srx revisited

[Peleus]

Thanks, JimL!
I get your point and be appreciated your work of SRX+.

[JimL]

9 hours ago, joehpj said:

for AC coupling caps, any suggestions?

I used Audiocap Theta polyprop caps for my build because they are relatively affordable and relatively small, but any polyprop is fine, e.g. Mundorf, etc.  Polystyrene should be OK also but be careful with them because they can melt if directly touchéd by a soldering iron. 

[Peleus]

Hi~ Joe, 

I recommend this one, Mundorff supreme 0.33uF/1000V.

You can buy it from local web store, Tube Fan Studio.

And the most important, Its price is not too high to buy four of them in once. And more...the quality of sound is very good!

[Blueman2]

On 12/27/2015 at 1:04 PM, JimL said:

You HAVE to have two 6.3 volt windings because the 6SN7GTA cathode sits at around -320 volts and the 12AT7 cathodes sit at around 0 volts and above.  You cannot get away with one 6.3 volt winding, due to exceeding the filament-cathode ratings resulting in premature tube failure.  This is one of the screw-ups in the SinglePower ES-1.

 

On 12/28/2015 at 6:15 PM, JimL said:

Now if you use the KGBH supply which is cap input, then divide that by 1.4 to give around 640 vct, but you should also go with around 100 mA current or more.  Those should give you an idea of the transformer size.

JimL,

I have spent a few days reading, and am learning a lot!  I see on your board design you have separate Filament inputs for 6SN7GTA and 12AT7.  One labeled "F  F" (for the 6SN7GTA tubes) and other labeled "Filament" (for the 12AT7 tubes).  I assume each gets 6.3VAC from separate windings on the transformer, as you alluded to above, right? 

Also, with respect to using the KGBH PSU (kgbhultraminipsv4 design) with your AMP.   Sounds like I can keep the default +/- 350V outputs.  But I see near the "Filament" input for the 12AT7s a -20V input and GND. What is the -20V for?   Is that simply to drive the test points?   The KGBH PSU does output -15V.  Will that work?  Or should I replace the 7915 in the KGBH PSU with a 7920 and use that output?  Also, do I even need the +15V for anything in that case?  Maybe I could use a 7805 and get +5v for led or other usage?  

 

[JimL]

2 hours ago, Blueman2 said:

 

JimL,

I have spent a few days reading, and am learning a lot!  I see on your board design you have separate Filament inputs for 6SN7GTA and 12AT7.  One labeled "F  F" (for the 6SN7GTA tubes) and other labeled "Filament" (for the 12AT7 tubes).  I assume each gets 6.3VAC from separate windings on the transformer, as you alluded to above, right? 

Also, with respect to using the KGBH PSU (kgbhultraminipsv4 design) with your AMP.   Sounds like I can keep the default +/- 350V outputs.  But I see near the "Filament" input for the 12AT7s a -20V input and GND. What is the -20V for?   Is that simply to drive the test points?   The KGBH PSU does output -15V.  Will that work?  Or should I replace the 7915 in the KGBH PSU with a 7920 and use that output?  Also, do I even need the +15V for anything in that case?  Maybe I could use a 7805 and get +5v for led or other usage?  

 

Correct on the filaments.  The 12AT7 filaments sit about 60v above ground, the 6SN7GTA filaments sit about -315 volts, so they MUST be separate windings.  This is to prevent the filament-cathode voltage limits from being exceeded which would lead to filament-cathode leakage and early tube mortality.  Note that Kevin Gilmore's board (it's his design, as my original used 6.3 volts for both) uses 6.3 volts for the 6SN7GTA and 12.6 volt connections for the 12AT7s, so you'll need two separate filament voltages as well.  This prevents a builder from thinking he can get away with a single filament winding for input and output tubes, because they are two different voltages.  However, it also means that the transformer has to supply both voltages - or you need a separate filament transformer for one of them.

 

Yes, you can keep the +/-350 volt outputs.  I actually was going to use those voltages but my transformer didn't quite get there so I dropped the voltages to +/- 325 volts.

