Crack & LOW impedance phones?

[Todd R]

Guys,
I got the Speedball update coming.
I have also bought some nice low impedance phones that I would like to use in the big system with the Crack.
Any bright ideas out there for some kind of adapter or circuit mod I could switch in when I'm not not in a Sennheiser mood?

[Dr. Toobz]

Depending on the phones, you may need to do nothing. I have a pair of 32 ohm Grados that are just as loud and bassy on the Crack as my 250 ohm Beyers because they are far more efficient and require less current to begin with. I've heard that AKG's also work, yet those are 62 ohms and not terribly efficient.

I suppose you could try swapping the 6080 for a 5998 tube, which I think would lower the output impedance to about 70 ohms (mu is 5 vs. 2 for 6080/6AS7). But you may not need to!

[Todd R]

Jerry Harvey twisted my arm and made me buy some JH16 Pro's at CanJam last weekend. They run at 18 ohms. http://www.jhaudio.com/store/index.php?main_page=product_info&cPath=1&products_id=34

[Paul Birkeland]

I would try the Crack as-is.  I think you might be well served by switching to a 330-470uf 160v capacitor at the output of the Crack.  Also change the 2.49k resistors on the headphone jack to more like 1k each.

[Forte]

  I think you might be well served by switching to a 330-470uf 160v capacitor at the output of the Crack.  Also change the 2.49k resistors on the headphone jack to more like 1k each.

Would it still work fine with higher impedance phones after these changes?

[Grainger49]

Yes, the lower source impedance is what helps it drive lower impedance headphones.  If you were to upgrade the output cap that will also change the sound.  The problem with that change is upgrade caps, mostly film, at this voltage rating and value are expensive.  

Just increasing the value at the same voltage rating will help with lower impedance headphones.  Do try to stay with as good an electrolytic capacitor as Bottlehead uses.  They are particular in the selection of their components.  Sometimes upgrades are hard to find.

[Paul Birkeland]

Would it still work fine with higher impedance phones after these changes?
[/quote]

Yes, it will still work fine.

[Todd R]

Yes, the lower source impedance is what helps it drive lower impedance headphones.  If you were to upgrade the output cap that will also change the sound.  The problem with that change is upgrade caps, mostly film, at this voltage rating and value are expensive.  

Just increasing the value at the same voltage rating will help with lower impedance headphones.  Do try to stay with as good an electrolytic capacitor as Bottlehead uses.  They are particular in the selection of their components.  Sometimes upgrades are hard to find.

Got my Speedball kit today.
My new low impedance phones won't arrive for a few weeks yet.
I may do as has been suggested here and change those caps and the resistors on the headphone jack.
Question is, if this won't affect the operation of the high impedance phones (cuz I plan to use both), what was the downside of not doing this in the first place?
Was it just due to the size & expense of the bigger caps?
Any suggestions for the new caps BTW?

[Paul Birkeland]

Yeah, a lot of the issue was sourcing larger, more expensive caps.  Also, the charging time of that capacitor needs to be very short.  To achieve this with larger output caps, the 2.49k resistors need to decrease proportionately.  If you go to a 300-ish uf cap and roughly a 1k resistor at the output to keep the charging time short and charging voltage low, this resistance isn't such a big deal with low imedance headphones, but suddenly a high impedance set of cans will "appear" to be a lower impedance load, as 600 ohms in parallel with 1k is suddenly a 375 ohm load.  With the stock circuit, a 600 ohm set of headphones will appear as a 485 ohm load.  Decreasing the load impedance will reduce output levels, so we chose these resistors carefully to provide the best performance with the toughest to drive (normally) headphones. 

You may not notice any significant changes with your high impedance cans when switching to the bigger caps, do let us know!

[grufti]

The value of the capacitor determines the low frequency cutoff. A large value cap and a high impedance headphone will give you a very low -3 dB point. It doesn't normally hurt to have a very low value like 1 Hz for example.


(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg808.imageshack.us%2Fimg808%2F6843%2Fcouplingcap.jpg&hash=76ad0b3ab4e7303f822fd7592a3e0234b50abd76)

The formula 1/(2*Pi*R*C) will give you the -3db point based on the capacitor value in Farads and the headphone impedance in ohms.  Going to a smaller cap will raise the rolloff point according to that formula.  Plug in your headphone's impedance and 100uf (100*10^(-6)) so see what frequency you get out.  I would suggest shooting for anything between 3 and 10hz.

[Todd R]

Awesome info guys. Thanks so much for the chart!

[Todd R]

Got the new phones (18 ohms) and tried them with the Crack.
The bass seems fine, but the midrange is way louder than the other parts of the music. I was expecting a bass issue.
Any thoughts?

[ironbut]

Hey Todd,
I'm sure that has more to do with the individual driver's impedance. What do the JH16s have,.. 6 or 8 drivers per pair?
The midrange drivers are probably a better match. It could be that things will even out with burn in.

[Todd R]

Hey Todd,
I'm sure that has more to do with the individual driver's impedance. What do the JH16s have,.. 6 or 8 drivers per pair?
The midrange drivers are probably a better match. It could be that things will even out with burn in.

8 drivers

[tsingle999]

Hey Steve, I saw some new AT ADH2000 on ebay for $400. I just wanted to know if you have tried your pair with the crack?