Welcome! Need support, you got it. Or share you ideas and experiences.
24.18.240.124
In Reply to: Admittedly minor hum (1.1 vs 0.75 mV) posted by dnewman on April 26, 2007 at 18:59:58:
Those are the usual variables, once you have eliminated the obvious as Nutube talks about. Compare the two sides, especially looking at signal current loops and twisted pairs.
Glad to hear everyone's thoughts as they confirm at that I was
doing the right sort of checks already. Since Nutube's list checks
out okay (and I had previously reflowed ground paths), I'm
suspecting it's an induced current picked up from either a twisted
pair or proximity to the psu. I'll break out the oscope and pickup
coil. But first, I may try injecting a small signal into each of the tp's
and see if I can pick it up that way with a tone tracer or the scope.
I'm mostly curious if the technique will work. (Safer than doing it
with the amp plugged into the mains!) However, if this small hum
is coming from the filament wiring, then my puny injected signal
probably won't show it.Thanks,
Dan
Reduced hum from 1.1 mV to 0.90mV at which point (and as suggested), I will
chalk the rest up to component variances and that the other channel with 0.75mV
hum has a slightly different layout as it is a mirror layout.First twisted pair I played with was the AC input from IEC connecto & switch to
the power tran. I coerced it into lying closer to the chasis plate which also got
it a tad further from the pair exiting the PSU board going to the filament choke.
I then did some empirical adjustment with the twisted pair filament supply to the
12AT7.Now to find something else to futz with whilst enjoying my Paramounts.
Cheers,
Dan
I'm embarassed to say, but I have not tried the following. Parampounts are selling pretty well, and I have yet to get my hands on a pair for extended lab and listening...Anyhow, the LR filter on the filament of the 300B is not a "pure DC", it reduces the AC component by about 16dB relative to AC heating. I chose this because of the bad reputation that capacitors have in filament supplies. However, it is quite possible, even probable, that a large electrolytic cap across the heater pins (A1-A4) will result in hum reduction. You may also lose some "life" from the sound, or maybe not - there arer too many different and incompatible opinions on the web to be confident. Worth a try just to see what you hear though! I'd start with 10000uF, 6 to 16v rating.
There will always be some hum pickup from the driver heaters, even with a grounded center tap as in Paramount and Paramour. Some 12AT7s will be more immune to this than others. We are also working on a DC supply for the driver (hope to use it for some delay relays as well).
So there is plenty to futz with when you need it! :^)
