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Apheared 47 project

post #1 of 10
5/5/11 at 8:46pm
Thread Starter 

Hello

 

I'm building a variant of the Apheared 47 based on the scheme provided by Ben Feist. I have made some changes to the design according to datasheets and forum discussions. There are, however, a couple of questions that I have came up with. My current scheme is provided below.

 

  1. Ben didn't use opamp between TLE2464 as opposed to Tangent's guideline. Is one recommended?
  2. Are the bypass caps of the BUF634 connected properly? I mean that the BUF bypasses the noise from supply rails to itself.
  3. Are the bypass caps sufficient (0.1uF ceramic for opa2134, 0.1uF ceramic + 10uF tantalum for BUF, 470uF+10uF electrolytics and 470nF+100nF film for regulator)?
  4. Should I use the CGND ground for something or is it better to have only one common point (AGND)? E.g. Should I direct the input ground to the CGND and the speaker ground to the AGND?
  5. How much output capasitance is needed? Is it connected like this or should it go from V+ to V-? Or is this needed at all?
  6. Are there sufficient amount of capasitance before the rail-splitter/active ground buffer (currently 2x470uF)?
  7. I'm (currently) going to use regulated 24V switching supply (that seems to be isolated) to power the amp. Is there any help from this e.g. considering supply noise? Or is just a disadvantage?

 

APheared 47 scheme

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post #2 of 10
5/5/11 at 10:33pm

Hi the TLE2426 is a resistor divider followed by an opamp its just in a small case, so you do have an op-amp before the buffer

 

cheers

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post #3 of 10
5/5/11 at 10:55pm
Thread Starter 

Oh, I wasn't aware that TLE includes op amp. However, currently there is no additional op amp (e.g. opa134) between TLE and buffer and thus no feedback in the ground buffer. But should there be an op amp (and feedback) as suggested in Tangents virtual ground tutorial: "Getting more complicated"? Is there some disadvantages if the op amp is left out?

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post #4 of 10
5/6/11 at 4:37am

Take the output from the TLE2426 and use that as the ground reference for the board.

Take the output from the BUF634 and connect it straight to the ground on the output

jack. Do not connect any capacitors to the output from the BUF634. Use your bypass

caps from rail to rail and not from rail to ground. If you feel you need some extra capacitance

in the rail splitter, use something like a 3.3uF Wima on the output of the TLE2426.

Your power supply looks good.

The BUF634 does not need to be wrapped in a feedback loop.

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post #5 of 10
5/6/11 at 1:14pm
Thread Starter 
Quote:
Originally Posted by Avro_Arrow View Post

Take the output from the TLE2426 and use that as the ground reference for the board.

Take the output from the BUF634 and connect it straight to the ground on the output

jack. Do not connect any capacitors to the output from the BUF634. Use your bypass

caps from rail to rail and not from rail to ground. If you feel you need some extra capacitance

in the rail splitter, use something like a 3.3uF Wima on the output of the TLE2426.

Your power supply looks good.

The BUF634 does not need to be wrapped in a feedback loop.


Thank you for advices. I changed the bypass caps of all ICs to rail-to-rail configuration and dedicated the BUF out for output ground. I got a little confused as the datasheets always suggest connecting the bypass caps to ground. However, as there is no actual ground the rail-to-rail configuration seems more meaningful.

 

Are those Wima caps somehow superior to others? They are used just about in all designs.

 

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post #6 of 10
5/6/11 at 1:52pm

Wima makes some of the better quality film caps.

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post #7 of 10
5/6/11 at 1:55pm

I suppose the A47 is something to build but philosophically I'd just recommend using op amps with higher output current ratings to begin with

 

then if your headphone's impedance, sensitivity numbers justify the next step is a high current output buffer/amp in a multiloop - especially if you're already buying BUF634

 

 

 

Avro_Arrow's advice has you changing the amp to "3-channel" from the author's active gnd split supply - I think that is a technically bad move

 

the amp will be more accurate with only a single gnd for input and output - without the errors of the "3-channel" gnd buffer in the way

 

The "3-channel" topology is fundamentally flawed - most technical reasons given for it are simply not true or can be bettered by proper layout

"active supply splitter" can be useful when wanting to use single V supply - the single active gnd should be used for input, feedback and load gnd

but "3-channel" puts the gnd “buffer” amp in series with the load - with the input and feedback grounds separated from the load gnd - this means all of the error of the "output gnd" buffer is in series with the headphones - giving distortion and "output gnd" impedance crosstalk that is worse than with a good "passive" gnd of heavy conductor with signal input gnd, feedback gnd "star" connected with the load common ground pin of the TRS connector


A "active gnd" showing star at output - can better "3-channel" performance:
[
star.jpg

 

star gnd at the output jack gnd is "better"

 

 

there are other issues with the A47 circuit shown - the BUF634 has significant output impedance so the return current will drag your gnd voltage around with respect to the power supply V midpoint - it can cost some output range

 

the worse cost of output swing is the 47 Ohms current sense/sharing resistors - they can and should be much lower value - 10 Ohms or less

 

the spec'd op amps don't swing very close to the power supply rails and do not have "rail-to-rail" input common mode either so you can't use all of the supply V

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post #8 of 10
5/9/11 at 7:55am
Thread Starter 

Nice, now there are options to chose from (considering the grounding issue). Not really sure which way to go, both alternatives seem to have a justification.

 

Quote:
Originally Posted by jcx View Post

 

star gnd at the output jack gnd is "better"

 

Does star ground here refer to actual PCB star grounding or just the principle of single common ground reference? I was initially going to use ground plane so i was wondering if this grounding scheme really needs to be implemented with true star topology.
 

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post #9 of 10
5/9/11 at 1:14pm

I'd connect the active gnd to the gnd plane at the trs gnd connection - nearly all of the driver return current will then flow in the wire to the active gnd and not in the plane used for Feeback, Signal, Volume Pot reference

 

since the plane resistance is so low you can consider it as "one point" for the sensitive gnds


Edited by jcx - 5/9/11 at 1:16pm
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post #10 of 10
5/15/11 at 9:25pm
Thread Starter 

Thanks for replies so far. New ideas and new questions.

 

Would it be beneficial to use dual supply (such as this one) over the single supply, which is currently being employed. This would remove need for virtual ground splitting but are there any drawbacks (other than a bit more complex circuit)? I have understand that the best case is to have dual supply with center tapped ground. However, quick search for center tapped wall wart transformers produced zero results. Thus center-tapped version is probably out of scope. So is non center tapped dual supply any better than single supply in my original scheme?

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