[DIY Stax Tube Amp] Rolled the dice on an auction, what exactly did I just buy?

[astromaddie]

Hi, been lurking for a while but finally signed up!

Quick background on my gear: I'm in the Stax ecosystem with an L700(Mk1), SR-3, and an SRM-003. I also have a D10 and T1S, the latter of which was on a shelf unused in the original box for 25 years or so and last year I bought it. I'm still planning to recap it and upgrade it with the constant current mod, but that's to come soon.

In the meanwhile, I came across a curiosity online, a homemade normal-bias tube amp. No real info provided, but it was a reasonable price so I took the shot in the dark, and I don't have the electronics knowledge to grok what I'm looking at. Can anyone more educated help me?

[Craig Sawyers]

Looks like a DIY version of an early STAX unit. Whatever it is, it is really nicely made with high quality components. The ceramic tagstrips are the ones used in 1960's Tektronix tubed oscilloscopes, which is a sign that the builder was very aware of their quality in a point-to -point wired build.

[Pirx]

For security reasons I see missing series c.a 5 kΩ resistors with output capacitors.

[astromaddie]

I see, thanks for pointing that out @Pirx — can you go into more detail for someone with less circuitry experience? Is this something I should fix?

Also @Craig Sawyers thats good to hear! Sounds like that bodes well that this will be a worthwhile, interesting purchase at least. I’m wondering how it’ll pair with my L700, or if I should just keep it with my SR3.

[Pirx]

My mistake, there are no output capacitors, but series resistors should be placed with output that is coming from those big red resistors.

For each channel two resistors are needed, just like on this example  schematic, where I marked them.

If you are not familiar with electronics it should be easy to do for any DIY guy around you.

 

 

 

[spritzer]

Yup, that's the SRX circuit. 

[astromaddie]

Thanks @spritzer! That’s cool to hear, I’ve read good things about the SRX— how difficult would it be to modify it for Pro bias? (or not worth the added effort?)

[spritzer]

You would have to make a bias supply from scratch as there is nothing you can simply tap into here. 

[astromaddie]

That’s what I figured, thanks!

The amp arrived the other day and, wow, it sounds really good. I think I need to recap my T1S much more than I realised, because the sound on that is all wonky compared to this— peaky upper mids, way diminished bass.

For the time being though, what actually is the downside of using Normal bias outputs with Pro Stax (particularly lambdas)? I noticed a rolled off treble and more smoothness, but I assume that’s moreso because of the tubes rather than the Normal bias— or is it?

[spritzer]

NB just gives you lower potential, so a weaker electrostatic field, hence less control over all.  The top end is the hardest to reproduce with electrostatics so it will have a large impact. 

[JimL]

If you modify it with the CCS mod (will need heatsinks), you'll have a really nice sounding amp, not that far off from much more expensive ones.

Edited January 2, 2021 by JimL

[astromaddie]

On 1/2/2021 at 5:04 PM, spritzer said:

NB just gives you lower potential, so a weaker electrostatic field, hence less control over all.  The top end is the hardest to reproduce with electrostatics so it will have a large impact. 

Makes sense. I wasn't sure if it would effect the overall output on extreme frequencies, or fine-control between frequencies.

On 1/3/2021 at 5:12 AM, JimL said:

If you modify it with the CCS mod (will need heatsinks), you'll have a really nice sounding amp, not that far off from much more expensive ones.

Hey Jim, thanks. I actually just got components in to put together the CCS mod. I was holding off on replying here until I got hold of your articles from AudioXpress in Nov/Dec 2015, for the SRX Plus, and I just started reading them. Do you think I'd have better returns putting the CCS mod into this SRX or the T1S I also have and will be recapping? I've been doing a lot of electrical studying and am really trying to wrap my head around everything conceptually here.

[JimL]

My friends who have listened to both say they think the SRX-plus sounds better than the T1. However, note that my SRX-Plus version uses the more powerful 6SN7GTA/B tubes which run at about 7 mA, whereas with the original 6CG7/6FQ7 tubes which appear to be what is in your version, you are probably going to be running them closer to the T1 standing current, around 5 mA. Both amps benefit from the CCS.

[Mach3]

Was there ever a commercial STAX SRX-Plus, or did it only exist in the DIY community?

[mwl168]

The "SRX Plus" is what Jim (JimL) named the circuit he modified based on the SRX schematic which I believe Stax released to the public decades ago. The modification, among other things,  adds CCS to some critical positions which significantly improves on the performance of the original circuit.   

I don't believe Stax ever produce an amp based on the SRX circuit. 

I built one and I love it!

