Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: JackOfAll on October 25, 2009, 06:01:34 AM
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Now admittedly the IHF standard is old, and specifically it predates the widespread use of headphones with portable devices like iPods. These devices run on low voltages, and in order to obtain sufficient loudness the headphones need to be low impedance and highly sensitive. Some of these headphones do not work at their best with the IHF standard 120 ohm source impedance, being designed for a near zero source impedance and low voltages (around 1vRMS is not uncommon). Unfortunately, few headphone manufacturers will tell you this. But if you do have such a set of phones, the ideal would be a low-impedance source of even lower voltage - in other words, for the SEX amp a tap of less than 1.0 ohms, with no series resistor. It is easy enough to modify the 120 ohm series resistor to an L-pad which will reduce the output voltage and impedance for such phones, though I don't think we have documented such a modification yet. It would merely involve changing the 120 ohm resistor for another value, and adding another resistor from the "hot" headphone signal to ground. I would suggest 6.8 ohms (5 watt) and 1.2 ohms (1 watt) as widely available and suitable starting values.
PJ, hope you don't mind, but the text above is quoted from a post on the old forum. I just tried to add a reply there but when I try to post it tells me that the forum is archived.
The L pad is probably a very good idea for anyone running the SEX with low imp phones.
The comment about some low imp phones not sounding their best with 120R source impedance is a little bit of an understatement. All of the Japanese low imp (20-50R) high-end phones, (that I've listened to), sound just about as bad as it's possible for them to sound when driven by 120R! ;)
I meant to post on this topic last time it came up when someone was talking about "mushy" bass and GS1000's. Having used the SEX with just about every consumer high end phone out there, including low imp phones from Grado, Denon, AT and JVC, you at least need to jumper out or remove those series resistors. Driving these low imp high-end consumer phones with 120R source impedance is not going to allow you to either a) hear the phones as they are intended to sound, or b) give you a good impression of the SEX.
IMHO, the days of 600R studio monitors and IHF standard 120R source impedance are long gone.
NB. Senns and high imp Beyers sound just fine with the SEX as is. The 120R source impedance makes little difference to sound quality.
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JackOfAll, many thanks for sharing your wide experience. Not being a headphone junkie myself, I naturally just followed the old standards. We did have some issues early on with certain low-impedance headphones that were too sensitive to use directly on the 8-ohm outputs, which led us to implement that standard.
I'm hoping for a revision of the SEX amp one of these days, where we will include several minor changes that have accumulated since it came out. One of those will I hope be a new output transformer with a wider range of possible output impedances, which might adequately address this problem.
Meanwhile the L-pad idea sounds like it may get more play. The L-pad as I initially suggested it creates an 8-ohm load on the amp, making it impractical to use with speakers. I'll give it some thought and see if I can come up with a more universally useful approach. Of course, the first step would be to short out the 120 ohm resistors. If the phones are not so sensitive that the amp's self-noise is a problem, then you are done.