Hi there,
got a strange hum problem with my DUAL 721 turntable.
Good thing first: It plays fine and is absolutely quiet when you lift the cart off the disc during play. No hum.
But: As soon as the signal-shorting switch activates during auto start/stop, it starts humming noticeably in both channels. After switching off, the hum is gone again.
So it only hums when the signal leads are shorted to ground.
Anyone ever seen that behaviour?
Kind regards,
Andreas
got a strange hum problem with my DUAL 721 turntable.
Good thing first: It plays fine and is absolutely quiet when you lift the cart off the disc during play. No hum.
But: As soon as the signal-shorting switch activates during auto start/stop, it starts humming noticeably in both channels. After switching off, the hum is gone again.
So it only hums when the signal leads are shorted to ground.
Anyone ever seen that behaviour?
Kind regards,
Andreas
> shorted to ground.
That's a dirty word.
The cart Hots should short to cart wire Shields.
Not turntable chassis.
I do not know that machine. If the mute switch leaf is riveted to the metal, and the signal is quiet left open, then I might just cut the mute switch wires. But if I could move the "ground" so it connected to signal shield instead of base metal, that would be best.
That's a dirty word.
The cart Hots should short to cart wire Shields.
Not turntable chassis.
I do not know that machine. If the mute switch leaf is riveted to the metal, and the signal is quiet left open, then I might just cut the mute switch wires. But if I could move the "ground" so it connected to signal shield instead of base metal, that would be best.
I don't know about Dual, but on Technics and Pioneer turntables that I have opened that have it, it's a NC switch that shorts the phono leads (hot to shield) when the turntable is off and/or the tonearm is at rest position. If this is the same arrangement in your Dual, then hum is when the phono leads are in fact not shorted.
It is exactly as Rundmaus said: The Dual 721 is an automatic TT that lifts the tone arm from it's rest when the 'Start' lever is pushed, then moves it over the spinning record, then slowly releases the stylus into the groove. At the record side's end, or when pushing the lever to 'Stop', it performs the same procedure vice versa. Simultaneously to these tone arm movements, a dual pole switch shorts the cartridge's both channels' hot and ground connections. As factory standard, both ground terminals are connected to chassis ground also.
@Rundmaus: Does your 721 still feature it's standard DIN cord?
Best regards!
@Rundmaus: Does your 721 still feature it's standard DIN cord?
Best regards!
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Kay, it already had an RCA cord when I bought it 15 years ago...
There is an official procedure for the DIN to RCA conversion by Dual, which removes the connection between L and R grounds and adds a separate ground wire for chassis ground. So if the conversion was done correctly, L ground, R ground and chassis ground should be separated and the short-circuit switch should only connect the signal lines to the respective grounds. Easy enough to check, I'll get my meter and have a look...
Regards,
Rundmaus
There is an official procedure for the DIN to RCA conversion by Dual, which removes the connection between L and R grounds and adds a separate ground wire for chassis ground. So if the conversion was done correctly, L ground, R ground and chassis ground should be separated and the short-circuit switch should only connect the signal lines to the respective grounds. Easy enough to check, I'll get my meter and have a look...
Regards,
Rundmaus
Yes, do it.
Besides these official instructions by the manufacturer, I know many who did aother procedure successfully: They just cut/desoldered the DIN plug and soldered a pair of RCA plugs to both channels' leads.
Funny enough that I have exactly the opposite issue with my Dual 1229 turntable in my Wega 3220 console - slight hum when the signal is not short circuited. Still have to search for the cause, maybe even by swapping the TT with another one. We'll see.
Best regards!
Besides these official instructions by the manufacturer, I know many who did aother procedure successfully: They just cut/desoldered the DIN plug and soldered a pair of RCA plugs to both channels' leads.
Funny enough that I have exactly the opposite issue with my Dual 1229 turntable in my Wega 3220 console - slight hum when the signal is not short circuited. Still have to search for the cause, maybe even by swapping the TT with another one. We'll see.
Best regards!
Crazy world. Go home physics, you're drunk.
Did some further testing, here are the results:
1. The turntable wiring seems correct. The L and R grounds are not connected to each other or to chassis ground.
2. The amplifier is not the culprit. It's dead quiet with both inputs shorted to the respective grounds.
3. If I leave the separate ground wire for the turntable chassis unconnected, there is a loud hum in playback mode. With the short-circuit switch activated, it is dead quiet.
4. If I connect the separate ground wire, the turntable is dead quiet in playback mode, but there is noticeable hum with the short-circuit switch activated.
5. WTF?
Did some further testing, here are the results:
1. The turntable wiring seems correct. The L and R grounds are not connected to each other or to chassis ground.
2. The amplifier is not the culprit. It's dead quiet with both inputs shorted to the respective grounds.
3. If I leave the separate ground wire for the turntable chassis unconnected, there is a loud hum in playback mode. With the short-circuit switch activated, it is dead quiet.
4. If I connect the separate ground wire, the turntable is dead quiet in playback mode, but there is noticeable hum with the short-circuit switch activated.
5. WTF?
could be a ground loop...........i guess.
when you play back the record,the shielding and the - (minus phase of the cartridge) meet only at the phono input of the amp (screw nearby the input).
if you activate the muting switch there is a second connection between shield and minus ........and you can have a closed loop ( cartridge - to amp input and back along the shielding wire through the switch to cartridge - again......)
put the wires close together or so what , perhaps it helps.
it is only a litte flaw i think.
greets
when you play back the record,the shielding and the - (minus phase of the cartridge) meet only at the phono input of the amp (screw nearby the input).
if you activate the muting switch there is a second connection between shield and minus ........and you can have a closed loop ( cartridge - to amp input and back along the shielding wire through the switch to cartridge - again......)
put the wires close together or so what , perhaps it helps.
it is only a litte flaw i think.
greets
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