Benefit of 600 Ohm Phones (vs 250/300) with Crack + Speedball?

[fromnowon]

Hello, I've ordered a crack and speedball. 
It's my first bottlehead kit.  First post here too.  Looking forward to being a part of this community!

So now that I took the plunge, I'm trying to sort out all the things I've read about headphones and make some decisions.
 
After much reading here is what I'm still debating and I would love some input:

- Per specs "output impedance 120 ohms, gain about 15dB into a 300 ohm load like a Sennheiser HD600, max output about 10V into a 300 ohm load", does this imply that 300 ohms is sort of an ideal load match to the Crack's output?

- Many people love the Sennheiser HD 600/650/800s with Crack, which are all 300 ohms, so this is a great pairing . . . maybe 300 ohms (or say 250 ohms) is the ideal impedance pairing for Crack? 

In general, with a Crack/Speedball, is there some benefit to 600 ohm phones vs 250 or 300 ohm phones?  If so, in what way and I wonder what kind of trade-offs there are.  For example if a person had both DT880 250 ohms and 600 ohms, with the Crack, would it drive both equally as well?

I know it's subjective and will take lots of experimenting, just trying to make a good starting decision.

THANKS!

[Paul Birkeland]

Welcome!

The Crack's performance will improve as headphone impedance rises.  The difference between a 300 ohm and 600 ohm headphone in terms of what the amp is doing will be overshadowed by the differences between the headphones themselves.

If you're set on getting the DT-880's, I would recommend the 600 ohm version if possible, but maybe not if you find a good deal on the 250 ohm ones!

[fromnowon]

Thanks for the welcome and reply!

I'm not set on the DT 880s, just using that as an example of something available in both 250 and 600.  (Besides Beyer's I don't know of any that give you an impedance option.)

"The Crack's performance will improve as headphone impedance rises" - I wonder in what way does its performance improve, and how a listener would experience the improvement.   

[Paul Birkeland]

The amount of available power from the Crack will increase as headphone impedance increases.  Distortion performance should also improve, though I haven't taken the time to measure it (to get a really good set of data points, comparing something like the DT-880/250 and DT-880/600 would provide better data than just resistor loads).

Again, the listener is far more likely to experience the large differences between two very different models of headphones rather than the amp's behavior itself.

[bigfatpaulie]

Out of interest, and I am sure this has been covered before but I couldn't find it, as anyone added resistors to the headphone jack of the Crack essentially upping the resistance of any headphone?  IE, add 300 ohm resistors so that HD600's become (in the Cracks view) 600ohm headphones?

[Paul Birkeland]

Out of interest, and I am sure this has been covered before but I couldn't find it, as anyone added resistors to the headphone jack of the Crack essentially upping the resistance of any headphone? 

That doesn't increase the resistance of the headphone, it just adds to the output impedance of the Crack (99.9% of the time this is a very bad thing).

[bigfatpaulie]

I have no doubt you are correct, I'm just wondering, being a newbie, how so?  Two 300 ohm resistors in series is the same as one 600ohm one, correct? 

[Paul Birkeland]

I'm just wondering, being a newbie, how so? 

The output impedance of an amplifier will be in series with the resistors you're suggesting.  They add together to form a new, higher output impedance.

Two 300 ohm resistors in series is the same as one 600ohm one, correct?

That is correct.  A 300 ohm resistor in series with a 300 ohm headphone is not the same as a 600 ohm headphone though.

[Paul Joppa]

I'll offer an example: Below is a graph of the HD650 impedance as a function of frequency. As you can see, it rises in the treble, and even more around the resonance at 85Hz. If the source resistance is large then the high-impedance regions will be increased in level. Without going into the math, the Crack's 120 ohms gives a 1.1dB boost at 85Hz, but with an extra 300 ohms resistance that boost is 2.3dB. The 1.2dB difference is not very large, but because it's over a broad frequency band it will be audible to most people. In particular, it will be significantly more audible that the small reduction in distortion.

https://cdn.head-fi.org/a/9557608.png

[bigfatpaulie]

Thank you both for taking the time to try and explain this to me, but I must be a little think in the head...

I'm not talking about adding a resistor at the source, I am talking about adding it only after the output.  This could even be a dongle/adapter that plugs into the headphone jack of the Crack, and then your headphones into the adapter.

If it wouldn't benefit high impedance headphones, would it work for low impedance ones? 

-Paul

[Paul Birkeland]

I'm not talking about adding a resistor at the source, I am talking about adding it only after the output. 

Yes, that's what we have been talking about.  Notice the red resistor in the drawing that says "output impedance in series with output".  Again, any resistor you add between this output impedance and the headphones will add to the output impedance of the amp.

If it wouldn't benefit high impedance headphones, would it work for low impedance ones? 

The effect PJ mentioned would be worse/more pronounced with low impedance headphones.

[bigfatpaulie]

Basically, it's a crackpot idea... 

Thank you again.

[fromnowon]

Thanks!

Paul's answer was great.  Seems that using the Crack with 600 ohm phones vs. 300 ohm phones might mean a  "1.2dB difference . . . over a broad frequency band".  My interpretation as a listener would be a slight benefit in output and fullness of sound but not really major, you could notice but not huge difference and as stated different models of phones could drive a much more noticeable difference than just changing impedance of a given model. 

Interesting that doubling the phones' impedance (seems like a huge increase) doesn't have a bigger effect.

Maybe I'll keep my eyes open for some used 600 ohm phones like AKGs to try out.  They seem to be getting more rare in new models. 

This is a great forum.  I'm looking forward to getting my kit and having lots of questions to post!  Thanks.