Fisher KX-100 / X-100 B headphone power output rating?

[Bklynbass]

Hello all

Does anyone know if there is any way to calculate or know the actual power output on the fisher KX-100/X-100 B headphone output? My understanding is that amp is rated at 24 watts per channel with the headphone jack wired off the the speaker output with a 330 ohm (?) resistor in series.

Thanks

 

[fred soop]

This would depend on the impedance of the headphones. 24 watts would correspond to 14.1 volts rms out. If the phones are 8 Ω, maximum voltage would be 0.024 V and power would be 0.1 mW. If the phones are 50 Ω, voltage would be 1.855 V and power would be 69 mW.

 

[Bklynbass]

Thanks for that... I forgot to mention that the main headphones in question are 14 ohms...

 

[Tim D]

I seem to remember that the headphone jack had some power resistors connected to it when the speakers were switched out. It might mean that some of the power is being thrown away.

But why ask that kind of question? Would you really want ???W of music power pumped directly into your ears? You wouldn't have hearing for very long. My uneducated guess is that you never get the full power of the amp via headphones.

 

[fred soop]

With 14 Ω headphones, the maximum power would be 23 mW. While the switched load resistors would draw power, they would not reduce the output voltage enough to affect the headphone power.

 

[Bklynbass]

I seem to remember that the headphone jack had some power resistors connected to it when the speakers were switched out. It might mean that some of the power is being thrown away.

But why ask that kind of question? Would you really want ???W of music power pumped directly into your ears? You wouldn't have hearing for very long. My uneducated guess is that you never get the full power of the amp via headphones.

The whole point of my original post is to try to ascertain what the output of the headphone jack is. I recently got a set of cans that are particularly insensitive (90db) and I'm trying to figure out if I need to get a dedicated headphone amp to drive them. Of course you never get the full power of the fisher amp when you're listening to it through headphones. 24 watts into 8 ohms fed directly into just about any headphone would be a disaster... As I originally stated, there is a 330 ohm resistor which knocks down the voltage wired in series from the speaker output to the headphone output, and that resistor is engaged whether the speaker switch is on or off, and I'm trying to figure out what the resultant maximum voltage/wattage would be into 14 ohm headphones. When the speakers are turned off, I believe the amp has a dummy load that is engaged to protect the transformer in case no speakers are attached which is why some folks buy these units to use primarily as a headphone amp (there are a lot of posts about the KX-100 on head-fi). That may throw some power off but it's not audible to me.

 

[Bklynbass]

With 14 Ω headphones, the maximum power would be 23 mW. While the switched load resistors would draw power, they would not reduce the output voltage enough to affect the headphone power.

Thanks for that! Wow, 23mW seems really low! Are you sure it's not 230? Most headphone amps these days are at least 100 mw into 16 ohms.... Would you mind showing me how you made the calculation? I've tried to read some tutorials about ohms law, etc but I'm too mentally challenged to figure it out

 

[fred soop]

24 watts out is 14.1 volts out at whatever impedance the 24 watts is rated.

The internal switched load resistors are not going to reduce the 14.1 volts.

Headphones are 14 ohms. Dropping resistor is 330 ohms. So headphone voltage will be 14.1 volts divided by the load which is 14 ohms divided by (330 ohms + 14 ohms).

14.1 volts multiplied by 14 / (330 + 14) = 14.1 x 0.0407 = 0.5738 volts to the headphones.

0.5738 volts divided by 14 ohms = 0.041 amps.

0.5738 volts X 0.041 amps = 0.0235 watts = 23.5 milliwatts.

My Koss Porta-Pro headphones are rated at 101 dB SPL at 1 milliwatt. 23.5 mW would be +13.7 dB or almost 115 dB SPL. This is very much into the long term or permanent hearing damage category, so you would have plenty of power.

 

[Bklynbass]

24 watts out is 14.1 volts out at whatever impedance the 24 watts is rated.

The internal switched load resistors are not going to reduce the 14.1 volts.

Headphones are 14 ohms. Dropping resistor is 330 ohms. So headphone voltage will be 14.1 volts divided by the load which is 14 ohms divided by (330 ohms + 14 ohms).

14.1 volts multiplied by 14 / (330 + 14) = 14.1 x 0.0407 = 0.5738 volts to the headphones.

0.5738 volts divided by 14 ohms = 0.041 amps.

0.5738 volts X 0.041 amps = 0.0235 watts = 23.5 milliwatts.

My Koss Porta-Pro headphones are rated at 101 dB SPL at 1 milliwatt. 23.5 mW would be +13.7 dB or almost 115 dB SPL. This is very much into the long term or permanent hearing damage category, so you would have plenty of power.

Wow, thanks for that very clear explanation. So unless I did the math wrong, if I put in 90 ohm resistors, I'd increase the output to 257 mW. The headphones I'm using are only rated at 90 db and the recommended minimum power is 250 mW at 14 ohms. Only thing I'm not clear on is what wattage they should be rated for.

 

[fred soop]

Without changing the resistors, you should still be able to get 105 dB. Start with the existing resistors and see if a lower value is needed. Then experiment with values.

 

[Bklynbass]

The problem is not the volume level. It’s the sound. Above a certain level I start to get clipping because of the low sensitivity (90db SPL @
1 mW) of the headphones. Kind of like when a low wattage tube guitar amp starts to overdrive. I have another set of cans that are a higher load (20 ohms) but have a sensitivity of 103 db and they sound amazing through the same amp. In its current state it seems that the amp just has to work too hard to drive the lower sensitivity headphones. The manufacturer recommends a 500 mW amp or at a very minimum 100 mW, so 23 is way too low.

 

[fred soop]

Got it. You need to reduce the voltage drop to the phones to avoid having to turn up the volume to clipping.

 

[Bklynbass]

UPDATE ALERT!

So after a very long 11 months I finally got around to doing this mod... well, actually my friend, who's a recording engineer and much handier than I am, did the soldering. The switch and output jack wiring is densely arranged and tucked very tightly under the power indicator lamp assembly and I don't trust my ham fisted soldering skills to get the job done without at least a minor disaster. I took the KX-100 over to his shop, opened everything up and showed him exactly where the operation was to be performed, aided by a screenshot from the assembly manual I downloaded off hifiengine.com. We swapped out the 330 ohm resistors for 60 ohm, which, by my calculation, raises the effective wattage of the headphone output from a very low 23 mW up to about 500 mW. I also had him reverse the connection of the resistors so the headphone jack would finally be wired correctly.

So I finally have this beautiful thing operating more or less in line with modern headphone amps, (at least in terms of power), so I can enjoy the all tubey, gooey goodness from the legendary fisher headphone output we all know and love, even with low sensitivity cans like these amazing Audeze LCD-5s. I recommend it if you also have low sensitivity headphones and have great soldering skills like my friend or have someone you know who can pull it off for you.

One caveat: there's a hum on the left channel that can be noticeable now, depending on how high I have the volume turned up, due to the higher output on the headphone jack. So I may now need to sort that out at some point...