From: mynamespacedbydots@xs4all.nl
"Alex Pogossov" wrote in message
news:4e82d9da$1@dnews.tpgi.com.au...
>
> "Engineer" wrote in message
> news:9a294d45-dee5-48f0-968a-f3f6a14965dd@q26g2000vby.googlegroups.com...
>> Hi, Vcuumlanders,
>> Perhaps this is just a minor point, or even common knowledge, but it
>> has merit...
>> I had put a pre-set 1 meg log gain pot at the front end of my recently
>> built monobloc (the Williamson near-clone that some here may
>> recall...) It worked, of course, but I was aware that near the mid-
>> point setting the amp had more hum and noise than at either end (with
>> a pre-amp connected.) With no pre-amp connected the hum and noise
>> just went up as the pot was tuned up. The penny dropped! The grid of
>> the first audio tube was picking up "under-chassis" hum and noise when
>> the first stage grid had a high impedance to ground, i.e. 1/2 meg, or
>> so. I had not specifically shielded any under chassis AC wiring and
>> there was a 60 Hz power transformer under the chassis, too, for B+
>> boost.
>> The solution: use a 47 K log pot for the preset input gain. Now the
>> first grid never sees more than 23 K to ground - very quiet. But now
>> the amplifier now has 47K input impedance. This is OK if the pre-amp
>> has a cathode follower output, i.e. has a Thevenin impedance of, say,
>> 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid
>> state - 47K is easy to drive.
>> I promptly changed my two other monoblocs, a home-brew and a Pilotone
>> commercial unit, to the same pre-set pot input value with some noise
>> improvement - less than the first one, but the idea is a sound
>> one.
>> Cheers,
>> Roger
>
> Just by the way: If a pot is set to midpoint, and source output impedance
> is low, then output impedance of the wiper is R/4, not R/2.
Just by the way: R/4 at wiper midpoint only when it's a linear pot, not log.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
