Hi all,
I've been listening to my 2A3 amp for about a year, and love it. But I recently plugged my TV into it to watch a film, and the hum is unbearable for movies because of all the silence in films.
I just wondered what the typical hum figures are for these types of amps.
I have 10.8mV left and 10.6mV right at the speaker outputs, when loaded. Is this more than should be expected from sovtek 2A3's?
I know its originating in either the 2A3's or the output transformers and reckon this is typical filament hum, so I thought I'd just check.
It's 100Hz and is there whenever I switch the power on. It remains when I pull the rectifier, so it must be the AC filaments right?
It also remains when I pull the driver tube, and only disappears when I pull the 2A3's. It is an octave above the 50Hz hum I get when I wind the hum pots to one end.
Any information would be greatly appreciated, if only to put my mind at rest.
Thanks, Bob
I've been listening to my 2A3 amp for about a year, and love it. But I recently plugged my TV into it to watch a film, and the hum is unbearable for movies because of all the silence in films.
I just wondered what the typical hum figures are for these types of amps.
I have 10.8mV left and 10.6mV right at the speaker outputs, when loaded. Is this more than should be expected from sovtek 2A3's?
I know its originating in either the 2A3's or the output transformers and reckon this is typical filament hum, so I thought I'd just check.
It's 100Hz and is there whenever I switch the power on. It remains when I pull the rectifier, so it must be the AC filaments right?
It also remains when I pull the driver tube, and only disappears when I pull the 2A3's. It is an octave above the 50Hz hum I get when I wind the hum pots to one end.
Any information would be greatly appreciated, if only to put my mind at rest.
Thanks, Bob
Thanks SvErD,
The problem is the same whatever the source, or even without one, so I don't think the TV is the problem.
Bas Horneman,
Yes, the amp is home built. Where should I look first for ground loops?
It's such a simple circuit (JE Labs/Angela 6SL7/2A3) and as far as I can tell there should be no ground loops.
It is however, built on an aluminium chassis. Could the ground plane resistance (one ohm end to end) play a role in this?
Thanks
The problem is the same whatever the source, or even without one, so I don't think the TV is the problem.
Bas Horneman,
Yes, the amp is home built. Where should I look first for ground loops?
It's such a simple circuit (JE Labs/Angela 6SL7/2A3) and as far as I can tell there should be no ground loops.
It is however, built on an aluminium chassis. Could the ground plane resistance (one ohm end to end) play a role in this?
Thanks
Hi Bob,
I'm still learning about grounding..it's not easy. What does your current grounding scheme look like? Star ground has worked well for me...except in the last amp which I built for someone else...someone else took over and fixed my mess... and made a seperate star earth for signal and psu and joined the two star earths..worked better...because you don't want large currents near small currents...
I'm still learning about grounding..it's not easy. What does your current grounding scheme look like? Star ground has worked well for me...except in the last amp which I built for someone else...someone else took over and fixed my mess... and made a seperate star earth for signal and psu and joined the two star earths..worked better...because you don't want large currents near small currents...
Bas,
Every component which requires grounding is grounded by it's lead (or short wire) directly to the chassis, at the closest socket or transformer mounting nut.
The chassis layout is (from left to right):
Mains In
Mains Tx
Mains switch and fuse
Rectifier
Choke and HT caps
Speaker terminals
Output Tx's
2A3's
6SL7's
Volume and inputs
Its fairly conventional in order to eliminate such problems.
Every component which requires grounding is grounded by it's lead (or short wire) directly to the chassis, at the closest socket or transformer mounting nut.
The chassis layout is (from left to right):
Mains In
Mains Tx
Mains switch and fuse
Rectifier
Choke and HT caps
Speaker terminals
Output Tx's
2A3's
6SL7's
Volume and inputs
Its fairly conventional in order to eliminate such problems.
He..he Bob ...not mocking you..seriously...
I build my stuff in wooden enclosures..that way one is ensured that where you want earth to be that is earth.
A tip by someone was also to have the transformers isolated from the topplate except.
At a glance I see no problems.. Are the RCA plugs isolated from the chassis?
An externally hosted image should be here but it was not working when we last tested it.
I build my stuff in wooden enclosures..that way one is ensured that where you want earth to be that is earth.
A tip by someone was also to have the transformers isolated from the topplate except.
At a glance I see no problems.. Are the RCA plugs isolated from the chassis?
Two resistors make up the divider network, lower resistor bypassed with 1uF capacitor to ground.
