[terriblepaulz]

Bump! I am noticing the same thing with my recently acquires amp. It runs two 6SN7s for input and a single 6BX7 or 6BL7 for output. This is the second version of a DIY amp, and the hum is greatly reduced from the first version. The builder said that is probably due to the cleaner layout, larger chassis, and better quality parts. The builder told me I could always expect a little hum (w/ Grados - his choice of cans) with this design (an integrated [5W/channel] using NOS tubes and some vintage parts.

As stated by others above, it's only noticeable with Grados. It's usually so quiet after about 10 minutes of warm-up that I can barely hear it. I have tried rolling the output tube, to no noticeable effect. I am using a pair of matched "side-getter" GE tubes. Are these known as noisy? Any 6SN7s known for being particularly quiet? Any other suggestions for generally making simple tube amps quieter?

 

[Rob N]

SP amps are quiet and run 6SN7's

 

[Skylab]

Quote:

Originally Posted by Rob N /img/forum/go_quote.gif SP amps are quiet and run 6SN7's

Indeed - I get *NO* hum from either of my SP amps, with any headphones - and the MPX uses ALL 6SN7's. It's not that tube.

A number of tube amps I have hum with low impedance headphones (the SP and DV amps I have do not, blissfully). But MANY tube amps are going to hum with low-impedance Grados.

 

[Uncle Erik]

Quote:

Originally Posted by hypoicon /img/forum/go_quote.gif I think that's mostly bull. Vintage tube equipment frequently has two-wire cords to begin with and the shock hazard is pretty minimal. The main reason for a ground lead is to accelerate the response of a circuit breaker in case of a failure-- there aren't generally stray voltages drifting around just for the heck of it, particularly lethal ones. That happens during a short that causes a failure (i.e. smoke).

A lot of vintage tube gear has a two prong plug, but a lot of them also have "hot" chassis. They are not terribly safe.

Every time I work on a vintage set (I collect and restore old radios) I change it over to a grounded plug and ground on the chassis. If there's a ground loop or other issue, you deal with it directly. Proper grounding is mandatory.

There are lots of reasons equipment hums. It can be a ground loop, improper chassis layout, a little AC hum from the mains, RFI radiation (from fluorescent lights, AC motors, dimmer switches, etc.), and running AC on the tube filaments. Probably a few other things I can't think of at the moment, either.

I'd recommend ruling out the easy stuff, like getting it away from a dimmer switch, etc. Stuff like converting filaments from AC to DC is a little more involved.

 

[Rob N]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif But MANY tube amps are going to hum with low-impedance Grados.

The Earmax Anniversary has no hum with Grados

 

[nikongod]

Quote:

Originally Posted by Rob N /img/forum/go_quote.gif SP amps are quiet and run 6SN7's

SP amps all (as far as I know) run DC heaters
And all bias the heaters above ground.

Heater-hum dosnt stand a chance. DC heaters are decent, although as applied often bring their own interesting drawbacks as the caps charge up.. Floating the heaters is pretty sweet.

In the silver ghost, I floated (actually tied...) the heaters 75V above ground, which is the limit of the 6sn7. The amp has AC heaters, and except for a few bad tubes, the amp runs silently.

 

[scootermafia]

Some tubes will hum in some amps no matter what, unless you mod it. Three new-production (tungsol, sovtek, electroharmonix) 6SN7s all hummed in my 336SE, but when I tried an older RCA cleartop and a very old Crosley both sounded great.

 

[daleda@sbcglobal]

I used to own the 300SEI and it went to back to Cary twice for this problem (and after many tube changes later) - I sold it! It is absolutely ridiculous that an amp of this price point have hum issues (nothing less than a black background is acceptable. If yours is an early model - you'll never get rid of the hum - it is a design flaw - the tech at Cary admitted as much (especially with the headphone tap)! So many better alternatives now - YMMV.

 

[Brandon_Ottawa]

Quote:

Originally Posted by daleda@sbcglobal /img/forum/go_quote.gif I used to own the 300SEI and it went to back to Cary twice for this problem (and after many tube changes later) - I sold it! It is absolutely ridiculous that an amp of this price point have hum issues (nothing less than a black background is acceptable. If yours is an early model - you'll never get rid of the hum - it is a design flaw - the tech at Cary admitted as much (especially with the headphone tap)! So many better alternatives now - YMMV.

