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My First Amp: choosing a design?

post #1 of 10
Thread Starter 
Hello!
I've decided to build my first headphone amp. I have very little electronics knowledge. I mean, I have built many circuits, but I know very little about how the work.

I have spent alot of time the last two days reading the forums here and at HeadWize and looking at the projects there. I want to build a portable amp. I think there are two options: the popular cmoy amp and the Lindesberg Portable Headphone Amplifier with Crossfeed by Toni Kemhagen. I know alot of people recommend starting with the simplest design, but first I'd just like to ask some questions:

1) What are the differences in sound quality between the two designs?
2) How does the sound of the Ohman crossfeed filter in the Lindesberg amp compare to other crossfeeds (Meier, modified Linkwitz)?

And some questions specifically about the Lindesberg amp:
3) The author uses four 9v batts. Does it run fine with just 2 (using 1 18v supply instead of 2)? Or can it even be run from just 1 9v (+/- 4.5)?
4) This would be obvious to anyone with electronics knowledge but... what is the purpose of the resistor and inductor? Could they be omitted or some other inductor substituted?

Thanks.
post #2 of 10
I think that those questions would require having built both amps and that's why no one has responded yet! The Cmoy amp is easy to build and a very succesful design. The Lindesbergh is (in my opinion) a more sophisticated design. The Lindy may have a technical edge in performance, but as to what the audible difference is is a tough question. For some people, it may not be significant, for others it may.
The power supply voltage for the Lindy is probably not something I would change without refering to the data sheets for all the chips involved. The inductor and resistor on the output would be there for stability, and I would think you would not want to fiddle around with them without seeing what the manufacturer suggests. There may be some flexibility there, but layout of the design is also an issue for stability. I hope I have helped!
post #3 of 10
Hi mizterbob

1)
Well, I have never build the cmoy so I really don´t know.

2)
There is more crossfeed in the Ohman filter, fore my taste the Ohman is perfect. Specially with classical music I get a perfect soundimage in depth and right to left.

3)
Yes, the amp runs on 18volt and 9volt supply also but then you need a splitter (fore example TL2426).

4)
The resistor and inductor is there to isolate capacitance from the load.


Regards Toni
post #4 of 10
Thread Starter 
Quote:
I think that those questions would require having built both amps and that's why no one has responded yet!
I was afraid of that. I can't believe nobody has built both of these!

Would anybody be interested in a comparison of the two amp designs? I might just want to try both... One thing though, where is the best (preferably no minimum order) place to get Analog Devices opamps in the US?
post #5 of 10
mizterbob,

i think you will find that the basic cmoy amp has been built many many many times and modified even more.

the Lindesburg design looks a little more complicated, but has probably been built as well.

I too am contemplating what to build as a first amp.

I have chosen the basic cmoy with no crossfeed. I plan to build this large and messy with enough space in the wiring area to add switches for crossfeed (which i will build later on separate boards). this way i can test different types of crossfeeds and see which one(s) i want to add to my final setup.

the Lindesburg design looks like fun, but it uses 4 of the AD823 chip which i believe is only available in "soic" format (small outline integrated chip) and when they say small, they mean small, i.e. more difficult to work with.

why 4 batteries, i think the author is very clear on that in his description, to have lots of available clean power. and he has separate power for the input and output. the chips should work with 2 or just one battery, but you may not get the best sound in more intense parts of the music. also, you will be changing batteries every 2-4 hours depending...

i wouldnt remove anything from the design if i were you. i think the author knows his stuff and put the output resistor and inductor there for a reason. besides, if you are going to build this, you might as well put in these 2 pieces (per channel) you can buy inductors as little resistor like thingys.

if you want a good sounding amp, but dont want to carry around a 4 battery powered box, dont shave down this amp, just build a one battery cmoy and work up from there!
post #6 of 10
Personally, I think you should build the simplest amp first. If you're like me, you won't build it correctly the first time, and this will be educational. That education will serve you well when you later build more complicated amps.

