No more 60 Hz hum in a PL2000 preamp.

[George S.]

Testing the preamp on the QuantAsylum QA-400. THD is less than .003%

 

[George S.]

THD+N is less than .01% I do have a noisy room. Many computers and Allman Bros playing in the background on another system!

 

[George S.]

SNR greater than 80 dB.

 

[George S.]

Frequency Response tested with White Noise stimulus. Very similar results with Impulse, ExpoChirp, and FarinaChirp stimulus. I see a issue here that Joe has talked about in past. Close up follows.

 

[George S.]

FR closeup. I'm thinking I need to increase the output capacitance.

 

[J!m]

See what Joe says but it looks pretty damn nice for an antique preamp!

 

[e30m3mon]

So was it worth it? Oh hell yes! Absolutely no hum with volume cranked, noise floor is so low I'm hearing things I've never heard before. Bass is super tight and defined. The lower frequencies are now being passed more fully to drive the subs. Transients are sharp, sharp, sharp.
Best upgrade to the PL2000 I've made so far! Thanks Joe!

Awesome work, George!

Did you by chance summarize what you ended up doing - which toroid transformer, snubbers used? I can look back but just in case there was anything else modified that did not get picked up in the notes.

I scanned back - missed all the updates from yesterday as I did not get on much yesterday. Thanks for capturing all your notes, George.

 

[e30m3mon]

Question now is how other preamps might benefit from an upgraded transformer - C1, 4000, 3000, others. Seems this upgrade has a lot of potential across many preamps - as well as signal processors, etc.

 

[gene french]

way to go george!!!

 

[J!m]

I think I commented on the Aragon preamp already. Big improvement there.

 

[George S.]

I've had these 36 uF Panasonic polypropylene AC filter caps sitting here for some time.
If I remove the accessory AC plug assemblies from the chassis back wall, they'll fit. They won't be soldered to the board, but that's OK I think, as the stock output caps are fed by a wire off the switch PCB. I can easily run that wire to one of these, then another wire to the RCA.
The output caps now installed are 4.75 uF Panasonic ECW. It'll be interesting to see what difference these 36 uF make.
In the meantime, I have reading to do. Not sure what a preamps frequency response is supposed to look like on a RTA. Later y'all.

 

[George S.]

Been doing some reading. The frequency response issue may be related to a engaged "high pass filter".
Next weekend I'll put it back on the bench and do some investigating. I need to make sure the filter switches and circuits are working correctly before subbing output caps.
Joe's #1 rule of "it's your test equipment" also applies. Oh well, work week starts tomorrow morning. Later all.

 

[Vintage 700b]

So was it worth it? Oh hell yes! Absolutely no hum with volume cranked, noise floor is so low I'm hearing things I've never heard before. Bass is super tight and defined. The lower frequencies are now being passed more fully to drive the subs. Transients are sharp, sharp, sharp.
Best upgrade to the PL2000 I've made so far! Thanks Joe!

George,
That is a fantastic install ! Thanks for sharing the whole plan, concept, research and the work you went through.
It's got to feel fabulous kicking back now and appreciating the work you've done!

 

[George S.]

Just picked up a cheap Acurus L10 to use as a bench mark for comparative testing.
A future retirement project will be cloning that board to the PL2000 chassis, so having one on hand is a plus. Been picking up the NOS Toshiba transistors for some time and have enough for several builds.

 

[laatsch55]

What Toshibas George?

 

[George S.]

Lee, if you look at the schematics, and research the part#s, you'll see all the transistors are Toshiba production. The 2SJ109GR and 2SK389BL dual FET are long out of production and there are no current direct replacements. There are work arounds, but nothing beats the original dual FET package.

 

[Gepetto]

Lee, if you look at the schematics, and research the part#s, you'll see all the transistors are Toshiba production. The 2SJ109GR and 2SK389BL dual FET are long out of production and there are no current direct replacements. There are work arounds, but nothing beats the original dual FET package.

Nothing is a pretty strong word…

 

[George S.]

Nothing is a pretty strong word…

Well, there is a company that makes a replacement for one of them, but it's a different package.
Then there's a way to use 2 FETs for the other, bonding them together.
From what I've read, the best solution is using a original.

 

[Gepetto]

Well, there is a company that makes a replacement for one of them, but it's a different package.
Then there's a way to use 2 FETs for the other, bonding them together.
From what I've read, the best solution is using a original.

Whenever a Japanese transistor is involved, there is always a better solution...

 

[George S.]

Joe, you've said that many times before!