[SilverTrumpet999]

The U70 is an orthodynamic headphone, similar to ones built by Stax, and designed by MB (according to a previous head-fi poster in a different thread). I recently acquired a pair. Documentation/info on these is scarce at best.

Some specs for the headphone and a picture are at the other end of this link: http://www.beoworld.org/prod_details.asp?pid=917

The important parts for my purposes: 140 Ohm impedance, continuous input power of 2 Watts (!) tolerated. From my initial tests, these are quite difficult to drive with standard sources, and I am feeling the need for an amplification upgrade. I could stick with one of my preamps' headphone out, but they are decidedly not mobile and likely not optimal for this use.

I'm looking for recommendations for a reasonably priced (college student here!) portable amplifier to do these justice. I am savvy enough to put an amp together myself, given a circuit diagram and some spare time, so that is an option as well - especially if it will perform better/save some money. Thanks!

 

[ericj]

One of the great things about orthos is that they present an essentially resistive load to the amplifier, so the amplifier need not be at all exotic.

Be forewarned, due to lack of backwave control, you will never hear much bass come out of these. It's not that the drivers can't do it, it's because the backwave cancels out the front wave. So far, none of our resident U70 owners have been brave enough to figure out how to open up the casing and get at the driver to stuff some damping materials behind it.

You don't need a ton of power - any of the buffered amps you can power with two 9v batteries should do the job. Possibly even 1 9v.

I'm not an expert on amps you can buy. if you want to build something, sijosae's "simple class a-b" version 1 may do a fine job. one opamp, a dozen transistors, handfull of resistors.

You'll want to start with a gain around elevenish, probably.

 

[facelvega]

Yep, MB ortho drivers are power-hungry, more so than Yamaha drivers but not so bad as the east german knock-offs. A 9V or 18V portable amp will get them loud, but still not squeeze all the potential out of them.

The bigger problem though, as ericj points out, is the absence of damping. There's no point in worrying about amplification until you've solved that one. Supposedly with earpads they're bassy but lose the upper register entirely, and without them the bass disappears. Rather than damp behind the driver for these, I'd recommend pulling off the pads and substituting some thick felt with a hole cut for the driver face. Might not be as comfortable as the earpads, but it should solve the bass and treble problem at one blow. The other option is to try to damp behind the driver if possible, but then this still leaves the problem of the earpads. If you post a couple of pictures of the way the original earpads look, we might be able to help more.

 

[SilverTrumpet999]

As I have heard is typical for the U70s, mine have the original black leather (pleather?) layer removed, leaving the white cotton pads underneath. These remaining pads are in good condition.

This is NOT my picture (pulled from a completed ebay.co.uk auction) but the condition of my pair is virtually identical to the ones depicted below:


The velvet option sounds interesting, facelvega, and I would love to damp behind the drivers too - if I knew how to get back there. Ideally I'd find a second pair, sans pads, and experiment on those... if it were a perfect world.

 

[benherron]

Would any old B&O tuner/amp work *thinking 7000 model adm* the same as a dedicated headphone amp? Going for a bit of a retro buying spree and there's a few knocking about fairly cheap on ebay. It would look much cooler than a new headphone amp even if I didn't use the tuner part of it, but would they be as good for powering my U70s?