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Old 1st October 2013, 12:16 AM   #211
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Hi agdr

You also may want to consider using single opamp which is capable to deliver desired current instead of paralleling multiple lower power devices. It can bring some flexibility into design.
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Old 1st October 2013, 01:12 AM   #212
agdr is offline agdr  United States
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Hello Sergey888! I've been thinking about that, or something in the middle with paralleling fewer higher-current chips in keeping with the "O2" design theme.

I believe that NwAvGuy may have been considering OPA551's at one point since they were being used in a Violectric amp. The slew on those may be high enough to wrap them in a loop with something (NJM2068?) to linearize them a bit more. The noise would probably be about the same, 14nv/(sqrt(Hz) vs the RMS sum of 10nV/sqrt(Hz) each with all those NJM455A's. There is always the LME49600/LME49610 but I'm kind of hesitant to have to have the output stage wind up looking like OPC's Wire amp (no point in reinventing the wheel). Could use the BUF634 but there have been a ton of amps based on that chip. Then there is a discrete output stage, but I would like to stay with chips that have published datasheet numbers, in keeping with the O2 theme. There is jcx' favorite, the TPA6120, which is a great chip but needs that 10R in series on the output.

I'm open to suggestions on output chips! Any chip used would have to have on-chip current limiting, like the NJM4556A does, to prevent the TRS jack shorting thing from destroying them. The OPA551 does have current limiting and it is unity gain stable.

Last edited by agdr; 1st October 2013 at 01:34 AM.
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Old 1st October 2013, 01:29 AM   #213
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Well, I was thinking about something a tiny bit faster, like LT1206/1210 or even THS6022/6012, depends how much current you need. The first two exist in TO-220-7, which is extremely DIY friendly
I have experience using all of them with pretty satisfying results.

Last edited by Sergey888; 1st October 2013 at 01:44 AM.
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Old 1st October 2013, 01:36 AM   #214
agdr is offline agdr  United States
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Quote:
Originally Posted by Sergey888 View Post
Well, I was thinking about something a tiny bit faster, like LTC1206/1210 or even THS6022/6012, depends how much current you need. The first two exist in TO-220-7, which is extremely DIY friendly
I have experience using all of them with pretty satisfying results.
Interesting! I don't think that I've heard of either of those chips. I'll look them up. Lol - I deleted my comment about DPAKs before reading your message here when I realized the OPA551 was DPAK too.
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Old 1st October 2013, 01:43 AM   #215
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TPA6120 is pretty much the same as THS6012.
Was a mistake in the previous post, LT not LTC
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Old 1st October 2013, 01:56 AM   #216
agdr is offline agdr  United States
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Ah yes, I have looked at that LT1206 before. My issue with that one was more of a video chip that didn't have audio range THD distortion numbers published. Just took a look again and all they have is 2nd and 3rd harmonics starting at 1mHz, I think. Still, that is a chip worth a closer look.

I do think I recall reading once that the TPA6120 was a re-branded ADSL line driver. That THS chip may have been the original.

Both of those chips are definitely good suggestions for a closer look!
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Old 1st October 2013, 02:12 AM   #217
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Quote:
Originally Posted by agdr View Post
didn't have audio range THD distortion numbers published. Just took a look again and all they have is 2nd and 3rd harmonics starting at 1mHz, I think. Still, that is a chip worth a closer look.
It likely has rather high distortion in audio band due to thermal effects. It does have an input stage which is more sensitive to it, compare to "audio" opamps. The key point is to create the design where you will be using another opamp's front end as an input stage. This way you will get an excellent performance in audio band and good driving capabilities of powerful video device.
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Old 1st October 2013, 05:41 AM   #218
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There are also LM7372, THS3111 THS3021, THS4211 etc.
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Old 2nd October 2013, 01:24 AM   #219
agdr is offline agdr  United States
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Sergey888 - thanks for the output chip suggestions! I don't believe I know about any of those. I'll take a look at all of them.

I've just spent some time Googling and searching on the forum for an OPA551 headamp with the chip wrapped in a linearization loop with some low noise, low distortion thing like a NJM2068 or LME49990. So far nothing, although I have found quite a few amps with just a straight OPA551 output from years back. I see that theAnonymous1 built one 6 years ago here:

DIY headphone amp not loud enough

and the Violetric V90 uses them in the output stage

Violectric HPA V90 Reviews

which gave me a heads-up on something I missed, that chip has an operating voltage up to +/-30Vdc (60 volt swing!). Hmmm......

Now there would be an interesting "voltage select switch". Run that OPA551 output stage at a gain of 2x on +/-30Vdc rails so the driver circuitry could still be LME4990s running off +/-15Vdc rails. I have the place for the attenuator resistor in series with the volume control anyway, easy enough to cut the input in half if needed. I could easily generate +/-32Vdc supplies by switching in a couple of diodes and caps in the power supply to make a dual rail voltage quadrupler as opposed to the existing "O2" dual rail voltage doubler. I already posted that desgin in the mod thread sometime back. +/-15Vdc and +/-30Vdc would both be available at different tap points in the quadrulpler. A 60V swing should take care of every single high impedance headphone out there.

Also some interesting comments from NwAvGuy/RocketScientist during his chip tests with the OPA551:

NwAvGuy: O2 Design Process (search in there for OPA551)

For fun I may have a small PC board fabbed with just a NJM2068 and a OPA551, then do my best with reactive loads to try to get it to oscillate.

Also some small progress on the second order comp network. I was able to finally get my arms wrapped around this one from a post in the solid state forum right now, which I think may be similar to one of yours, from memory:

two pole compensation for lme4702 lme49830

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Feeding the network from the CC2 end I get 3dB down at 414kHz for the pole, plot below, exactly as advertised.

Thanks again for all your compensation ideas and suggestions!

Unrelated: if anyone has a pointer to measurements of organic solid polymer electrolytics' capacitance vs. voltage, please post or PM me. I'm always looking for reasonable alternatives to that expensive pile of WIMA 4.7uF film caps on each channel. Maybe nonpolar (back to back) solid polymers are a cheaper solution. I know that regular aluminum electrolytics have unacceptable nonlinear properties, but I don't think I've ever seen signal path measurements done with the new solid polymer caps, either aluminum or tantalum.
Attached Images
File Type: png Original filter.PNG (25.9 KB, 258 views)
File Type: png original -3dB point 414khz.png (7.9 KB, 252 views)

Last edited by agdr; 2nd October 2013 at 01:52 AM.
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Old 2nd October 2013, 01:39 PM   #220
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it is not a problem to do something based on OPA551, but it is slightly less convenient because of rather limited bandwight. of the chip.

Here is a small example how you can approach this design.
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