Portable amp for low impendence phones Design options & component selection

[Nicwix]

Hi all

I started a thread on Headwize (id 2703): Best opamp-buffer combination for a portable multi-loop amp for low impedence phones which is developing into a discussion on the best design for a battery powered portable amp for low impendence phones (such as Sony, Etymotic and Grado, certain Beyerdynameic and Sennheiser models, etc).

There are obviously a number of conflicting design goals, such as:
1. high-end sound quality
2. small, portable package (board and battery)
3. good battery life (say 20 to 40 hours)
3. high current capability
4. as minimalist as possible (I like to know what each component is there for)
5. reasonable cost and easy to source components
6. relatively easy to build and package

What makes things a bit easier is that only a low voltage swing (say 2V p-p) is sufficient and little gain is required (say 2x = 6dB) for efficient phones.

Contributions from people on Head-Fi would be valued - either directly in the Headwize thread (click link above) or here in this thread.

Nix

 

[NotoriousBIG_PJ]

Tangent built me a cha-47 that runs off of 3 AAA batteries (rechargeables). It has 40-50 hours battery life and sounds pretty nice with my beyers. I can't tell you what is under the hood though, so you would have to ask him if you care.

Biggie.

 

[tangent]

Quote:

I can't tell you what is under the hood though, so you would have to ask him if you care.

Well, there's a CHA47 under the hood, of course.

The reason you can get so much battery life out of that CHA47 on just 3xAAA (4.5V) is the AD823 op-amp I used will run down to 3-4V without distortion, depending on the headphones, the source, and the music. The AD823 is also quite current-efficient. The whole amp probably draws on the order of 20mA at peak. AAAs are ~1100mAh, so you'd expect 55 hours of run time in the best possible case. So, 40-50 hours is about what you'd expect in the real world.

I don't know of a comparable chip from an electrical perspective that also sounds as good. Surely they must exist, but they haven't been discovered by us DIYers yet.

I was doing some tests on the fine new META42 design, and because it uses half the number of op-amps as the Hansen and CHA47 amps, it uses nearly 1/2 the current. There are some additional current draws in the META42 not present in these other amp designs, but they're small.

Quote:

There are obviously a number of conflicting design goals, such as...

The META42 meets criteria 2, 3a, 3b, 5b, and 6 for sure. Points 1 and 5a I have no business making a public statement about. That's for someone else to decide.

Point 4 is variable with the META42: you could build an über-CMoy on the board if you wanted, or you can set it up for every option the board allows. That's entirely up to you. There is an article under way right now that will help you choose among the options the board allows. The article describes every section of the circuit -- why it's there, what the result is if you enable that section, how to do it, what parts you need, etc.

I'd give you the URL to the article, but it's still under construction, and I recently learned a lesson about giving out URLs before what they point to is finished.