Hair Trigger volume control on 2.1

[Oberst Oswald]

Using either Sennheiser HD-800 (300 ohm), Audez'e LCD 2 (60 ohm) or Denon D-7000 (25 ohm) there is only a little difference in the "gain" between them. The volume control is at @ 7 o'clock with the slightest movement causing an extreme increase or decrease in volume.  All three headphones sound great to my ears and even with this "hair trigger" control I can easily find a sweet spot but I don't think this is working properly.  Any ideas on this issue?

[Jim R.]

What are you using for a source and what is it's output voltage?

-- Jim

[Oberst Oswald]

X-Fi Titanium Fatal1ty, Stello DA100, SEX 2.1.  The analog output is 1.2 Vrms. It maybe that my DAC is too hot for the present gain on the SEX.   I sort of fixed the problem by lowering the volume on the X-FI which gives me more travel but usually I have the volume set to 100% when using my other amps.  What is the gain on the SEX 2.1?  In the SEX 2.1 kit I have two 120 ohm resisters that are not used for anything.  Looking at some builds for older SEX's I see resisters on the volume control which are not used or shown on the SEX 2.1 instructions.  Is this something that I should be concerned about?  Anyway this is not a big deal as the amp is more than I expected it to be in "stock" form and I will improve it as time goes on. 

[JC]

Were these 120 Ohm resistors "extra"?  Do you have two 120 Ohm resistors installed on the headphone jack?

[Jim R.]

Yes, what Jim said -- those should help, and also the output le vel should not be a problem with the s.e.x., which I belive is 15 db gain, btw.

There is probably a sonically optimum level setting for the dac, so once you get the 120 ohm resistors in place, experiment with that.

-- Jim

(the other one)

[Oberst Oswald]

No... they were not used because they were not shown or written about in the 2.1 instructions.  If they need to be used where exactly do they go and what are their purpose.

[Oberst Oswald]

Looking at the schematic it shows a 120 ohm resister used on the headphone jack but it is not shown in the pictures or print.  I do not have 120 ohm resisters there.  Is this where they need to go or am I completely messed up and they need to go somewhere else?  If so what numbers on the jack do I connect them to?

[JC]

You know, I have Version 2.0, so I'm very probably not the one to advise you on that.  On Version 2.0, they were wired in in such a way as to be "in series" with the tip and ring connections on the headphone jack when 'phones were plugged in.  This, I understand, was/is the IHF standard for headphone jacks.

Perhaps Doc or Paul will see this and be able to tell us what's up.  Please understand that I am not trying to be evasive, but your S.E.X. will have different output iron, among other things, and possibly a different headphone jack.  So, I don't know that it would be worthwhile to try to wire it like my version.

  

[Jim R.]

Same here -- not sure what's up with the new design so PJ or Doc will have to chime in.

The new kit I belive has the shorting headphone jack, and not sure that will be wired the same way as the ones in the original.

-- Jim

[Doc B.]

The 120 ohm resistors were originally wired in series with the V2.0 output transformer secondaries ( 4 and 8 ohms only) to even the output levels when changing from high impedance (usually not so loud) to low impedance (sometimes too loud) headphones. This was a standard recommended by the IHF long ago and it has not not always been followed by manufacturers.

Some users posted that they thought the amp sounded better without the 120 ohm resistors installed, even with low impedance headphones, so we left them out of the 2.1 assembly (and also the 2.1 version now has a 32 ohm tap that should help to increase output levels from the past version - this may be contributing to increased sensitivity if you have wired the secondary for 32 ohms).

If you are not hearing much difference between headphones it is likely that the 120 ohm resistors will not help much and that you simply have a lot of gain ahead of the S.E.X. amp. Output signal level for the DA100 is rated at 2.4Vrms on their site, which is a little bit on the high side, and that may be contributing. Since you can turn down the signal level ahead of the DAC it seems like that is a simple solution.

[Oberst Oswald]

Thanks for the replies and your help is appreciated.  I'll wait to see what Doc or Paul have to say before I do anything.  But now another thing that bothers me is that I also have two "extra" Nichicon 220uf 50v caps that are also not used anywhere.  I checked the instructions over & over again to find where these extra parts were used but could not find where so I assumed they were "new kit blues" and completed the kit.  I did find one mistake with the instructions (which is now a sticky) so maybe there are others.  As I said the amp works very fine with all my headphones other than the volume issue which is minor and is easily cured by adjusting the X-FI.  In some ways I like the SEX better than my Beta 22 so these extra parts missing can't make much of a difference either way (and I'm pretty sure I built it correctly) on the proper operation of the amp.

[jslabovitz]

Thanks for the replies and your help is appreciated.  I'll wait to see what Doc or Paul have to say before I do anything.  But now another thing that bothers me is that I also have two "extra" Nichicon 220uf 50v caps that are also not used anywhere.  I checked the instructions over & over again to find where these extra parts were used but could not find where so I assumed they were "new kit blues" and completed the kit.  I did find one mistake with the instructions (which is now a sticky) so maybe there are others.  As I said the amp works very fine with all my headphones other than the volume issue which is minor and is easily cured by adjusting the X-FI.  In some ways I like the SEX better than my Beta 22 so these extra parts missing can't make much of a difference either way (and I'm pretty sure I built it correctly) on the proper operation of the amp.

I, too, have both left-over resistors and capacitors, along with three terminal strips and a tube bracket. And a lockwasher, though that one was likely a result of my confusion between #6 & #8 washers.

--John