Headphone impedance matching with tube head amp

[albertli]

Guys,
I lately was gifted a headphone amp which was built with the 6BX7 as output tube and it says it's output impedance is 300 ohm.
I have a Sennheiser HD555 with the input impedance 120 Ohm, and don't have extra money to invest for the new headphone at the moment. Is there a simple way to make it work.
Thanks in advance
Albert

 

[sgrossklass]

Please clarify: Is the output impedance 300 ohms or is that the recommended load impedance? Output impedance for an OTL cathode follower (sans feedback) tends to be a little under 100 to several hundred ohms, depending on tube.

Since the 6BX7 seems to be a medium-power tube and the early "120 ohm" HD555 (if it truly is one) is closer to 150 ohms and quite efficient, I wouldn't be overly worried about volume. Output impedance could mean things get a little more bassy than they should though.

 

[albertli]

Please clarify: Is the output impedance 300 ohms or is that the recommended load impedance? Output impedance for an OTL cathode follower (sans feedback) tends to be a little under 100 to several hundred ohms, depending on tube.

Since the 6BX7 seems to be a medium-power tube and the early "120 ohm" HD555 (if it truly is one) is closer to 150 ohms and quite efficient, I wouldn't be overly worried about volume. Output impedance could mean things get a little more bassy than they should though.

Thanks for your reply.
Yes, this is a OTL with no transformer at the output. My friend has a pair HD600 and this is 300 ohm.

 

[Stixx]

Just connect your headphones... there is no real impedance matching with an OTL-design. As sgrossklass says it will sound with an emphasis on bass but that can be pleasant.
To tighten up the sound you can try things like Oscon as cathode bypass (when there is any) for instance.

 

[albertli]

Just connect your headphones... there is no real impedance matching with an OTL-design. As sgrossklass says it will sound with an emphasis on bass but that can be pleasant.
To tighten up the sound you can try things like Oscon as cathode bypass (when there is any) for instance.

Thanks for the advice.
I did try hooking it up and listen. The sound was murky with No bass. This leads me to think about matching. What if this tube amp was purposely designed for 300 ohm headphone. I have to raise the impedance of my hd555 to match. Is there any easy hook up I could try?

 

[DF96]

Maybe the output coupling cap needs to be bigger? This may increase bass a little.

 

[jameshillj]

There's the old trick of adding a series resistor inline with the signal o/p that's sometimes useful to 'trim-up' the sound of a 'phones - sound a bit dumb, I know, but easy one to try - a 100R in each + signal line.

My Cavalli OTL prefers a 120R in series for the Senn 650s, for example and it balances up the lower mids/bass region quite a bit - we all have different tastes in headphone sound so don't be surprised if this doesn't work, but worth a try.

 

[albertli]

There's the old trick of adding a series resistor inline with the signal o/p that's sometimes useful to 'trim-up' the sound of a 'phones - sound a bit dumb, I know, but easy one to try - a 100R in each + signal line.

My Cavalli OTL prefers a 120R in series for the Senn 650s, for example and it balances up the lower mids/bass region quite a bit - we all have different tastes in headphone sound so don't be surprised if this doesn't work, but worth a try.

Sound like a easy way to try. Thanks n will post for result.