Bottlehead Kits > Smack

Why no balanced inputs?

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adamct:
Why doesn't the Smack have/need balanced inputs? I had always understood that in order to get the full (possibly theoretical) benefit of balanced headphones, it was necessary to not only use a balanced amp, but also a balanced source, in order to have a "fully" balanced setup. Is there some reason why this is true of solid state amps, but not tube amps?

Also, will the Smack be powerful enough to authoritatively drive something like a pair of HiFiMAN HE-400s (Sensitivity: 92.5 DB, Impedance: 35 Ohm)? I have a mix of headphones, with impedances ranging from 25 Ohms - 300 Ohms and sensitivity ranging from 92-106 dB.

Best regards,
Adam

Doc B.:
The idea of running headphones in "balanced" mode simply means that the signal cold for each channel is isolated from the other channel. With a standard single TRS stereo plug this is not the possible, the left and right channel signal cold connections are common. I believe that what you are referring to as a balanced amp and balanced source may actually be fully differential. That's something different, but the terms are often confused. IMO it does not sound as good as single ended circuitry, and that is why our stuff is single ended.

I have no direct experience with the HE-400 and specs don't seem to tell the whole story with respect to which amp sounds best with which headphone. I did just hear the HE-6 with a S.E.X 2.1 amp and that was a great combination.

adamct:
Thanks. How big is the power difference between the SEX and the Smack? I assume it is significant? Is the difference reduced if using the balanced outputs on the Smack?

How hard is it to switch the output impedance of the Smack? If I go back and forth between my Denons (25 Ohms) and my Sennheisers (300 Ohms) -- something I actually do VERY often -- can I change the output impedance on the fly, or does it require me to open the amp and fiddle with the switches on the bottom of the top plate?

Thanks for your help, Doc.
Adam

adamct:
Bonus question: will the SEX with the C4S upgrade essentially be the equal to the Smack in terms of audio performance, while providing more power (but with less flexibility in terms of changing output impedance)?

Paul Joppa:
Unfortunately the questions are a little complicated.

1) SEX can put out 2 watts into 4. 8. 16, or 32 ohms. Smack can put out 0.2 watts (200mW) into 16, 32, 64, or 128 ohms. Both put out the same power, balanced or unbalanced.

2) Above the maximum impedance listed above, the power falls proportionately with increasing impedance - in other words, it is limited by the voltage available.
* A SEX set for 32 ohms can make 8 volts, which would be 213 mW.
* A smack set for 1238 ohms can make 5 volts, which is 85mW.
* An IHF/IEC standard headphone output which is 5v/120 ohms, would make 3.6v or 43mW.
* Crack, which is current-limited, might make 120mW.

3) Switching the Smack impedance involves turning the amp off, waiting for the power supply to discharge for safety, turning it over, and flipping the switches. You could install the switch board in a more accessible place if you like; the only reason not to switch it while the amp is running is safety.

4) Switching the stock SEX impedance involves re-wiring the output transformer. However, the switch board from Smack can be retrofitted - Doc is finishing up the kit now, I think.

5) If you have no need of speakers, the SEX amp could use the Smack output transformer. You'd have to ask Doc about a substitution, there is no specific kit but the transformer looks the same and is wired the same, it just has 4 times the impedance on the outputs.

6) The Smack is superior in terms of audio performance, due to being only a single stage, shunt regulated, and C4S loaded. SEX is two stages cap coupled, unregulated, and (with the upgrade) C4S loaded on only the first stage.

7) There is no particular reason not to use high impedance phones on the low impedance tap - i.e. set the amp for 32 ohms and use both 25 ohm and 300 ohm phones. The problem comes when you go the other way - low impedance phones on a high impedance amp. Depending on the respective sensitivities, there maybe a significant level difference. And if you have very inefficient high impedance phones then you'll want to use a higher impedance output setting.

So ... long complex answer; I hope it helps!

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