Modding SEX2.1 closer to Stereomour-like power to run planars off speaker outs?

[spaniard02]

I've been doing some preliminary lurking on this forum, trying to gather a sense of whether I can get what I want out of the S.E.X. 2.1 in stock form, and am really getting confused by some of what I'm seeing.  Long story short, I want to run high impedance dynamic phones off of the headphone jack (i.e. Senn HD650, Beyer 250/600ohm models) and a really demanding/difficult planar off of the speaker outs (balanced), e.g. the HiFiMAN HE-6/4 (recommended 4 watts).

I have tried the more-efficient but heavier (~500-gram) HiFiMAN & Audeze planars before, and while the SQ is certainly 'choice', they aren't a great for me for an everyday, desktop-only, extended-listening experience. 30% lighter weight is ideally where I want to be.  I would buy the Stereomour for it's 3.5 watts, but the related threads on the topic seem to generally suggest that the power design is ill-suited for low-impedance headphone use.  No indication in that discussion if the suitability would change if you're talking about planar magnetics vs. dynamics.

The reason I started considering the S.E.X. is because I saw 2 watts of output as a low-cost interim compromise, but as I've started searching in the forum, it seems like I would only be getting that full 2 watts off of the 32-ohm tap, making for a compromised damping factor.  Some like DocB talk about this as if its a complete non issue, others like Caucasian Backplate urge people to run low-impedance planars off the 4-ohm tap "for the best signal to noise ratio, lowest distortion, and optimal damping."  Meanwhile, I'm sitting here scratching my head and wondering which way is up, amid all the calculations & parts references being thrown around. 

So can anyone either clarify for me what I could expect to squeeze out of the stock design by optimizing for max power off the speaker outs for a 38ohm headphone, and how I might go about doing so?  Or possibly share ideas for select component upgrades for achieving closer to 3.5-4watts, for something under $200?  A quick glance at the Magnequest site seems to suggest that upgrading the transformer in the S.E.X. would run at least $300... ... but then again, I have no idea what I'm looking at there. 

[Paul Birkeland]

There are a few things going on here.  I recommend the 4 Ohm tap for high sensitivity headphones, as this gives optimal noise floor performance.  This is a recommendation I make when our customers mention something like the amp seeming noisy using the 32 Ohm tap driving Grado headphones.  Selecting the proper impedance requires both considering the load impedance and the load sensitivity.

You are asking about low sensitivity headphones, so the noise floor will not be an issue.

If the manufacturer calls for an amp that make 4 Watts, this may be 4 Watts at 8 Ohms (solid state), which will be significantly less power at 38 Ohms (maybe more like 1 Watt).  Running the SEX amp on the 32 Ohm tap into 38 Ohm headphones will yield about 2 watts. 

Changing the output transformer will not increase the output power of the SEX amp. 

-PB

[spaniard02]

There are a few things going on here.  I recommend the 4 Ohm tap for high sensitivity headphones, as this gives optimal noise floor performance.  This is a recommendation I make when our customers mention something like the amp seeming noisy using the 32 Ohm tap driving Grado headphones.  Selecting the proper impedance requires both considering the load impedance and the load sensitivity.

You are asking about low sensitivity headphones, so the noise floor will not be an issue.

If the manufacturer calls for an amp that make 4 Watts, this may be 4 Watts at 8 Ohms (solid state), which will be significantly less power at 38 Ohms (maybe more like 1 Watt).  Running the SEX amp on the 32 Ohm tap into 38 Ohm headphones will yield about 2 watts. 

Changing the output transformer will not increase the output power of the SEX amp. 

-PB

Thanks for your response.

