SEX Kit, fairly loud hum

[Geographer]

So I've jumped through the various hoops and finally have both channels working. I also purchased a pre-amp so now the music is nice and loud. Unfortunately in my left channel I am getting a very audible hum. For reference, it's not nearly as noticeable when music is playing.

• If I swap the tubes the hum swaps to the right channel which leads me to believe it's the tubes.
• The hum in the right channel is there but significantly lower, maybe 1/3 as loud.
• If I cut the power while listening to music the hum goes away completely.
• I'm using the 6DN7 type tube though had to replace them as the ones that were shipped were faulty.

So 2 questions...

1. Is this normal to have any hum at all coming through?
2. Is this something that can be fixed? I ordered two new tubes from ebay to see if I can get the hum lower?

[Paul Birkeland]

If I swap the tubes the hum swaps to the right channel which leads me to believe it's the tubes.

That is likely the case.

What kind of speakers/headphones are you running with your SEX amp?  Which impedance did you end up selecting for your output transformers?

-PB

[Geographer]

I am using GRADO RS2's and SR125's. I went with low imped.

Disclaimer: I'm a complete neophyte when it comes to this equipment.

I was looking at the sex 2.1 C4S kit. Is this designed to alleviate the issue that I described above? Also I assume I have the 2.1 unit since mine was shipped in sept 2013.

Thanks for your help!

[Paul Birkeland]

By low impedance, do you mean 4 Ohms?

[Geographer]

Yes, if memory serves me, that is correct.

[Paul Birkeland]

With headphones that are that sensitive (you are powering headphones that don't need an amplifier with a 2 Watt amplifier), you can either plow through a bunch of 6DN7's to find the quietest pair, or there is a modification that was present in the SEX 2.0 that can lower the noise floor a bit.

This modification comes at the expense of damping with low impedance headphones, and it disables the speaker cutoff when headphones are plugged in.

-PB

[Geographer]

Is that the 2.1 C4S that you're talking about?

[Paul Birkeland]

No, the SEX C4S addresses a different aspect of the circuit.

[Geographer]

Ah, how do I go about attaining the modification route?

[Paul Birkeland]

You'll need a pair of 120 Ohm resistors.  Metal film or carbon film resistors will work well here, and 1/4 Watt or 1/2 Watt are adequate.

On the headphone jack, remove the red wire from the top outside contact of the headphone jack.  Reattach this wire to the top inside contact of the headphone jack.  Attach and solder a 120 Ohm resistor between the top inside and top outside headphone jack contacts.

Repeat these steps with the next level down, moving the red wire attached to the outside contact to the inside contact, then attach and solder a 120 Ohm resistor between each of these contacts.

-PB

[Geographer]

Great! Thank you so much, I think I'll try this modification out as well as rolling the tubes.

Do you have any suggestions for alternate tubes or just stick with the 6DN7?

Thanks again for your quick responses. Your help has been hugely appreciated.

[Paul Birkeland]

The 6DN7 is the only tube that works in the SEX, which is an advantage and a disadvantage all at the same time.
-PB

[Geographer]

Quick update:

I cycled 2 other tubes through the bottlehead and lucked out finding one with low noise. Sounds great now! Still a slight hum but I imagine that's normal.

Do you think the 120ohm bypass that you recommended will remove the rest of the noise?

Thanks again for your help!

[Paul Birkeland]

Yes, the 120 Ohm resistors will lower the noise floor.

[mcandmar]

Quick update:

I cycled 2 other tubes through the bottlehead and lucked out finding one with low noise. Sounds great now! Still a slight hum but I imagine that's normal.

Do you think the 120ohm bypass that you recommended will remove the rest of the noise?

Don't worry with a few tweaks this amp will run dead silent with Grados (i have a few).  Try the resistors as Paul suggests, if that doesn't do it for you there are a few other tricks you can try. S.E.X. and Grados are a wonderful combination IMO, you will be pleased..