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humm and buzz

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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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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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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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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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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
 
Is this "ST70" a stock Dynaco Stereo 70? Are its input jacks isolated from chassis? If so, is there a wire that connects from the input jack 'ground' to chassis at some point? If so, where exactly is that connected to chassis?

Now check the input jacks on the solid state amp. Are those isolated from chassis?

How is the preamp's output grounded? Is that RCA jack isolated from chassis?

How is the preamp's input RCA jack isolated from chassis? Where is the ground lug on the RCA input jack connected to chassis? Or is it connected to chassis?

--
 
Is this "ST70" a stock Dynaco Stereo 70? Are its input jacks isolated from chassis? If so, is there a wire that connects from the input jack 'ground' to chassis at some point? If so, where exactly is that connected to chassis?

Now check the input jacks on the solid state amp. Are those isolated from chassis?

How is the preamp's output grounded? Is that RCA jack isolated from chassis?

How is the preamp's input RCA jack isolated from chassis? Where is the ground lug on the RCA input jack connected to chassis? Or is it connected to chassis?

--

on the Triodelectronics stereo 70 the inputs are grounded to a common star ground via 470k resistors.

on another amp where the input is not grounded the preamp is quiet.

if a turntable is plugged into the st70 via a phono preamp there is humm but if it is plugged into the fred nachbaur miniblok pushpull amp which does not have a grounded input there is no noise.

so the problem is not in the pre-amp or the phono preamp since it is quiet with the miniblok. the problem is with the st70 i guess.

the CD player through a passive preamp into the st70 quiet.
 
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on the Triodelectronics stereo 70 the inputs are grounded to a common star ground via 470k resistors.

on another amp where the input is not grounded the preamp is quiet.

if a turntable is plugged into the st70 via a phono preamp there is humm but if it is plugged into the fred nachbaur miniblok pushpull amp which does not have a grounded input there is no noise.

so the problem is not in the pre-amp or the phono preamp since it is quiet with the miniblok. the problem is with the st70 i guess.

the CD player through a passive preamp into the st70 quiet.

Try the RCA screen cable ground loop breaker: just add 1 ohm resistor on one channel to lift the ground. This resistor value being higher than the screen copper resistance forces the ground loop to go out from pre-amp chassis into the chassis of amp and CD player instead through it's own chassis or ground, thereby eliminated the hum and noise. I have used on my power amp and it works.
 

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The added 1r0 is an interference current attenuator.
Fit one to each channel of the pre-amp.

Fit one to each channel of the Power amplifier.

These added resistors are referred to as HBRL and HBRR in other Threads.

These added resistors are NOT loop breakers.
They do not break any loops, the loops still exist ! look at the sch !!!!!!
 
imho your preamp as drawn has flaws,
the varable pot at the second stage changes the operating points
of the 2nd stage as you turn the pot, to me this is the cause of your difficulty...
why not put the volume control at the input of the first stage instead?

this is the first time i see this preamp topology,
why not do a strarightorward Broskie CCDA?
CCDA: constant-current-draw amplifier

my tube preamps using the broskie ccda are dead quiet,
no hum, no hiss, just music....;)

why not post a picture of your actual build so as we can do away with speculations....
 
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imho your preamp as drawn has flaws,
the varable pot at the second stage changes the operating points
of the 2nd stage as you turn the pot, to me this is the cause of your difficulty...
why not put the volume control at the input of the first stage instead?

this is the first time i see this preamp topology,
why not do a strarightorward Broskie CCDA?
CCDA: constant-current-draw amplifier

my tube preamps using the broskie ccda are dead quiet,
no hum, no hiss, just music....;)

why not post a picture of your actual build so as we can do away with speculations....

Ya agree, just happen to be another variation, it does change the bias resistance, it would affect bias if DC coupled. The builder is ChopChip, the sch is attached as post #1, he has vol pot in front I believe, ChopChip please you're ask to send a picture of actual build.

The problem is not uncommon, please read the forum for a similar problem http://forum.allaboutcircuits.com/threads/humming-sound-in-stereo-amplifier.56682/
 
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The added 1r0 is an interference current attenuator.
....
These added resistors are NOT loop breakers.
They do not break any loops, the loops still exist ! look at the sch !!!!!!

Ya I really mean the same. "ground loop hum breaker". I prefer to add resistor to one channel, as the ground resistance should be kept as low as possible, and this is offered on the other channel which is dead short for DC, and also for effectiveness of hum current attenuation, AC flows from high ground to low, not via visa, if go both way AC flows and so no attenuation.
 
here are some pictures of the build.

i installed a 1.5 ohm, couldn't find a 1 ohm, and it seems to have gotten rid of the humm but there is still bzzz.

thanks again for the help.
 

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