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The Headcase Stax thread


thrice
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That is it plus any energy storage in the bias supply in general is a poor idea.  It has to be a super high impedance supply from a performance and safety perspective so storing anything after the ballast resistor is just poor engineering.  If one was going to do that (no idea why though) you would need a rather high value series resistor to negate the effect of the charge on the diaphragm. 

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As an illustration, when I was young and innocent I once decided to discharge a power supply capacitor by putting a screwdriver across its terminals. The resulting spark produced a 2mm divot in the blade. A plastic diaphragm is a bit more delicate than a tool steel screwdriver blade.

 

To quote somebody or other: "Good judgment comes from experience. Experience comes from bad judgment."

Edited by JimL
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Electrostatic headphones and speakers have the least distortion when they have a constant charge on the diaphragm. This was established at least as far back as PJ Walker who designed the original Quad electrostatic speaker. 

 

The "rationale" for having a "large" capacitor in the bias supply, e.g. 1 uf or more, is to smooth out any residual AC so the diaphragm charge and voltage remain constant. However, once the diaphragm is charged, the amount of current needed to maintain the charge is minuscule, so that in fact even a "small" capacitor (Stax uses 0.1 uf) is more than sufficient. This is illustrated by the fact that if you use an old Stax converter box, which contains a step-up transformer and bias supply but no active electronics, charge up the headphones and then disconnect the bias supply from AC, the headphones will continue to play for some time. The large value safety resistor used by Stax (about 5 megohms) between the last bias capacitor and the diaphragm ensure that any current traveling to the diaphragm is very low so the diaphragm cannot get zapped. Companies like iFi and Woo that don't follow these design parameters either don't understand or ignore these facts. iFi Audio's use of a high voltage battery to "power" the bias supply is, IMHO, an expensive solution in search of a non-existent problem.

Edited by JimL
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In other news, the Monolith Electrostatics from Monoprice are now available and a set is already on its way to Kevin for a full dissection.  While we wait, we finally have some specs:

33455_03.thumb.jpg.ed94f76fee8c12c4545ed9b08511d786.jpg

Now I have some doubts about some of this, the capacitance is probably just the drivers and not the cable included as it is really low.  Now the cable is pretty short but still, 60pf is not probable.  The drivers are also quite small so comparable to the old Stax designs (SR-5 etc.) which are about 120pf so yeah...half of that is not probable. 

Now the only thing that makes me pause is the amp...let's look at the specs:

33455_06.thumb.jpg.147373b8c201094fa431d311f9fac1e9.jpg

For first off, no volume control so one wonders what is going on in that small box.  It has a battery which powers something for some 5 hours but why no volume control?  It is just odd... 

Now to further add to the mix, I found this on ebay:

https://www.ebay.com/itm/223221739939?ul_noapp=true

Now here they are for a lot more money with no markings at all.  I looked at what other items the seller has and it is a lot of monoprice stuff for a lot more money.  Very odd... 

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SR-001_sche.gif.aa309b7b0e6f3a7747b15de3d108c129.gif

Turns out the amp is a clone of the SRM-001 with some issues so yeah... not ideal.  This is a schematic I found in my archives so no idea where it came from. 

Kevin also notest that the player didn't have a high enough output to properly drive them so something is wonky. 

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yep, pretty much that schematic. pull up resistor is 1 meg. false advertising, shows a rca to mini, not supplied, shows 1/4 jack not supplied. comes with usb cable.

bias resistor is 20 meg. power supply is fully regulated, something the sr-001 was certainly not.output stage bias is 200 microamps.

I think that this needs a lot more input voltage than it should. also no volume control, so designed to hook to something with a volume knob.

made in Taiwan. no names anywhere. after a few hours of warmup, still sounds like absolute crap. No way is this worth $700. or the $1k+ the guy on ebay is selling it for.

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We could do a portable amp but there are some issues there such as the PSU, custom chassis perhaps and stuff like that.  I'm not really interested in portable amps as I find them a silly novelty for open phones that bleed as much as Stax do but we could do something for the Shure's.  The stock amps are pretty good but could be improved upon... 

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