Universal upgrade board for Stax SRD energizers

[ericj]

9 years ago i started tinkering with the idea of a board to upgrade both of my SRD-7 to SRD-7 MkII, and then eventually a lot of stuff in my life went sideways and the project got shelved with a lot of other stuff. 

It did not work! it kind of worked. I screwed up something in the way the voltage multiplier was hooked up to the extent that there was only 26vac across the zener. I re-drew it yesterday. It's possible that aliexpress 1n4007 diodes were a factor. I'll use uf4007 sourced from mouser for the next revision. 

I know this shouldn't be hard. I'm kinda dumb for a smart guy. Sometimes i get a brain cramp. At the time, 9 years ago, my only high-bias stats were my esp-950. It wasn't a high priority. I have other ways to listen. I seem to be diving back in, though I'm still a cheap bastard. Yesterday i received an SR-L300 w/ brand new pads w/ all the original packaging and documentation from Japan for $250 shipped. I think I'm still in no danger of ever owning any kind of omega. 

So i have a couple DIY amps, two SRD-7, an SRD-4, an SRD-X in pieces, and today i received an SRA-3S. 

The other day while i was revising my SRD-7 Mk2 board i decided to see whether it would fit inside the SRD-4 and yes it would, just needs different mounting holes. I would be tickled pink to turn it into some kind of SRD-4.73 Pro/SB. Not ideal, sure, but i am a strict adherent of Duke Ellington's law, plus it would be funny. I could also swap sockets between the SRD-4 and one of the SRD-7 so i have a legit 5 pin socket on the 7 and a legit 6 pin socket on the 4, and just make it a 6/sb with some modern adjustments. 

It looks to me like the SRD-4 and SRD-6 share the same chassis. The other night i bought an SRD-5 from someone on the auction site for $25 shipped. 

If i keep it small, I bet i can have a board that can be used to convert any stax energizer to modern, potentially high bias spec. Probably the early 80s low-dollar stax oem (magnavox, etc) boxes, maybe micro-seiki stuff and whatever else as well, wall or self bias. 

I don't have the patience for commerce so i am considering just publishing gerbers and BOM. 

[ericj]

I could swear i had the whole schematic sheet for the original SRD-7 but i don't seem to have it at hand. Anyway, just the drawing of the SRD-7 schematic, SRD-7 Mk2 and SRD-5 official schematics below, and someone's hand drawn SRD-6 schematic. 

Notably the SRD-5 has 5k resistors on the outputs. I had sworn i saw spritzer or someone say they were only used in later amplifiers. At any rate these seem to be of not much important and will probably not be included. 

The 5, 6, and 7 all have a 30-ish ohm resistor across the transformer primary while the 7 Mk2 (and the 7 Pro) do not. My SRD-4 doesn't either. I have seen some people advocate for these to be present. I presume it is to provide an alternate path for ultrasonics. It seems like the board should support these, but the BOM should list them as optional. I'll make sure there is room to just harvest them from your original board, as there's a good possibility that the old Stax corp used nonmagnetic resistors. Haven't checked. 

A lot of people feel that the PTH devices are unnecessary and a potential source of distortion. The 5, 6, and 7 had 4.7-ohm PTH devices and the 7 Mk2 (and pro, and SRD-4!) have a pair of 10-ohm in parallel. Many people would prefer to just use a 4.7 ohm 5W resistor and be careful with the volume knob on their amplifier. Maybe new PTH devices are less noisy? I can't say i hear the distortion. I think perhaps the board should have options for both, perhaps overlaid so that it is impossible to install both of them. 

I have identified modern equivalents of the PTH devices. 

ZL01-ZL04 are MOVs. These are only found in the 7 Pro and 7 Mk2. A lot of early stats and trets had these inside the ear cup. I have heard that back in the day some people wondered if they were harming the sound somehow. I think the board should accommodate these and the BOM should specify that they are optional. 

There's some variation on the capacitors used in the voltage multiplier and based on prior discussions with spritzer and others i think we can just specify 0.1uf 250v, wima MKS4 is what i have been using so far. 

The 0.01uf cap between the transformer center taps and the bias is in some and not others, and there seems to be a consensus that they should be removed. 

Some schematics specify 4.7M or 5M for the bias resistor, some specify 2.2M. I am pretty sure the consensus is that 4.7M is the better value. A resistor that is specified for high voltage may be warranted here. It's not much more expensive than others from Mouser. 

The voltage multiplier diodes are nothing special, 1n4007 will be fine. There are reports of leaky diodes from china, so, use diodes from a major supplier. 

The zeners. In the official schematics that i have seen, they always use the symbol for a bidirectional zener, what is more commonly called a TVS diode these days. There is some confusion about these, but Semitec manufactured them until 2005 with that part number. There are modern equivalents, or we can use a pair of 100v zeners. Some people are saying that the SB energizers should use a unidirectional zener but i don't know what their logic is. At any rate, if upgrading a 5, 6, or 7, you have the option of harvesting the original component and using it on the new board, strapping a wire across the position for the other zener. 

[ericj]

From another thread here, a drawing of the SRD-6/SB circuit. 

 

I am not too sure about this one. The 100nf cap between the 100k resistor from the transformer secondary and the zener seems wrong to me? But we may as well provide accommodation for this kind of modification.