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reliability and replacement of Mag ribbon tweeters?
chung wrote in message ...
wrote: The difference is that there may be little or no resistance in that path, whereas in your case there is the attenuator in the path. In my box, the attenuators are not in the tape loop. The input signal from each source splits. One branch goes to the tape loop, the other goes through the attenuators to the power amp. The tape loop device is in parallel with the attenuators and power amp. But even though they're separate branches of the signal, they're not isolated from each other so clamping in one branch could affect the signal in the other branch. As I said earlier, the likely culprit is the analog switch or op-amps, or protection diodes on the PCB, in your recorder. The analog switch is typically a pair of CMOS transistors. When no power is applied to the unit, these switches act like diodes to the supply voltages, which are at ground level. Therefore you get the clamping action, meaning gross distortion starting at a few tenths of a volt. That was why when you were listening via headphones, you did not hear the distortion, since the level is much lower. Actually I did hear the distortion on headphones. I was using HD-580s which are relative inefficient 300 ohm phones so the voltage needed to drive them is a little higher than most other headphones. On the speakers, the distortion wasn't "gross" or obvious. Several different people listening to the system never heard it. It was audible only on certain well recorded acoustic music passages, during big dynamics. But it's true that low level signals did not produce the distortion. When I used my portable CD player as a source device, the distortion wasn't there. The line out of the CD player is only 0.7 volts instead of the 2.05 volts of my Rotel. The fact that you have a passive attenuator makes the distortion fairly obvious, but even if you have an active preamp, you will still have the problem, since those diodes clamp the voltage going to the power amp. You need to leave the recorder powered up, or switch it out so that the preamp does not see it, using a mechanical switch when the recorder is powered off. RS and Sony both sell simple mechanical switches for very little money. Or you can build your own, using a pair of mechanical switches from RS. Yes and active circuits can veil subtle distortions. When I ran the signal through a high quality headphone amp the distortion was still audible, but not quite as easy to hear as it was through the passive preamp. Even a preamp that measures well and sounds clean and neutral still veils the sound slightly compared to a straight wire. |
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