Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: bjorgens on March 02, 2014, 06:21:19 AM
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Could use some assistance to remedy hiss/ white noise on my paramounts
This is not hum, it is white noise that is present regardless of volume level. Likewise, it is present even with source/ pre amp disconnected and amp inputs shorted with shorting plugs.
Amp is setup with relatively new EML 300Bs and some NOS WE396A tubes. Coupling caps are Vcap, parafeed caps are mundorf Silver in oil. Noise was present before these components were installed.
Interestingly enough, the hiss is worse in left channel. There is also an inrush sound when amp is first powered on.
Plate voltage is right at 175VDC on both, and hum is at 2.0mV on both
Any recommendations on where to first start looking?
Bryan
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Hi Bryan,
Hiss is often tube related and is not easy to get rid of. Try swapping the tubes to see if the hiss level is linked to your tubes. And try using them for 50 hours to see if it goes away. I had that with my stereomour 12AT7.
I have to say that hiss never completely disappeared from my system and that it is quite common to audio gear. I have a Naim solid state amp with higher hiss level than my stereomour.
Maybe someone at bottlehead has a circuit change that could help you with definitively removing hiss.
Fabien
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Thanks
Just flipped over my left channel amp again, and noticed that there appears to be oil leaking from one of the 270uF electrolytics on the power supply board. .. Not a ton, but presumably any is too much. Not the case on the right channel amp
Aside from me needing to address this for obvious reasons, could this be source of the inrush and louder hiss on this channel?
Doc B
Am I better off replacing the PS board entirely?
B
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Those caps need replaced if they are bulging or leaking though they should be electrolytic caps and don't have fluid in them.
The constant white noise could be tubes but if you have high sensitivity speakers, you are hearing the regulator on the driver board. It's normal for this circuit.
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Thanks
Just flipped over my left channel amp again, and noticed that there appears to be oil leaking from one of the 270uF electrolytics on the power supply board. .. Not a ton, but presumably any is too much. Not the case on the right channel amp
This is likely worth posting a photo of.
"Oil" could just be some solder flux. The power supply capacitors in the Paramounts will last a very, very long time.
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This is a bit of a story...
Re: the oil...It was a lot more than I would have ascribed to solder flux... More on that below
Incidentally, this is the same amp that I sent in to have you guys help me get working after initially attempting to build with what we subsequently learned was a mislabeled transformer.
You fixed it, and I put it happily into use. The inrush noise/ constant hiss was there at that time on that channel only, but I just figured it was what it was. I should note that I did not check plate voltage after getting the unit back from you... Again... More on that in a sec.
About a month ago, I read that when changing tubes to reset plate voltage to 175VDC. Since I had changed the 300Bs some time earlier, I checked the voltage and discovered it was at 215VDC on the left channel (!) The other amp was at ~179 IIRC... ie big difference between the two, and for all i know, it could have been that way the entire time. That was reset to 175, and this was when I also first noticed the oil. At that time, I thought my coupling cap (a Duelund cast PIO) had failed, so I decided to change that to the Vcap now installed. I now realize that it was oil from the electrolytic leaking down onto and saturating the coupling cap. At same time I also decided to change out the parafeed caps, and swapped to the EML 300Bs and WE396As. Plate voltage was reset to 175VDC on the dot.
So... What to do? A picture won't show much at much at this point unfortunately, as I wiped said oil away already.
Could there have been more initial damage elsewhere than we thought from the mislabeled transformer?
Was my failure to check and set plate voltage after getting the amp back from you a potential culprit?
Or is it just a bad cap?
Thanks as always !
Bryan
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Hello Bryan,
None of this sounds like anything that would damage the amplifier. The mislabeled transformer would result in lower B+ voltages - so no harm to the caps.
IIRC - I put a new power supply board in there anyway (I think).
The plate voltage of the driver tube should be 175 +/- 20% (tighter for the 2A3). If you had 300V on the plate of your driver tube, then we'd be rocketing into that as a problem.
Can you swap tubes to see if this might follow one of the tubes?
BTW - the startup noises are normal to the circuit.
2.0mV of hum is quite high from a Paramount, are you by chance on the 16 Ohm setting?
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Thanks Paul!
Does not follow the tube on previous tries, but I will double check on that and the impedance setting and advise ASAP
Cheers
Bryan
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Paul.
confirmed that noise does not follow the tubes, and OT is wired properly for 8ohms.
b
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Hello Bryan,
OK, this is helpful information.
In the amplifier, there is a 249K resistor on the terminal strip by the 300B tube. Can you temporarily solder a piece of wire across this resistor, then listen for hiss again?
This will remove the contribution of the driver stage and isolate the source of the hiss to one half or the other of the amplifier.
-PB
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Brian,
IIRC, that kills the input to the output tube. So you will hear no music, just the noise.
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to ensure I understand, leave the 249KOhm resistor attached but ALSO add a jumper between the same terminals (7U and 10U)?
b
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Yes, he just wants to separate the driver from the output tube for this test.
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to ensure I understand, leave the 249KOhm resistor attached but ALSO add a jumper between the same terminals (7U and 10U)?
b
Yup.
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Ok... Soldered in the jumper, powered back up, and amp is dead quiet with shorting plug in input. No hiss and no inrush sound on startup either.
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That says your problem is between the input and the output tube. Quite possibly in the driver tube.
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Noise did not follow the tube
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You can put a big cap across -reg and Kreg on the A side. I would try 10,000uf at 6-10V.