 

The -20v output goes to the input stage tail constant current sources - the resistors are chosen so that the combined current to both of them drops the voltage from -325 (or -350) to around -20 volts.  Actually for -350 volts you need a about 10k higher resistance to get to -20 volts.  -15v would do just as well, in which case you could simply tie the 6SN7GTA filaments to -350 volts, or just let them float.  The +15 volt isn't needed, unless you want to use it to power a high voltage delay circuit.

[Blueman2]

On 1/12/2016 at 7:33 PM, JimL said:

The -20v output goes to the input stage tail constant current sources - the resistors are chosen so that the combined current to both of them drops the voltage from -325 (or -350) to around -20 volts.  Actually for -350 volts you need a about 10k higher resistance to get to -20 volts.  -15v would do just as well, in which case you could simply tie the 6SN7GTA filaments to -350 volts, or just let them float.  The +15 volt isn't needed, unless you want to use it to power a high voltage delay circuit.

I see in your article on AudioXpress, Figure 3, that you only had one such circuit for both input stages.  But the srx6 board layout has separate circuits for each channel. Is that something Kevin added for balancing reasons?  (EDIT: Doh!  I misread the schematic. Nevermind).

If do go with B+/B- of 350V, specifically which resistors should I change?  Alternatively, I can just put in different zener diodes and drop my KHBH PSU to +/- 325V.  Which would you do?  I already found a nice toroid transformer that can drive up to 400V if needed (it is 2x 350V), so that is not an issue.  

Edited January 13, 2016 by Blueman2

[Blueman2]

14 hours ago, JimL said:

Correct on the filaments.  The 12AT7 filaments sit about 60v above ground, the 6SN7GTA filaments sit about -315 volts, so they MUST be separate windings.  This is to prevent the filament-cathode voltage limits from being exceeded which would lead to filament-cathode leakage and early tube mortality.  Note that Kevin Gilmore's board (it's his design, as my original used 6.3 volts for both) uses 6.3 volts for the 6SN7GTA and 12.6 volt connections for the 12AT7s, so you'll need two separate filament voltages as well.  This prevents a builder from thinking he can get away with a single filament winding for input and output tubes, because they are two different voltages.  However, it also means that the transformer has to supply both voltages - or you need a separate filament transformer for one of them.

OK, I made a mistake then.  I should have seen that the 12AT7s needed 12.6V and the 6SN7GTA needed 6.3.  But in my mind, I had thought both were driven by 6.3. So the 2x 6.3V transformer I bought will not be enough.  

I cannot just combine the 2x 6.3 for the 12.6V, right?  (Since one leg of the 6.3 is driving the output stage).  So I will need to buy another 12.6V transformer to drive the input stages(?)  Would a single 25.2VCT transformer be OK to drive the 12.6V input stage filaments as well as providing 25.2V for the -20V supply?  I was not sure if there would be any issues with one winding of the transformer being used for the 12AT7 filament and the -20V supply at the same time.   

Edited January 12, 2016 by Blueman2

[JimL]

7 hours ago, Blueman2 said:

I see in your article on AudioXpress, Figure 3, that you only had one such circuit for both input stages.  But the srx6 board layout has separate circuits for each channel. Is that something Kevin added for balancing reasons?  

If do go with B+/B- of 350V, specifically which resistors should I change?  Alternatively, I can just put in different zener diodes and drop my KHBH PSU to +/- 325V.  Which would you do?  I already found a nice toroid transformer that can drive up to 400V if needed (it is 2x 350V), so that is not an issue.  

Take a closer look at the srx6 board.  There is one input for the -20 volts.  Figure 3 shows a single channel, so there are two of everything. 

 

Either 325 or 350 volts is fine, your choice.  For the KGBH PS, the voltages are set by the zener diodes.

