a47 distortion - not enough current...or something else?
Jul 18, 2009 at 7:22 AM Thread Starter Post #1 of 4

Zaubertuba

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EDIT--Problem fixed. Read on only if you're looking at clues to troubleshooting your own builds.

I finished one-half of my balanced a47 build, using the original values from Apheared's design, OPA2134's, and adding Benjamin Feist's TI Rail-Splitter/Buffered power supply. I wired it up as a "traditional" single-ended amp just to check out how it sounded.

When I first powered it up using a cheap 12v, 350mA wall-wart, and did a brief listening test with my HD 202's, it distorted horribly at all volume levels.

Changing to a 12v 2000mA *switching* power supply I had sitting around (I expected it to be noisy--but it wasn't at all), it actually sounds great at low volumes, but begins distorting again as I increase the volume to normal listening levels.

Here's a few shots of the prototype:

5375_1181987878165_1482242182_504384_6561102_n.jpg

5375_1181987918166_1482242182_504385_7833262_n.jpg

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Sorry about the spread-out layout--I know I could probably get all four channels on that single board, but packing it that tight makes it really tough to troubleshoot, especially for my failing eyes.
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Do I just need yet more current? Or is there something else I should be checking?
 
Jul 19, 2009 at 1:13 AM Post #3 of 4

hiker101

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What happens when you power it with a single 9V battery? That would at least eliminate the power supply as the source of your distortion.

I have an Apheared 47 sitting on my desk right now in plug board format, and it runs fine on a single 9V. I have also added the same rail splitter mod that you have.
 
Jul 19, 2009 at 4:30 AM Post #4 of 4

Zaubertuba

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Quote:

Originally Posted by MASantos /img/forum/go_quote.gif
Please post close up photos(preferably not cellphone pics) of the underside of the board and the schematic you're using, and parts used.


LOL--20 shots with a borrowed Cannon G3 and this was the best I could do--not much better than my cellphone camera.
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5375_1182455489855_1482242182_506399_5337278_n.jpg

...this was my "prototype" on this splitter, it's really ugly.
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5375_1182455529856_1482242182_506400_2844171_n.jpg


The schematic is part of a rather large image I probably shouldn't post directly, it's here.


Quote:

Originally Posted by hiker101
What happens when you power it with a single 9V battery? That would at least eliminate the power supply as the source of your distortion.


Thanks--yeah, I think something must be haywire with my circuit--I'm getting the same distortion with a 9v battery hooked up that I had with the 2000mA supply.

EDIT--Oh, MASantos: I'm not deviating at all from Feist's schematic, as far as parts, except I did parallel two 470uf caps on each rail instead of just one per rail. On the a47 I'm using Ratshack 1% Metal film resistors (no change from apheared's schematic), WIMA .47uf 250v poly caps and OPA2134PA.

I did check the output voltage of the rail splitter from negative to vground (~6v), from vground to positive (~6v), and from negative to positive (~12v), so at least that part of the circuit seemslike it's working fine.

EDIT 2--Swapped both opamps out. Still distorts. I also double-checked the solder pads on the a47 circuit to make sure I didn't accidentally bridge between pads.

EDIT 3--Speaking of bridging pads, I looked again at Boettcher's building schematic and noticed I missed jumpering pins 1-2 and 6-7 of the second opamp. Sheesh--gotta get my eyes checked!
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It's working great now! Sorry for the redundant posting!
 

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