[rcgrant]

I'm planning to build a PIMETA for use with my future Etymotics. I have some questions regarding portability issues:

How significant are they benefits of using an 18V power supply over a 9V power supply, if any?

If the 18V power supply is worthwhile, what would be a good enclosure for the board and two 9V batteries?

Should I NOT bias the opamps into class A if I'm using battery power?

Any answers or other advice are appreciated.

 

[n_maher]

All I can tell you is what I know from personal experience so here goes:

If I was going to build another portable PIMETA I'd:

1. Use a dual 9V supply to maximize battery life.
2. Use a Bulgin battery drawer or an enclosure with a built in drawer/door.
3. Use single buffers.
4. Not bias into class A.
5. Make sure it's a rugged, semi water resistant enclosure.
6. Use the panasonic or smaller Alps pot, blue velvet will limit your enclosure choices.

That said here's a couple of pics of one of the "portable" PIMETAs that I built:

HTH,

Nate

 

[DobsOnly]

n_maher,
Which Hammond case is that?

DobsOnly

 

[skyskraper]

i put my pimeta's in to the standard serpac h65. i preffer portability rather then transportability, and for me, nate's setup is still too bulky. the h65 is even a bit bigger then id like but its the best option ive come across for a pimeta so far. no problems running off a single 9v so long as you choose the right op amps, you can also get more battery off it by not using the class a biasing, i didnt notice a major difference in my portable use not using class a biasing.

these are pics of an amp i built for a member in canada...

 

[mono]

Serpac H65 was my choice too, it is a very good match for single-9V use but you would have to decide the needed runtime and design around that regarding the multiple buffers, class A biasing and opamp choices.

While there is a "little" bit of wasted space in the H65, it's still about as good as it gets for a Pimeta and it's rounded corners also make it more desirable for carrying in a pocket or other limited-space. Often I see twin mini jacks on the front but I put the input on the side and use a short 90' mini-mini cable... mainly because I like a little free space on one side of the volume know and I used a short knob.

If you're a glutton for punishment you can even file and chisel away some PCB and casing on the H65 to squeeze 6 ("maybe" 7) prismatic cells in it. I'll post a pic of mine if I get a chance but it isn't "pretty", I usually go for function over form... not that anyone else is sacrificing function, I'm just not as particular about form.

Towards the end of function, I recommend opamps chosen for sound quality, not low current, and the class a biasing. My feeling on this is that if you're going to too many lengths to limit current usage you might as well just build an (inherantly smaller) Mint instead.

 

[n_maher]

Quote:

Originally Posted by DobsOnly n_maher, Which Hammond case is that? DobsOnly

1455N1201BK

And skyscraper raises a good point, the word "portable" means different things to different people. For me it meant that I could easily take it with me, not it can fit in my pocket. Neither is any more correct than the other, just different interpretations. My original plan was to have a matching STEPS for that PIMETA so I had two identical Hammond cases. And as you can see that's about the smallest enclosure that you can fit the battery drawer in, without modification that is.

And to my ears there was a plainly audible difference between my PIMETA (pretty maxed out) and my MINTs (also maxed out).

HTH,

Nate

 

[looser101]

rcgrant,

Have you seen Tangent's own portable Pimeta with his Ety's in the "Post Pics of Your Builds" thread?

Maybe he knows something about what you are asking....

Cheers,

-R-

 

[skyskraper]

tangents pimeta is in a similar enclosure.

i chose not to stack buffers but keep the class a biasing. i like the opa134/132 series in my portable pimeta's, an enjoyable sound and low current draw!

i really like the pimeta for a portable amp, in the h65 its a bit big for my pocket, but doesnt take up much room in my bag at all. usually when im commuting i have a tonne of books, or if im travelling o/s i like to use a small bag, so size is a concern

 

[DobsOnly]

[size=medium]--n_maher--[/size]

Thanks for the Part number.

[size=medium]--rcgrant--[/size]

I have started assembling my pimeta with 4 1000uf FC caps, stacked buffers and to conserve a little on the batteries, no class A biasing. If you’re going to try and go with the smaller case be careful on your parts choice. Look at the layouts the others have posted for ideas on minimizing space.

Sorry I can't advise on the audible or operational difference if any between 18v and 9v. I would suspect there are several factors including OpAmp choice, stacked or single buffers and Class A operation or not.

My current build is a little big for the Serpac Case at over 1" tall. Any way I'm thinking something big enough for the board and 2 - 9v batteries for this one. Thats why I like n_maher's case and am looking for something along those lines.

If this one turns out Ok I might go for the portable version like skyscraper and the others. Those guys are pretty good at neatly stuffing these things into small areas.

Good Luck

DobsOnly

 

[rcgrant]

Thanks to everyone for the advice. For the sake of simplicity, I'm favoring a single-9V design using a Serpac H-65 enclosure and AD8610/20 opamps, of which Tangent speaks very highly for battery-powered use.

I'm not sure yet about class A biasing. How much life can I expect from a typical 9V battery with and without biasing? I plan to use rechargables, so I really only need enough time to get through the day.

 

[mono]

It's not just simplicity to do that but being realistic too.

There is very little to no gain in using thousands of mfd of bulk caps on a Pimeta. Maybe everyone is simply getting confused about the differences in power requirements between a multi-watt amp and that needed for headphones. In a Pimeta the faster film caps are far more significant until you REALLY crank up the volume.

Point is, there might not be any gain to a large case from a functionality standpoint unless you NEED more power or I/O jacks, more batteries for runtime or voltage. The safest way to make these choices (IMO) is build the amp, then try it with one 9V. You might be VERY surprised at how low a voltage a Pimeta can use for typical cans. Set up a variable voltage power supply, reduce the voltage while listening... make up your own mind how low the voltage can go before you hear a difference.

The statement that will provoke some:
Most people using 18V are simply wasting space and a battery. If they have cans that are hard to drive it's a different matter but those are the minority.

 

[mono]

If you need it to run a whole day on a typical 9V (which is still under 250mAH IIRC) then you do need to be conservative with current... single buffers and no class-A. "Whole day" is pretty vague though, it could mean you do need to parallel a pair of 9V.

 

[rcgrant]

Is the current draw so great that a 9V will only last for a few hours? If that's the case, maybe I should be considering another CMoy instead. The one I have at my office seems to last very long on a single battery.

 

[n_maher]

Sounds like you're going through a lot of the same debate that I did during the past 6 months. Instead of just doing another CMoy, maybe think about a MINT. I have one built with a OPA2227 and another built with the AD8620, gotta have the senn and grado focused amps!

Both sound significantly better than even a tweaked Cmoy. Here's an example of one of the MINTs that I use as a portable:

Both of them run for at least 20hrs on [2] 9V rechargeables.

 

[rcgrant]

Shouldn't a PIMETA have comparable battery life to a MINT, seeing as how their circuits are virtually identical?