7 hours ago, Blueman2 said:

I cannot just combine the 2x 6.3 for the 12.6V, right?  (Since one leg of the 6.3 is driving the output stage).  So I will need to buy another 12.6V transformer to drive the input stages(?)  Would a single 25.2VCT transformer be OK to drive the 12.6V input stage filaments as well as providing 25.2V for the -20V supply?  I was not sure if there would be any issues with one winding of the transformer being used for the 12AT7 filament and the -20V supply at the same time.   

Yes, you need separate windings for 6.3 and 12.6 volts when using the srx6 board.  The filament windings are AC, the -20 volts is DC, you cannot mix them up.

Edited January 13, 2016 by JimL

[Blueman2]

It just hit me that I am asking a lot of build specific questions in a thread that is more about the SRX-Plus design.  Sorry about that.  Mods, maybe move these to the Plan to build SRX. Where to find BOM, Boards? thread?  I will continue my questions over there. 

Edited January 13, 2016 by Blueman2

[JimL]

Update to the shunt regulated power supply.  The larger value resistor for the C- tap should be decreased from 133k to 100k.

[mwl168]

On 1/13/2016 at 0:08 AM, Blueman2 said:

It just hit me that I am asking a lot of build specific questions in a thread that is more about the SRX-Plus design.  Sorry about that.  Mods, maybe move these to the Plan to build SRX. Where to find BOM, Boards? thread?  I will continue my questions over there. 

A suggestion - maybe rename the thread to something like SRX Plus build thread? 

[Blueman2]

 

11 hours ago, mwl168 said:

A suggestion - maybe rename the thread to something like SRX Plus build thread? 

Done!

[Blueman2]

On 4/4/2015 at 5:13 AM, congo5 said:

  Hi; I built this amp, boards and case. And while it looks homemade, it works so well that I want to know why?

What am I comparing too?......  KGSSHV, Megatron ,KGST,Egmont.

 

 

pongo5, do you have any more pictures of your build you can share?  I would like to see it along with the other components, and how you mounted them into a case.  And what case you used!  Thanks.

 

Blueman2

Edited January 18, 2016 by Blueman2

[congo5]

[JimL]

So, wanted to update this thread with the recent modifications to the schematic.  Basically, this consists of running a resistor from B- to the input stage tail current source, and changing the top MOSFET in that current source from a DN2540 to a 10M90S, which increases its voltage limit.  This should protect this current source from frying if one of the input tubes goes bad.  With this change, C- in the power supply is no longer needed, so the only connections from the power supply are B+. B- and ground.

[JimL]

I have made a few revisions to improve the power supply – see schematic.   First, the 15 volt zener at the bottom of the two MOSFET cascode shunt was deleted as unnecessary.  Second, the 431 output filter capacitance was increased to 1000 μf to roll off its output noise above 0.8 Hz. Third, I now recommend running a 220 kilohm resistor from B- to each input tail current source, so I deleted the –C supply resistor chain.  Fourth, I recommend floating both filament supplies rather than tying them to the high voltage supply, so I modified the voltage setting output resistors to all have the same value.  Fifth I decreased the resistor chain values for the 431 chip the schematic shows the original values in parentheses.  These are mostly deletions or changes in values but not in topology, i.e. if you leave out the component values the circuit looks pretty much the same as the original.  Finally, I moved the capacitor from between reference and anode terminals of the 431 to across its cathode and reference terminals.  Although this appears to be a very simple change, it significantly improved both stability and noise performance.

 

The 431 has an internal op amp with a gain of about 55 dB and a bandwidth extending to between 5 kHz to 50 kHz before rolling off.  This op amp is not unity gain stable, which means it can and will oscillate.  Despite this, the 431 chip is widely used as a voltage regulator in switch mode power supplies in all sorts of consumer electronics, such as PCs, laptop chargers, cell phone chargers, solar panel charge controllers, etc., so various techniques have been developed to stabilize it. 

