Suggestions to make the SEX quieter without destroying the sound?

[Todd R]

As long as I use my higher impedance headphones (HE-560 & 500) I have no issues at all.

I would like to use my JH Audio 13's  with the SEX amp http://www.jhaudio.com/content/jh13-pro but even with the impedance switches set to the 4 ohm position, there is still hum present.

I have checked all grounds and used premium Dale/Vishay resistors which do reduce the noise somewhat. I also replaced the volume pot with a nice Alps pot which also improves the sound and noise level, but I need to take it further. The C4S is installed as are the impedance boards. FYI, shooting some contact cleaner in the slide switches and operating them got rid of some random noise.

The noise is independant of volume setting and does not change if there is nothing plugged into the inputs.

The amp is plugged into a Furman power conditioner.

i have also rigged up an adapter so I can put the 120 ohm resistors on the output. This gets rid of the noise, but makes the sound worse.

Any thoughts about other things I can try?

[Nathan]

I have a Crack amp and the SEX amp. On my Crack I found that using a shielded power cord made a useful difference in how quietly it ran. I've only used a shielded power cord on the Crack. Worth a shot- the cord I use is the "Asylum" power cord which uses Belden 19364 cable. You can make it yourself, or there's an outfit in Canada- I think it's called Take 5, that makes and sells the Asylum power cord.

[mcandmar]

Try adding two 100r resistors to the heater supply, see the end of this post http://bottlehead.com/smf/index.php?topic=5316.msg51191#msg51191

[Todd R]

Try adding two 100r resistors to the heater supply, see the end of this post http://bottlehead.com/smf/index.php?topic=5316.msg51191#msg51191

100r? Do you mean 100 ohm?

[mcandmar]

Yes, 100ohm, half watt at a minimum.

[Paul Birkeland]

Those IEM's are incredibly sensitive.  When you put a headpone that wants less than 1mW of power on an amplifier that puts out 2,000mW, then you can expect these issues. 

Since the 120 Ohm resistors are functioning well, you can try them and add a 10 Ohm resistor from ground to tip and ground to ring on the headphone jack. 

[Doc B.]

I concur with Peebs. The best thing you can do is put a brake on the amp (that 10 ohm resistor), because the S.E.X. kit is not really intended for use with super sensitive IEMs. That will allow you to turn up the volume knob and thus improve the S/N.

[Todd R]

I have some good Vishay resistors on the way, however....
The 120 ohm resistors I have in my adapter do eliminate the noise for the most part, but they also cut some of the bass. What will adding the 10 ohm resistors do for the sound?

[Paul Joppa]

I have some good Vishay resistors on the way, however....
The 120 ohm resistors I have in my adapter do eliminate the noise for the most part, but they also cut some of the bass. What will adding the 10 ohm resistors do for the sound?

Probably, they will restore the bass.

[Nathan]

After reading this and a related thread, I decided to add 100 ohm 1 watt resistors to the heater supply. Tight fit with a fully built kit but oh my, what little hum I had, and it wasn't much, has now vanished. I spent $1.50 at Radio Shack for these resistors- and I know that's overpaying- but to the powers that be- this very simple addition which would add pennies to the parts cost should be made part of the basic build.

[Todd R]

So I want to understand this completely before installing the resistors
"Connect one 100r resistor between terminal C1 to C3, and C2 to C3"
Is what I drew in the picture correct?

[Chris65]

Commonly known as an artificial centre tap: http://www.valvewizard.co.uk/heater.html

[mcandmar]

Something i have always meant to try was removing the .1r resistor as with 240v mains the filaments are only running at 5.7v DC.   Or alternatively replace the .1r with a .05r on the + and - rail to better balance the supply.

[Todd R]

Commonly known as an artificial centre tap: http://www.valvewizard.co.uk/heater.html

I'm wondering if this is so effective at reducing noise why is it not part of the build? Is there a downside to doing this?

[Paul Joppa]

I'm wondering if this is so effective at reducing noise why is it not part of the build? Is there a downside to doing this?

I can only give my theory, which is based on the apparent fact that this noise issue is somewhat new - the incidents as reported on the forum have mostly been in the last year or two - and sometimes a new tube will seem to resolve the problem.

The heater winding is grounded at the center tap already, which allows it to serve as an electrostatic shield between the primary and the high voltage secondary. But the filament voltage is only connected to the winding for the short intervals when the rectifier bridge is conducting. Most of the time it is floating, and if there is a lot of leakage current between the heater and the cathode, that would cause the common-mode voltage to drift until the next connection, which happens 120 times per second. So my theory is that, as the supply of 6DN7s is drying up, more of them have excess leakage. Lots of suppliers cherry-pick the good ones and return the bad ones to the marketplace. The artificial center tap on the output of the power supply drains the leakage current to ground.