I recently bought a used Cary CAD-300 SEI and have experienced a hum just like you. I hear no hum through the headphone jack, but I do have hum with the volume at zero if I put my ear up to the speakers. What really has me scratching my head...I'm currently trying the CAD-300 SEI on a PS Audio P300. On a P300 you can increase the fequency in the SIN mode. As I increase the frequency from 60Hz to 61, 62, 63,...90, the hum will increase in frequency too and become a buzz. There's no choke filter on the CAD 300-SEI power supply right?, so What?

Anyways, I can't kill the hum. My used set-up is as follows and is overkill in power filtering...

Elco HPC-10 SP 8 ft PC >> PS Audio Ultimate Outlet 15 A >> Elco HPC-10 SP 5.5 ft PC >> PS Audio P300 MW II >> PS Audio XStream Statement 1.5 m PC >> modded Sony SCD-1 >> Enterprise NW Music Timbre 1.0 m IC>> Cary CAD-300 SEI (with PS Audio XStream Statement 1.5 m PC) (Tubes: NOS Tung Sol 6S7NGT round mica & round plates, NOS 1578 metal russians, Sophia Electric Mesh Plate 300Bs) >> PS Audio Xstream Reference 3.0 M Speaker Cable >> Totem Mani-2 Sigs on Target-2 stands

I love the Cary's even with the hum. They're magic with the Totems. Maybe I'll try a PS Audio humbuster.

 

[takezo]

what kind of hum is it? low 60hz or higher 120hz?
the lower hum may be caused by AC heater current to the tube...
insufficient twists or if leads to the heater are not wired correctly.
higher 120hz hum may be insufficient ripple filtering by the power
supply capacitors... most easy to identify is the latter; if the hum
is audible when volume is zero and doesn't increase when volume
is turned up.
it may just be a bad solder joint...

 

[Brandon_Ottawa]

Quote:

Originally Posted by takezo /img/forum/go_quote.gif what kind of hum is it? low 60hz or higher 120hz? the lower hum may be caused by AC heater current to the tube... insufficient twists or if leads to the heater are not wired correctly. higher 120hz hum may be insufficient ripple filtering by the power supply capacitors... most easy to identify is the latter; if the hum is audible when volume is zero and doesn't increase when volume is turned up. it may just be a bad solder joint...

I have no amp chassis hum. The majoirty of my hum comes through my speakers' 6 1/2" woofers. The tweeters add in some noise too for good measure, but the woofers are what's most audible within a foot of the speakers.

The hum I hear is a 60Hz hum when the P300 sinwave is set @ 60. I can make it a higher 120Hz hum if I set the P300 sinwave @ 120. If you'd ask for 75Hz, I could make it hum @ 75Hz too.

 

[takezo]

does the hum increase when you increase the volume knob? if not,
it's most probably a powersupply problem, though i can't be sure
since i don't know the layout of your amp.

if it's a powersupply problem and if you're handy with
electronics and soldering, try bypassing each of the caps in the
power supply line, one at a time with an equivalent cap, and if
your amp has it, try bypassing the coupling caps as well. if you
notice the hum decreasing after that, then you've just found your
humming source, or at least one of them.

 

[synthmad]

As for hum issues, all my tube amps (Woo, LD) have had some slightly annoying low level hum issues. I had tried many things to reduce it by changing tubes, adding power conditioner, moving wifi router, phone base station etc.

I relocated one of the tube amps from my bedroom to my living room and found it was totally silent with regards to hum, I have since found all the amps have no hum in the living room, even with the same source equipment and interconnects etc.

I assume that my bedroom is on a noisier mains ring, likely shared with air-conditioner.
As this is an apartment complex, who knows what is connected to what...

So I wouldn't single out the amp for hum issues before trying different mains outlets etc.
My solid state amps do seem more tolerant of dirty power.

 

[mironathetin]

I use Sennheiser HD 650 (300 Ohm). No hum. But then I tried my Sennheiser ie7 in-ears (16 Ohm). They produce a hum with the earmax-pro and a hiss with my x-can. I also tried them once with a creek obh-something. Also a hiss.

So my guess is the impedance. Could you try an attenuator?

 

[BryanP]

For me, it's also a matter of impedance/spl. My EF2A hisses when no music is playing at high volume levels (not for a prolonged amount of time, I was just testing the volume pot) only for low impedance headphones (40-70 ohms) that are reasonably sensitive (around 105 dB @ 1 mW). This is only when the volume levels are 80% or so.

I get zero hiss with my HD600s which are 300 ohms and 97 dB at 1 mW. It's not as sensitive as the other hps, and /much higher/ in impedance. Background is super black.

Therefore, it's most likely a combination of inherent amp noise along with the impedance and sensitivity of the headphones connected to the amp.