76.5v reference to one side of heater winding only.
As I originally said though, I can pull the 6SL7's and the hum remains the same so I don't think this is where the hum is being introduced.
Any other idea's?
76.5v reference to one side of heater winding only.
As I originally said though, I can pull the 6SL7's and the hum remains the same so I don't think this is where the hum is being introduced.
Any other idea's?
Bob,
You said your grounds were by wire to the closest screw or bolt connected to the chassis. I think I would do that a little differently. Keep in mind I'm a relative newbie having built only five or six tube amps and never a DHT amp.
So... here is my general idea of grounding.
1- Connect the mains earth wire to the chassis near the point where the mains enter the chassis.
2- Ground all the power components to a single point (not to the chassis).
3- Ground all the signal components to a different single point (not to the chassis).
4- Run separate heavy gauge (12 to 16 AWG- sorry, don't know the mm equivalent in millimeters) copper wires from each ground point back to the chassis ground. I tend to use solid core for this but have also used stranded.
5- Use isolated RCA jacks for the input and don't ground them to the chassis but to the 'signal ground point'.
6- Use shielded wire to connect the RCAs to the volume pot (or input selector if you have one) and from the volume pot to the input tube.
Ground loops can definitely be caused by the chassis. The voltage potential between two points on the chassis with all the components mounted and powered up can be significant. By not using random points all over the chassis for grounding you can reduce the loops.
You said your grounds were by wire to the closest screw or bolt connected to the chassis. I think I would do that a little differently. Keep in mind I'm a relative newbie having built only five or six tube amps and never a DHT amp.
So... here is my general idea of grounding.
1- Connect the mains earth wire to the chassis near the point where the mains enter the chassis.
2- Ground all the power components to a single point (not to the chassis).
3- Ground all the signal components to a different single point (not to the chassis).
4- Run separate heavy gauge (12 to 16 AWG- sorry, don't know the mm equivalent in millimeters) copper wires from each ground point back to the chassis ground. I tend to use solid core for this but have also used stranded.
5- Use isolated RCA jacks for the input and don't ground them to the chassis but to the 'signal ground point'.
6- Use shielded wire to connect the RCAs to the volume pot (or input selector if you have one) and from the volume pot to the input tube.
Ground loops can definitely be caused by the chassis. The voltage potential between two points on the chassis with all the components mounted and powered up can be significant. By not using random points all over the chassis for grounding you can reduce the loops.
I built my JE Lab 2A3 monoblocs with very minimal hum. It's less than 2mVp-p.
I had some buzz noise when the amp was connected to the Cable TV box (even with it switched off). The ground from the Cable TV box causing it to buzz or hum a lot. Finally, I had to isolate the ground wire from the Power Sockets in the 2A3 amps. The system ground will now be connected from the amps to the cable box, then to the cable wire's grounding.
Did you try to remove all the sources cables and see if it still hums???
Johnny
I had some buzz noise when the amp was connected to the Cable TV box (even with it switched off). The ground from the Cable TV box causing it to buzz or hum a lot. Finally, I had to isolate the ground wire from the Power Sockets in the 2A3 amps. The system ground will now be connected from the amps to the cable box, then to the cable wire's grounding.
Did you try to remove all the sources cables and see if it still hums???
Johnny
Bas,
If the tubes are pulled, then there is no current flowing through the heater wires, and since the wires are twisted together, how could this influence the output?
Yes, the RCA's are connected to the chassis, not isolated, but again, how can a ground loop be caused when the rest of the signal path is broken by pulling the tubes?
Sherman,
In hindsight I should perhaps have arranged the grounding differently, but I'll try and isolate the problem first before rewiring the grounds, as this might not be the problem. Thanks for info though, as it will be useful if I have to rewire it.
What could cause hum on the speaker outputs with the input tubes AND rectifier pulled out?
Thanks for the input.
If the tubes are pulled, then there is no current flowing through the heater wires, and since the wires are twisted together, how could this influence the output?
Yes, the RCA's are connected to the chassis, not isolated, but again, how can a ground loop be caused when the rest of the signal path is broken by pulling the tubes?
Sherman,
In hindsight I should perhaps have arranged the grounding differently, but I'll try and isolate the problem first before rewiring the grounds, as this might not be the problem. Thanks for info though, as it will be useful if I have to rewire it.
What could cause hum on the speaker outputs with the input tubes AND rectifier pulled out?
Thanks for the input.
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