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The DIY Electrostatic amp thread


spritzer
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Given the outcry for some documentation on how to build some of these ESP amps I think it is time for a thread on the subject. Now the plan is to post here a summary of the parts needed to build a KGSS amp using the boards from Headamp.com plus what ever build/mods of Stax amps come to mind.

KGSS

Now for some time people have claimed that the KGSS is somehow hard to build when it really couldn't be further from the truth. My first DIY amp was a KGSS with those horrible old PCB's using offboard heatsinks and caps but after blowing up a couple of boards and PSU's (I really didn't have a clue as to what I was doing) I had two working amps. Now the new boards make life a lot easier and they are pretty much self explainatory (as all the values are printed on the boards) but there are a few things that are good to keep in mind. Given the way the boards are made (i.e. with a full groundplanes and traces carrying very high voltage running right under the heatshinks) it is a very good idea to raise the boards a bit using plastic washers (or nylon or some other non conductive material) on the pins before soldering them in place. Doing this you aren't simply relying on the solder mask to be the insulatior as it really isn't a good idea.

A second point would be to pick the correct LED's for the amp and PSU boards. You need LED's which have a nominal voltage drop of 1.7V. Mouser part# 859-LTL-307ELC is the one I use (also for the BH) but you can use other LED's but it will have an effect on the standing power of the amp and thus how much heat it gives off. With large enough heatsinks you could go crazy but lets not stray off the path. :)

So lets start off with the PSU and here is a picture of the PSU board which shows all of the parts needed. Not shown is the transformer needed which has to be custom made. KG and I use SumR in Canada with great results and here are the specs (two 0-280V/140mA windings, one 36VCT/200ma) plus I'd go with a core band and two 0-117V primaries for international use.

For the diode bridge in the LT supply (+/-15V) I use the RS402 unit as it fits and I've always used it with these boards. The small caps are 0.1uf/50V with a 5mm pitch such as these. 1N4007's are very common but you can also use UF4007's here. As for the transistors, the board offers two good choices for the first spot (and you only have to use one, the two spots are there just to offer a choice given the different pinout) but I use a third, cheaper option (at least when I bought my stash), the 2SA1831. It is alsmost identical to the 2SA1968 but has less voltage tolerance but that's not an issue here. It also goes in the spot marked for the 1968. As for the Mosfet, I use the IRFBC30 but there are other options. Both of these transistors should be easy to find but I get mine from bdent.com

The caps for the bias supply are 0.1uf/1000V MKP with a 22.5mm pitch and the bias adjustment pot is the standard Bourns 3296W (straight line of pins on the bottom and with top mounted adjustment). For the bias resistor I use Xicon's which are rated at 500V but you could use a 1/2W unit which is rated at 700V. Anything between 4.7M and 5M is just fine for a Stax headphone. For Sennheiser's they need an additional 5M resistor but you can add that inline later on or find a 10M resistor at Mouser.

To adjust the bias you must measure before the resistor so raising the bias resistor is a good idea so you can addatch a measuring probe to it. One should also know that not all multimeters are created equal and you need one with a real 10M input imedance to measure the bias accurately. The cheap VC99 appears to work ok for this but a Fluke is a good choice.

For the rest of the parts they are all pretty easy to find. There are dozens and dozens of choices when it comes to terminal blocks so pick one which fits you budget. The mains PSU caps are all the standard size, I've used Nichicon and CDE which both fit well. The resistors are Vishay RN60D for the smaller ones which I'll cover in the next section. For the larger units I've never actually used 180K/5W but rather 100K/10W units I had here. Mostly due to me reading the board wrong :palm: but they have been fine for years of heavy use.

AMP section

Here is a picture of the amp board. The majority of the parts here are Vishay RN60D resistors which you can get from pretty much anywhere. Mouser is a good choice and there is a rather good way of buying all the values you need. Just do a search for 71-RN60D-F- and find the values you need from the drop down box and you are set. I always order resistors in multiples of 100 but you don't need so many here.

The larger 3W resistors are also Vishay such as this one and this one. The 22pF caps on the board are 500V silver mica caps from CDE such as these. The heatsinks used in this amp are these and you also need something like this to insulate and mount the transistors. Trimpots are 3296W Bourns like the PSU.

Now the only real problem here is the input transistor but as it often is, it's not a problem at all. The dual 2SK389 has been discontinued and the current LSK389 uses a very different package. The good part though is that you only need matched 2SK170's and the spot for them on the board has already been marked. You can buy these matched from AMB so it isn't a problem.

Now lets say you have built the amp and then you need to adjust it. Like any of the SS amps you measure at the Stax terminals and there are two pots on the board, one for balance and one for offset. The one next to the 2SK389/2SK170 is the balance pot so put the probes between the + and - output and adjust for 0VDC. The pot towards the "back" of the board is the offse so put a probe between the + output and ground and adjuse for 0VDC.

