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

[digger945]

So the Beta output is going to be connected directly to the stax jack?

Will the signal need to be capacitively coupled to the bias board in any way, like it is in the srd wiring diagram via the center tap on the secondaries?

Using balanced all the way from dac to srd5>404, with a Dynamite with gain of 11/22 as amp, I still go half way with the volume(dact 50k) just to begin quiet listening levels.

Is the aim here to increase the bias on the diaphragm to compensate for lack of voltage swing on the stators?

[spritzer]

The bias supplies are fixed so no way to increase the bias.

As for the caps, they are there but not to couple, they are bypass caps. The ground would have to be taken from the B22.

[digger945]

Beta PS ground if used unbalanced, and positive/negative if used balanced. Correct?

I am still leery about how the amp will react when directly connect to the stators. I was under the impression that the transformers role was two fold, one being to increase the voltage of the signal, and the other being to match impedance to the amp connected.

[spritzer]

There is no way to use this unbalanced, a single ended ES driver will sound like crap with very high THD. It has to be driven by a balanced amp or at least a balanced output stage which has a phase splitter in front of it. The transformers are center tapped so they take care of this plus impedance matching. The transformers are really just tube output transformers used backwards.

[digger945]

Very well. Really like for this chap to experience the same level of performance as I have using the stax that you sent me.

I suppose it would be OK to test this with my current listening setup to make sure it will have a decent sq, and if not then add trans. later if necessary.

[Spiug31]

according to amb a 100K+ resistive impedance is "super easy" for my beta22 to drive. The capacitive load however may require an output inductor on each channel in order to stabilize the amp.

I will only ever be driving this application balanced.

p.s. been a while since I was called a chap

[spritzer]

The ES drivers are a reactive load so the impedance varies wildly with frequency but the B22 should cope with that. Perhaps the best way would be to have two outputs, one direct from the Beta, the other fed through transformers.

[Torpedo]

FWIW I used the B22 with a SRD-7mkII to feed the O2mkI for several weeks until the 717 arrived. The combo sounded quite right. The B22 never had a problem and the only thing to complain of is the lack of power when listening at loud levels. Using bigger trafos than the ones built in the SRD7 will likely take care of that.

My B22 has speaker outputs, which I used to connect the adapter. The speaker outs have some sort of protection or impedance matching solution. I can't recall but it's whatever is described at Amb's site, according to Nate -the builder of my B22-.

[spritzer]

Our B22 has a Zobel network which helps the amp deal with inductive loads. It certainly wouldn't hurt when driving the transformers.

Also what we are talking about here is driving the Stax phones directly from the Beta for extremely low level listening.

[digger945]

If I didn't already have a good head start towards the KGSS, I would definitely build a custom transformer coupled box using the Lundahls and Birgir's bias board.

I still think that transformers would be the way to go for stability and sound quality, just turning the volume down on the amp to reduce the listening level. It will be interesting to see how it turns out.

[Torpedo]

Our B22 has a Zobel network which helps the amp deal with inductive loads. It certainly wouldn't hurt when driving the transformers.

Also what we are talking about here is driving the Stax phones directly from the Beta for extremely low level listening.

Using the B22 without output trafos for low level listening and just a gain of 2 Extreme low level listening I'd say

[spritzer]

Yeah, with a gain of 10 it would be possible but with a gain of 2 it would be extremely low level indeed. Running them from the Pro output could work...

[digger945]

Well, it's really hard to hear, but it works. Using the Dynamite with gain of 11(22 balanced) and it takes turning the knob all the way up to really get an appreciable amount of detail. The amp outputs are connected directly to the stax jack, nothing in between.

Got a hum somewhere. Reversing the wall plug made it worse.

Checked the bias with my cheap fluke. Got 254v to earth. I can't find any info on the impedance of my meter anywhere, so I guess it's low imp.

[spritzer]

The direction of the power plug is crucial as the power systems aren't star wired anymore. With it pointing in the right direction you shouldn't get any hum so I'd test the bias supply with the transformers in the circuit and see what happens.

As for measuring the bias, you need a VTVM to accurately measure it since the impedance is so high.

[Spiug31]

[digger945]

I'm ready to wire this thing up. Do you use the series resistor on the yellow wires for your stuff Birgir? The two parallel resistors on the SRD-7 schematics, primary side of the transformer.

[spritzer]

No resistors, just direct connections to the connectors on either side of the transformers.