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stax mafia electrostatic portable

kevin gilmore

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I couldn't stop listening to this yesterday :D 

I was using my SR-007's.  It drives them very nicely.  Very rich deep bass and good imaging / sound stage, which is usually where the 007's suffer without the proper amp. 

This was running this off of a bench supply at 11.1V and it was drawing .4A very consistently.  The top board is the +/-300V supplies (with input EMI filtering) and the one below that is the bias supply set to 580V.

Each amp section is 1" x 2" and each of the +/- PS sections are 1" x 1".  Bias is a little wider.

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the 300 volt supplies are about 110ma each which is acceptable.

the bias supply is 200ma. This needs to get fixed. there is a resonant hvm module that does the bias for 25ma, but its a $135 part.

if i could get someone to make us a custom switching transformer, we could do better. but no one will even return my calls or emails.

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so the idea is a 10 hour runtime. so a 3 cell lipo battery with 2.8 amp hour batteries is the target. if it has to be a bit bigger then it has to be a bit bigger.

the amplifiers are 200 microamps output stage bias with 1.5ma current limit. Could certainly bump up the current limit a bit.

the benefit of kerrys power supplies is that they are extremely small, max component height is .3 inch. if we change to a transformer based supply then the transformer is .8 inch. if we use the hvm supply for the bias it is .5 inch high.  Smaller is better! For a given chassis size more room for the battery is a good thing.

The final product will likely be a single circuit board with the power suppies on one side and the amplifiers on the other.

The components with the largest height will be the stax jack and the volume pot.

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A little bit of playing with some options/config on the bias supply and we got it down to 40mA draw on the batteries :)   Might be able to do a bit better still.

All in draw now is 260mA.  We have some ideas on how to maybe bring this down a bit more.

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  • 2 weeks later...

Thanks Alan.  I'll give that a read.  Interesting that it's from back in the day :) 


Here's a rendering of what the amp will look like.  



It will be about 70 x 150 x 24 mm.

We're looking at using USB-C PD so we'll be able to run off of most chargers (at least 20W).  Power input will have a boost/buck converter so it could run off of 5/9/15/20V chargers.

Still some work ahead, but we think we'll be able to fit a DAC in as well.

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  • 3 weeks later...

Making some progress on the main amp board.  It will have a USB-C PD port so it will accept 5, 9, 15 & 20 volts.  I think a 20W charger should be enough, but I'll have to test that once the prototype is built.

This is getting pretty dense.  We'll have to have this partially assembled since the PD stuff is going to be a pain to do.  There are 0402 components in addition to some of the larger chips with multiple pads under the chips.  

This has 4 amp channels, +/-300V supplies, 580V bias and now the PD stuff with boost / buck on the input and includes the battery charging components as well.  Looking at using 3S lithium batteries.  The board is currently 161mm x 61mm and I think I can fit what's left.

There's still more to do, but here's where I'm at...



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I'm considering that.  It would be nice to have one board for each component so they could be tested separately as well.  Each supply, and the USB PD/battery mgmt could have separate boards.  Everything else is DIY solderable and could go on the main board.

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That's what I was thinking.  If I used 1mm boards I think I would be fine.  

The power supplies have parts only on one side, but the PD / battery mgmt needs parts on both sides.  The cutouts would be easy enough.

I'm not sure what I'm going to do for the initial prototype.  I'm pretty close with the current board and I may just build one.  Seems like I should sleep on it :) 

PS  Interesting.  Are you working on a balancer / protector or the management components? 

Edited by Kerry
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  • 4 weeks later...

Making some good progress on the amp...

I’ve got the USB-C PD all soldered on and ready for testing


Here’s the face plate with the jack 


Here’s the amp assembled, but I still need to finish the power supply and amp sections of the main board


Edited by Kerry
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This project looks very interesting. The two KGs have certainly done some great work!

I may have missed some earlier threads on this, but from what I have read, you plan to offer this a DIY kit, correct? If so, will purchase info be posted on this thread?

Keep up the good work, Gentlemen.

Best regards,

Linear (Doug


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makes no sense to make this a diy kit. the main board is more than 200 components on both sides of the board. The board house wants about $60 to assemble this board.

no one i know can hand stuff that board for $60. The dac board is likely going to use the ess chip which has nda agreements associated with it. So no gerber files or schematics either.  Still should be much cheaper than the d10 or kse1500.

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