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KG Balanced Dynahi build discussion thread


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I did on the other two. Only the amp units had all the holes....I mounted it on the PS board for fit. :)

I'm thinking of putting it in this single chassis:

http://www.diyaudio.com/store/amplifier-chassis/full-width-40mm-heatsinks/standard-amplifier-chassis-maximally-vented-4u.html

You could also have left out about half of the holes.

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Try this:

 

looks like my 2sc2705/2sa1145 are good enough, Ill get another batch of 2sc3421 for better match.

 

Also Im planning to use diyaudio 4U case, would you mind to share your design after you ve finished?

 

Thanks

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Yes, JB weld has better conductivity. It does not conduct electricity, it is an insulator. :)

I may use that on the other board...got a bunch of it.

Although it is probably splitting hairs - JB weld probably has better thermal conductivity than superglue.

JB weld is certainly electrically conductive, so be careful not to bridge any pins.

Edited by GrindingThud
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JB Weld contains iron particles. It is not safe to rely on it for any sort of electrical insulation. A bunch of people fried computers with this when it first started to be fashionable to JB-weld computer chips to heatsinks. 

 

Just be careful not to get any JB weld near pins on low voltage parts and you will be fine. 

 

If you do need electrical insulation there are readily accessible options. Devcon 2-ton epoxy is available just about anywhere you would find JB weld and lists dielectric strength in its datasheet - 600V/mil.

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Ok, I've got power (one board assembled). The smoke did get out the 1st time though. Q22 (BC556B) died on power up and the Negative rail sat at ~-5V. Powerup was in the dual track config. I swapped to the untracked config and changed the transistor and it came up with some mystery smoke but otherwise ok. Then swapped back to tracking config and it still works. I looked all over the board and can't figure out where the mystery smoke came from.....but it did smell transistory.

The 1K resistors are not on this board either...should those be spliced in? or only necessary if the current limit transistors are in (not installed now and no plans to put them in).

post-4334-0-75122900-1421011300_thumb.jp

Nice strong +/- 30.0. :)

----

Edit: had the time and went and did a quick amp board checkout. I don't have full sized heatsinks yet, so these were short tests. I was able to tweak the offset across positive and ground and negative and ground to be symmetric and about 12mV. After I put the servo in it brought it to around 1mV balanced and each side to ground. Very nice. When I went and measured the bias current (voltage across the 10R output resistors) I was getting a pretty consistent match for all N and P devices in the same channel (6mA device to device), but across channels overall (O+ to O-) there is about 150ma difference. Right now one side is 550mA and the other is 400mA. Is there a way (or is it even necessary if output DC is nil) to get the sides the same?

post-4334-0-60539100-1421026076_thumb.jp

Edited by GrindingThud
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As I wait for the enclosure, I tried hooking headphones up and running some sound through it. Seems I may have a problem with noise (hiss)...huge hiss. The hiss is much quieter listening across the balanced output than a single side to ground, but still hissy. When listening to a side, the noise is less if I short the same side input to ground directly. Input grounded through the pot makes no change in noise. Thoughts?

One thought I had was that my input sand is crap. I may try to spin a small mezzanine board to drop that340 in place of the jfets and use mpsa in the associated input leg like in the new board.

Edited by GrindingThud
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I'll need to double check gain values and position and try again. I tried dropping the 200K to 20K to get I think 3:1 (kept the original cap) and it did not seem to change the noise any....so I assumed I dorked something up along the way or have a noisy part somewhere in the front end. I'm assuming weird symmetric noise because it's much much quieter on the balanced output than from either leg to ground. I may try singles for the 2sa1349/2sc3381 before I tear it down too far. What is the recommended singles for those and are they unobtanium too? I'm liking the THAT340 more and more but come this far on this boad and determined to figure it out. :)

I appreciate your help on all these amps and for making them available, I'm having a blast building them up.

reduce the gain of the amplifier by lowering the feedback resistors

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  • 2 weeks later...

Did more testing today and built up the second board, but used MPSA and the THAT on perfboard instead of the Chinese fooo I have in the other. Board came up ok and tweaked easily to a couple mV all around prior to installing the servo. This was a little easier to troubleshoot on the big heatsinks. Seems the balanced output is dead quiet with low gain (20K feedback) and either SE or balanced input, but SE output hisses loudly between either side and ground regardless of input. I think I'm missing something obvious.....where's captain obvious when you need him. Anyone have any ideas or experience using the amp with single ended output? :)

post-4334-0-58227400-1422837445_thumb.jp

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