 

The regulator voltage is controlled by feeding back a portion of the output voltage into the reference terminal of the 431.  Often a wide bandwidth is desirable to control both the DC voltage and cancel any AC variations across a broad spectrum.  However, this design uses local feedback to the lower MOSFET via a capacitor to its gate to cancel any AC changes and noise, so we want to restrict the 431 to controlling the DC only.  Broskie did this by rolling off the feedback signal using the capacitor between reference terminal and ground.  But that did not affect the feedback loop gain, leaving lots of opportunities for oscillation.  However, instead of limiting the feedback signal, if we limit the bandwidth of the 431 sufficiently, we both restrict its effect on AC voltage variations and stabilize it against oscillation.

 

This is done by removing the capacitor between the 431 reference and anode terminals, and instead connecting it between the 431 cathode and reference terminals.  In combination with the resistor chain between B+ and the reference terminal, this forms a negative feedback compensation loop that rolls off the gain of the op amp at 6 db/octave above its corner frequency.

 

Since I modified the 431 resistor chain and output RC filter, I used trial and error to determine the needed feedback cap value.  There were large oscillations with a 1 μf cap, while 3.3 μf produced smaller, slower oscillations but still managed to fry the 431.  A 10 μf/50V polyester capacitor was stable with generally low noise, but random bursts of higher noise – however, the levels of the bursts were still below the 2 mV noise level in the original design. Increasing to 20 μf decreased the magnitude and frequency of the random noise bursts by about half.  

 

With the original 4 megohm resistor chain for the 431, a 2.2 μf cap should give the same gain compensation curve, so the simplest modification is to remove it from its original position across the reference and anode terminals and connect it between reference and cathode terminals.  On the PS PCB there isn’t room to mount a 2.2 μf cap next to the regulator chip but you can drill a couple holes next to the 200 ohm resistor on the opposite side from the 22 μf output cap and run insulated wires to the two outer leads of the 431 chip. Note that if you are using a polarized cap, the + terminal should be connected to the cathode and the - terminal to the reference.  This is the opposite from the previous connection where the + terminal of the cap was connected to the reference terminal.

 

Replacing the TL431 with the quieter SPX431 dropped the noise voltage another 10 dB or so.  With these changes, the broad-band power supply noise with a Fluke 189 DMM was decreased to around 0.16 mV into a resistor dummy load on the kitchen counter, with occasional bursts up 0.4-0.5 mV, so call it a -120 dB power supply.  Not too shabby considering its simplicity.  DC voltage overshoots by a volt or so but then remains within 0.1 volts of its target voltage due to the excellent temperature compensation in the 431 chip.

Edited December 3, 2016 by JimL

[mwl168]

Since the input to the two channels' tail current sources are on the same connector on the amp board marked as -20VDC input, how do you recommend wiring the two 220K ohm resistors from B-? Also, I assume the 220K resistors would need to be 1W or higher given the voltage drop across them?

[JimL]

Yes, they would need to be at least 1 watt.  I used the same 2 watt resistors as are used in the output stage resistor chain.  The reason for doing this is to further isolate the two channels for purposes of trouble shooting, but practically, it makes no difference if the circuit is working properly.  

 

As you note, with the first run PC board you would have to cut the traces from the C- supply - I think Kevin has updated the board to the newer circuit, but that doesn't help people with the first board.  I built mine point-to-point so more difficult to build but easy to modify.  Circuit boards are easy to build but more difficult to mod.

Edited December 3, 2016 by JimL

[Sorrodje]

Hey there,

I'm not that much well know down there on Head-Case. Suscribed a few times ago , I never posted and I'm use to lurk only. @JimL asked me to give some impressions about the srx+ i commissionned to a reputable builder a few month ago and received in last october. So here am I.

