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Old 17th September 2011, 07:22 PM   #1
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Default Preamp / buffer suggestions for TA2020 / TA2024 boards

I'm doing some experiments with various low-power Tripath boards from quite a few sources, and I am noticing that most of them reach their max clean volume potential when fed by a proper preamp as opposed to the output of a phone or MP3 player.

The headphone output on my 5.5G iPod Video can somehow push most of these boards to / past the brink of clipping, but my Sansa Clips and my Android phone do not have nearly as much output, and I am always missing a few precious clean watts of potential unless I connect a full-size preamp between those sources and the board.

I am not a circuit-design wizard or anything approaching it, would just like to find a simple to extremely simple circuit I could insert as a buffer or preamp before the T-amp inputs. Ideally it would be something that could sit in the same small box and maybe even use the same power supply. I have no idea if the latter is possible, even with a beefier 12-13.5V supply with 4-6A on tap, but it would be nice.

Maintaining the quality / clarity of the signal as much as possible would of course also be ideal, but I do not need the circuit to be audiophile-grade, since I am not feeding the amps audiophile-grade signals in these more casual configurations. I would just like the circuit to be as clean-sounding as practical and, preferably, add very little noise to the circuit.

I've been looking for several weeks for viable designs, here and elsewhere, and am not finding much that doesn't require a full separate-box build and/or discrete power supply of 24-35v. I can't help but wonder if there's a good, basic IC-based implementation out there for what I need to do.

Any good ideas / starting points?
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Old 17th September 2011, 07:40 PM   #2
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Use a line trafo, or just change the feedback resistors in the amp to the correct vlue.
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Old 17th September 2011, 07:50 PM   #3
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Thanks, Saturnus... what is the "correct value" for a headphone-level app, and how do I definitively spot said resistors?
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Old 17th September 2011, 10:45 PM   #4
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Follow the chip leads. Feedback resistors are by necessity in a switching amp extremely close to the chip.

20K Rin / 100K Rfb is what I'd consider optimal.
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Old 17th September 2011, 11:18 PM   #5
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OK, looking at the datasheet for the 2024 I'm guessing I'm looking for the resistors bridging pins 10 / 11 and 14 / 15. The Tripath datasheet application circuit calls for 20K resistors here. On both of the 2024 boards I'm staring at right now - one Helder MK II, one Sure - the SMD resistors that appear to bridge these pins are marked 20K. So if there's a "correct" value, it must not be the one called for by Tripath.

I'm also not super-stoked about having to replace SMD components on these boards.

I've looked around for info on an appropriate transformer and I'm not coming up with much there either. We did try out a moving-coil-to-moving-magnet in-line transformer a few weeks ago for giggles, and it boosted output to right about the similar levels as a preamp, but also drastically increased the source idle noise floor. Granted, this was not a transformer designed for the application, but I'm concerned that any transformer is going to boost noise in a similar fashion, if not as greatly.

Could perhaps a simple headphone amp circuit like the Cmoy be adapted to serve the purpose, at least with an op-amp selected to run on a lower voltage than 18V?

edit: Saturnus, sorry, I didn't see your posting of 30 min earlier-- I started this post a while ago. So by Rfb, I assume you mean the resistor bridging the VP and IN pins?

Last edited by lossfound; 17th September 2011 at 11:20 PM.
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Old 17th September 2011, 11:27 PM   #6
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Quote:
Originally Posted by lossfound View Post
So if there's a "correct" value, it must not be the one called for by Tripath.
Suit yourself. It's by far the best way to do it.

Rin : Rfb

56K : 22K (for very high output sources like car audio crossovers)
22K : 22K (for standard line sources)
22K : 56K (for low output sources like old fashioned portable cd-players and first generation mp3-players)
22K : 100K (for very output sources like most smartphones and modern portable media-players)
15K : 82K (for extremely low output sources. Only consider this if you're sure the above is not enough gain)
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Old 18th September 2011, 12:18 AM   #7
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I'm definitely not questioning whether the Tripath specs will give the best results, but the stuff I'm doing will require me to mass-replace the resistors by hand on quite a few of these boards and I've never had any luck with SMD soldering. That's why an off-the-board solution would be nice, even if not nearly as simple / practical / fast on the surface of things

Thanks for the input. I'll order some 50-100k SMD resistors and pick up a bottle of Jack at the store to build up my courage.
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Old 18th September 2011, 12:32 AM   #8
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I guess the other remaining question, if I take the feedback-resistor approach, is what I would need to do to the value of the input volume pot, presuming I am using one. A 50k pot feeding the inputs is the recommended value for most of these amp boards in stock form. Would I need to go with a different value for typical performance or sweep once the feedback resistors have been changed?
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Old 18th September 2011, 01:21 PM   #9
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(Semi off-topic? Sorry! Anyway:

Replacing surface mount resistors is actually pleasant and easy!

To desolder, you just add blobs of solder to both joints, and heat both joints alternately. The blobs are there to store heat - pretty soon both blobs are molten and the part slides right off.

Suck the solder off the joints with solder wick. Leave a little solder on one pad if you like.

Use liquid flux and normal solder to solder the new part on; This is a good method: Surface Mount Soldering 101 Video ... although a little bit overkill. These guys are soldering TINY chip leads - you won't need the loupe, and you'll be able to touch the solder directly to the board. But the video shows the relationship of joint, part, flux, and solder.

Here's a couple of great videos:
Surface Mount Soldering Techniques
How to Desolder Electronic Parts
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Old 18th September 2011, 02:56 PM   #10
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Thanks, kristleifur. Saves me quite a bit of time with the Goog.
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