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SRX-Plus Build Thread


Blueman2

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12 minutes ago, wink said:

Can Birgir interest you in a nice KGSSHV Carbon....?      :D

After completing the 2 SRX-Plus builds, my mind is already wandering to what is next.  I am leaning towards a solid state amp just to have something on the other side of the spectrum from the wonderful SRX-Plus.  If anyone has any spare boards they can part with, let me know!!!  Otherwise, I will keep my eyes out for the next group buy.  I have caught the Stax bug in the best possible way.....

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I don't, but could potentially start working on layout in a few weeks, given enough people want it and Kevin is okay with it. Do beware that even though the boards can be shrunk much smaller using SMD parts, the heatsinks obviously need to be the same size as before to dissipate the same amount of heat. I need to experiment more with forced convection to really get things small.

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I used 300×100×40mm sinks for my Carbon, and they don't get very hot at all at 20mA. I guess that with lower current I could get away with sinks a quarter of the size. You will retain the through-hole Cree SiC MOSFETs, right? Designing the boards so the devices attach directly to sinks rather than through angle brackets should help with dissipation also.

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I had two ideas. One is just to make the board as small as possible so no mounting bracket and you mount the PCB directly to any size heatsink you want, just like you said. The other is using these new heatsinks I found: https://www.mouser.com/ProductDetail/Wakefield-Vette/OMNI-UNI-41-75/?qs=sGAEpiMZZMttgyDkZ5WiulvLQlijNHCBZqUtUbH5nOM%3d The datasheet says you put two back to back and they have mounting holes for a fan. The problem with that is the bigger ones say they can only fit bigger packages, no TO-220. I sent them an email inquiring, but if people just want the first option then I don't need to pursue this any further. 

Edit: sorry to JimL for crapping on your thread. Any new posts should go in the Carbon thread.

Edited by mypasswordis
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19 minutes ago, mypasswordis said:

Edit: sorry to JimL for crapping on your thread. Any new posts should go in the Carbon thread.

I was going to suggest the same thing, thanks. That's probably where anyone interested in an SMD Carbon would look first, anyway.

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I thought I would provide an update on the sound of my 2 SRX-Plus amps.  I used new Penta Labs quad 12at7s that were balanced and matched in both builds.  I used some existing NOS 12AT7s I had laying around and could not balance the amp correctly.  With new balanced and matched Penta Labs, it was a breeze to get the amp dialed in. Then for one amp, I used new Tung Sol 6SN7GTBs, and in the other, NOS GTA Sylvanias.  The Sylvanias sounded better out of the gate. cleaner and crisper sound.  Tung Sols seemed a bit muddier.  I have been primarily listening to the Tung Sol amp for the past week, and it appears that over time, the amp has become more clear and defined; less muddy.  I am not sure I believe everything I read about break in periods for headphones, speakers, tubes, etc.  Much of the break in talk is not supported by objective measurements so I am skeptical.  However, it sure seems that something has changed in the Tung Sol amp over time.  And I don't think it is just my ears because I have been using an HD650 Sennheiser as reference for comparison.  Both driven from the same O2 DAC/AMP source, with line out of the O2 DAC going into both the O2 Amp (for HD650s) and the SRX-Plus (for SR-507s).  Of course, the SRX/507 combo always won, but at first, the HD650s just seemed cleaner less muddy.  There was and is a surprising amount of bass coming from the SRX/507 combo, but the bass seemed too muddy and was somewhat fatiguing.   Now, that muddiness in the SRX/507 seems mostly gone.  Still not perfect, but better.  

Not sure if this is a combination of the amp settling in, the Tung Sols breaking in, the Penta Labs breaking in, my final dialing in of the amp outputs (my offset was about 70V or so before I did all the tuning to get it to essentially 0V after warm up), or maybe my ears getting more used to the sound.  But the SRX seems to sound a bit better every day.  I have about 30 hours of listening time on it so far.  I will report back when I cross 100 hours. 

I did notice that when I put the NOS Sylvania SN7s in my amp instead of the Tung Sols, it does sound noticeably cleaner and clearer. Perfect, really.  No trace of muddiness at all.  And that is without retuning the amp for these tubes.  So perhaps there is something to be said for either the quality of those old tubes, or the fact that new tubes need an extended break in period?  Why do I continue to use the Tung Sols, you ask?  Well, because my son's ears are much better than mine, so he gets the good Sylvania tubes!  

