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After exhaustive searches here - I've decided to post. I use this SET amp with headphones only (Senn HD650 w/ Mobius 300 ohm and JVC/Victor DX1000 60 ohm cans). There is the very annoying mechanical hum in the noise floor - more noticeable with the lower ohm cans). I've checked all tubes - I use WE (new) 300B and Tung Sol Black Plate 6SN7 driver and Sylvania Chrome Top 6SN7W Short Bottles. This baby even went back to Cary recently for a bias resistor that blew - and while there checked for this hum - and it returned with, you guessed it, the same mechanical hum. I've hooked up a small set of monitors and the same low-level noise is still there also. I've made sure the transformers are tightly screwed down and pretty much traced down every possible source. I use a high grade PC into an Audio Magic Stealth PC (or even straight into the wall) - hum still there. I have seen multiple posts that this is just the SET Faustian bargain - and some inmates have gone as far as to just sell the damn thing! I'm almost there - even thought I must admit to a midrange and dynamic audio spectrum unmatched to these old ears. Cary has been most unhelpful - and don't seem to want to admit a design flaw (even though there are many, many complaints as to the contrary. Any input would be much appreciated before she goes to Agon soon (a last resort).
Follow Ups:
sorry it did not work out for you. I've never had a problem with my 300B SEI which I also use strictly as a headphone amp.
Does the level of hum change if you turn the volume control? What about if you short the inputs to ground? Do you have more than one source connected to the amp?
The volume control has absolutely "no effect" on the hum - at 0 or 100 - still the same
(does not decrease or increase).! The only source to the amp is my Xindak SCD-2 and it is dead quiet. I use HT Pro Silway MKIII IC's. I haven't tried shorting the inputs to ground - just have them covered with Cardas Caps for stray RF.
1. Do you have any dimmer switches or flourescent lights in the listening room? If so, they could send a "buzz" into the system
2. Your interconnect cables could have a compromised connection and that could couse some buzzing from the speakers
3.If the buzz is in the amp and not the speakers, you could try a power line filter. They can quiet transformers on ocassion.
I do have halogen lights under the kitchen cabinets - but I always have them turned off (and even on, they don't seem to effect the rest of my system (which is black). I've changed IC's and still no luck. There is a slight "hum" in the power transformers (but Cary has admitted "they do that" as stated below in another response). I'm beginning to believe this is the culprit - but no way to relieve the situation. I use an Audio Magic Stealth PC (which works as advertised with the rest of my system).
A friend of mine used to have a pair of big/impressive looking 211 special mono-blocks and quite happy with it....A Class A amp from XXXXXXphile magazine. One day a friend of mine(& his) brought his DIYer 45 amp. to his place and compared with the gaint 211 special....Wow !!!! The tiny 1.5watt 45 amp. walks all over the 211 ! It sounds better and have better bass control than the Cary!!! The CARY was dumped the next day !!! A few years before this incident. I was in an audio shop listened to a pair of JBL Hartfield and all Cary system...tube preamp./2A3 mono-block or something. The sound was really dead, flat and not interesting. I thought the store didn't how to do the set-up back then but the recent 211 Special/45 comparison confirmed that it was the Cary. I just wondered how the 211 Special got into the Class A catagory in the XXXXXXphile in the first place!
What do you mean by the term "mechanical hum"?If you mean the chassis is vibrating, then there are a few things that migh help, like rubber-mounting the pwer transformer, reducing the power line voltage if it is on the high side, cleaning up the power if it is not a very good sinusoid, switching to sealed headphones, etc.
But from your post, I wonder if you mean a hum voltage at the output terminals which is audible in the headphones. In that case, the mitigating approaches are quite different. The first step is to measure the hum voltage across the output terminals. The second step is to identify the cause of the hum - this may take some experimenting and measuring, but it is possible and many have resolved problems of this nature.
It is not ground loop type hum - no CATV anywhere in sight. It is an "audible buzzing" sound in the noise floor which is always there (either usig headphones or monitors). I've adjusted bias - no luck. So, if not tubes, bias, or transformer - then may be as you say. How do I measure for hum voltage across output terminals? I would have thought Cary could have done this while the amp was with them - they simply deny it's a problem = very frustrating!
OK, so it's electrical - it appears in the electrical output signal. And it's a buzz rather than a low-pitched hummmm. That helps identify the problem - it's not going to be power supply filtering or magnetic field pickup.The first question then is, does it in fact meet the specs? You can measure the output voltage with a voltmeter set on "AC volts". Most meters are flat up to several kHz at least, so the measurement is likely to be accurate. The 300 SEI is specified to have hum and noise 90dB below full output. Full output is specified at 15 watts, which is 11 volts rms. 90dB below that is 0.35mV. This is quite a good hum level, though it may still be audible on sensitive headphones The point here is to determine if the unit is operating correctly - that affects what solutions might be possible or appropriate. Note that the meter leads can pick up hum; check the voltage reading with the meter and leads in the same position but the leads connected to each other. You may need to twist the leads together to make this measurement accurately.
I'm assuming from Cary's response that they think the amp is operating correctly, so if it does not seem to meet specs there is likely to be some environmental source of the problem outside the amp itself. This may or may not be true, but in either case you'll have to determine the answer in order to make progress. If the measured noise is louder than it should be, it's time to isolate the amp. Disconnect everything - all input cables, speaker cables, and/or headphones, and repeat the test. Other things connected to the amp can introduce buzz from ground currents (most common) or by acting as antennas.
If it still buzzes, it is always possible the source is on the power line. Do you have a power line filter? The alternative is to run around the house makeing sure all light dimmers, flourescent lights, computers, and other possible power-line pollutants are turned off.
In my experience, by the time you've done all these things the odds are at least 9 out of 10 that you will have found the problem, and can start working on solutions.
Thanks Paul for taking the time to offer suggestions - I really appreciate it. I'll try to give it my best and report back my findings.
Another possiblity is that tube gear can pick up RF noise. Do you live near a tower? Its a bit of a hassle, but you could take your amp to a friend that lives in a "clean" area, and try it there? Since you dont have to move speakers, it would not be all that hard.
I've moved this puppy all over the house (and yes, also across town). I don't live near a tower and I don't have any RF noise in any other part of my system (2 channel and HT noise floor is "black") - so, it's just isolated to this amp. I'm beginning to think (based on comments above) and from other inmates, that this is just how the 300SEI operates. The 60 ohm phones are more sensitive to it than the 300ohm ones. My wife thinks I'm crazy (maybe I am)! Thanks all!
Well it seems you have worked the problem from just about every angle! When you say Cary has not been too helpful, have you asked to speak to Dennis? He is usually pretty good at troubleshooting I have found.
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.
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".
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