In building my cmoys I have learned:

o enclosure machining
o the 422 incorrect ways to handle grounding
o the most effective way to use a soldering iron
o how to translate a schematic to a protoboard layout
o how to pick substitute components when the ones spec'd aren't available
o how to build an amp in testable sections to simplify troubleshooting

All that experience will make my next amp that much easier to build. In the end, I'll have more amps than I really need, but the later ones will work better than they otherwise might if I had simply built them from the start, because of the experience I gained building simpler amps first. This is already true of the four straight cmoys I've built. I expect to continue improving for a long time to come.
post #7 of 10
Quote:
Originally posted by joeyliao
mizterbob,
if you want a good sounding amp, but dont want to carry around a 4 battery powered box, dont shave down this amp, just build a one battery cmoy and work up from there!
I think so to.
It´s best to start with a easy project if you feel that my Lindy is a bit complicated.
Build a cmoy and try different crossfeeds to find out which you like.

Toni
post #8 of 10
Hello,

I am not fond of having many opamps in a signal path. I know many preamp does it to sandwich tone control stage and other filters. Less components the better unless you use more components to biase/stablize the circuit.

I think crossfeed filters are not all that necessary. For me, having crossfeed filter fuzz up the details sometimes. The benefit is small compared to the much fresher sound without the filters. I only use the crossfeed circuit for long term listening over 10 hours.

CMOY amp would be much better choice not only because it is easy but it is educational. You will learn how to use opamps. You should also read some textbooks on electronics on opamps. From that point, you can understand descrete designs which are more complex. (Note opamps are linear while transistors and mosfets are not so latter are much more difficult to grasp. ...)

Tomo
post #9 of 10
Thread Starter 
Thanks for all the comments. Everyone seems to agree that a cmoy would be a better first amp because it is easier to build. I'll take your advice. A couple questions:

1) If I decide to play with opamps, where do you guys order Analog Devices products?

2) tangent: what are "the 422 incorrect ways to handle grounding"? Or better yet, what is the right way(s) to handle grounding?
post #10 of 10
Quote:
What are "the 422 incorrect ways to handle grounding"?
Of course I was exaggerating about the number, but here are some mistakes I've made recently:

1. If you melt one of your I/O jacks with your iron, check with an ohmmeter that the signal and ground connections are still isolated from each other. I once melted a jack badly enough that the ground line running from the jack's contacts to the solder lugs on the outside of the jack "swam" within the melted plastic and connected to one of the signal lines. You couldn't see this from the outside. That mistake cost me many hours of troubleshooting.

2. If you're using RCA jacks and a conductive case, you must either use insulated jacks or you must add insulation yourself. By "insulated jack" I mean that there's insulation between the metal parts of the jack and the part that would otherwise touch the case's mounting hole. You don't have to worry about this with typical mini stereo jacks, since they insulate the signal ground from the case ground already.

3. Vulcanized rubber grommets seem like a good solution to #2 if you use uninsulated jacks with a conductive case but (surprise!) vulcanized rubber is actually conductive.

4. Don't complicate an untested and therefore possibly broken circuit -- always add complexity only after you have verified that what you have so far is correct. How does this relate to grounding? There are a few ways, but consider just one: what happens when you add a DC power jack for a wall wart without knowing if your ground circuit is okay? This multiplies your potential for ground loop problems, which you certainly don't need if you're not yet sure that what you have is grounded properly to begin with. The moral of this particular story is to test with batteries first whenever practical, but there's a broader lesson to be found here.

5. Always ground in a "star" pattern if you're building on protoboard -- all ground wires should go to a single trace running through the circuit. If your protoboard design doesn't have a convenient bus trace, make one by tying together several pads. If you're etching your own boards, make your ground trace a single line running between all the components that need to be grounded.

6. Don't add input and output jacks or the volume pot until you've verified that the base amplifier is working correctly. These three components are the biggest single sources of ground fault problems in my experience. If you have a ground fault in the basic amp, but add all the jacks and the pot anyway, how do you put your finger on where the problem actually is without unsoldering everything and starting over?

7. In addition to checking your soldering job visually, check with a meter. If, for example, you find a low resistance between sections of the circuit that shouldn't be connected, you know you have a solder bridge somewhere or your layout is incorrect. Or, if you are using a single 9V battery and find +9VDC within the circuit when the battery is plugged in, you have a solder bridge between one of the sides of the power supply circuit and ground, which sends the full unsplit 9V through the circuit.

That should get you started.
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