For someone coming from Dynamics or even Stax driven by SRD-7, I would rate the sound as 

SRX-Plus/507 >>> SRD-7/507 >>>>>>>>>> O2Amp/HD650 

So I would strongly recommend the SRX-Plus build.  Just be aware of the importance of good tubes both in dialing in the amplifier and in quality of sounds.  And if using new tubes, given them time to break in.  

Edited by Blueman2
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Nice work, @Blueman2!

One thing I observed: the ODAC (which is what I assume you mean by "O2 DAC") clips. At least, mine does. It only works properly (for some loose definition of "works" and "properly") with digital input volume on the source dialed down to ~50% (and no, I'm not sure that it doesn't throw away bits of dynamic information in this state). The clipping is substantial and easily audible compared to every other DAC I have available. I don't know much about tube swapping, but make sure your DAC is not causing you any grief first.

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12 minutes ago, gepardcv said:

Nice work, @Blueman2!

One thing I observed: the ODAC (which is what I assume you mean by "O2 DAC") clips. At least, mine does. It only works properly (for some loose definition of "works" and "properly") with digital input volume on the source dialed down to ~50% (and no, I'm not sure that it doesn't throw away bits of dynamic information in this state). The clipping is substantial and easily audible compared to every other DAC I have available. I don't know much about tube swapping, but make sure your DAC is not causing you any grief first.

Wow, you are absolutely correct about clipping.  But I thought it was more related to not enough power being provided by the USB PC driving the ODAC.  But in any case, yes, I had to reduce volume (digitally) from the computer that is driving the ODAC via USB.  I spent about 4 hours figuring that out.  :wacko:

I actually ended up connecting a proper high end DAC built into my Integra receiver.  It made everything sound much better.  Now I know why!

Edited by Blueman2
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  • 3 weeks later...

James, have you (or anyone else) considered the option of increasing the output current loads above 7ma?  Since I using an SE setup for my SRX, there is the option to greatly reduce the current for the o- (say to 2ma) and increasing the current for the o+.  Given the 5 W limit for the 6SN7GTB, I think 12ma is possible.  The combined would also be below 7.5W combined limit.  Since I am not using the O-, might as well give more to what I am using, right?  I got this idea after increasing the Carbon from 17 to 20ma and really noticing a difference at higher volume levels.  Curious if this might provide good results for the SRX.

Just thinking out loud really.  Probably something I am missing in doing this.  Would have to change some resistors, of course.  

Oh, and don't tell my wife I am even thinking this.  She likes the SRX just as it is.  Sssshhhh!

Edited by Blueman2
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  • 1 month later...

I treated myself to a new SR-007mkii and a Schiit Gumby for Christmas, and have been doing different tests/mods to see what I like the best.  Because of the darker nature of the -007 and the need for lots of power, I was assuming I would like the Carbon amp better.  But after a couple weeks of listening, I am finding the SRX-Plus to be a better match with the -007.  I initially did the blue-tak and spring mod on the -007, but reversed them when it did not seem to help the bass much (or at all that I could tell).  With the -007mkii in its original state again, I find the SRX provides a bit more bass and an overall more pleasing sound. I tend to listen for 2 instruments to evaluate sound: bass guitar and snare drum.  For bass, it is about, well, the bass.  How much punch and depth.  I found the SRX better than the Carbon.  For the snare drum, I listen to see if I can clearly hear the metallic sound of the wire mesh under the drum.  This is my 'acid test' for good source material.  It is the only way, for example, I can consistently win blind A/B test of 160kbps mp3 vs flac.  The -007 really brings this out, much better than the SR-507 I was using.  But I was happy to hear that the SRX was just as good as the Carbon in correctly presenting the snare sound on my new SR-007.  Not better, but just as good to my ear.  That really surprised me because the Carbon is just amazing at the higher frequencies in this regard.  So saying the SRX is just as good is amazing praise.  

I still like the Carbon for electronic music, for example, but am finding myself using the SRX-Plus as the go-to amp for my new SR-007s.  

But just just so that Jim L does not get too big of a head, I going to start this weekend on my BH build.  The SRX is about to get some more stiff competition.  

B)

Edited by Blueman2
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  • 2 weeks later...
On 1/21/2018 at 9:23 AM, JoseA said:

Hi,

I have just discovered this thread and I'm very interested to build this project but I'm not able to download any of the links provided here. Where I can get the PCB ?

Thanks

Not sure if any members have extra SRX-Plus boards, but you can have one made, try to start another group buy, or just do point to point wiring.  Attached are the board files.  Group buy was SRX6 version, as I recall.  Check the thread to see what the rev2 changed.  

srx6.zip

srx6rev2.zip

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