Question on output hum on tube amp?

[andy2]

I wonder how high output amplitude would cause a noticable hum?

At 60hz, what would the voltage amplitude do you think would begin to give an audible hum?

Above 10mv?

[bembel]

P=U²/R ! , just figure out how much dB vs signal

hope this helps !

PS: In silence, as a rough guess I wouls say near audible with my ears (as 10mV@8ohm= 10µW). though I didn't measured

[andy2]

Originally posted by bembel
P=U²/R ! , just figure out how much dB vs signal

hope this helps !

PS: In silence, as a rough guess I wouls say near audible with my ears (as 10mV@8ohm= 10µW). though I didn't measured

So I guess your guess would be if you place your ear close enough, you will probably here it.

[bembel]

yes, indeed

PS: just try to send an AF generator to a speaker to estimate (@60Hz or 120Hz for you) and just see ... (haven't time to do it now, but if you don't have a generator I can do it for you somewhere during the week).

[andy2]

Originally posted by bembel
yes, indeed

PS: just try to send an AF generator to a speaker to estimate (@60Hz or 120Hz for you) and just see ... (haven't time to do it now, but if you don't have a generator I can do it for you somewhere during the week).

I would appreciate it very much since I don't quite have the equipement.
Thanks.

[andy2]

By the way, one more quick question, would 10 - 13mv of output noise be acceptable in a tube amp?

[bembel]

have to measure, it depends, do you mean output on 8 ohm load, if not, then witch kind, RMS @ 60Hz ? How measured ?

[andy2]

Originally posted by bembel
have to measure, it depends, do you mean output on 8 ohm load, if not, then witch kind, RMS @ 60Hz ? How measured ?

Measured @ 8ohm peak to peak @ assuming 120Hz.

Thanks.

[SY]

OK, so that's about 3mVRMS, about -60dB below 1W. That's probably OK for speakers of normal efficiency, but I'd want to do better, especially if horns or high eff single drivers were being used.

[kevinkr]

I traditionally design my personal amplifiers (where cost is not a consideration) for 1mVrms or less of residual noise including hum/ripple and hiss. This is acceptable in my experience with speaker systems to 103dBspl or so.

My current diy Onken speaker system is >96dB efficient and 300B based SE amplifier output noise is <0.5mVrms, obviously the noise is just about inaudible.

My noisiest design is around 3mVrms and with over 20Wrms per channel was designed for speakers in the low 90dB range. Hum was not obtrusive with a pair of high efficiency speakers (100dB+) but was audible during silent passages from a meter away.

I don't use global negative feedback in any of my designs so this has to be achieved by careful layout and good power supply design.