First of all , let me give you a few elements about where I come from. I'm a french hobbyst ( that explains my average english and I apologize for that) involved in personal audio since 2012.  I began with a Koss Porta pro from my Laptop and now  my reference system is a Schitt Yggdrasil , a DNA Stratus , a Sennheiser HD800 (modded with the mod I created last year to make a HD800 sound a bit more like a HD800S : the Superdupont Resonator maybe a few people know here ) and an Old sennheiser HE60 driven by the SRX+ we're talking about in this thread. Actually , even if I'm a HD800 nut , I appreciate a lot Electrostatic headphones i discovered with a classic combo SR303 + SRM323. I owned or listened carefully almost all Stax including vintage ones  but despite their excellent sound , I still prefer the HE60 I bought one year ago.  My preference usually goes more to brighter/clearer sound sig hence the fact my Favorite Stax is the SR009 followed by the lambda Signature.  I'm listening mostly to Jazz ( modern jazz mostly from 90s to nowadays) , Classical and Electronic music. Not much Rock/Pop.  You can check my head-fi profile if my whole pedigree matters for you. 

 

so.. SRX+ .  here's a pic of mine : 

 

 

The Builder choosed to insert a Blue Hawai Power Supply , an Antek transformer, an alps RK27 potentiometer and mundorf mcap supreme capacitors . I asked him to respect a size ( especially depth) because i had some rule to respect in my living room.  . I'm delighted by the quality of this build and the look and feel offered by this amplifier. I choosed this amplifier because @n3rdling told me it could maybe a great and very valuable choice. I trust Milos so I took the plunge.  

I'm not that much a tube roller. I bought a quad or french NOS Laradiotechnique 12AT7 and a pair of NOS GE but replaced it lately by a pair of 6SNGTB Chrome top Sylvanias and they improved the sound ( tighter , clearer , more transparent) significantly .  

I had a trip in US in november and went to San Francisco and LA to meet US hobbyists. I brought this SRX+ with me and it seems listeners enjoyed it a lot . I didn't take myself enough time to compare to well known contenders ( Especially n3rdling's BHSE for example) though. We met a few guys from head case. @blubliss I think and another guy I don't remember the name. Sorry.  

So... When @JimL asked me to give impressions , i wasn't convinced to be able to provide valuable informations. Fortunately, a meet was organized in France at @Ali-Pacha 's place last week. Ali-pacha already allowed me to listen and measure his huge stable of Stax headphones last year and a BHSE joined his stable recently. It was the perfect opportunity for both of us. We added a SRM-727 (usual feedback mod done) in the shootout to have a more affordable contender. 

We used my HE60 and the two Stax Omega (007mkI and 009) to do our listening sessions. During the first sessions i was unable to detect any difference between amplifiers and it was the sign my ears wood... too tired and I had a 2nd sessions during Day 2 of our meet.

Let's go straight to the conclusion : the modded SRM-727 is not in the same league than the two others and the SRX+ is a true bargain in E-stat world.

First Session was with my HE60 I know well and at least much better than the two Staxes. HE60/BHSE is truly a match made in heaven. I love my HE60 but I know well his flaws : it's kinda brightish and sometimes, with a few recordings, it's borderline unbearable. Bass can be punchy but still too diffuse/soft by today standards, mids are wonderful but compared to Milos HE90 clone, they're undoubtly a tad colored (in a good way but still) and overally the HE60 show a bit his age with a kind of slightly "vintage" soft sound. It's really obvious when compared to modern headphones. Nevertheless, it's a superb headphone I love to use for a few genre and when I want a change of pace from my HD800SD.

The BHSE with its kinda cold uber-technical abilities and its slam/power makes the Senn sounds younger that it really is. The low end expecially is clearly improved with much more tightness down low and an overally cleaner, more controlled, more focused sound. The Baby Orpheus gains some 009 qualities but still with the typical Senn Stat sound and still that extremely good staging that offers both a good sense of focus and a lot of air hence the feeling of cohesive but endless staging. In one word, BHSE/HE60 is still HE60 but better everywhere. Think to a hand (the HE60) and its glove (the BHSE). 

Considering how BHSE and HE60 works well together, there's no surprise here : my SRX+ (I bought it especially for my HE60) can't compete and the Senn sounds noticeably more blurry and kinda soft/ limp dick on the SRX+. Let's be honest, it's still good enough for me but compared to the BHSE, the loss is significant. No surprise here. For my personal use it's not that bothersome because I use mostly my HE60 when i'm listening to intimate vocals, chamber music and that kind of music. For Orchestral, EDMs I always choose my HD800.

