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Old 25th October 2016, 09:21 PM   #1
davym is offline davym  Scotland
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Default Dual Chip Cmoy

I have one of these - Ultimate CMOY Dual chip rechargeable KIT : Fred's Headphone Amplifiers

Can i can drop in a JRC4556 in place of the LME49720? I'm looking for more current to feed my Grado's and was going to build a RA1 clone but if i can modify my existing cmoy it would save me the trouble.

I've looked at the dual chip cmoy circuit but can't make much sense of it, i don't even know it's output impedance.

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Old 26th October 2016, 12:28 PM   #2
davym is offline davym  Scotland
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All the RA1 clones i've come across use 2 x 9V battery's so i suppose it's not worth trying....
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Old 26th October 2016, 04:38 PM   #3
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No. The LME is not even providing the current.

If your Grado is like all the other Grados then it has that standard 32 Ohm, 112 dB SPL @1 Vrms driver. I don't see why you'd need any special cmoy. At the point you're hitting 10-20 mA the headphones will basically already distort badly on their own.
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Old 26th October 2016, 06:03 PM   #4
davym is offline davym  Scotland
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Thanks i cant seem to follow the circuit and have no diagram.Your right in that it can drive the cans to a high volume, almost anything can, it's the current i was concerned with. I have virtually no info on my dual chip version and was going by the RA1 specs, i read it's supposed to be the best Cmoy type amp for Grados due mainly to it's higher current output. My dual chip version seems bass light and i'm constantly charging it, roughly 3 - 4 hrs from a fully charged battery is not good for a Cmoy. Maybe i'm being unrealistic in my expectations but i'll probably try an RA1 clone and reuse the old enclosure (if i like it)
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Old 26th October 2016, 07:20 PM   #5
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Sounds more like a broken amp.
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Old 26th October 2016, 08:13 PM   #6
davym is offline davym  Scotland
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Nah it works too well for it to be broken, it's just not up to the job.
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Old 26th October 2016, 11:13 PM   #7
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Then an RA1 clone won't give you different sound. You'll need to charge batteries less often however.
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Old 26th October 2016, 11:37 PM   #8
davym is offline davym  Scotland
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That would be nice. Wish i could double up the battery's but it already takes it half a day to charge one. Don't suppose you've any idea what output current or impedance this thing has? I cant find info online about 2 chip cmoy's.
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Old 27th October 2016, 12:14 AM   #9
wwenze is offline wwenze  Singapore
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LM6171 is the virtual ground buffer. Apart from that this thing is identical to a CMOY.

Output impedance is negligible if looking at specs and applying theories.

Low bass... maybe because of the 0.47uF coupling capacitors. Differential input impedance of LM4562 is only 30kohm. Maybe because there are no power supply capacitors after LM6171. Maybe just placebo.
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Old 27th October 2016, 12:46 AM   #10
davym is offline davym  Scotland
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Thanks, the rest of it did look like a normal Cmoy to me. The RA1 has 5uF coupling caps but i doubt it does much other than maybe add some warmth. If output impedance is low enough then i suspect your right about placebo cos it can sound good with decent source material. I love my percussion, drums need to have good weight or i loose interest. What set me off was reading advice about how current hungry Grado's are, this amp uses LME49720, +/- 26mA vs the 70mA 4556 i was thinking about trying. I'm probably expecting too much from this rig to be honest. I might just treat it to a new battery and live with it a while longer.
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Last edited by davym; 27th October 2016 at 01:07 AM.
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