kambo

single ended with 2:1 and 4:1 out hum problem
« on: August 08, 2019, 12:42:45 PM »
simple mic pre with single ended  6SN7 gain stage, (B+300v,  Ra15k, Rk 750r, 4uf output cap to 4:1 output transformer sounding just fine
about 1% distortion... way less on normal usage.
no noise problem, no hum, all good.

if i replace 4:1 with 2:1 out trx, i am getting way less distortion, more gain, but noticeably more hum 50hz...
its almost audible on high volume.
if i add additional  white cathode follower to reduce output impedance  and still same 2:1 out trx,
all that extra hum  goes away!

any idea why, i have more hum, with 2:1 trx and WCF is taking it away!
is impedance mismatch  causing this extra hum ?



abbey road d enfer

Re: single ended with 2:1 and 4:1 out hum problem
« Reply #1 on: August 08, 2019, 01:16:42 PM »
simple mic pre with single ended  6SN7 gain stage, (B+300v,  Ra15k, Rk 750r, 4uf output cap to 4:1 output transformer sounding just fine
about 1% distortion... way less on normal usage.
no noise problem, no hum, all good.

if i replace 4:1 with 2:1 out trx, i am getting way less distortion, more gain, but noticeably more hum 50hz...
its almost audible on high volume.
if i add additional  white cathode follower to reduce output impedance  and still same 2:1 out trx,
all that extra hum  goes away!

any idea why, i have more hum, with 2:1 trx and WCF is taking it away!
is impedance mismatch  causing this extra hum ?
I would think the 2:1 is less well shielded than the 4:1. Maybe the 4:1 is humbucking.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

kambo

Re: single ended with 2:1 and 4:1 out hum problem
« Reply #2 on: August 08, 2019, 01:19:09 PM »
2:1 is not shielded at all, its cinemag 2820, 4:1 is jensen 10k:600 shielded nicely... but WCF killing the 50hz hum with cinemag...
i dont think its the shield!

its not like 2:1 rising the whole noise floor, because i have more output... its only 50hz sticking out... rest is pretty much the same!


abbey road d enfer

Re: single ended with 2:1 and 4:1 out hum problem
« Reply #3 on: August 08, 2019, 01:23:52 PM »
2:1 is not shielded at all, its cinemag 2820, 4:1 is jensen 10k:600 shielded nicely... but WCF killing the 50hz hum with cinemag...
i dont think its the shield!

its not like 2:1 rising the whole noise floor, because i have more output... its only 50hz sticking out... rest is pretty much the same!
Do you understand how driving the primary with a very low impedance kills hum? You can experiment, just take the xfmr, connect the secondary to a monitor and compare open primary with shorted primary.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

kambo

Re: single ended with 2:1 and 4:1 out hum problem
« Reply #4 on: August 08, 2019, 01:27:02 PM »
Do you understand how driving the primary with a very low impedance kills hum?
nope  :(

You can experiment, just take the xfmr, connect the secondary to a monitor and compare open primary with shorted primary.

i did that last night.  i thiought i had some leak from a cap or something... aaaahhh now i got it, while i am typing  ;D ;D ;D


 

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