking is to stay within the original designed voltage?
And, I have to say, the transformer upgrade is a major improvement. It really tightened up the bass. Getting the attenuators out of the amps sharpened the treble, the transformer tightened the bass. It's never sounded this good. Sitting here listening to old albums, the bass is amazing,
 
Joe, yes, that's what they were talking about, equal output. That was the concern.
Pretty damn amazing what that transformer has accomplished. Sitting in the "sweet spot" and listening to a Joe Walsh album. Volume is low enough I could hold a conversation with someone, yet I'm feeling the bass sound pressure. From a album! Actually getting output from the subs. From a album! Amazing! The preamp has been transformed, getting closer to my goal of a great sounding system at low volume. It sounds amazing, from a old album.
 
The Aragon preamps, and their phono pre (47k) both benefitted greatly when the outboard power supply was "beefed up" significantly to "well beyond stupid" levels for the expected usage- massive transformer and tons of storage capacitance.

So, for a linear power supply, it seems to matter, even with very low draw.

Nice to have the quantified somewhat as you've done here.
 
Jim, yes, I agree. The new transformer is so large that it's "beyond stupid levels". Yet it fits like the preamp was designed around it with plenty of clearance.
Was just listening to some digital files. Very, very happy. No reason for me to ever get a different preamp. Without a doubt it's the best upgrade I've made to it.
 

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Well, the upgraded transformer is a failure. Turned on the system this morning and heard a slight hum. Cranked up the volume, and yes, it's definitely there.
I have a early 70's class A bipolar amp upstairs that has always been very susceptible to hum issues in anything connected to it.
Brought the preamp upstairs and connected it to the bipolar. Yup, really pronounced hum. Went through every troubleshooting procedure I could think of, no joy.
Pulled the large transformer, reinstalled the origonal. No more hum with the units seperated, very slight hum with the units stacked. Typical of the approximately 50 years I've had the amp. The amp has two big transformers and induces slight hum in anything stacked on it.
I really liked the way the preamp performed with the big transformer, but won't tolerate hum in the main system. Guess I'll look at mounting it off board. Really don't want to, but that's the only way it will work. Need to research enclosures and connectors.
No big deal in the large scheme of things. Learned a lot trying to make it work.
 

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Mount it in a completely separate box on an umbilical/Amphenol and send only DC up to the main box.
 
George S. said:
Well, the upgraded transformer is a failure. Turned on the system this morning and heard a slight hum. Cranked up the volume, and yes, it's definitely there.
I have a early 70's class A bipolar amp upstairs that has always been very susceptible to hum issues in anything connected to it.
Brought the preamp upstairs and connected it to the bipolar. Yup, really pronounced hum. Went through every troubleshooting procedure I could think of, no joy.
Pulled the large transformer, reinstalled the origonal. No more hum with the units seperated, very slight hum with the units stacked. Typical of the approximately 50 years I've had the amp. The amp has two big transformers and induces slight hum in anything stacked on it.
I really liked the way the preamp performed with the big transformer, but won't tolerate hum in the main system. Guess I'll look at mounting it off board. Really don't want to, but that's the only way it will work. Need to research enclosures and connectors.
No big deal in the large scheme of things. Learned a lot trying to make it work.
Click to expand...
Try this George

AN-0212 - 25VA 12V Transformer - AnTek Products Corp (antekinc.com)
 
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