I'm beginning to wonder about that. I'll pull a cd player out later and use that instead of my laptop. I'd still expect muting to work, even with no input at all though.
Still no change using a cd player. Voltage at the anode of D607 & D608 remains at -6.6V regardless of whether the emitter and collector of Q602 is shorted or not.
Logically, if I lift R613 out, the amp should stay stuck in mute
Logically, if I lift R613 out, the amp should stay stuck in mute
Using min/max on my meter, I got a peak of 17.96V which quickly settles down to 0.1V. It peaks at switch on and switch off. At no point did I see 0V.
How can you get that much voltage if the head unit is grounded?
Confirm that you read 0 ohms between the shields and the case of your head unit.
Confirm that you read 0 ohms between the shields and the case of your head unit.
Could the use of mains powered kit as the source be the cause of my problems?
I checked the cd player (tascam CD-RW901) and the RCA shields were definitely 0 ohms to the case of the unit and the mains earthing pin.
Would running a head unit off the same supply as the amp be the best solution?
I checked the cd player (tascam CD-RW901) and the RCA shields were definitely 0 ohms to the case of the unit and the mains earthing pin.
Would running a head unit off the same supply as the amp be the best solution?
Mains powered equipment (in the US) is isolated from ground unless it has a 3-pin power plug. Even then, the audio may have isolation.
For now, simply connect the input RCA shields of the amp to the primary ground on the amp. Then it shouldn't make a difference what you use as long as the shield ground in the signal source is intact.
A normal head unit with its ground connected to the power supply feeding the amp would do this. This is what amplifiers expect to see as a source.
For now, simply connect the input RCA shields of the amp to the primary ground on the amp. Then it shouldn't make a difference what you use as long as the shield ground in the signal source is intact.
A normal head unit with its ground connected to the power supply feeding the amp would do this. This is what amplifiers expect to see as a source.
That seems to have solved it. I'll have to do this as standard practice from now on as floating input grounds seem to cause all manner of issues.
It mutes ok both at switch on and switch off but it does seem to un-mute as the caps discharge after switch off which allows some noise through the speakers.
It mutes ok both at switch on and switch off but it does seem to un-mute as the caps discharge after switch off which allows some noise through the speakers.
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