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Cheap DIY CMOY

post #1 of 14
Thread Starter 
I haven't seen this listed anywhere, so forgive me if it's common knowledge.

I came across the Banzai v2 CMoy DIY Kit when I was looking around at the different portable headphone amps listed here at head-fi. It seems to have been put on a pretty hefty sale since that article was written.

From my reading, the Op Amp might not be top of the class, but it does come with a socket, so changing it shouldn't be a problem.

I've never built anything like this before, but for $20 shipped, I couldn't pass it up.

If anyone has built one, I'd love to hear your thoughts. I will be building it when the kit arrives and using it with my iPhone (3GS) to drive my new cans (Beyerdynamic DT770/80, which are in the burn-in phase now).
post #2 of 14
I haven't tried that kit in particular, but I've heard mention of it before. Some thoughts regardless:

In an amp, the PCB layout is important, so it's hard to make statements about quality without knowing what's going on there. I'm relatively clueless at electronics and PCB layout, so I wouldn't be able to guess just by looking at it, whether or not it makes sense. The parts used are probably relatively cheap, including the potentiometer, which may mean flaky channel balance issues at lower volumes.

The diagram says the gain is 6x (15.5 dB), kind of large unless you have a weak source and need to power very insensitive headphones. To be honest, I don't think most people would want more volume with a DT 770 Pro 80 out of an iPhone, but it's definitely possible. It's just a CMoy so the performance out of it may be worse than from the iPod directly, considering the gain.

You could just get two different resistors to change the gain though. As you mention, you could also change the op amp. Usually op amp swaps are pretty useless, but in such a sensitive application doing double duty to provide the gain as well as drive the headphones, it really does matter which op amp you use. JRC4560 is considered outdated these days I think. A cheap and superior alternative for such a purpose is the JRC4556 (NJM4556).

It looks like there's no circuit to charge the batteries. You would need to keep replacing them. Also, note that for a minimal dual-battery design like this with what looks like no protections, if one battery dies before the other or gets disconnected, the amp is going to output a lot of DC and stop working. The DC voltage output can theoretically fry headphones if large enough, depending on the headphones.
post #3 of 14
Thread Starter 
Well, this will be my first CMoy, and my first soldering job in quite some time.

You say I can change the resistors to reduce the gain. Which resistors would I change and to what values?

I can change out some of the cheaper components, I don't have an issue with that. No, you're right that it doesn't have a charging circuit; but I'm now a bit concerned about what you said about the DC output if one of the batteries dies before the other. Is there a way to prevent that?

I'm hoping that this will be a decent project. At some point I'll likely build another one, but that will have to wait until I save some money.
post #4 of 14
You can follow this:

http://tangentsoft.net/audio/cmoy-tutorial/

I got all the stuff, but still haven't made time to build it:D
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post #5 of 14
Thread Starter 
Cheers!

I didn't think of looking there...though I undoubtedly should have.
post #6 of 14
Quote:
Originally Posted by mikeaj View Post

The diagram says the gain is 6x (15.5 dB), kind of large unless you have a weak source and need to power very insensitive headphones. To be honest, I don't think most people would want more volume with a DT 770 Pro 80 out of an iPhone, but it's definitely possible. It's just a CMoy so the performance out of it may be worse than from the iPod directly, considering the gain.
You could just get two different resistors to change the gain though. .
Quote:
Originally Posted by weebeast View Post

You can follow this:
http://tangentsoft.net/audio/cmoy-tutorial/
I got all the stuff, but still haven't made time to build it:D
Quote:
Originally Posted by liquidzoo View Post

Cheers!
I didn't think of looking there...though I undoubtedly should have.

I used the tangent website to build one a little over a month ago for my little brother and was actually pretty simple.

I built it with all the recommended values for the components, which meant it had a gain of 11 and would get someannoying hiss when the pot was turned more than 20% volume in my brothers ATH-AD 700 headphones. Changed theR3resistors in the schematic to 2.5k for a gain of 5 and would only get slight hiss once it was around 60-70% volume but it was only noticeable when nothing was playing so what mikeaj says about the gain being too high sounds right.