No, not a 4-watt amp - an amp that can deliver 4 watts into 38ohm headphone impedance (e.g. a Lyr, EF-6, WA5), and this is the consensus from the community (manufacturer somewhat understates the amping reqs, though still with a warning).  Other amp manufacturers quote an amp model's max power output at certain headphone impedence levels given their single fixed output impedance, such as:

Output Impedance: 1 ohm
Max Power, 32ohms: 6watts
Max Power, 50ohms: 4watts
Max Power, 300ohms: 650mW
Max Power, 600ohms: 320mW

So, is there any sort of information/measurement for comparable power figures to be expected for the S.E.X. amp for the other fixed output impedance levels (4/8/16ohms) given a 32ohm & 50ohm headphone impedance, for example?  Or, if one has to set the output impedance to 32ohms / run off the 32ohm tap to get close to 2 watts, at what headphone impedance does 2 watts max power turn into 1 watt?  Or 1 watt turn into 600mW?  I'm not really looking for numbers at those specific points, just trying to clarify what I'm trying to understand.

And concerning mods, if not that one part, then can anything else be changed about the stock amp to raise the power ceiling off of the speaker taps?

[Paul Birkeland]

So, is there any sort of information/measurement for comparable power figures to be expected for the S.E.X. amp for the other fixed output impedance levels (4/8/16ohms) given a 32ohm & 50ohm headphone impedance, for example?  Or, if one has to set the output impedance to 32ohms / run off the 32ohm tap to get close to 2 watts, at what headphone impedance does 2 watts max power turn into 1 watt?  Or 1 watt turn into 600mW?  I'm not really looking for numbers at those specific points, just trying to clarify what I'm trying to understand.

And concerning mods, if not that one part, then can anything else be changed about the stock amp to raise the power ceiling off of the speaker taps?

As the headphone impedance increases relative to the transformer tap used, the power will decrease, the distortion will decrease, and the damping will increase. 

You could redesign the SEX amp to use a tube that's a triode and pentode in one bottle, run the pentode as a pentode, employ some global feedback, etc., and you could get to 4 watts.  This would come with less than desirable sonic consequences.  In the end, the actual SPL difference between 2 watts and 4 watts is only 3 dB...

[Paul Birkeland]

Paul, can the loudness rule be also applied to headphones?

It can, but PJ's white paper uses the 1 Meter sensitivity of the driver as one of the governing variables.  Headphones tend to have a rated sensitivity, but it's incredibly rare that you actually get the distance along with the sensitivity. 

[spaniard02]

I have an LCD-2, HD600 and DT990-600 and I can say that the Sex amp can drive these cans to loud levels with no problem. I have not tried a Hifiman though.

There's huge variation in the HE series - I would expect results in 'trying a HiFiMAN' to vary significantly depending on the model.  Some might see you say the S.E.X. amp drives the LCD-2's to loud levels and jump to the conclusion that it should work fine for the hardest HE-series planars, but those LCD-2's have a 60ohm hp impedance and are rated I think at better than HE500-like efficiency - I would fully expect 1wpc (or whatever) from the 16ohm tap to produce decent volume and a respectable listening experience.  But as I think you're implying, volume/loudness alone doesn't prove they're being driven well. 

Anyway, I suppose I've got some things to work out, but thanks.

[Doc B.]

You can only do so much by speculating with numbers. Borrow a S.E.X. amp and try the headphones in question - that is the sure way to know. I have heard a couple  different HiFiMan cans with the S.E.X. and they both sounded very good. Sorry to say I don't remember which ones, but I thought one pair (HE-500s maybe?) was a particularly good combination, that I liked quite a bit better than LCD2s.

On the other hand if you are going to blow $1300 on the headphones, why scrimp on the amp? Buy a pair of Paramounts. That's what I use with K1000s.

[tdogzthmn]

Doc,

Do you still listen to your K1000's frequently, or spend more time with the HD800?  I've never really missed my HD800 but I do miss the K1000 from time to time.  Right now my STAX  SR-507 is keeping my very happy, but I miss the expansive and natural sound of the K1000 with certain songs.

[Doc B.]

Well, I have a four way all SET driven speaker system in a dedicated room, so I don't listen to either all that much. Because the mainline is fairly new i've been listening with it and the 800s when I listen in my office. I also like that setup for editing work. But if I want to better reproduce what I hear in the listening room I think the k1ks have some advantage.