Hi,
I built a preamp based on EF86 tubes. The PSU is equal to the one on this preamp .
At first it had a lot of hum. After careful checking I could not identify any ground loop or something and just cured it with a big electrolytic after the PSU.
But now when I put the volume down, when it is very close to the limit it starts to hum like hell. I am using an alps blue pot. When I put the volume up a bit it no longer hums. Any suggestion how to fix this?
Miguel
I built a preamp based on EF86 tubes. The PSU is equal to the one on this preamp .
At first it had a lot of hum. After careful checking I could not identify any ground loop or something and just cured it with a big electrolytic after the PSU.
But now when I put the volume down, when it is very close to the limit it starts to hum like hell. I am using an alps blue pot. When I put the volume up a bit it no longer hums. Any suggestion how to fix this?
Miguel
Ground loop
Maybe it is a ground loop but I can't find it!
For ground I have a very fat piece of wire and all ground connections go to it. Transformer, then first the first regulation cap, then the second, the error amplifier, the pass tube and then the amplifier ground. I believe that now I have the chassis connected after the caps, although I tried other places and with resistors too.
I will try some low value resistors on the volume pot
.
Miguel
Maybe it is a ground loop but I can't find it!
For ground I have a very fat piece of wire and all ground connections go to it. Transformer, then first the first regulation cap, then the second, the error amplifier, the pass tube and then the amplifier ground. I believe that now I have the chassis connected after the caps, although I tried other places and with resistors too.
I will try some low value resistors on the volume pot
. Miguel
You are deeling with a hi impedance circuit so it can get touchy.
I don't have much experiance with pentodes but if is not a ground loop you could also try adding a 100 Ohms resistor from the plate to pins 8 and 1 instead of a direct connection.
Anyway, I feel that other on this thread have more knowledge on this type of circuit so their advise will be more helpful.
I don't have much experiance with pentodes but if is not a ground loop you could also try adding a 100 Ohms resistor from the plate to pins 8 and 1 instead of a direct connection.
Anyway, I feel that other on this thread have more knowledge on this type of circuit so their advise will be more helpful.
While wainting for news of the hum!
Hi folks!
Have you see the schematic in the link that Miguel gave in the first post!!
One more High Hend preamp!!...what i see.!!..
A catode folower ,feeding a comum grid stage,another folower and another folower....
So much processing for a line level amp!
Any coments!
Cheers
Jorge
Hi folks!
Have you see the schematic in the link that Miguel gave in the first post!!
One more High Hend preamp!!...what i see.!!..
A catode folower ,feeding a comum grid stage,another folower and another folower....
So much processing for a line level amp!
Any coments!
Cheers
Jorge
As Frank suggested, I unmounted the pot from the chassis. I turned the preamp on and put the volume in the minimum and the hum was gone! I noticed that when I grabbed the pot with my hand a somewhat loud hum appears. Then I connected a wire between the pot case and the chassis but heard no hum again!! I mounted the pot in the chassis and no hum at all in the zero volume position!!!
But after a minute or so I put the volume in the minimum again and the loud hum came back again. If I tap the pot with my fingers there is some differeces in the hum noise. Maybe there is some deffect in that pot.
Jorge, I am not sure I understand what you mean. I believe I have already the wiper of the volume pot directly connected to the grid of the EF86, one end to earth and the other to the input signal. Can you give some more detailed information about this?
Ground loop:
I hear a low hum in the speakers when the preamp is on. So I do believe some ground loop is present. I have no earth connection in this preamp as the sockets in my house don't have any earth points. So all earth connections inside the preamp go to the 0V on the transformer (except the heaters). The chassis is directly connected to the earth wire at the point of the error amplifier, through a thick wire. No other earth connections from chassis to earth, except the volume pot. I really don't know where else to look for loops.
Miguel
But after a minute or so I put the volume in the minimum again and the loud hum came back again. If I tap the pot with my fingers there is some differeces in the hum noise. Maybe there is some deffect in that pot.
Jorge, I am not sure I understand what you mean. I believe I have already the wiper of the volume pot directly connected to the grid of the EF86, one end to earth and the other to the input signal. Can you give some more detailed information about this?
Ground loop:
I hear a low hum in the speakers when the preamp is on. So I do believe some ground loop is present. I have no earth connection in this preamp as the sockets in my house don't have any earth points. So all earth connections inside the preamp go to the 0V on the transformer (except the heaters). The chassis is directly connected to the earth wire at the point of the error amplifier, through a thick wire. No other earth connections from chassis to earth, except the volume pot. I really don't know where else to look for loops.
Miguel
Persistent hum!
Olá Miguel!
First conect the ground side of the 1M resistor to the some point as the 1,5k and the 1000uF capacitor.
This point must be conected to the ground side of the pot...as the chassis is already hearted don´t conect any wire at the pot case...finaly and more important...put a resistor of 2,2k betwen the midle point of the pot and the grid of the tube..this resistor must be the closest possible to the tube pin!...
Any doubt?
Jorge
Olá Miguel!
First conect the ground side of the 1M resistor to the some point as the 1,5k and the 1000uF capacitor.
This point must be conected to the ground side of the pot...as the chassis is already hearted don´t conect any wire at the pot case...finaly and more important...put a resistor of 2,2k betwen the midle point of the pot and the grid of the tube..this resistor must be the closest possible to the tube pin!...
Any doubt?
Jorge
Olá Jorge,
Let's see if I understood...
All earth connections of the preamp section go to a 5 cm wire that has a connection in the middle to the earth of the PSU. So your first point is already ok. BTW I am not using the 1000uF cap.
Now I have a wire from the pot ground conections to this earth wire of the preamp section - you say to cut these, right?
Third, instead of the direct connection between the middle point of the pot and the grid of the tube I put a 2,2kohm resistor. Is this value important or something like that is ok?
Um abraço,
Miguel
Let's see if I understood...
All earth connections of the preamp section go to a 5 cm wire that has a connection in the middle to the earth of the PSU. So your first point is already ok. BTW I am not using the 1000uF cap.
Now I have a wire from the pot ground conections to this earth wire of the preamp section - you say to cut these, right?
Third, instead of the direct connection between the middle point of the pot and the grid of the tube I put a 2,2kohm resistor. Is this value important or something like that is ok?
Um abraço,
Miguel
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