Newbie!!
Hi, everyone 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
Hi, everyone 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
There may some bass roll off, due to the lower than design load impedance.
Distortion could be a bit higher as well. But it will work ok.
Distortion could be a bit higher as well. But it will work ok.
Thank you 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?
First try a series 180R resistor in each channel, as in post #4.
The bass should improve, and the noise will definitely be less.
You may be able to get a jack adapter that you can install the resistors inside.
What are the plug and socket that you have, 1/8" or 1/4" diameter?
If this is not good enough, post the schematic here.
The bass should improve, and the noise will definitely be less.
You may be able to get a jack adapter that you can install the resistors inside.
What are the plug and socket that you have, 1/8" or 1/4" diameter?
If this is not good enough, post the schematic here.
Last edited:
It’s a white cathode follower OTL amp. Depending on design choices, it has jumpers on the PCB to switch between two output impediances or between output tubes. More info on EBay where the PCB is being sold. Seller is me, hvforless.
You are looking at optimizing R17 (1 per channel). Formula is R17=(rp + 2Rload)mu. You have either 6BX7 or 6BL7 output tubes. B+ is 300VDC. You'll also have to see what resistor was chosen for R11 to set the idle current. There are jumpers at R11 and R17 to give two options.
That's all the info you need (and tube datasheets) to calculate the ideal resistor values.
That's all the info you need (and tube datasheets) to calculate the ideal resistor values.
Typo above. Formula is R17=(rp + 2Rload)/mu. Rload is the headphone impedance.
rp and mu change with the idle current and voltage, but you can get close enough on resistor values with datasheet averages for both variables. For example, for a 6BL7 use 15 for mu and 2000 for rp and for a 6BX7 use 10 for mu and 1500 for rp. That will get you about +/- 5 or 10 ohms for R17.
It's also easy enough to calculate the values of rp, since that's the one that's mover variable than mu. Looking at R17 and using a fixed mu of 10 or 15 (6BX7 and 6BL7 respectively) you can calculate what rp is for the operating point of the circuit. Then just redo the formula for your headphones.
rp and mu change with the idle current and voltage, but you can get close enough on resistor values with datasheet averages for both variables. For example, for a 6BL7 use 15 for mu and 2000 for rp and for a 6BX7 use 10 for mu and 1500 for rp. That will get you about +/- 5 or 10 ohms for R17.
It's also easy enough to calculate the values of rp, since that's the one that's mover variable than mu. Looking at R17 and using a fixed mu of 10 or 15 (6BX7 and 6BL7 respectively) you can calculate what rp is for the operating point of the circuit. Then just redo the formula for your headphones.
Last edited: