So many Stax DIY Amplifiers, which to choose?

[Juggernaut1101]

Hi Everyone,

 

First of all, I'd like to thank you guys for all of the information you have posted on this site. Without all of this, I would probably be screwed, and not able to find enough information on how to make a Stax amp.

 

I have been wanting a sizable upgrade from my pair of HD 595s and after getting the Fidelio X2s which I wasn't too impressed with, have decided to go all the way and get the STAX SR-407  . I decided that I would also like to take this as a learning exercise and build my own amp. I've built a Millet SS Hybrid before, and know my way around electronics a bit, although probably not at these voltages. I have been doing some research over this past week trying to decide which AMP to build. The most attractive options I have found have been the SRX and KGST.

 

Unfortunately, it seems that no one has any boards built that I can find. I was wondering if anyone had any other suggestions. I was looking for a cheaper amp ($300 or so), and preferably one that I could just buy a board for, which would make my life quite a bit easier.

 

There seem to be many smart people on this board, so I was wondering if anyone had material I could go through for more of the theory behind amplification too, beyond just simple op-amp circuits. I've started looking at BJT cascodes. I have a mechanical engineering degree, so I am not completely at a loss when it comes to the technical side of things.

 

If you have any advice, or suggestions for me, feel free to give them!   

[nopants]

SR-X, point to point

[xianghao]

As far as I know, SRX has still lots of things waiting to discuss so it is not a good idea to built it.In fact, KGST KGSSHV or MEGATRON will more essay to built and they all have good performance.

 

Here my Personal views：

 

KGSSHV + 007 ------------PERFECT            KGST + 009 ----------- PERFECT 

 

KGSSHV + 009 ------------NORMAL             KGST + 007 ----------- WELL 

 

MEGATRON（PSVANE EL34 AND 12AX7 U7） + OMEGA OR 009 -------- PERFECT

 

OK,i know. Just a personal point of view. 

[TMoney]

after getting the Fidelio X2s which I wasn't too impressed with...

 

Blasphemy! Wearing mine at work right now.

[DefQon]

Build the kgst if you want tubes. The parts for it isn`t high compared to the kgsshv or megatron. I`ve got 3 sets of kgsshv to get through and my kgst boards is on the way have done a quick BOM calculation for the kgst and its much less then my kgsshv.

I think JimL and Kevin are still working n ways of improving the SRX circuit in the other thread.

[nopants]

SRX board is done

 

If you are committing to the 407 I would say play around with the SRX, the cost is going to be less than a KGST. You have the option of building the original circuit or Jim's hotrodded version. I'm opting for the latter

[FrankCooter]

I have some spare power supply boards for the KGSS. You're welcome to one if you want.

 

I second nopants recommendation for the SRX. It's easy, cheap, rugged, uses readily available common parts and can be built point to point.

 

Any design can always be improved, but there's nothing at all wrong with the circuit as is.

[JimL]

I don't know about Dr. Gilmore but I have no further plans to improve the SRX Plus circuit - it's good to go, and pongo5 has built one using the board files on Dr. GIlmore's site (its file SRX6), so the circuit board has been tested.  I've discussed the reasons for the modifications in the SRX revised thread, and the reasons for using current loads in the output current requirements thread, so the technical discussion is basically complete.  The only thing I haven't discussed is the shunt regulator PS - waiting for AudioXpress to get back to me on when they plan to publish my submission, but the KGST PS would work just fine.

 

I also agree with Frank Cooter that the basic SRX is about as simple a high-performance tube circuit as there is - 3 tubes, 3 or 4 capacitors, 13 resistors and 1 trim pot.  If you build the basic version point to point, it is easy convert it to a simplified Plus version by substituting current sources (on heatsinks) for the output load resistors, which is probably the biggest bang for the buck improvement.

[FrankCooter]

When I was referring to the the SRX circuit, I meant Jim's SRX Plus circuit. The original is pretty good, but Jim has taken it to an entirely new level. I don't often build other people's designs anymore, but the original SRX always intrigued me, and the SRX Plus has renewed my interest. I have a reputation for building "over-the-top" tube projects, but I actually find it a lot more fun to see how much you can do with simple circuits and limited resources. In the not too distant future I'll be building Jim's  SRX.

[nopants]

What Frank said, and my QQQ sockets just arrived, waiting on the turret strips

[xianghao]

JimL and nopants is right.

 

Ignore the stupid thing what I said........

 

and i wonder where can find the PSU of SR-X.

[nopants]

In the audio xpress article. Jim already said the standard BHSE supply works,  though I did want to check that 280V@80 ma HV secondaries would be ample for the SR-X. That would make it a single-transformer build.

[JimL]

280v should give around 380-390 volts DC which should be enough depending on how much of a margin you need (e.g. how much does the voltage drop in the summer when the ACs are running full blast everywhere).  In terms of current, the SRX only pulls about 40 mA for both channels, but since the KGST or mini KGBH are cap input filters you need some extra because of the high current draw at turn-on until the caps are topped off - I think the KGSSHV thread had a discussion about transformer requirements in terms of current, or you could just use the same transformer as the KGST 350V version as the voltage and current demands are quite similar, except the SRX doesn't need the 15 volt transformer - the -20v can be derived from the -350 volt supply using a 110-120 kilohm dropping resistor. 

[spritzer]

It would also be easy to use the PSU built on the KGST boards.  Just use a LM7918 and use a diode to lift up the ground for -19V or so. 

[JimL]

Yup, that would work, but it's a bit of overkill - not that there's anything wrong with that.   Actually -15 volts would work perfectly well since it's supplying the current source for the tail of the input stage and a few volts here or there makes no difference whatsoever.

Edited June 11, 2015 by JimL

[xianghao]

thanks a lot,everyone.i will make a plan to built SRX after i get all the detail. 

[spritzer]

Yup, that would work, but it's a bit of overkill - not that there's anything wrong with that.   Actually -15 volts would work perfectly well since it's supplying the current source for the tail of the input stage and a few volts here or there makes no difference whatsoever.

 

Good point.  A KGST psu and transformer could be used with the +15V just driving the power LED.  A bit more opulence just for the fun of it... 

[JimL]

Ooh, I like your style!

[JimL]

Just thought I'd mention that AudioXpress has published both parts of my SRX Plus article, and I've completed the discussion of its design in the SRX revisited thread.  Dr. Gilmore has board files on the amplifier.  One of these days when I've got the time I might design a board for the power supply but it's simple enough to be built point-to-point.