[R-390] 390A audio

[2002tii]

Bob wrote:

>Electrolytics of any sort are not a real good idea on the diode load
>test point. They leak, and the leakage current can drive the radio a
>little nuts. A 0.1 uf ceramic bypass capacitor is a reasonable
>choice. A 1 uf plastic (mylar or what ever) cap would be better, but
>low leakage comes first and then larger values.

 From the standpoint of sound quality, ceramic capacitors are the 
very worst (much worse than electrolytics).  Use a good film capacitor.

The preamplifier you are feeding needs to have a VERY high input 
impedance, or else the load will produce distortion in the 390A 
detector, so you don't need much capacitance.  For example, if the 
load is 100 k ohms (the absolute very least you should ever connect 
to the diode load point), a 0.1 uF capacitor will give you a -3 dB 
point below 20 Hz.  For higher load impedances, you need even less 
capacitance.  Capacitors much larger than this will only subject your 
amplifier and speaker to unnecessary subsonic thumping as you tune 
around.  Note that many hi-fi preamplifiers have input impedances in 
the 10 k ohm range, and are not suitable for connection directly to 
the 390A diode load.  Older tube preamps may have higher input 
impedances, but not necessarily.

Best regards,

Don