Simple really... :D

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SRM-XH

I've gotten a few PM's asking how to convert the small Stax amps to balanced operation and there really isn't much to it besides the issue of a 4 gang pot and finding inputs which fit the chassis. Now the SRM-XH I had used the green double layer PCB but some have a singe layer board and I haven't tested that as if yet. The whole point is to mimic the input as it is now and copy that to the other side of the 2SJ109. Here is how the input looked in my amp:

p1000277.th.jpg

Now I just ordered some 1/8W resistors and a cap and mounted this underneath the board using the holes which the 100R resistor used previously. I also replaced all the electrolytic caps in the amp with higher rated units and ran the amp at 14VDC and people quite liked it at CanJam even with it sitting next to the T2. :)

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Now the only real problem here is the input transistor but as it often is, it's not a problem at all. The dual 2SK389 has been discontinued and the current LSK389 uses a very different package. The good part though is that you only need matched 2SK170's and the spot for them on the board has already been marked. You can buy these matched from AMB so it isn't a problem.

This is a great thread and that is a great tip. 2SK389/LSK389/2x2SK170 always was/were my main concern. I did not know Ti-Kan was matching them. Fantastic!

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dont mean to rain on anyone's matching parade, but for this application, i don't find matching the JFETs by Idss to be effective. we won't be using them at their full current. instead, i find it far better to setup a test circuit at the current that will be used in the circuit, and match by Vgs. you may or may not find a correlation with Idss.

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I gather that on the amp board that the two circles marked with a + sign next to the 499k resistors are LEDs, and also that the RLED is provision for a ballast resistor for an external LED.

Also, where can we get stax sockets worthy of this project?

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About the sockets, take a look at these. :)

dont mean to rain on anyone's matching parade, but for this application, i don't find matching the JFETs by Idss to be effective. we won't be using them at their full current. instead, i find it far better to setup a test circuit at the current that will be used in the circuit, and match by Vgs. you may or may not find a correlation with Idss.

You are absolutely right so simply finding some 2SK389's is well worth it or just bending the legs of a metal can LSK389.

I gather that on the amp board that the two circles marked with a + sign next to the 499k resistors are LEDs, and also that the RLED is provision for a ballast resistor for an external LED.

Yup, those are LED's and the RLED can be what ever you want it to be. It runs off the +15VDC line so a 1k5 resistor would work or something like the 3k3 which I use since I hate bright blue lights.

I'm in no hurry, but will there be a run of the KGSSHV boards?

Yup. :) No need for chassis mounted heatsinks either so any box large enough will do.

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Farnell, RS Components and a few others don't go near the resistance value at 5 watts.

When buying in Australia, I always found it was cheaper to buy everything from Mouser. Particularly if you can get the order over $200, and qualify for free shipping.

Farnell is a ripoff, with poor local stock levels.

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  • 1 month later...

Some updates to the Stax DIY amps. As soon as I get the SRM-727 here then I'll post a guide how one goes about altering the feedback loop but for now we'll have to settle for something less fun.

SRM-Xs

These are pretty uncommon as they were only sold with the SR-80Gold headset and being electret units, have no bias supply. Now this being Stax it doesn't mean there isn't a space for one or two on the board. :) Now this design is very different from the SRM-Xh I modified above even though they share the same chassis and there were even some SRM-Xh's which used this PCB. Just take a look at these pics:

p1000522n.th.jpg

p1000309o.th.jpg

The core design is similar but very different layout and same goes for the output devices. The Xs is a bit unique as it uses Mosfets for output devices, 2SK216's to be exact which can be found in the Blue Hawaii as well. Now the whole point of me writing this was lack of a bias supply in the SRM-Xs. In the pic above it should be between the large blue caps and the transformer. The spaces are for 3 caps (10mm pitch), two diodes and one resistor plus the terminal for the connecting wire. For the caps I just used what I had at hand, 10nf Wima caps rated at 630V as the exact value of the caps isn't important. You can also leave out the one next to the resistor as it isn't really needed. Now for diodes I used UF4007 (1N4007 would also do just fine) and the resistor is the standard Stax 4.7M (I used 5M). This is just a voltage doubler off the B+ so the bias isn't exactly correct but it works just fine into my SR-404.

Here is also a teaser for something I'm working on, converting a SRM-1 Mk2 to balanced operation.

p1000535f.th.jpg

p1000534u.th.jpg

I found this upgraded PP unit in Japan and it was missing the main heatsink/output transistors and in a pretty sorry state inside. These will be replaced but the main attraction is that these amps sound great, are fully DC-coupled and are a doddle to convert to balanced operation. Just replace one resistor on the board (910K) and add one resistor beneath the board (1K) and you instantly double the voltage swing of this thing.

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  • 5 months later...

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