No more 60 Hz hum in a PL2000 preamp.

Got lowest noise with a X cap between Line and Neutral. A Y cap from Line to star ground screw raised noise slightly so removed it. Green wire added from IEC ground lug to star ground screw made no difference in noise, so left it installed.
Getting the paint out from under the transformer retaining bolt lowered noise. Trimmed the original shield away from the transformer bolt where it was touching and lowered noise some.
Burn't out working on it for now. Going to put it back in system, listen to some music.
Getting good time in using the QA400. Some of the tests aren't automated, more to learn, more reading.
 

Attachments

  • PXL_20230220_165740520.jpg
    PXL_20230220_165740520.jpg
    2.3 MB · Views: 16
  • PXL_20230220_234204175.jpg
    PXL_20230220_234204175.jpg
    3.3 MB · Views: 8
Last edited:
Getting ready to test WIMA MKS 0.1 uF bypass caps on the BrownDogs.
These have OPA2134 opamps installed. Data sheet specifies decoupling caps from plus and minus voltage pins to ground. Unfortunately PL included no nearby ground plane, and these are isolated from ground, so I'll try this. Doubt it'll make a difference, but will give it a try.
 

Attachments

  • PXL_20230227_180004335.jpg
    PXL_20230227_180004335.jpg
    2.3 MB · Views: 10
The WIMA bypass caps were worthwhile. THD was reduced.
Now trying the same on a identical set of BrownDogs using 0.1 uF Kemet MLCC. These are supposed to be superior to film caps according to some highly technical posts.
 

Attachments

  • PXL_20230227_202053770.jpg
    PXL_20230227_202053770.jpg
    1.8 MB · Views: 6
The MLCC bypassed units have a issue. May have a bad opamp. Might be awhile until I get back to it due to other issues requiring attention.
Sitting here listening to a album with the WIMA units installed. Excellent. Was worthwhile.
Now to order parts to fix the wife's Buick 1.4 Turbo. Seeping coolant at the connections to the turbo. Impossible to get to without pulling the exhaust manifold and turbo as a unit. Next weekends job. Time to order parts.
Ronnie Montrose, produced by Edgar Winter.
 

Attachments

  • PXL_20230227_213604368.jpg
    PXL_20230227_213604368.jpg
    1.9 MB · Views: 3
Gene, rebuilt my 240 VAC water heater yesterday. Tripped the breaker. Blown element. Two elements and two thermostats replaced and it's good as new(fingers crossed). It's always something! :)
 
George S. said:
Gene, rebuilt my 240 VAC water heater yesterday. Tripped the breaker. Blown element. Two elements and two thermostats replaced and it's good as new(fingers crossed). It's always something! :)
Click to expand...
you are kidding....
i am rebuilding mine!!!
saw water on floor...it seeped down inside the walls could not see it...by passed it...i have 2 others but very seldom used...i have solar hot water .just finished opening it up...i see no leaks ..i think a fitting was leaking inside the shell . i see a resty spot in top...will pressurize tomorrow...it is full of water...but not leaking...glad i didnt toss ..it is an on demand tank with maybe 2 gallon tank...dual heating elements...240v .. will check the corrosion rod tomorrow!!!
yes...dad always said ...dont matter where you sit in the house....you can always see something that needs fixin...lol
 
I've listened to 6 or more albums this evening while figuring out my car parts order. I really like the bypass caps on the BrownDogs.
Using the Acurus for comparative testing has been worthwhile.
Looking to improve the QA400 setup. Have ordered some 50 ohm BNC terminators to put on the QA400 outputs. Going to try generating the 1 kHz sine wave with the DDS signal generator and directly drive the preamps. The QA400 internal signal generator is producing harmonics at settings above -40 dB, and I think this is a issue. Going to find out first hand as there is little information available about this.
QuantAsylum has posted they may have new units next month. Looking forward to it happening.
 

Attachments

  • PXL_20230228_004616240.jpg
    PXL_20230228_004616240.jpg
    2.5 MB · Views: 6
George, does (or can) the QA400 account for the harmonics of the generated sine wave such that it can take it out for the UUT (unit under test). I know nothing about the QA400, so this is kind of a general question. Back in my old testing days, we'd first account for the harmonics of the source, then test the UUT, and finally subtract the effects of the source. More modern systems can automatically account for this.
 