Also my brother rarely has it at over 50% volume as at that range it is more than adequate for his listening needs so I suppose that a gain of 3 or maybe 4, depending on the audio source, is probably a more ideal range.
Edited by Rhystic - 4/5/12 at 10:31am
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post #7 of 14
Thread Starter 
Here's another question, since I can't seem to find the info on the tangent site.

Would this potentiometer work to replace both the pot and switch in the kit? I can't find the specs on 2 of the 3 listed on the Tangent site to compare the amp/VDC ratings.

If not, no big deal. Just looking out for some tweaks I can make.
post #8 of 14
Quote:
Originally Posted by liquidzoo View Post

Here's another question, since I can't seem to find the info on the tangent site.
Would this potentiometer work to replace both the pot and switch in the kit? I can't find the specs on 2 of the 3 listed on the Tangent site to compare the amp/VDC ratings.
If not, no big deal. Just looking out for some tweaks I can make.

The diagram shows a 20 kohm potentiometer. A smaller value puts a larger strain on the source you're connecting to the amp. However, if you're connecting through the headphone output of the source device, it can handle lower impedances just fine so that's no problem. A line output may not fare as well, depending. Generally about 10 kohm is fine anyway though. A lower value also means lower noise I think.

Note that the potentiometer you linked may not necessarily be better, and it definitely has a different pin layout than the one they use, so it's not a drop-in replacement. You would want something that fits in the same holes and hopefully uses the same cap as well...all in all, I don't think that's worth replacing. Again, the weakness of a poorer potentiometer will show up only really in the lower ranges of the volume control, so if the gain is okay then you probably won't need to go there unless you listen pretty quietly.

To change the gain, you want to change the 5 kohm resistors to something lower. Given the circuit, the gain is (1 + R / 1000) where R is the resistance of the resistor we're talking about, which has a default value of 5 kohm. That's why the gain is 6x by default. In theory you want to be using high-quality resistors like low-noise metal film types, if you want lower noise overall. These are several cents per resistor from something like Mouser or DigiKey I think, but shipping/handling costs can be a killer.

If you want higher safety, battery charging, or something else, you probably want a different CMoy-variant kit. You're not going to need a ridiculous amount of power for a DT 770 Pro 80, so a single-battery CMoy is okay. However, you get lower performance with those than with an otherwise equivalent dual-battery type.
Edited by mikeaj - 4/5/12 at 12:20pm
post #9 of 14
Thread Starter 
Thanks.

I asked about that particular potentiometer after looking at the parts list on the tangent site. Their default is a 10k pot, but their Radio Shack alt is a 100k model; hence my question.

I will be looking into getting some different resistors to lower the gain setting.

As to it not making a huge difference, you're likely right; but it will give me a more complete sound than just running them from the iPhone by itself. Turning on the built in headphone amp on my Xonar DG makes a huge difference in what I can hear (please keep in mind, my last set was a Senn HD203, and before that a Creative Fatality model that is at least 5 years old; so anything sounds miles better to my ears).

I posted this just to point out its availability to beginners into high(er)-fi audio like myself. I did a lot of research and testing before I bought my DT770s and they sound fantastic to my ears (which is all that really matters); though the discussion about the resistors is very useful to myself and likely others that look into this kit.
post #10 of 14
Anyway, it could be a fun project.

In terms of price/performance and overall features and usability, I'd probably pick a FiiO E5 or E6 over that because those are smaller, lighter, and more convenient in terms of the battery and charging. Audio performance driving headphones shouldn't be much different, though a CMoy with the right op amp, a lower gain, and a decent layout, probably will outperform cheap amps. A dual-battery CMoy can swing more voltage than anything else in that price range though, that's for sure. It's just that the extra voltage is not needed for your headphones.
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