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Old 1st November 2015, 03:41 PM   #1
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Default humm and buzz

i plug in the preamp to the transistor power amp and it is quite as a mouse but if i plug it into the st70 it humms and buzzes like a hive of angry bees.

any recommendations?
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Last edited by chopchip; 1st November 2015 at 03:43 PM. Reason: schematic
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Old 1st November 2015, 04:13 PM   #2
kaputt is offline kaputt  Germany
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Check the connection of signal ground to PE in the transistor amp and the ST70. Is there a difference?
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Old 1st November 2015, 04:35 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by chopchip View Post
if i plug it into the st70 it humms and buzzes like a hive of angry bees.
It could be poorly fitting RCA plugs on the Stereo 70 sockets. Try another pair of cables.
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Old 1st November 2015, 05:26 PM   #4
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If power amp is on the full volume like end stage, You have make very good filtering CLCLC for tube pre stage.....Your pre amp is two stages. And You have too much gain, one prestage is enough. Every micro noise will be like monster bzzzz and hmmmm.
If You plug to the audio input, the effect is almost the same.
Tube prestage is for SS end stages, and Your amp needs only one stage pre tubeamp, or two stages with cathode fallover stage.
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Old 1st November 2015, 05:31 PM   #5
tomchr is online now tomchr  Canada
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The loose RCA connectors sound like a winner to me.

If the power amp is quiet with the inputs shorted and the preamp's output is free of hum when it's on its own, it could also be that you have a ground loop from the mains ground through preamp, power amp, and back to the mains ground. You can test this hypothesis by TEMPORARILY converting one of the 3-prong mains plugs into a 2-prong, buy lifting the ground connection. If this solves the hum/buzz issue, I suggest looking at ground loop breakers - or better yet changing the circuit to move the ground loop out of the signal path.

Tom
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Old 2nd November 2015, 01:26 PM   #6
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Quote:
Originally Posted by tomchr View Post
The loose RCA connectors sound like a winner to me.
If this solves the hum/buzz issue, I suggest looking at ground loop breakers - or better yet changing the circuit to move the ground loop out of the signal path.

Tom
would an isolation transformer solve this problem? i have one but it is has a ground on its secondary. you think i should disconnect it?
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Old 2nd November 2015, 02:21 PM   #7
Koonw is offline Koonw  Malaysia
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Originally Posted by chopchip View Post
would an isolation transformer solve this problem? i have one but it is has a ground on its secondary. you think i should disconnect it?
You should determine where ground loop first so you apply ground lift in that problem areas:

1) in common earth ground loop simply by temporary disconnecting the earth from either preamp or amp side. If so a isolation transformer can be used to break the ground loop.

2) in RCA cable screen ground, any hum if only plug in one cable? If so, you'll need a ground lift of 10 ohms resistor on screen signal ground and ground on both input, see attached
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Old 2nd November 2015, 04:10 PM   #8
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Originally Posted by kaputt View Post
Check the connection of signal ground to PE in the transistor amp and the ST70. Is there a difference?
what is "PE"?

thanks
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Old 2nd November 2015, 05:08 PM   #9
AndrewT is offline AndrewT  Scotland
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PE = Protective Earth.
The third wire in your mains cable that gets permanently and mechanically fixed direct to the Chassis.
It's there to save the lives of any operators.
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Old 2nd November 2015, 05:26 PM   #10
tomchr is online now tomchr  Canada
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Quote:
Originally Posted by chopchip View Post
would an isolation transformer solve this problem? i have one but it is has a ground on its secondary. you think i should disconnect it?
Depends on the kind of isolation transformer. If it's a line-level isolation transformer that you can insert in the signal path between the preamp and the power amp, then it will help. A mains isolation transformer, so 120:120 or 230:230 V, probably won't do much.

Tom
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