Jim, issue is the QA400 is itself producing harmonics with it's built in sine wave generator. It's good up until approximately -40 dB. At higher output I start to see harmonics.
If this is proper or not, I don't know, and I see no way to "subtract" them from the results.
Understand, I'm a hobbyist and have had no formal training, just reading what I can and trying to learn "hands on".
Thought I'd sub the DDS unit and see if results are same or different.
The QA400 unit I have is the first model QuantAsylum made and may well have issues they improved on later models.
Going to do further reading this week and try to learn more. Just seems wrong to drive a DUT with harmonics of 1 kHz. But then music is composed of harmonics, they're essential. I just don't know and need to learn more.
Guess the question is, should the 1 kHz test frequency driving the DUT be pure and free of harmonics?
I see no harmonics at low FFT resolution, but they are there at high FFT resolution. Perhaps I'm testing at too high a resolution.
 
Going to download the manuals for later models, perhaps the answer is there. The QA400 manual is rather sparse as it was evidently quickly superceded by the QA401.
 
Ideally you'd like your source signal to be as free of harmonics & noise as possible. That's what the HP8903B does, i.e., it's output signal has very low noise and THD so that when measuring a DUT the readings are attributed to the DUT. That just makes everything easier. But you could measure your source and determine it's harmonics. When you apply that signal source to the DUT, you could calculate (knowing its gain) how much of the harmonics at the output was due to the harmonics in the input signal. This is complicated to do by hand, but a computer could do it easily.

But have a pure signal source is the best of all possibilities.
 
BlueCrab said:
Ideally you'd like your source signal to be as free of harmonics & noise as possible. That's what the HP8903B does, i.e., it's output signal has very low noise and THD so that when measuring a DUT the readings are attributed to the DUT. That just makes everything easier. But you could measure your source and determine it's harmonics. When you apply that signal source to the DUT, you could calculate (knowing its gain) how much of the harmonics at the output was due to the harmonics in the input signal. This is complicated to do by hand, but a computer could do it easily.

But have a pure signal source is the best of all possibilities.
Click to expand...
Got it. Thanks Jim. Been doing some reading. Appears my set up of the software parameters for the sine wave generator is incorrect. I'm overdriving the generator and causing unwanted harmonics. Found a post about using the next model QA401 to measure distortion and they discussed settings that aren't documented in my QA400 manual, yet I believe are in the QA400 software.
Hopefully I'll find time this coming Monday to look at this.
Like you say, the sine wave should be spectrally pure.
The DUT produces the harmonics, and THD is the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
Wish I had more free time to pursue this, I find it really interesting. Thanks!
 
Al, those dual pots I was cleaning are from my very first PL 2000 S2 that I was using to learn on. That one got the single 100k balance pot, and was then retired after I built two more 2000's based on what I learned on that first unit.
I kept the dual pots in these as the pots were good enough to use at that time.
The pots I just disassembled and cleaned, were bad, very bad, from that first unit.
So, how'd they turn out? The balance pot dual tracks consistently within 2k ohms of each other. The volume pot tracks within 3k ohms.
Not good enough. Going to investigate those 24 position attenuators on Ebay that are supposed to be within 1%.
 

Attachments

  • PXL_20230424_204848880.jpg
    PXL_20230424_204848880.jpg
    2.9 MB · Views: 10
Last edited:
The original pots are 50k, and have a issue with out of spec carbon tracks. Whether they were made that way, or it's a aging issue, don't know.
The first balance dual pot I overhauled had carbon tracks with a 3k difference between them.
The second balance pot has a 7k difference between them.
I could have saved myself a lot of work by measuring the resistance of the 50k carbon tracks(the outer lugs) first.
Obviously, mismatched tracks will never be in perfect balance, though they will be equal in resistance at one point if testing at the properly wired pigtails. Then the pot can be rotated and tightened down, and said to be correctly indexed.
I'll wire them up with pigtails next weekend and see how they compare, though using a Bourns 100k single seems the best choice for the balance pot.
However, the volume pot is critical, and as Jim has said, those stepped attenuators with resistors are probably best. Will probably pursue those for the volume and perhaps the balance also.
 
George S. said:
The original pots are 50k, and have a issue with out of spec carbon tracks. Whether they were made that way, or it's a aging issue, don't know.
The first balance dual pot I overhauled had carbon tracks with a 3k difference between them.
The second balance pot has a 7k difference between them.
I could have saved myself a lot of work by measuring the resistance of the 50k carbon tracks(the outer lugs) first.
Obviously, mismatched tracks will never be in perfect balance, though they will be equal in resistance at one point if testing at the properly wired pigtails. Then the pot can be rotated and tightened down, and said to be correctly indexed.
I'll wire them up with pigtails next weekend and see how they compare, though using a Bourns 100k single seems the best choice for the balance pot.
However, the volume pot is critical, and as Jim has said, those stepped attenuators with resistors are probably best. Will probably pursue those for the volume and perhaps the balance also.
Click to expand...
That's why the 2000 has a balance pot George :-)
 
You must log in or register to reply here.
Back
Top