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will give it a try Paul...thanks
Acknowledging my own ignorance, could you briefly explain what you think is going on and what the cap will do?
WRT to the leaking electrolytic on my PS board...any sage wisdom on how to get that thing off so I can replace it without pulling the whole board? :)
b
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will give it a try Paul...thanks
Acknowledging my own ignorance, could you briefly explain what you think is going on and what the cap will do?
WRT to the leaking electrolytic on my PS board...any sage wisdom on how to get that thing off so I can replace it without pulling the whole board? :)
If the TL431 biasing the tube is being a little noisy, the cap across it will present a low AC impedance that will calm that down.
I'm still suspect about the leaky electrolytic, does it exhibit any signs of a bad cap? The top of it should bulge out and rupture the plastic wrap if the cap is bad.
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While we're at it, did the jumper solve the hum problem? I know, two things at once is confusing, but it's less long-winded. :^)
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PJ.
the jumper eliminated the hiss. There was still a SMALL amount of background hum. I did not have a chance to compare to the other channel, but will check tonight.
PB
WRT to the electrolytic, don't think the top is bulging (will reconfirm tonight), but the amount of fluid that cumulatively has come out seems pretty substantial. Mind you... I'm not hell bent on pulling that PS board again. :) I may just monitor it for a while and see if the seepage dissipates. In the interim, I have some replacement 270uF's on their way form mouser along with the 10000uFs you recommended
b
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The remaining hum might be banished by a fine tuning of the hum pots.
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Hi Grainger
you mean with a DMM connected to speaker outputs and inputs shorted (as per normal), or is there another way to tune even further?
bryan
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Grainger and I have ten turn pots in our builds. They won't get it any lower but they can find the sweet spot easier. You can tune by volt meter but I find it easier to tune by ear with my ear next to the driver. I have the faintest of hum with my 95dB sensitive speakers.
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Interesting... What pot(a) are you using?
Bryan
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PJ
WRT hum... In amp with jumper between terminals 7 and 10 near 300B, hum is negligible at best. Is substantively less than other channel without jumper... This said, the other channel was pretty low to begin with.
I.e., the jumper completely eliminated hiss, and seems to have reduced hum as well.
PB
Electrolytic is not bulging so far as I can tell.
Bryan
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Re: hum, that means the 2A3 is fine, and the hum is coming in somewhere earlier. Useful to know. Check the connection of 12AT7 pin 9 to the chassis ground as a first step. Ater that it gets harder, so maybe it's time to concentrate on the hiss first.
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If that cap has leaked at all you really need to replace it.
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Ok... Will replace the cap... Should have parts Saturday hopefully.
Will check the ground connection on the 9 pin and advise tomorrow
Thanks again folks for the help.
B
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Bryan, PM Sent.
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PJ,PB, Doc
Replaced the one electrolytic on PS board. On closer inspection, top was bulging slightly. Added 10000uF 6.3V caps across -reg and Kreg, and hiss is gone... I mean gone.
Retuned for hum, and system is very quiet
Haven't had much time to actually listen, but MUCH better in terms of noise floor.
Thanks again for all your help
Cheers
Bryan
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Bryan, did you use a bipolar or standard 10,000 uF cap? If standard, did the neg side go to the - neg point?
I too would love to eliminate that hiss and this seems like an easy thing to try. For the team, does this mod have a potential to adversely effect the sound?
Also, did Grainger PM you about the hum pot?
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Howdy
Neg lead went to -reg. mounting the cap is a tad ticky, as the through holes are too small to accept more than one lead. I simply bent and soldered the cap leads across the top if the pads... May choose to change the mounting later if I feel ambitious.
So far so good re: sound. No deleterious effects I can hear... Only good ones.
Re: the hum pot, did not get a part number from Grainger... The 10ohm 10 turn pot I could find was rather expensive (~$50), so I might stick with what I have for now... :)
Bryan
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Thanks Bryan. Yes the pots are pricey not to mention large but you pay for precision.
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... For the team, does this mod have a potential to adversely effect the sound?
[This was the 10000uF cap from kreg to -reg] Yes it does. Let me clarify: many people (including the Bottlehead team) find that a normal cathode bypass capacitor slightly degrades the sound relative to no capacitor. Presumably this has to do with the signal current that flows through the capacitor. In the case of bypassing the 431, the regulator chip has a very low AC impedance (unlike the normal situation where a resistor is bypassed) so the signal current through the capacitor is reduced. So when I agree it has the potential, I mean I don't know whether there is a residual loss or not. At this level of resolution, calculations of current are IMHO not going to predict the subjective effect; only careful listening can do this. Of course it will be confounded with the reduced noise - but at least that's the realistic choice - does it sound better with the cap or without? Results are likely to be different with different speakers as well.
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Thank you Paul. It's a cheap experiment :)
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Paul(s)
Ignorant question #74
would a relatively lower capacitance (film) cap do anything with less potential for adverse effect? I.e. I'm not smart enough to understand why a 10,000uF cap is needed in this particular case.
Even if so, I don't intend to change anything any time soon... I have so many new parts in my system right now, it just needs time to settle down so I can establish a "new normal"
bryan
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The dynamic impedance of a 431 set for 2.5 volts is about a half ohm. The capacitor has to have a lower impedance if it is to reduce the chip-generated noise. 10000uF is 0.5 ohms at 32Hz, so it will be effective above 32Hz.
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Thanks Paul... Much appreciated explanation
B