Need assistance addressing hiss

[Paul Joppa]

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.  :^)

 

[bjorgens]

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

 

[Grainger49]

The remaining hum might be banished by a fine tuning of the hum pots.

 

[bjorgens]

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

 

[johnsonad]

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.

 

[bjorgens]

Interesting... What pot(a) are you using?
Bryan

 

[bjorgens]

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

 

[Paul Joppa]

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.

 

[Doc B]

If that cap has leaked at all you really need to replace it.

 

[bjorgens]

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

 

[Grainger49]

Bryan, PM Sent.

 

[bjorgens]

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

 

[johnsonad]

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?

 

[bjorgens]

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

 

[johnsonad]

Thanks Bryan.  Yes the pots are pricey not to mention large but you pay for precision.

 

[Paul Joppa]

... 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.

 

[johnsonad]

Thank you Paul. It's a cheap experiment

 

[bjorgens]

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

 

[Paul Joppa]

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.

 

[bjorgens]

Thanks Paul... Much appreciated explanation

B