Hi

I have a "technical" question about an amp.
Well, I have a Sony TA-FE330 and, interestingly, when I turn it off the headphones continue to go on for a couple of minuites - they fade out back in and then go off (the amp has been off for a good 7 seconds or so). Any ideas on that one?

Secondly, i think this is to do with ohms - When i have my headphones in-i know it is normal to have to turn the volume up on the amp a "touch" but with this you have to turn it from say 2quater to nine" exactly (which is pretty loud throguh the speakers, but I have to turn it to "8.59" as it were which is quite a bit for the headphones, anyone know why?

Cheers
Alex

Could be something to do with your headphones not being sensitive or something.

The 'fade out and back in and off' thing is sometimes found in large PA systems after the amp has been turned off when the (i think) capacitors or transistors or some other electronic component in the amp drains. I think thats right, couldn't remember it properly, but anywho.

The first part is due to the power supply in the amp. It will have at least a couple of large capacitors whose primary purpose is to aid conversion of the AC mains to the DC voltages needed by the amp's electronics.

They are in effect storage devices for electrical charge, as anybody who has had the misfortune to connect themselves across a fully charged one will testify...

The "fade out" you describe is simply the charge on these capacitors draining away. You may not hear the same effect on the speakers because the amp probably has protection relays that disconnect the speakers when the amp powers off.

The second bit is that headphones don't require the kind of power needed to drive a loudspeaker so the feed to the headphones is attenuated or may even be driven by a separate low power amp. Obviously there is no way for Sony or whoever to know what kit you will use with the amp so have to "guess" how much attenuation to use. Most manufacturers err on the side of caution and bump the level down quite a lot to save you from going instantly deaf at full volume.....

Couple that with the fact that the drive units in headphones are less efficient at converting electrical energy into sound energy in the main and it's no surprise you have to adjust the volume upwards.

Nearly all amplifiers have a separate circuit for headphones which uses purpose made integrated circuits (costing about 5p each) similar to those built into CD players and tuners. It is a lot cheaper for them to do that than switch and then attenuate the high power from the speaker circuits.

Unless using very efficient 'phones, they will nearly always sound distorted at higher volumes. I've noticed this even with £1,000 amps.

If you have decent headphones then it is always worth buying a separate headphone amplifier to drive them.

True, but is/could be poor circuit design or a small fault thats developed.

You only need a 'bleed' resistor to prevent this effect. After all, there is plenty of amps that do not do this.

StereRowe