Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Smack => Topic started by: adamct on February 20, 2012, 12:36:15 PM

Title: Why no balanced inputs?
Post by: adamct on February 20, 2012, 12:36:15 PM
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,

Title: Re: Why no balanced inputs?
Post by: Doc B. on February 20, 2012, 12:46:12 PM
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.
Title: Re: Why no balanced inputs?
Post by: adamct on February 21, 2012, 11:27:18 AM
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.
Title: Re: Why no balanced inputs?
Post by: adamct on February 21, 2012, 11:46:05 AM
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)?
Title: Re: Why no balanced inputs?
Post by: Paul Joppa on February 21, 2012, 12:13:57 PM
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!
Title: Re: Why no balanced inputs?
Post by: Doc B. on February 21, 2012, 12:35:23 PM
Impedance switching is done by powering down the amp, flipping it over and sliding a couple of switches. It's easy, takes about 15 seconds. I do it all day long at trade shows.

The S.E.X. 2.1 will soon have an upgrade option that installs the same impedance switching boards. We're still struggling to keep up with kit orders at the moment, but that will will be released when we get a breather.

Sonically the Smack is superior - to my ear. It is a single gain stage and employs shunt regulation. This gives it a better sense of resolution and speed. I only recommend the S.E.X. over the Smack in the case of demanding headphones like K1000, LCD-2, HE-6 etc. For the broad range of easier to drive headphones I prefer the Smack.
Title: Re: Why no balanced inputs?
Post by: adamct on February 21, 2012, 06:12:00 PM
Thanks for the very helpful replies. I think I'll go for the Smack. I have other amps I can use if I need more power.

Title: Re: Why no balanced inputs?
Post by: Grainger49 on February 21, 2012, 09:57:57 PM

The problem is that you have too many headphones of such a wide variety.  Sadly one size doesn't fit all, just ask your wife about pantyhose.

The thing is that you have moved away from solid state because of the sound and detail of tubes.  Sadly with the gains is the fact that they don't drive all impedances equally well.

So start collecting the whole set!
Title: Re: Why no balanced inputs?
Post by: Paul Birkeland on February 22, 2012, 06:22:18 PM
You could add a set of 1:1 input transformers to allow the Smack to accept balanced connections at the input (consider changing the potentiometer resistance depending on the transformer).
Title: Re: Why no balanced inputs?
Post by: adamct on March 11, 2013, 10:51:38 AM
This is funny...

A while ago I was interested in buying a Smack, but never got around to it. I eventually bought a Crack, and have been very, very happy with it. Since then I have always regretted not picking up a Smack when I had the chance. Well, this weekend I managed to buy a Smack from someone else (unfortunately, fully-built, not in stock form).

While I haven't received it yet, I figured I would read up in the Bottlehead Forums. I came across this thread and read it with great interest. Would you believe that it wasn't until I got to the very end that I realized I was the OP?!?!?!?  ;D ::) :P