Edited January 8, 2021 by mwl168

[astromaddie]

On 1/8/2021 at 8:05 AM, JimL said:

My friends who have listened to both say they think the SRX-plus sounds better than the T1. However, note that my SRX-Plus version uses the more powerful 6SN7GTA/B tubes which run at about 7 mA, whereas with the original 6CG7/6FQ7 tubes which appear to be what is in your version, you are probably going to be running them closer to the T1 standing current, around 5 mA. Both amps benefit from the CCS.

  I think that's got me interested in modifying this to be more like your SRX-Plus, though I realise I'll be keeping it normal biased.

On 12/30/2020 at 9:20 PM, Pirx said:

Which brings me to my next question, after doing a lot of research.. Curious, Pirx, where did you get this schematic? I was comparing it with Jim's published schematic from the 2015 issues of AudioXchange and noticed a few differences... like the use of a 10M90S instead of a DN2540, and some other resistors are different (102ohm vs 250ohm, 1k vs 500, etc) -- plus the addition on the left taps into the -340V power supply with a 220k resistor. Can you help me understand these additions/modifications?

Also, @mwl168, that is a beautiful build!!

[jose]

That´s a bit modification did by Jim, I have been using (10M90S instead of a DN2540, etc...) for years and I have had no problems.

[mwl168]

JimL explained the details of the cascode CCS in the thread below. It's a great read. In short, using the combination of 10M90S and DN2540 is intended by Jim. There is also a SRX Plus build thread in the DIY forum that also contain much information you'll find helpful.

 

 

Edited January 13, 2021 by mwl168

[JimL]

The 10M90S/DN2540 cascode combination makes a very good constant current source for a simple and easy to build circuit. One drawback is that the current setting resistor can be quite variable due to the variability in the DN2540 from sample to sample, so a bit of experimentation is necessary. The original SRX Plus circuit used a separate -18-24V supply constant current source (CCS) on the tail of the input diff amp, but I realized that there was no need for a separate supply, since the this could be derived from the B- supply by using a 220k dropping resistor, which was added to the schematic. This eliminates the need for a separate -18-24v supply and also balances out the B+ and B- currents.

[JimL]

Actually, since the T1 and SRX sound somewhat different, and the basic CCS mod is pretty similar (if you don't do the CCS on the output tail, which makes things more tricky, you could listen to both and modify the one YOU like better.

[astromaddie]

On 1/15/2021 at 10:39 AM, JimL said:

Actually, since the T1 and SRX sound somewhat different, and the basic CCS mod is pretty similar (if you don't do the CCS on the output tail, which makes things more tricky, you could listen to both and modify the one YOU like better.

Nice idea, James! The only issue concern I have is that the SRX only has Normal-bias outputs. I actually just picked up a 007A (still coming in the mail) so I'm not sure I'd get the same kind of ROI from the CCS mod on the SRX as I would from the T1S. Am I on the right track with that thinking, or would the normal-bias SRX (+ CCS) still be powerful enough?

Also, thanks for the explanation earlier regarding the 10M90S/DN2540 cascode combo. That's interesting, nice optimisation using the B- supply!

[JimL]

Well, you can always build a pro bias board and install a pro bias jack for the SRX to use the 007A. The amount of available power from the SRX depends on the power supply voltages and the amount of current that the output tubes are running. The basic SRX circuit uses 50k plate resistors vs 66k for the T1, so about 30% higher current-to-voltage ratio, but lower voltage supplies. Assuming +/-250 volt supplies for the SRX, that means about the same standing current but somewhat lower voltage swing - however still high enough for most listeners - probably peak SPLs around 110 dB. Not sure what you mean by ROI - the CCS mod more than doubles the effective power for less than $100 in parts for either amp, which is a pretty cheap to me. The difference between 250 volt and 320 volt power supplies is about 2 dB in ultimate volume, which you will likely never reach unless you are looking for permanent hearing damage.

[astromaddie]

So, from reading your SRX Plus diagrams, it looks like the pro bias line could be created by adding a few zener diodes to reach the 580V bias-- is that right or am I over-simplifying things? (This is still new for me.) I'm asking because, if I converted this, I'd probably just convert the normal bias jack into a pro bias. As for what I meant by ROI, I didn't mean cost-based ROI (since the CCS mod is cheap enough, as you said) but I was asking under the pretense that I couldn't convert my SRX to Pro bias, so I thought the CCS in the SRX with Normal bias may not be worth it. Though, I do think my particular SRX amp is pretty powerful, as it noticeably sounds better than my stock T1S. How do I determine the power supply voltage? That would be coming from the Tango transformer block, depending on which input/output terminals are being used, right?

[JimL]

IF you have access to the power supply capacitors, you can just measure the voltage on those. For the positive power supply you can measure the voltage at the "top" of the first or second stage plate resistors. For the negative supply, you can measure the voltage at the "bottom" of the output stage differential amp tail resistors.

[garageboy]

So the most "up to date" SRX+ would be that schematic above, with 

power supply?