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6EM7 Headphone Spud - Please comment

6L6

Moderator
Paid Member
2010-10-22 6:43 pm
Denver, Colorado
Hello all!

I recently came across a pair of Hammond 125CSE for such a deal that I could not pass it up. (Free...)

Anyway, I have decided to make a 6EM7 SE 'Spud' amp for headphones. Total overkill in the output department, but what the heck, right? That's what volume controls are for.

Primary impedance will be as high as can be made, 8-10k, to help with coupling, linearity and to keep output a little lower. The 30ohm resistor across the secondary is to help keep the transformer loaded properly and attenuate the output a bit as well.

PSU voltages are not written in stone, I need to go to my project box and see exactly what I have to use. It will be tube-rectified with a normal CLCRC filter.

Schematic is based from a project of Tom McNally.

6EM7SpudHeadphone.jpg


Any comments are welcome from anybody who has experience with this sort if thing! I have never made a Headphone amp and am very open to pointers and suggestions.
 

artosalo

Member
2010-02-16 9:00 am
How much gain do you need ?
What is typical output voltge and the impedance of the headphones ?

At first the output section seems to be biased very hot !?
What is your target plate dissipation ?

Hammond 125CSE has quite low inductance which in typical SE-output stage causes the loss of bass response, but the ra of 6EM7 (2nd section) is very low (750 ohms).
Therefore I believe the op-transformer will work fine even at bass frequencies (< 100 Hz).
 
It's a pretty straight forward design that should work OK. What headphones is this for? For low Z, gain will be very high. You might leave out the cathode bypass cap on the first stage to keep this under control a little. Also, loading with something smaller than 30R might also be useful for soaking up some power.

The biggest challenge will be noise, and you may find that AC heaters won't work, or that a B+ regulator is in order -- even just a few zeners and a pass transistor of some sort can work wonders.
 

6L6

Moderator
Paid Member
2010-10-22 6:43 pm
Denver, Colorado
Thanks for the feedback so far!

Gain - I will be using a CD player with 2v output.

I am planning on using 32-64ohm headphones. Grados some day... I have a set of 64ohm Sennheiser that will get replaced - I don't find them to be very comfortable. I thought that 30ohm on the secondary would be appropriate with that kind of headphone - please correct me if I am wrong.

Voltages as drawn are too high - I'm thinking that 250 on the power stage and 200 for the voltage gain stage...

Could bias on the first stage be done with a diode?

PSU - I have an 8H 150ma choke that I am wanting to use. Tube rectified with 5Y3, then CLCRCRC I have a number of 47uf 450v caps to use up. DC heaters seem to be mandatory as well.
 
Last edited:

artosalo

Member
2010-02-16 9:00 am
Gain - I will be using a CD player with 2v output.

Then you need no gain, I think.
What would be sufficient power input to headphones ?
Would 30 mW be enough ? One volt across 30 ohms headphones is 33 mW.

Could bias on the first stage be done with a diode?

It can be done, but I do not see any reason why to use diode instead of resistor. Please, somebody tell me.

I would try a topology something like this:

[IMGDEAD]http://kuva.termiitti.com/image/14590.gif[/IMGDEAD]