[weeghel]

Hi everyone!
I'm back again after having built AMB's cavalli kan amplifier last year

I want to build another amplifier. This time I'm looking for a portable design. I'm not looking for an amplifier that I can use "on the move", but one that I can take with me to work and use there.

It should be small, battery powered (re-chargeable is a plus), run for at least 8 hours on a single battery charge and be able to drive earplugs (not a big demand, I admit ). The ability to drive big (high impedance) headphones is not needed, since I won't be wearing those while working

Off course, the AMB Mini-3 looks ideally suited for the task, and I might end up simply building that one. But this time, I'm looking for a bit more challenging build. I'd like to design my own PCB for example.

So if you know of any more designs that meet these demands, I'd like to hear from you.

Thanks!

 

[TzeYang]

LISA III FOR EPIC!!!

Otherwise, just wait for Nelson's discrete portable amp. It should sound pretty good.

 

[weeghel]

oww, that's fast

Could you give me a link to these amps? I'm a bit rusty, here on head-fi (shame on me

)

 

[MASantos]

If your looking at designing your own board, you could go with an opamp plus discrete buffer design. You could search for schematics of popular amp designs such as the PIMETA, M3, PPA, JISBOS, and so on. Be aware that discrete buffers will draw more current than an IC, but taking all the factors in consideration when designing your own amp, should give you lots of learning experience.

Good luck and keep us posted.

 

[NelsonVandal]

3 ch JISBOS. If you make your own board, it can be made small enough for a Hammond J1201 with 10 AAA's. Class A and all discrete - what more could you ask for? Biased to 20 mA or something like that, you still get your 8 h of playing time. I have a stripboard 2 ch JISBOS in a C1201 burning in my pocket. It used to be 3 ch, so I know it's working. I've made mine with gain.

It would be cool to see something like a 3 ch JISBOS opitmized for lower supply, with gain and SMD parts except for the input JFETs and have it in a fairly small box. I think it would be a success.

AD797 with discrete buffers will also make a good amp. Since it'll have a high current draw, you'll have to use AAA's for this as well. AD797 needs a higher supply, at least 10 V, so you can't make it smaller than to fit inside a J1201, I think.

 

[weeghel]

Thanks for all the tips, those discrete amps look very nice.
I'll study them and I'll post here when I know what I want to do.

 

[NelsonVandal]

I've just finnished a stripboard prototype before I went to work this morning, so I haven't done any extensive listening or measurements but it plays music without major faults and seems fairly stable. I can't get the bias as high as I want, so it needs some tuning. I also have to box it up in a Hammond J1201. It's more or less a 3 channel Hiraga le Classe A for 9V supply. I'll let you know how it goes.

Nico Ras at DiyAudio.com has designed a PCB for this amp but with passive ground. I'm about to order some boards. The boards fit a J1201 and looks very nice with star ground.

I'm happy to see more people interested in doing portable discrete amps. I hope there'll be a spin-off effect now. There's a guy at Headwize doing a portable JISBOS-like amp for lower supply.

 

[weeghel]

I think I have decided on what I want to do.

I like the 2ch jisbos design, and I'd like to add an opamp for the gain. Not because I think it's better, but because I'd finally get to use the opa627/637 I have lying around

I'll start a thread on headwize when I've studied the schematics and get a better idea of what I'm dealing with.

I'm not experienced in designing circuits, so I hope I get a lot of help.

For now, this is my plan: opamp input, jisbos buffer, battery operated with a built-in charging circuit.

Ambitious, I admit, but where's the fun in starting a project which you know you can easily finish?

 

[NelsonVandal]

You can actually have JISBOS with gain, and that makes OPA627 redundant, sorry about that

. If you plan on making something with JISBOS, make sure to use powerful batteries because of the high current draw. AAA's are fine. A 9V NiMH can't stand this load, it drains in no time. It's not only the output transistors that are biased high. If I understand it right, the idle current of the second stage is 14 mA with 24 or 30 V supply. In my amp with 12 V supply it's 10 mA. The JFET's idles at 2.3 mA in my amp and with the higher supply it's supposed to be 3. So if you set the output transistors to idle at 20 mA, you'll have a current draw of 32 mA/ch at 12 V supply.

A good way to use OPA627 is to use it in closed loop with a discrete buffer as in LISAIII or PPA.

About 2 vs 3 channels, I prefer 3 ch amps since the soundstage is truer and the sound is overall clearer. The drawbacks are 50 % more current draw and size.

 

[infinitesymphony]

NelsonVandal, how does your portable amp design compare with the old standard Tangent CMoy (and its variants), in terms of sound quality, difficulty of construction, and overall cost?

 

[NelsonVandal]

Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif NelsonVandal, how does your portable amp design compare with the old standard Tangent CMoy (and its variants), in terms of sound quality, difficulty of construction, and overall cost?

I don't really like the original CMOY with OPA2134. I don't really like any amp with that opamp, I think. Having a better opamp in a CMOY can make it a good amp. But... I think a 3 channel topology is better (active ground). The spatial information and the overall clarity improves with it. I'm not entierly happy with my amp yet. The treble is a bit harsh and "edgy" and the voltage swing is a bit low. It has a very good soundstage and lots of energy and is fun, but I like a smoother sound. I don't know if I can tweak it to sound smoother but I'm going to try. Maybe it's misbehaving or maybe this is just the way it sounds

. I don't know if there's anything to do about the voltage swing other than add more batteries.

So far I'd recommend building a portable JISBOS, it works excellent with 12 V supply. I'm going to try it with 9 V sometime and do some trial and error with the resistors. It's very smooth and almost neutral.

The parts cost is of course higher than a CMOY. If you build a JISBOS, you've got to order the JFETs from amb. The most expensive parts are the box, volume pot, trim pots and jacks. There could be some transistor cost depending on how well matched pairs you'll get. I had to buy transistors of different batches to find good matching pairs.

 

[weeghel]

Do you have a link to a forum post or webpage that cover a jisbos with gain?

 

[NelsonVandal]

Quote:

Originally Posted by weeghel /img/forum/go_quote.gif Do you have a link to a forum post or webpage that cover a jisbos with gain?

Instead of using a jumper from output to the midpoint of the 100R offset pot, insert a feedback resistor and another resistor from the point between this resistor and the offset pot to ground. I just picked some values, I can't remember exactly which, I think it was 4k/1k. This could probably be optimized though, but it's working and seems stable.

Like I said, I'm going to tweak the values to find the right ones at lower voltage supply. I can convert my bipolar amp to JISBOS since I have the transistors and resistors on sockets.

 

[weeghel]

I have ordered the parts for both a mini3 and a jisbos from AMB today.
I'll build the mini3 so I'm guaranteed to have a working portable amp

I'll do some experiments with the jisbos to learn about the circuit and use that knowledge to try and design my own circuit and PCBs

 

[NelsonVandal]

Good luck with your amps, hope you enjoy them. As always, report your findings especially with the JISBOS, since there aren't many reviews of it yet as a stand alone amp.