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There is no ded­i­cated line out­put as such on the SR15, but it doesn’t re­ally mat­ter be­cause there’s a ‘line out’ mode us­ing the reg­u­lar head­phone socket. You en­able this in the set­tings menu and choose the pre­ferred level — 0.7 volts, 1 volt, 1.25 volt and 2 volts are avail­able. Then when you change the vol­ume us­ing the knob, a line out but­ton ap­pears on the screen (in ad­di­tion to the vol­ume chang­ing nor­mally). Tap the but­ton and the SR15 switches to fixed out­put mode. (Why those lev­els? I don’t know. There are in­ter­vals of 3dB, 2dB and 4dB be­tween the pairs above.) I mea­sured the line out­put with the 2 volt set­ting into a 47.1kHz load, and it came to 2.01 and 1.99 volts RMS for the two chan­nels ac­cord­ing to my os­cil­lo­scope. In other words, spot on the claim. (Even if the slight di­ver­gence is in the unit rather than my mea­sur­ing rig, it amounts to less than 0.05dB.)

Set­ting the unit to head­phone out­put mode, play­ing the same 0dBFS 1kHz sine wave into the same load, with the vol­ume con­trol set to 150 — the max­i­mum — the re­sult was iden­ti­cal. I’m guess­ing all the line out­put set­ting does is fix the out­put level. In other words, it’s a con­ve­nience for use with other au­dio sys­tems.

Switch­ing from the 47.1kHz load to 295 ohms, there was no clip­ping even at max­i­mum out­put, and just about no re­duc­tion in out­put (only in the third sig­nif­i­cant fig­ure, from 2.01 to 2.00 volts). Into a 15.9-ohm load there was mas­sive clip­ping. I turned the level from the max­i­mum of 150 down to 130 to elim­i­nate it. You can safely say that play­ing into just about any head­phones at an in­di­cated 130 will guar­an­tee no clip­ping (although it may well guar­an­tee ear dam­age!). The out­put was still 0.565 volts, which is 20 mil­li­watts into that load. And that works out to 13dB above the sen­si­tiv­ity rat­ing of what­ever head­phones you’re us­ing.

Switch­ing the load back to open cir­cuit while re­tain­ing the same vol­ume level, the out­put in­creased to 0.637 volts. From that it was easy enough to cal­cu­late an effective in­ter­nal re­sis­tance of pretty much spot on 2 ohms. Per­haps a touch higher than with other high qual­ity por­ta­ble play­ers, but still suf­fi­ciently low that it should re­sult in no fre­quency re­sponse anom­alies in any head­phones with vary­ing im­ped­ances across the au­dio band.

Sadly, the soft­ware I have been us­ing in re­cent years to con­duct other mea­sure­ments sim­ply re­fuses to work any more on any of my com­put­ers. That means I can­not mea­sure sig­nal-to-noise ra­tio or dis­tor­tion, but I’d note that to the ex­tent there is any noise or dis­tor­tion cre­ated by the SR15 (as there must be), it is be­low any au­di­ble lev­els.

I went old-school on mea­sur­ing fre­quency re­sponse (above), us­ing pink noise sig­nals and fil­ter­ing them af­ter record­ing to re­move the 3dB per oc­tave tilt. For 44.1kHz sam­pling, there’s a straight­for­ward clin­i­cal treat­ment: flat to 20kHz and a sharp fil­ter there­after. For 96kHz sam­pling, the treat­ment is much the same, ex­cept that it was flat to 42kHz. The same pat­tern prob­a­bly re­peated for 192kHz, ex­cept that my ADC was soft­en­ing the curve at the top end. Even then it was flat to 56kHz and down 3dB at 80kHz.

Those who swear by slow, gen­tle fil­ters are likely to be dis­ap­pointed. The rest of us can be con­fi­dent that what­ever is up there in the high fre­quen­cies, or in­deed, supra­sonic fre­quen­cies, is be­ing de­liv­ered.

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