SRX-Plus Build Thread

[Blueman2]

On 4/29/2016 at 2:45 PM, mwl168 said:

.....

4. The goal of this step is to set the output current of the 6SN7GTA/B to the desired value (7mA). Measure the voltage drop of the 100 ohm resistors of the output CCS (the 4 resistors between the large heatsinks and the .1uf/400V caps by the edge of the PCB). They should measure about 0.7VDC (0.7VDC/100R = 7mA). Adjust the 100 ohm trim pots to get the 0.7VDC voltage drop on these 100 ohm resistors.

5. The goal of this step is to set the output current sink to 17mA, 3mA higher than the output current. Measure the voltage drop of the 100 ohm resistors of the current sink (the 2 resistors between the 20 ohm trim pots and the single .1uf/400V cap between the two lower 12AT7 tubes). They should measure about 1.7VDC (1.7VDC/100R = 17mA). Adjust the 100 ohm trim pots to get the 1.7VDC voltage drop on the 100 ohm resistors.

6. The goal of this step is to set the two plates of each of the upper 12AT7 to be equal and about mid-way between B+ and ground. So if you are using B+ of 360VDC, the mid way is 180VDC.

• First attach the DMM probes to each of the two .22uf/1000V coupling caps where they are connected to the plates of the upper 12AT7 and measure the DC voltage between them. (in my case. I purposely left the leads of the coupling caps sticking out a bit so I can clip the DMM probes on them. With the 5K trim pot set at its mid-point, the measured DC voltage between the two plates tells you how well matched are the two triodes of the 12AT7 tubes used. If they are well matched it should be within 5 VDC or so. If it's more than 20VDC you may have problem**.) Adjust the 5K trim pot so the measured voltage is close to 0VDC.
• Next, move one of the DMM probes and clip it to the power ground (leave one probe still connected to one of the coupling caps) and adjust the 500 ohm trim pot so the measure voltage is about 180VDC (half of the B+). 
• The adjustment of the 500 ohm and the 5K trim pots will interact with each other so you'll need to go back and forth a bit.

7. Let the amp warm up for another 15 - 20 minutes for the tubes to reach their stable operating state then repeat the adjustments again. Measure the balance via the headphone jack like you do with other Stax amps. If the two triodes in each of the 6SN7GTA/B are well matched you should see around 1 or 2 VDC or so. And this is perfectly fine.

8. Measure the offset via the headphone jack like you do with other Stax amps. Adjust the 20 ohm trim pot (output current sink) to bring the offset as close to 0 VDC as possible.

Great summary, mwl168!  Finally finished the build of my PSU (KGBH) and the SRX+ amp board.  I dialed in the output current source, output current sink and input current sink using a pair of 9V batteries as described by JimL in response to your post.  I checked the PSU and it is giving good and stable +/- 370V.  Dialed in the bias voltage to 580v.  Using 7918 on the KGBH to supply -18V for the amp's -20v connection.  My transformer provides 2x 6.3V for the filaments (I modified the board to allow the 12AT7's to run on 6.3V to avoid getting another transformer; and yes, each set of tubes has its own 6.3 volts winding!).   

I just finished turning it on and letting it settle for about 15 minutes.  Everything seems fine, but I am not able to zero out the plate voltages in step 6.  Started off a 24V off with pot centered, and was only able to get it to about 10V off with pots all the way to their end.  Also, I was not able to get the plate voltages down to 185V (my PSU is putting out 370V).  No matter what I did to the 500 ohm pot, the plate voltage barely moved at all when adjusting the pot, and stayed at about 240V.  Any idea what I am doing wrong?  I am using a KGBH PSU and using the -18V from that PSU for the -20V input.  Is perhaps my 370 B rail voltage too high for this amp??  I guess I can swap diodes and reduce the voltage to see if that helps.  

I am also getting a very loud 60hz hum.  VERY loud.  It is mostly attenuated when I return the 5k trim pots back to near center.  Perhaps I have badly balanced tubes, causing the inability to zero out the plate voltage and causing hum when I try to turn the pots to extreme end of their range??  I carefully twisted the filament winding wires and they are nowhere near the inputs.  Moving wires around does not attenuate the hum.  

But even with the hum, I did listen to the amp for about 10 minutes.  I LOVE the sound!!!  Much richer and full bodied than the SRD-7 I am currently using in conjunction with my traditional amp.  Just the soft passages are ruined by that darned hum!

 

Edit on hum issue: for those who might be looking at how I solved the hum:

•  Used voltage divider to elevation the filament voltages.  +65V for 12AT7s and -315V (or in my case, I just tied it to B- so -370V) for the 6SN7s
• I switched to different set of tubes.  The ones I started with just hummed.  After switching, much less hum
• More careful grounding of components
• Finally, to get rid of slight hum when volume control was at 11:00 to 5:00 position, I switched from a 50K pot to a 10K pot, which totally eliminated any hum when the volume control was in the mid/high region.  

