Great information Elizabeth. Thank you and I'll give your recommendations a try. Even with the hum, it is a sweet sounding preamplifier, so I wan't to keep it in use.
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Jmho, it’s not DC offset on the AC mains. I agree it could be a ground loop hum.
For a test disconnect all source equipment interconnects from the inputs of the preamp. If you have a sub also disconnect any interconnect/s from the preamp.
Plug the preamp and power amp directly into the same AC wall duplex outlet.
Check for the hum and post back your results.
Oh, and it produces a thump when the amp and preamp are turned off together (I don't do this anymore).
That's not unusual.
Amp off first wait about 45 to 60 seconds then shut off the preamp. If you still hear a slight thump wait a little longer after shutting off the amp before you shut off the preamp.
See if you can open this. There is a little button cap right across the wall power in the preamplifier. If it is a capacitor, I'm hoping this is the issue. What do you think?
Listen to the 60Hz Hum and then the 120Hz Hum. Does either of them sound like your hum?
Also I forgot to ask you earlier, does the hum vary with the volume control?
The Audire amplifier and preamplifier are both old enough to have just a two prong electrical plug.
Do you have a multimeter?
I assume the AC power plugs on the preamp and power are not polarized? They will plug into the receptacle outlet in either direction.
If you have a multimeter you should measure the voltage from the metal chassis of each piece of equipment to the ground contact of the wall outlet.
** Interconnects must be disconnected/removed from the equipment.
With the equipment powered up measure the AC voltage from the ground contact on the wall outlet to the metal chassis. Make note of the AC voltage reading. Turn off the equipment wait a few minutes then unplug the piece of equipment, being tested, and rotate the plug 180 and plug it back in. Turn it back on and measure the voltage from the ground to the chassis again. The lowest voltage reading of the two readings is the correct plug AC polarity orientation for the primary winding of the power transformer of the piece of equipment tested. Mark the plug for identification. Test both pieces of equipment.
Reconnect the interconnects from the preamp to power amp. Plug in the preamp and power amp plugs with the correct identified plug orientation. Check for hum.
True. Except the OP said the hum sounded like a 60HZ hum and not that of a 120HZ hum. I assume the equipment has full wave rectification.
Also as a rule electrolytic caps don’t fail all of a sudden.
True if we were dealing with an AC balanced power system where both AC lines to ground were 60Vac each. We are not though. Or an isolated power system where both AC lines were floating above ground. But we are not. We have a hot to ground potential that measures 120Vac nominal to ground and a neutral conductor that is at ground potential.
The way the primary winding around the core of the equipment power transformer is wound is one lead of the winding is closer to the core than the other. The proper AC polarity orientation will yield a lower capacitance coupling voltage to the secondary winding than the other.
As for the voltage readings from the chassis to ground I have seen the voltage as high as 90Vac when the polarity is reversed. Current is quite low though.
(In reality you are actually reading from the chassis to the neutral conductor. The branch circuit equipment grounding conductor is connected to the main electrical service neutral conductor in the main service electrical panel. Or service equipment panel.)
I tried to find a wiring schematic diagram for the amps with no luck. Depending on how old the amps are it was not uncommon to install a capacitor between one of the AC power lines to the metal chassis of audio equipment back in the old days. In the 1970s Marantz for one installed a resistor from one of the AC power lines to the chassis. In either case the chassis voltage to ground would be lower if the capacitor or resistor was on the AC mains neutral conductor.
IF,.... a capacitor was used the cap could be going bad and leaking,passing, AC voltage to the chassis. In this case we may not only have an AC voltage but current as well.
It won’t cost the OP a dime to preform the AC polarity test, other that a little bit of his time.
Make certain the power cords are plugged in polarized. The Half of the cable with the grooves is the negative; the smooth one, the positive, i.e. small hole. I had a Diffet 1 and a Model 2. Hum can be caused by nearly invisible corrosion on the internal wiring, especially at the neutral. Don't ever run an input to this Amp more than 3 feet long. It picks up RF waves from radio broadcasts. As a real Rocket Scientist for the Apollo missions, Julius did not originally owner misuse. That is why the Model 2 got clipping indicators added. He also did not consider that people could plug in things backwards, and added the three prong, grounded plug with the Forte.