Hello Everyone,
I am having some difficulty with hum in a push pull amplifier I am working on. The amp powers up correctly, the B+ voltage reads about 10 volts above what I modeled during my design calculations, and everything seems OK with respect to component temperatures, capacitor polarities, etc. -- i.e. I didn't get electrocuted and the amp didn't catch fire.
But, there is a nasty hum present once I went to attach speakers (although I can hear a signal alongside the hum if I apply one).
If I ground the input grid to the first voltage amplifier stage (plate resistor loaded 12AX7) by turning the input volume potentiometer all the way to zero, there is no hum. The signal lead is taken off of the wiper, so turning up the pot increases the resistance between grid input and ground. As anticipated, the hum volume audibly increases as I turn up the volume pot. Can I interpret this as meaning that the hum is originating by some unwanted noise being imposed on the input stage's grids? Both channels exhibit similar hum.
I'm trying to rule out heater current inducing hum insubsequent stage's grid connections, tube imbalance, transformer shielding, etc., by virtue of the fact that grounding the first input stage's grid eliminates the hum.
Before I tear everything apart, some details about the circuit from input to 12AX7 grid:
-1k grid stopper
-100k volume pot, signal taken from the wiper
-0.2 uF film capacitors to attenuate DC
-26 AWG shielded cable from RCA jacks to input selector. Shields are connected at one end to the aluminum chassis. Cutting the shield connection didn't help anything.
-Both input selector and volume pot bodies are earthed by virtue of their contact to the aluminum chassis. Ditto the RCA jack's body. Initial signal inputs were with an iphone not connected to its charger. The hum is present without any input device connected to the RCA jacks.
-All signal grounds join at a single bus, which is then connected to a grounded bus in the power supply turret board. This is connected to IEC neutral. I am using an IEC plug with the earth connected to the chassis and no connection between chassis and neutral.
Any suggestions?
I am having some difficulty with hum in a push pull amplifier I am working on. The amp powers up correctly, the B+ voltage reads about 10 volts above what I modeled during my design calculations, and everything seems OK with respect to component temperatures, capacitor polarities, etc. -- i.e. I didn't get electrocuted and the amp didn't catch fire.
But, there is a nasty hum present once I went to attach speakers (although I can hear a signal alongside the hum if I apply one).
If I ground the input grid to the first voltage amplifier stage (plate resistor loaded 12AX7) by turning the input volume potentiometer all the way to zero, there is no hum. The signal lead is taken off of the wiper, so turning up the pot increases the resistance between grid input and ground. As anticipated, the hum volume audibly increases as I turn up the volume pot. Can I interpret this as meaning that the hum is originating by some unwanted noise being imposed on the input stage's grids? Both channels exhibit similar hum.
I'm trying to rule out heater current inducing hum insubsequent stage's grid connections, tube imbalance, transformer shielding, etc., by virtue of the fact that grounding the first input stage's grid eliminates the hum.
Before I tear everything apart, some details about the circuit from input to 12AX7 grid:
-1k grid stopper
-100k volume pot, signal taken from the wiper
-0.2 uF film capacitors to attenuate DC
-26 AWG shielded cable from RCA jacks to input selector. Shields are connected at one end to the aluminum chassis. Cutting the shield connection didn't help anything.
-Both input selector and volume pot bodies are earthed by virtue of their contact to the aluminum chassis. Ditto the RCA jack's body. Initial signal inputs were with an iphone not connected to its charger. The hum is present without any input device connected to the RCA jacks.
-All signal grounds join at a single bus, which is then connected to a grounded bus in the power supply turret board. This is connected to IEC neutral. I am using an IEC plug with the earth connected to the chassis and no connection between chassis and neutral.
Any suggestions?