Doing these 4 things, the amp is now virtually silent.  

 

 

Edited October 31, 2017 by Blueman2

[JimL]

Could be a couple of things going on.

 

First of all, what are your input stage plate resistors?  That plus the input tail current source sets the plate voltages.  If your B+ is around +370 volts, and you have 250k plate resistors, you need the tail current source to be running at 2 x 185/250k = 1.48 mA.  If you are running 300k plate resistors, then the tail current source should be 2 x 185/300k = 1.23 mA.  If B+ is at +340 volts, you want the upper plate voltage to be at 170 volts, which means the tail current source should be 2 x 170/250k = 1.36 mA for 250k resistors, and so on.  Once you have set the tail current source to the proper value, you should have the plate voltages close to the desired value.  If you are not able to balance the tubes, you need to either swap tubes around until you can achieve balance, or replace the tubes.

 

Not sure about the source of the hum, but if the tubes are bad that is a possibility so I would try replacing them.  Otherwise, there could be a ground loop, and those can be a b***h to track down.  You should begin by checking your grounds - the amp board should go to a star ground, as should the PS board.  Sometimes hum can come from the power cord ground, in which case lifting it from ground with a 10-100 ohm resistor can help. I seem to recall someone posting a diagram for the KGSSHV showing how to connect all the grounds, inputs and outputs, but I can't find it at the moment. 

 

I am working on an update to the shunt regulator so I wouldn't recommend building it at this point.  The KGBH PS board is well sorted out and works fine.

[Blueman2]

Thanks, Jim.  I ended up elevating the filaments and that got rid of 99% of the hum.  I put the output stage to B- (-370V).  For the input stage, I made a voltage divider using 470K and 100K resistors across ground and B+, which gave me 65V for the input stage.  Tweaking the 500K pots then removed what hum remained.  

I am using 300K plate resistors per the silkscreen.  I will double check the tail current source and adjust as needed.  I also did order a better set of balanced and matched tubes which hopefully will get rid of the last 1% of hum!  

I allowed myself about 3 hours of listening pleasure before I took it all apart and started the process of putting it all into a case.  I am about 1/2 of the way there.  I will post pictures when I get the system fully installed and running.  

Love my SRX-Plus!!!!  BTW, I guess I win the award to longest time to complete a build!  Started in November 2015 and finished in October 2017.  But the sound was worth the wait!

Edited October 14, 2017 by Blueman2

[jose]

If it can help you;

I had similar problems with 6 step and a f@$k Hum. I changed the ecc81 quads until I found a perfect mach quad at the same time, I separated the AC lines and did a ground loop breaker.

Now my amp is absolutely silent and the different between triodes is around 3~8 volt.

[Blueman2]

Thanks jose!!  I have better tubes coming in next week and will try those.  They are supposed to be very well balanced (the 2 triodes in each tube well matched to each other).  

What do you mean when you say "separated the AC lines" and "ground loop breaker"?  I am unfamiliar with those terms.  

And while on the topic of grounding, what is the strategy for grounding on this amp?  I see the following grounding items:

1. PSU ground
2. AMP ground
3. chassis ground
4. earth ground (via 120vac plug)
5. transformer shield wire
6. POT (volume control) ground from inputs

I was going to tie 1-5 all to a single ground point (star config) and leave POT ground floating since I cannot be sure all of my sources would use negative signal supply as ground.  Does that make sense??

 

Edited October 9, 2017 by Blueman2

[jose]

Sorry, English is not my language.

 

If you take a look at my photos, you can see that I separated as possible the heating lead to the rest. I did a star grounding with a simple loop breaker (with a bridge diode)

 

I'm sure that you know this but I recommend you read this

 

http://sound.whsites.net/earthing.htm

 

 

[Blueman2]

1 hour ago, jose said:

Sorry, English is not my language.

If you take a look at my photos, you can see that I separated as possible the heating lead to the rest. I did a star grounding with a simple loop breaker (with a bridge diode)

I'm sure that you know this but I recommend you read this

http://sound.whsites.net/earthing.htm

Great article!!!!  That is very helpful.  I wonder if I can buy a loop breaker pre-made?  

Thanks jose!!!

[jose]

Yes, I guess that on eBay you can find some PCB, but I only use a KBPC5010 bridge diode with a R10 as Nelson Pass recomends.

My ground way :

Psu Ground to Star grounding (before Bridge diode)

Amp Ground (Amp has G on LV and HV, really is the same plane so I only use one of them) to Star grounding (before Bridge diode) or PSU Ground.

