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Old 2nd January 2010, 02:58 PM   #1
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Default Cmoy headphone amp?

I've been looking at the cmoy headphone amp and it looks like a fun build to try...does anyone know if can I use LT1028 in the design.
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Old 2nd January 2010, 03:04 PM   #2
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Hi,

No way, use something else. Also, I found that the LT1028 can't drive 32 ohm headphones properly.

You can try the LT1122 instead. I liked this in mine, though probably I prefer the OPA132UA and the AD845.

Among dual opamps... give the TLE2062 a try
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Old 2nd January 2010, 03:11 PM   #3
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Ok thanks, I'm just trying to find a use for the LT1028's I have.. Any ideas, links, anything!
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Old 2nd January 2010, 09:25 PM   #4
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Are you going to build a portable or stationary amp? If you're going for the latter you could use LT1028 with some kind of buffer. There are many buffers to choose from. Either a discrete diamond buffer (look at eg PPA v2 or LISAIII), complementary MOSFET like M3, single end class A like Tori amp, monolithic buffer like PIMETA etc. Do some reading at tangentsoft.net and amb.org

If you use LT1028 be sure to compensate it properly. I've found that it overshoots and even oscillates without proper compensation at "normal" gain (for headphone use). The best sounding audio integrated opamps The first waveform is without input cap and compensation cap.
The picture below is the curciut from the example in the link.
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File Type: jpg LT1028 sch.jpg (80.5 KB, 763 views)
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Old 2nd January 2010, 11:01 PM   #5
jcx is online now jcx  United States
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you probably should also plan on using the datasheet "overcompensation" (page 8 graphs - not shown in example circuits) using pin 5

with the local feedback C shown in Nelson's schematic the LT1028 is running unity gain at high frequencies and it is not unity gain stable wihtout using the overcompensation C between pin 5 and the output - may require several hundred pF


also the headfi fashion for jfet ccs bias is not the best for these circuits if you want to swing close to the power supply V
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Old 3rd January 2010, 12:11 AM   #6
tvrgeek is offline tvrgeek  United States
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Might be a hint that the Grado amp uses a much higher power op-amp. Not as clean as some others, but higher power. About 50 mW where the best we can do with a good amp is about 35. The National buffer follower looks like a good solution. It can drive half a watt! Of course, arguments if it should be inside or outside the feedback loop.

If you have high Z cans, in other words not Grado's, then you have more options like the MCoy as it is. BTW, You can get a Grado clone off the auction for $35. Board, caps, resistors, and Alps pot. As it comes with the higher power op-amp, you are OK as is, or just swap in the op-amp of your pleasure and check compensation. I bought some to use as generic preamps. Cheaper than I can build.
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Old 3rd January 2010, 11:02 AM   #7
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Quote:
Originally Posted by jcx View Post
with the local feedback C shown in Nelson's schematic the LT1028 is running unity gain at high frequencies and it is not unity gain stable wihtout using the overcompensation C between pin 5 and the output - may require several hundred pF
Aha, that's why it oscillated I guess
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Old 3rd January 2010, 11:04 AM   #8
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Quote:
Originally Posted by tvrgeek View Post
Might be a hint that the Grado amp uses a much higher power op-amp. Not as clean as some others, but higher power.
The TLE2062 is FET input and has around 80 mA of output current, more than the 4556 in the Grado amp. It sounds very good too when it drives headphones.
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Old 16th January 2010, 09:03 AM   #9
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Quote:
Originally Posted by Andrea: View Post
Aha, that's why it oscillated I guess
No, it didn't oscillate with the compensation shown above. It oscillated without compensation. The bottom square wave is with the compensation above. I think it's close to perfect.

I've tried capacitors from output to pin 5. It doesn't make the amp more stable, produce a better looking square wave or sound any better.
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Old 16th January 2010, 02:38 PM   #10
jcx is online now jcx  United States
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Default but will you always be lucky?

I've used the LT1028 professionally in strain gage amps with a gain -1 bridge balance circuit and 50' cables - the over compensation cap does improve stability into the heavy capacitive load

the LT designers/apps writers are really the best in the business - its worth paying attention to their description of the chip as not unity gain stable
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