Crack + Speedball - ohm rating for headphones

[Neil]

As per the FAQ note below, what is the actual range for the Crack + Speedball? 150-300 ohm? or is the range wider than that?

6.  I want to use my Crack with low impedance headphones, why is this bad?

There are several reasons for this.  The first one is that the CR filter formed at the output with the 100uF capacitor and the low impedance headphones will result in a frequency response that is not all that optimal.  For 32 Ohm headphones, you'll be down 3dB at 50Hz.

The second reason is that the output stage of the Crack is a current amplifier, and Ohm's law tells us that Power=(Current2)*Resistance.  When resistance goes down, power also goes down.  While this may not be that big of a deal for a sensitive headphone that is low impedance, it is a big problem for power hungry headphones that are also low impedance. 

The last reason that comes to mind is that the output impedance of the Crack is not sufficiently low to drive low impedance headphones with good damping and distortion performance.  This can be circumvented with the implementation of a global feedback loop, but doing so tends to collapse the soundstage of the amplifier. 

https://bottlehead.com/smf/index.php?topic=4295.0

Edited to make the quotation a quotation

 

[Tom-s]

You can go higher if you want to.
I used some 800 Ohm philips 80's headphones with mine once, worked out fine.
Would not advise on using lower impedance headphones.

You could go to 120-100 ohm with some output tubes and a larger last powersupply + output cap setup.

 

[Paul Birkeland]

As per the FAQ note below, what is the actual range for the Crack + Speedball? 150-300 ohm? or is the range wider than that?

You aren't going to damage anything by going lower than 150 ohms; the amp just isn't designed to perform well into those loads.  You can go much, much higher if you like. 

 

[Neil]

You can go higher if you want to.
I used some 800 Ohm philips 80's headphones with mine once, worked out fine.
Would not advise on using lower impedance headphones.

You could go to 120-100 ohm with some output tubes and a larger last powersupply + output cap setup.

You aren't going to damage anything by going lower than 150 ohms; the amp just isn't designed to perform well into those loads.  You can go much, much higher if you like.

Even as low as 18 ohm (sennheiser momentum)?

And in terms of feeding the Crack - is it a "no-no" to feed it Line Out? Or is a Line Out feed the most ideal source signal?

The Schiit Modi 3 is working very well into the Crack. But I'm curious about using my Pioneer 300R as a source for the Crack - but not sure if I should enable the Line Out or just use normal gain to feed the crack?

 

[Doc B]

Even as low as 18 ohm (sennheiser momentum)?

You can do whatever you want. We think that is not going to sound optimal. There are only so many ways we can say that.

Use line out to drive the Crack.

 

[Neil]

You can do whatever you want. We think that is not going to sound optimal. There are only so many ways we can say that.

Use line out to drive the Crack.

Right on thanks Doc - just want to make sure I don't accidentally ruin anything!

 

[Doc B]

Don't crank it up very loud. Some low impedance headphones need very little power to overload.

 

[Neil]

yeahhhh just tried it and it really didnt sound good at all.

i'll try line out of the 300R later - thanks again!