Wanted: A Builder to construct and case-up a Stax Bias Supply

[Spiug31]

I was chatting to justin and cetoole last night, asking for a solution to produce low volumes of sound, max -40dB down into the -80`s and below.

Comfortably loud for me with my rs-1`s is up to -52dBu with pro mastered (+4dBu) material, using my donuts (yh-100`s) comfortably loud rises up to -44 dBu.

The suggestion that came up, apart from a vinyl source was to

justin: get stax

justin: and use the beta and a bias supply

I know very little about what is required to drive electrostatics so will really need a builder to construct this for me. A pre-built stuffed Stax bias supply PCB is available from spritzer. I`d really like to try this with my SR-3`s.

Anyone willing to construct this for me please let me know.

I have an SR-3 and SRD-5 here that I have yet to hear (, waiting for my beta22 to return from thrice). My apogee minidac outputs a balanced signal of 1.228V (+4dBU) and my balanced beta22 has a gain of 2. There are goldpoint stepped attenuators within my beta22 that usably cover the range 0 to -41 dB.

thankyou

Douglas

[kevin gilmore]

As goofy as it sounds, this will actually work.

Make absolutely sure you don't get the ac input

wires to spritzers board backwards as its not isolated

and you want to make sure neutral and ground are the same.

Which might cause a ground loop.

[spritzer]

I was thinking about adding a Hammond 229A230 to the input of the bias board just too be sure.

I've done some thinking on this and you might need more bias (if the volume from the B22 isn't loud enough) then the PCB can give you in the stock configuration. I might be able to fit 400V+ caps in there which means 1000V+ output which could then be cut down at will.

Btw. Kevin you have an email on a similar note...

[Spiug31]

As goofy as it sounds, this will actually work.

that is good to know

[digger945]

SRD-5 + Birgir's pro bias board = with 404's.

I would imagine the Beta22 would be (almost) as good as the Dynahi in a similar setup.

I know i'm lovin' it, alot. I'm gonna have to loan it to someone to ever get the KGSS built.

I'll put it together for you if Birgir officiates.

[Spiug31]

builder found

[spritzer]

To pick this up from the PM's, there are some issues with having a variable bias since depending on how it is implemented, it could put too much load on the voltage multiplier and thus pull down the voltage. There are two options here, put a pot on the series or shunt positions. The series has to be rather large in value to go all the way down to 100V or so and I don't think I've ever seen a 10M pot that is any good. The shunt can be smaller but if that is adjusted all the way down then the bias voltages could sag. The is also the issue of having a meter good enough to be able to get an accurate reading off the bias supply. Standard meters will not work though the VC99 talked about in the Fluke 179 thread seems to work ok and it is quite cheap.

[Spiug31]

VC99 is easy for me to get over here (via eBay) if an inbuilt voltmeter isn't feasible.

Is there a pot you have in mind for the shunt pot ?

[spritzer]

A built in voltmeter would never work, the input impedance is too low.

I really don't think that a pot would work in the shunt position, the more I think about it. If it were say a 5M pot and adjusted way down then that would be just asking for trouble. Putting a resistor in series would kill off most of the adjust ability while making sure the supply worked as it should. Now if you could find a 10M pot for the series position then that should work and give an adjustment range from 600V and down to less then 200.

[Spiug31]

VC99 it is then

And for the pot would this do ?

Panel Bushing Mount - Pots with Shaft, Single Turn - 500k Ohm & Up

(RCV) JJP97662

[spritzer]

That one would work if it is linear, not log. Freaking huge though...

[Spiug31]

I`ve sent an e-mail to inquire.

[Spiug31]

got a reply, it is linear and is he thinks rated for 500v.

[spritzer]

Then it should work well.

[Spiug31]

great

so SRD-* box, bias supply with Hammond 229A230, 10M ohm pot, VC99 voltmeter.

anything else I should look ?

[spritzer]

Some extra sockets would be a good idea since with the transformer you'd have to recase it and Stax sure loved to epoxy them in except on the most recent boxes.

[Spiug31]

does what you said above also apply to the large SRD-7 ?

if recasing is going to be necessary are there any advantages in getting an SRD-* box, apart from a source for stax sockets ?

[spritzer]

If you are just going to use the Beta to drive the phones then you have no need for the transformers in the SRD boxes and can just leave them out. How the B22 will behave into the load is something I'm not sure about. The transformers step up the voltage but also step down the impedance which the amps "see". We are talking 100K+... \

[Spiug31]

what is the usual step-up ratio of the transformers and what impedance do they usually present to the amp driving them?

I`m wondering if it might be possible to use transformers with a gain 100 times lower and therefor achieve -40dBu.

p.s. thankyou for all this help

[Spiug31]

without presenting the amp with such a high impedance load.

[spritzer]

The Stax transformers are around 1:25 so they step down the impedance 625 times. Lower ratios are certainly possible and the Lundahl LL1630/PP would do the trick.

[digger945]

You know what kind of transformers are used in the Illusion? They look like Hammonds.

Lundahls may be budget breakers here, so Douglas and I are shopping on ebay for SRD boxes to rob transformers out of.

[spritzer]

They are most likely something custom made. It's not like there is a drought of high-end transformer manufacturers in Japan. Hashimoto, Tango, Tamura are the high end candidates but they likely used something more basic. If you want to go custom then why not contact Electraprint? Kabeer ordered from them and for 10W units they are huge. To find the correct values to get a quate take an 8 ohm input, multiply it by the desired ratio and square it to find the impedance of the secondary. Stax used 1:25 transformers but less then that would also work.

[Spiug31]

You know what kind of transformers are used in the Illusion? They look like Hammonds.

Lundahls may be budget breakers here, so Douglas and I are shopping on ebay for SRD boxes to rob transformers out of.

I won`t need an audio transformer for my board (if my calculations are right). I am looking for inbuilt non-resistive attenuation of around -40dBu.

A 1:25 transformer gives +38dBu when fed +4dBu from my mini-dac via my amp with its gain of two. Missing out these transformers (provided my beta22 remains stable with the capacitive load and is happy with the resistive load) will achieve in one stroke a real 38dBu cut to final listening volume and should accomplish what is a main aim of this project for me.

[Spiug31]

the figures I am working with are

mini-dac output (+4dBu) = 1.228V

beta22 gain = 1:2

so max voltage output = 2.455V

Stax transformer gain = 1:25

Voltage on the transformer secondaries = 25 * 2.455V = 61.38V or +38dBu