Question here.. some time ago, I (believe that I) damaged an AC-powered MP3 player by connecting it to the line-in of a vintage Fisher integrated (Philharmonic IV).
What I believe happened, is that the Fisher had a 2-wire cord (with the polarity backwards..) and both a small value cap and high value resistor tied from one side of the AC line to chassis.
This had the chassis at 120AC - albeit at a fairly high impedance.. classic "death cap" issue. When I connected the MP3, that 120AC leakage was enough to fry the player. Make sense?
So I've switched to a 3-wire cord, ditched the cap & resistor. I've also checked the input caps for the AUX input to verify there is no D.C. leakage present.
Is there anything else I need to do before I connect a DAC / MP3, etc. so nothing is damaged? I think the bases are covered, but just want to be absolutely sure.
Thanks!
What I believe happened, is that the Fisher had a 2-wire cord (with the polarity backwards..) and both a small value cap and high value resistor tied from one side of the AC line to chassis.
This had the chassis at 120AC - albeit at a fairly high impedance.. classic "death cap" issue. When I connected the MP3, that 120AC leakage was enough to fry the player. Make sense?
So I've switched to a 3-wire cord, ditched the cap & resistor. I've also checked the input caps for the AUX input to verify there is no D.C. leakage present.
Is there anything else I need to do before I connect a DAC / MP3, etc. so nothing is damaged? I think the bases are covered, but just want to be absolutely sure.
Thanks!
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AudioWize's advice is sound. Apart from that, I can't think of anything further that must be done. ⋅-=≡ GoatGuy ✓ ≡=-⋅
Yes; it happened to me before. Your modification should avoid this for the future. But some tube designs also generate voltage spikes on the RCA connectors at power on/off that may damage the solid state device. Transformer coupling plus a voltage transient limiter (zener diode) is a good option to interface unknown/marginal tube-based devices at line level.
There is a Blueglow Audio video on YouTube describing how to do this. I think it was part of his Mac MC30 series. He also has one on removing the infamous "Death Capacitor" from the transformer primary circuit.
I guess this was considered "safe" because the chassis was internal to the console and should not be exposed to touch? I looked at the schematic and you could see the cap and resistor running to the chassis ground. Was the 2 wire cord even polarized?
Yes, the 2-wire cord was polarized and appeared original. I've seen plenty of 2-wire chassis with a grounding cap but not also a resistor. The values are such that the short-circuit current is very low, safe in most conditions.. and yes, the chassis is insulated by the console case -but- I'll bet the turntable deck was common to chassis?
Also yes, the 3rd wire is bolted firm to chassis now. Kind of the whole point, really.
Btw, these are really nice, warm, vintage sounding units with excellent FM stereo. You do need to convert the outputs from the near-impossible ELL80s to a pair of EL95s per channel, though.
Also yes, the 3rd wire is bolted firm to chassis now. Kind of the whole point, really.
Btw, these are really nice, warm, vintage sounding units with excellent FM stereo. You do need to convert the outputs from the near-impossible ELL80s to a pair of EL95s per channel, though.
Good question.. The chassis is insulated by the wood cabinet and plastic knobs. But the faceplate is aluminum, so if there's any galvanic contact the faceplate could be live AC. Same for the metal parts of the turntable, which is likely the entire frame & plinth, maybe even the arm.