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In Reply to: Re: Cary 300 SEI with HUM in Noise Floor posted by daleda on March 10, 2007 at 13:11:11:
I have a similar "issue" with my moth s2a3. That is just the way it is. The measured hum level on the 2a3 is .15mV, which is absolutely dead silent on my cicada speakers, but I can hear a slight residual hum through headphones. And yes, my lower 32 ohm grados pick up more of it than the 300 ohm sennheisers. You have to understand that when you couple transducers directly to your ears that require milliwatts of power, any presense of noise or hum will most definitely be picked up. The only way to remove the last bit of residual hum is to outboard the power transformer. There is another version of the moth 2a3 that has the transformer outside the main chassis, known as the si2a3H. Craig specifically made that model due to the higher demands of headphone usage for noise and hum levels.The hum in my situation is neglible, and when music plays, it cannot be detected. In fact the hum level is simply below the noise floor of most recordings that it should not really be an issue whatsoever. Is the hum impeding on your musical enjoyment? If not, don't worry too much about it, the simple fact is the power transformer is simply too close to the output transformers, its bound to pick up some hum.
Follow Ups:
If the issue is sensitive headphones, especially low-impedance phones, some attenuation between the phones and the amp will help a lot.There's a standard for headphone outputs, which is 5vRMS at 120 ohms source impedance. Headphones are usually designed to work correctly from this source, though many are also designed for low-impedance sources as well. Unfortunately, many amps with headphone jacks do not include the 120 ohm resistance. A 120 ohm resistor in series with 30 ohm phones will reduce the output - and any hum or buzz - by 14dB. In my experience, a 120 ohm resistor will make most phones about equally loud, whatever their impedance between 30 and 600 ohms. 5 volts into 8 ohms is about 3 watts, so for a more powerful amp you can attenuate the output even further with an L-pad, and retain the 120 ohm output impedance.
If an amp is working correctly, i.e. does not have some unusual hum or buzz due to a noisy electrical environment or an internal malfunction, this is where I would start.
I am not sure about that model, but I could have sworn the cary uses a 240 ohm resistor on the output to nullify any hum from the transformer. But it appears in this case that the output resistor is simply not there or was not installed in the 1st place. The only issue with raising the output impedance is that the bass would lose control and sound rubbery. Also, there would be no damping applied to the headphones, thus relying upon the headphone itself to be very well self damped, which in my experience, most headphones aren't.
I don't have a broad enough experience with headphones to comment, but it does not surprise me to hear that some headphones sound better with a higher damping factor. Since there is a standard (unlike speakers, alas!), it's my opinion that this is a headphone design problem, not an amp design problem. Anyone is free to have a different opinion of course!Having said that, I also think the standard is gettng out of date. It works well for headphones connected to power amps, because it equalizes the sensitivity of different phones. But many - possibly most? - headphones of modern design are aimed at i-Pods, where a low impedance phone will extract all the power that is available and it's reasonable to assume a low source impedance.
Really, there are two kinds of headphones: those designed for home stereo and those designed for portable players. Both kinds try to sell to both markets, but the requirements are quite different. Confusion is rampant. The ultimate headphone amp would have to cater to all of them, meaning impedance taps from 16 ohms to 600 ohms, and covering a sensitivity range of something like 30dB. What a mess!
That is pretty much what I have concluded. The folks at Cary insist it's the tubes (which clearly it is not). I'm a big believer in their products and am very happy with the 300SEI's
performance (other than this pesky problem). I only notice it between tracks - other than that - it is a non-issue. Thanks for the response.
I had Cary 300SE's years ago and the trannies hummed. I call Cary and Kirk said, "they do that".Les
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