The Stax thread (New) - Page 52

^ egads a slope filter switch. QUADtastic.

You just won that then Carl?

SRD7 Pro Update

Hello, just wanted to give a quick update on the scary Ebay auction I had with the SRD7 Pro (Ebay canceled the fellow's user account after I pad for the unit). Well, it showed up, but it had a bit of shipping damage due to a loose transformer (the bolt was missing):





Basically the speaker/earspeaker switch got totaled. I didn't realize until I looked at it that this switch actually switches AC voltage to the input of the bias supply while also switching between the speaker outputs and the input to the transformers.

Oh well, I say... no better time than now to bypass that switch altogether! I also hooked the AC input directly to the bias card. Now when I plug it in the bias circuit works right away:



Question: Does anyone know the best way to measure the bias output? I checked with a multimeter set to DC voltage and got 385 volts out of the bias board. I know the DMM pulls the bias signal down quite a bit... and since 385 is at least higher than the low bias spec, I figured I must be close to okay. The bias circuit on the SRD7 Pro doesn't use a transformer, but a voltage multiplier. I guess perhaps if I look at a schematic (I don't have one at the moment) I could see what value the output resistor is and figure out the effects of my 10k multimeter. Anyway, if someone has any guidance they can provide here I would greatly appreciate it.

Oh, and the sound? Excellent!! I gotta say I wasn't expecting to like this thing as much as I did... especially considering I used a neglected old Moscode 300 to drive it. I just felt the music more than I do with the SRM1 MK II. More dynamics, more presence, more musical. All I can say is... wow! I listened to quite a few CDs last night with it and it is just really good sounding.

I may look for a more appropriate amplifier now. What have you guys had good experience with? Tubes? I'd like to find a nice tube integrated to use. I have used the Moscode to drive transformer-coupled Acoustats in the past... and it is very good in that situation. Maybe that isn't such a bad amp for the SRD7 after all. It's just a bit noisy to have in a headphone system. Those have just a touch of background hiss and the power transformer is physically noisy, which is a problem with open-backed headphones.

Happy Listening!

I was going to bid on that but backed out. Congrats Carl.

If you measure after the output resistor you get a wrong reading. The output resistor is there just to limit all current the the headphones should short out. Put one probe on the input lead of the output resistor and the other at the beginning of the voltage multiplier and you should get +550-580v DC.

Cool. I knew I was doing something wrong!

I'm actually thinking of populating the circuit board with components to create a normal bias output as well... making this unit an SRD7 MKII. I want to do this because I REALLY want to hear my low bias Sigmas on this thing. Based on what I heard with my Lambda Pro's, I'm thinking Sigmas and an SRD7 may be close to sonic euphoria!

Does anyone have a schematic for an SRD7 MKII? The circuit board looks as though it isn't missing much to create a normal bias output:

It's been posted a few times here at Head-Fi. Do a search and you should be able to find it. I've got a pdf at home, if you have trouble finding it PM me and I'll send something to you when I get home tonight.

If your multimeter (probably digital) has a standard 1 megohm input impedence all you need is a 1000:1 High Voltage probe. B&K makes one for about $50.00.

I would let this board be as it is and make a new one for the low bias. For 117v AC you only need a simple voltage double i.e. 2 1N400x diodes and 2 0.1uF 300v caps and some resistors. Here is how Stax did it with their OEM stats



The Pro bias unit is basically the same but it uses zeners so it is universal and more voltage multipliers.

Here is the bias circuit for the Beyer ET1000. It uses 91v zener's that are then doubled for a 180v bias.


There is a larger one here

SRD7 and ED1?

I just had a thought... since I like my ED1 Diffuse Equalizer some of the time, I wonder if I would like it with the SRD7? Has anyone tried this?

I'm thinking it could go into either a processor loop in the preamp or just insert it en route from the source component to whatever amplifier is driving the SRD7. I wonder what that would sound like?

This is what I like about old Stax gear... the possibilities are endless!

BTW, sorry about the off-topic post. My ADD is getting the best of me today. Oh, and did I mention I'm really liking this SRD7?

Thanks, Spritzer... that looks easy enough! I may even be able to put those components inside the SRD7 chassis since they wouldn't take up much space. Hell, I could put them in place of the busted switch that I bypassed!

Coo, I wish I could see that it was this easy the other way round, i.e., to convert one of my SRD-7's (normal) outputs to Pro! I know I asked about this earlier, but there's a bit more I need...

...something between the afore-mentioned SRD-7 circuit board diagrams (which I'm afraid I cannot readily interpret) and myself buying an SRD-7 Mk 2/Pro!

Is there an idiot's guide anywhere, or at least a reference to symbols used in circuit diagrams?

You could put it after the source or on the tape loop and it will work exactly like if you were using direct drive amp.

It will fit on a 1" by 2" board and you can go a lot smaller if you have the resistors stand vertically.

Just google electronic symbols and that should held you along. A good text book is also a great guide and a resource for the future and you can always ask us. Take care though that while these things are simple enough they are not by a long shot a beginners project. High voltages can kill or hurt you badly and it's generally not a whole lot of fun to get zapped in the fingers.

Interesting. Ferrstein, could you post some higher-res shots of the top and bottom of the bias supply? I'd like to replicate it in a pcb layout.

Thrice, if you could post a screenshot or something of the SRD-7 mkII schematic, that would be great. As far as I know, only the SRD-7 Pro and the regular low bias SRD-7 schematics have been posted so far.

I can certainly post some high res pics of the boards. I'll try to get them up this evening. I may not be able to get the component side much better than I already have (the wires are attached and aren't very flexible), but I can get a nice square-on shot of the back side which shows the traces.

Yes, I have also only found the SRD7 Pro and SRD7 schematics, so Thrice if you have an actual SRD7 MKII schematic, that would be awesome!

Thanks a lot. I was asking because my 303's seem to sound significantly better after playing some music (about 30-45 min. I think), even if they were connected to the amp turned on a few hours before. At first they sound somewhat lacklustre and boring and only after a while all the details start to integrate together nicely and the sound seems to gain some brilliance and some, I don't know how to say, the equivalent of the depth of an engraving - as opposed to the initial "shallow" sound.