Pot for the S.E.X.

[boulos]

Hi,
I'm wondering what are good volume pot upgrades for the SEX.  I prefer the continuous kind, but could be convinced otherwise.  One question is whether an upgraded pot really makes a difference in the sound. But the real reasons I wish to change the pot are:

-- Slightly scratchy at very low volume (that wouldn't be a big deal if it weren't for the next point).
-- I have a very small usable range with headphones;  if 0 signal is 6 o'clock, I can barely go to 7.

I listen to both speakers and headphones. My speakers are 8ohm (and the output transformers are wired for 8ohm), and my headphones are 32ohm (and they're nothing fancy).

Would a better solution be to have a voltage divider at the headphone output?  If so, what would be good resistor values? Any other solutions?

Thanks!
Boulos

 

[Paul Joppa]

I recommend the divider at the headphone output. I'd suggest a 100 ohm pot to start with, possibly substituting fixed resistors when the best values have been determined.

I'm not up to speed on the current wiring, since we've changed the jack and removed the 200 ohm resistor. It does depend on what version of the amp you have. PB may have better insight.

 

[Paul Birkeland]

Hello Boulos,

In the current SEX configuration, the output from the output transformers heads to the headphone jack, then out of the headphone jack to the speakers.  For very sensitive headphones, you can move each red wire that connects to the headphone jack and comes from the output transformers to the other side of the jack (this disables the switching function), then add a 100 Ohm resistor from each of the two now empty headphone jack lugs to the lug on the opposite side of the jack.

This will disable the speaker switching function, but optimize the amp very well for low impedance/high sensitivity headphones.

Depending on the sensitivity of your speakers, you may also find some benefit to wiring your output transformers for 4 Ohms.

-PB

 

[boulos]

Thanks PB!  I have the Orcas (newer bamboo version). Will the 4ohm tap work well with them?

Can I get a headphone extension cable, cut it, and insert 100ohm resistors in series with each of the signal wires? I can shorten it and wrap it in heat shrink so that it looks acceptable. Assuming I understood correctly, this will be functionally equivalent to your solution (uglier, but keeps the speaker switching function).  What do you think?

If the 4ohm tap works, do you recommend I try that first, or try the 100ohm resistors first?

Thanks
Boulos

 

[goozy]

Following this with great interest. I have the same problem as bolus plus a slight channel imbalance at low audio levels.

Peter

 

[Paul Joppa]

...
Can I get a headphone extension cable, cut it, and insert 100ohm resistors in series with each of the signal wires? I can shorten it and wrap it in heat shrink so that it looks acceptable. Assuming I understood correctly, this will be functionally equivalent to your solution (uglier, but keeps the speaker switching function).  What do you think?...

Yes, but put a 33 ohm resistor in parallel with the headphones, and the 100 ohm in series upstream of that combination - it's an L pad. The 33 ohms keeps the source impedance low, avoiding the lumpy bass that some 'phones make if they are not well enough damped.

 

[boulos]

Yes, but put a 33 ohm resistor in parallel with the headphones, and the 100 ohm in series upstream of that combination - it's an L pad.

To make sure I'm less likely to mess this up, is this what each channel would look like? (Please see attached.)

Should the resistors be 1W rated or can they be less?

Thanks!
Boulos

 

[Paul Joppa]

Your connections look good. Use 2-watt resistors (minimum) to be sure it is still safe if you wire for 32 ohms.

 

[boulos]

Thanks Paul!  I'm going to go with the Vishay CPF3 (3 Watt) resistors. For the resistor housing, I found a neat Switchcraft jack (see picture).  I hope it will fit all 4 resistors   I will update the thread once I make this.

 

[Paul Joppa]

Sweet! If you can, post some pictures of the assembly process so others can use them - maybe start a new thread with a more specific subject so it's easy to find?

 

[boulos]

Definitely!  I will start a new thread with pictures and part list.  The parts will hopefully arrive by the end of the week, and I'll try to work on it the weekend.

 

[boulos]

I completed this. The improvement to the sound of my headphones is very noticeable (they're AudioTechnica Air ATH-AD700 open headphones). It's finally enjoyable to listen to them.  I will post the procedure in a new thread soon.  I made a mistake initially in the build where I connected the 33ohm resistors to ground at the point between the amp and the 100ohm resistors (as opposed to the point between the headphones and the 100ohm resistors). I discovered the error while doing final resistance checks and I was able to easily correct it, but unfortunately it rendered many of the photos I took during the build useless. So.. less photos in the new thread, but hopefully the instructions will still be clear.

 

[JamieMcC]

Looking forward to seeing a picture of this as I wonder at times if I am following such discussion correctly so a pic of your installation will be very helpful.

 

[boulos]

I started the other thread: http://bottlehead.com/smf/index.php?topic=7252.0
I'll complete it in the next day or two.