The Stax thread (New) - Page 53

The SRD-7 Pro uses a voltage doubling ladder. If you check the voltage at each diode in the ladder the voltage will go down as you go down the ladder. You can probably find a voltage close to 230 for the low bias. All you need to do is switch the wire from the last diode to the diode further down the ladder.

Audiod

We would have ripped out each others throats in that bidding war

I bowed out for the simple reason that I'm always more interested in the phones them selves rather then the amps driving them so it should go to a better home.

As modders go RAM are supposed to be very good but some of their stuff is a bit out there. On the other hand they've got the main points right, paper in oil caps, silver transformers and cotton covered silver wire.

The best I could do with my crappy camera is this shot:



If you want to see higher res photos of both the front and back, check here:

http://www.servodrivenacoustats.com/SRD7CBRear.jpg

http://www.servodrivenacoustats.com/SRD7CBFront.jpg

http://www.servodrivenacoustats.com/SRD7CBFrontb.jpg

I hope this gives you some idea what you need to do. It is actually a quite simple circuit board.

I had a quick question. J-Pak advised me in a separate thread to reserve judgment on the Sigma Pros until I heard them out of a Pro Energizer. I didn't want to hijack that thread, so I guess I'll ask my newbie question here. Sorry about that J-Pak.

What exactly is meant by the term pro bias? I thought that a "pro" jack was basically a pro bias output. Does pro bias refer to a large voltage and not the jack type? (400 V+ for example)

Thanks so much!

Pro bias means the amp/energizer will provide something close to 580VDC that is transferred to the diaphragm of the headphones. Any Pro output (5-pins only, not six) will provide that. A normal bias output (6-pin) will only provide 230V and that is not enough to adequately drive something like Sigma Pros. Of course the amplification of the sound signal is another matter entirely, clearly not all amps or energizers are created equal and the voltage swing on the signal lines can vary significantly between them without any relation to the bias voltage.

Since J-Pak especially mentioned a Pro energizer though, he may be referring to a transfomer box only such as a SRD-7 Pro, SRD-7 Mk2 or Illusion ESC-1001.

Ah, I see. Thanks for the help everyone!

Sounds true of both my Stax gear (Sigmas, Lambda Nova, 404, SR001) and my Koss ESP 950. I assume that it takes a while for the stators and diaphragm to get fully charged even though the amp may have been on. Also, for the Lambda at least, the ears pads get warmed up and a bit sweaty and seal better around the ear.

It is of course possible that the rest of the equipment is undergoing similar warm-ups.

After looking at the SRD-7 Pro schematic and looking at the circuit board pictures (courtesy of ferrstein) that shows parts missing (probably were used in the mkII) I drew a schematic that should be the bias supply for the SRD-7mkII with both Low and High bias voltages. I don't know the value of R103 but it should be the same as R104. Z101 is a BiDirectional Zener diode with a breakdown voltage of 90-110 volts. D101 to D106 are 1 amp, 400 volt Silicon diodes.
I am working on a parts layout now.

AudioD



If I tap off the rungs of the voltage doubling ladder in the SRD-7 Pro bias supply I could have 5 different bias voltages from 230 to 580. I would replace the front panel switch with a 5 position switch. Now I could use my SRX’s and SR-5’s with a bias boost without going all the way to 580. Cool!

Carl,
Oops... Typo. Fixed.
Thanks,
AudioD

When I get a spare moment I'll open up my SRD-7 Mk2 and take a look at the parts values. (Photos when I get my camera back too). The photos ferrstein posted look a lot more readable than my SRD-7 so hopefully the Mk2 will be nice to follow too.

One thing I notice in that circuit is the center tap which I assume is there for the purpose of creating a ground reference shared between the bias supply and the audio signal. In my SRD-7 Mk2 I get some 50Hz mains hum that is quite noticeable with my SR-X Mk3 (The O2s don't seem to be efficient enough for it to be very noticeble). There is no direct connection to the ground in the mains plug, it's only got two prongs. Considering these center taps are the only place the audio and bias signals interact I'm assuming that the hum is creeping into the signal through here. Is there a way I can minimise this? Perhaps running a line from the mains ground to this spot (i.e. on the center tap PCB traces) in the circuit to actually make it a true reference to ground?

On another semi-related note, I am planning to build my own custom transformer box along the lines of Andrea Ciuffoli's circuit but with a substantially higher ratio for the two signal transformers so it can be used with lower power amplifiers (under 10W). Now having more turns will increase the capacitance. I'm wondering if this will have any effect other than reduced high frequency performance? I'm thinking that as long as the transformer can maintain good performance (under +/- 1db) to near 25Khz or better this is not a concern. If it was a problem it would seem I'd have to scrap the idea and settle for a lower ratio.

Great to see that the SRD-7 Pro showed up! Even if it was partly broken...

But if you did not intend to use the speaker/earspeaker switch anyway, it might not have been to bad. Since bypassing the switch can only lead to positive results sound wise.