HE60/SRM 727 . No need of long comparison here. the 727 does not play in the same league. The immediate feeling of congestion struck me immediately. Something broken/dead in the sound as well whereas the two other amplifiers let the headphone breath naturally. Maybe something warmish but still steely/grainy in the treble in the tonality was a showstopper as well. I didn't expect so much difference to be honest, even more if we consider I already heard the 727/HE60 combination and it sounded OK to my ears. But compared to the SRX+ and even more the BHSE, the story wasn't the same ...

Summary for HE60 : BHSE >>> SRX+ >>>>>>>>> 727. if the HE60 was my main headphone, I'd order a BHSE right now.

Second session with the SR009.

OK , BHSE/009 is a known association and I won't be long . Sounded really good, clean, a tad cold, very technical to the point of maybe some sharpness but I see how performing is the combo. Seems the Midrange shout that usually bothers me with all Stax headphones is well controlled though. Good pairing but not as special as the HE60/BHSE one to my ears.

Surprise came from the SRX+ there... Honestly I think I'd choose the SRX+ over the BHSE for the 009, especially if I wanted to use a 009 on daily basis as my allrounder. Whereas the SRX+ made the HE60 a bit too blurry, it made the excess of sharpness disappear here and the 009 sounded a tad more rounded , more "human". SRX+ / 009 offered to my ears a very balanced sound with a hint of forwardness/intimacy, a touch of warmth and overally a kinda relaxed yet fun/pleasurable presentation.

Probably a very very good choice for those who listen a lot of modern music with their 009. I liked it a lot and definitely made me think more to add a 009 to my stable. I still think the BHSE is "better" in almost all point but the synergy goes to the SRX+

One more time, the 727 sounded much less good than the two others with the same flaws than with the 009. Both BHSE and SRX+ made the 009 sound more clear and crisp , more focused, tighter in all register. the main issues are congestion and Kinda loose low end I'd say. This and some grain in the treble...

With the 009 I'd see the ranking as this : BHSE>SRX+>>>>>>>>>>>>> 727 . the experience made me want a 009 to associate to my SRX+ ...more than a BHSE

We tried as well the 007 but I'm not familiar with this headphone. Moreover it's not my signature of choice (too mellow for me) so I prefer to let you read what @Ali-Pacha wrote about it.

Overally I Still think the little and kinda cheap SRX+ is a great option for people who want a very competitive amplifier without to spend an arm and a leg. At least, Im' really really happy with my purchase. 

Maybe @Ali-Pacha will add some of his own impressions . 

[mypasswordis]

The originator of the Superdupont mod makes his presence here! Nice first post. Was the SRM-727 feedback modded?

[JimL]

I asked sorrodje (Pierre) to post on the SRX Plus because I haven't had a chance to compare it much with other stat amps, so I was curious about other people's impressions of it.  If you're not familiar with him from Head-Fi, he has a nice measurements and sonic impressions post on SBAF in collaboration with Ali-Pacha of about 10 Stax headphones ranging from the SR-5 (my very first Stax headphone) to the L700, and including the SR007 and 009.

[Sorrodje]

Sorry I forgot to mention the 727 was indeed modded. My bad. 

@JimL Thks for your positive comment on our thread dedicated to Stax measurements. I just need to add to take them with the grain of salt. Distorsion plots aren't accurate due to some calibration issues. I should redo them to do justice to all headphones measured. but we're off topic here. 

[blubliss]

Thanks for the thoughts sorrodje.  Makes me want to redo the behemouth Single Power ES-2 I have in the srx manner, so it actually works.

[JimL]

blubliss, I thought you had a SinglePower ES-1 that had been modified to work properly?

[wink]

Even though I prefer the Stax SR-007 and SR-009 generally over the Senn HE-60, there are times when the Senns just sing and you just want to glue them to your head...