Bogen CHB35A Headphone Jack Mod?

[tbryanh]

I want to add a headphone jack to a Bogen CHB35A mixer/PA http://www.schematicheaven.com/hifia...gen_chb35a.pdf

I want to use the mixer to mix the output of the microphone from my isolation cabinet and the output of a CD player, YouTube Audio, of Google Video Audio etc.

I am not worried about where to connect the inputs to the mixer, its adding the headphone jack that I am concerned about.

I am considering removing V3, V4, and V5 and putting a headphone transformer on the output of V2B.

Sowter Transformers http://homepages.tcp.co.uk/~sowter/a...ORMERS_29.html has headphone transformers with primary impedances between 5-10k ohm.

I have not bought headphones yet, but the transformers will work with 40, 70, 150, 300, and 600 ohm headphones, so I am thinking I can find a transformer to match whatever headphones I buy.

I have heard that headphones need about a half watt or so of power.

Does it appear that the modification I am considering making will work well as far as getting the right amount of power and getting a good impedance match between the primary of the transformer and the V2B stage?

Also, I was wondering if Hammond Transformers or some other manufactuer has a headphone transformer that might work. I wanted to compare prices and availability. Sowter might not be the best, I don’t know.

Also, any feedback on some decent headphones that are not priced too high will be appreciated too.

Thanks.

[Steve Conner]

Hmm, if you have the complete unit and it's all working, I'd be inclined to just stick a dummy load on the speaker output and connect the headphones across the dummy load with some kind of attenuator. If you make this mess switchable, you can go back to jamming through some stereo speakers or whatever, when the neighbours are out.

Don't attenuate it too much, as you don't want your power tubes running hard, it'll only wear them out prematurely, and 7868s are rare. The 4 ohm tap might work fine with no attenuation at all (but be careful not to trash your headphones/ears if you try that!)

If you don't want to do that, I'd try the Sowter transformer, or any small single-ended OT from a table radio etc. would work. I don't see half of a 12AX7 giving you enough clean headphone volume though. Maybe the 6C4 would. You could connect the transformer primary in place of the 6C4 cathode load resistor (the 22k one) and also short out the 22k plate load resistor and unplug the power tubes.

Or maybe you need to add a 12AT7 or 12AU7 with the sections paralleled, or even a 6V6 or EL84 or whatever...

I like the Sennheiser HD215 headphones.

[tbryanh]

You could connect the transformer primary in place of the 6C4 cathode load resistor (the 22k one) and also short out the 22k plate load resistor and unplug the power tubes.

This mod sounds good. It looks similar to HeadWize's Two-Stage Transformer-Coupled Amplifier http://headwize.com/projects/showfil...ciuff3_prj.htm

Any reason why the transformer is in the cathode rather than the plate like a typical OT?

[tbryanh]

You could connect the transformer primary in place of the 6C4 cathode load resistor (the 22k one) and also short out the 22k plate load resistor and unplug the power tubes.

The DC bias point would shift I guess. But maybe that applies to common cathode, not cathode follower.

[Steve Conner]

Putting the transformer in the cathode ought to give lower output impedance for better bass damping. But YMMV.

[Jag]

Can't help much on the Headphone Jack mod.

I really like AKG240 headphones. I've got 5 or 6 pairs in my studio and a couple pairs I carry with me for listening to CD's, etc.

If you're going budget, I think the Behringer headphones are okay as are the Apex phones (they're Canadian, I don't know if they're available outside the Great White North).