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Old 11th May 2006, 07:59 AM   #1
cuibono is offline cuibono  United States
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Default Pluto hiss?

I'm the proud new owner of two pairs of Pluto speakers - SL's design. Their great, they raise the hair on my arms. The only problem is I am getting noise out of them, just barely audible at the listening position (all four of them, exactly the same). I've tenatively solved a ground hum problem, but the speakers hiss. It sounds exactly like tape hiss. I am using a passive pre, and Cambridge Audio's Azur 640C V.2. The hiss stays no matter what I connect the speakers too. Any ideas where the source is? The electronics are all IC based, and the boards fairly simple. I don't see any fault in assembly... Could I have hurt an OPA2134 or a voltage regulator? Poor quaility caps? I have no idea...

Patrick
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Old 13th May 2006, 02:03 AM   #2
cuibono is offline cuibono  United States
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Unhappy hmmm

no one has any idea how an IC based active xover/eq/power amp circuit could be indused to hiss? thats not a good sign...
peace all
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Old 13th May 2006, 02:21 AM   #3
tade is offline tade  United States
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hiss hmm.
Not 60hz humm?
maybe it is harmonics from that. Anyway, try onnecting the chassi of every component to ground. either that, or crank up the music!
good luck
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Old 13th May 2006, 02:39 AM   #4
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fwiw, this is a link to the design;
http://www.linkwitzlab.com/Pluto/electronics.htm

you can work backwards from the electronic XO and repeatedly ground the inputs to see from which stage the hiss may eminate -- also did you use a printed circuit board? did you use all the bypass caps which Siegfrid recommends?
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Old 16th May 2006, 08:52 PM   #5
cuibono is offline cuibono  United States
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Default here is the kicker

the hiss seems to eminate from the rca connector. when I remove it and hardwire a cable, the hiss is gone. wtf? is it possible that some small amount of capacitance is causing leakage from ground to signal at high frequencies? im still working, the hiss returns when connected to some components and not others, as does the ground humm. I wonder if these circuits are especially sensitive to noise. can anyone clue me into why there might be leakage across my connector?

I am thinking of building Jon Risch's AC line filter to clean up my mains, which are part of the problem. Does this circuit look like it would clean up the AC well?

http://www.geocities.com/jonrisch/surge.htm
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Old 18th May 2006, 06:04 PM   #6
kevinkr is offline kevinkr  United States
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Do you have an oscilloscope? Sounds suspiciously like there might be some high frequency oscillation going on. As a quick fix install some resistors (470 ohm - 1k) right at the rca output jacks of your x-over.

Without more information it is hard to really know what is going on.

RCA jacks and plugs don't make noise, although jacks directly grounded to the chassis can cause problems with internal and external ground loops. These should be isolated and grounded at one point - usually the star grounding point. Depending on the dielectric of the jack they can add a significant amount of additional shunt capacitance - if the circuit driving them is marginally stable that in conjunction with the cable capacitance may be enough to make the op-amp driving them oscillate.
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Old 19th May 2006, 06:11 AM   #7
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Have you asked Linkwitz?
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Old 21st May 2006, 07:59 AM   #8
cuibono is offline cuibono  United States
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Linkwitz said he had heard of low frequency oscillations around 7hz, caused by low quality voltage regulators. I suspect them, but replacing them hasn't fixed it. They are my main suspect, but the really odd thing remains, that if the rca input is disconnected, no hiss. That totally stumps me. I will figure it out.

He also said that using anything 'nicer' than the panasonic caps he suggests would lend no difference to the speakers preformance. He is the boss, but I doubt it - I feel the speakers lack detail, and am looking into switching in blackgates and auricaps for the 70+ panasonic caps. Any thoughts?
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Old 21st May 2006, 01:10 PM   #9
kevinkr is offline kevinkr  United States
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I'd pin down the problem with the cross-over first and then think about upgrades afterwards. Is it possible that the hiss you hear is actually noise from the pre-amplifier or some other source. Commodity op-amps like the TLO84 and LF353 for example are extremely noisy. What out of curiosity is used in the Linkwitz x-o?
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Old 19th July 2006, 07:45 AM   #10
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Quote:
Originally posted by cuibono
Linkwitz said he had heard of low frequency oscillations around 7hz, caused by low quality voltage regulators. I suspect them, but replacing them hasn't fixed it. They are my main suspect, but the really odd thing remains, that if the rca input is disconnected, no hiss. That totally stumps me. I will figure it out.

He also said that using anything 'nicer' than the panasonic caps he suggests would lend no difference to the speakers preformance. He is the boss, but I doubt it - I feel the speakers lack detail, and am looking into switching in blackgates and auricaps for the 70+ panasonic caps. Any thoughts?
Your Pluto lacks detail? I just built a Pluto for my friend. It is one of the best speaker system we ever listen. It is very clean and extremely detail. We are using the cheapest NAD preamp and twisted pair cat5e interconnect to Pluto without shielding.
You should fix all the problems before wasting money to upgrade.
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