Input rca (insulate)/XLR to Volume Pot

Volume Pot (insulate) to Amp.

Star grounding (diode/resistor/cap) to chasis

Transformer shield wire to chasis (On the same place where you put the star Grounding)

Earth ground (plug) to chasis. (On the same place where you put the star Grounding)

Very important:

If you are using an unbalanced input, you must bring the "cold" channel to ground. 

[Blueman2]

Thanks again Jose.  Finally got it all into my case and did the grounding per your recommendation.  Much better.  Only slight hum I get appears to come from one of my  6SN7GTA's because the hum is only from 1 channel, and it switches channels when I switch the  6SN7GTA's.  Also, with the grounding and with the elevation of my filaments per JimL's post, I was able to do all the adjustments per the guide and get to the right levels.  This amp sounds really nice.  A few pictures below.  I have not installed the volume control yet.  

 

Edited October 11, 2017 by Blueman2

[jose]

I recommended the new Tung-sol 6SN7GTB

[mwl168]

I have not cased up my SRX Plus yet but in its air-chassis mode it is dead silent when I am carful with the wiring arrangement.

I did experience tube-related noise at different time; once with a noisy 12AT7 and once was actually caused by dirty tube pins on a 12AT7 that just needed to be cleaned.

Maybe try cleaning the pins on the 6SN7GTA in question and see if it helps.

 

[Blueman2]

Great suggestions from JimL, mwl168 and jose.  I forgot to tie the case to ground (duh!) so did that.  I have tied PSU, AMP, case, and transformer shield to earth ground. I also cleaned the pins of all the tubes with electronics grade alcohol and  steel wool.  And per JimL, since I am running 370V PSU B+, I adjusted input current sink to .123V across the test resistor (I am running 300K plate resistor).  I am now down to a very quiet but perceptible hum when not playing any music on the input. Of course, the hum goes away (from a perception standpoint) when playing music even at low levels.  Comes from both channel, but is stronger on the right channel.  When I get my new tubes, I will see if those make a difference.  

I have not installed the volume control yet.  I assume that goes on the INPUT side, right?  I test installed it initially but it created quite a bit more hum, which varied by position of the volume control.  In reality, I am not sure I even need a volume control since since my supply devices all have them already.   

I am now going to do a side by side test with my SRD-7 setup to see what the quality difference is.  

Edited October 10, 2017 by Blueman2

[gepardcv]

Your hum problem sounds awfully familiar. I posted about this a few weeks ago, see https://www.head-case.org/forums/topic/12840-technical-assistanceadvice-thread/?do=findComment&comment=772776.

 

A few things I have found helpful in eliminating noise:

1. Do not attach volume pot grounds to anything. Makes things horrible.

2. Attach input XLR pin 1s to the chassis near the input. Many Neutrik jacks have a special pin or other feature to do this painlessly. (Check continuity to be sure.) Do not run grounds from the XLR inputs anywhere else. If you have loop-out XLRs, also tie their pin 1s to the chassis.

3. The safety earth should be separate from the star ground, and needs to use tooth washers and locking nuts, since it's the critical electrical safety feature. I have found that it does need to be tied to the star ground.

4. Do not allow any audio signal wires in any direction to run close to the transformer.

5. Do not allow any AC wires from the transformer (including heaters) to touch any signal wires.

6. With the mini-BH PSU, you may hear some hum with no input plugged in and the volume control turned up. I've empirically found that the mini-BH PSU hums in this situation and the GRHV does not. There should be no hum with a source plugged in, for any position on the volume pot.

Note that in my builds, I attach the amp boards to the PSU, rather than to the star ground. I have found it to makes no audible difference with amps using the mini-BH, GRHV, or GRLV PSUs (i.e., dead silence). YMMV.

[jose]

Hum problems are a b***h. 

You only need patience. My SRX-P is dead silent now but I needed weeks of work to find the cause of the problem. Basically trial and error.

 

Edit: Laowei put this layout on "Technical Assistance/Advice Thread" maybe It can help you too.

 

Edited October 10, 2017 by jose

[Blueman2]

Well, after all of the adjustments mentioned, and after warming up for about 1 hour, I can say the amp is pretty much silent.  My son, who has much better hearing than me, says he can still make out a hum, but it is really not bothersome.  If I listen to the SRX-Plus with no music playing, and pull the headphone plug while wearing them, I can barely make out any difference.  I am going to try different tubes when they come in this weekend, and see if that makes any difference, but I am a happer camper right now!

EDIT: I installed a quad set of Penta Labs 12AT7's and that eliminated the remaining hum. Odd, because it was the output stage tube that I thought was causing the hum because when I swapped the  6SN7GTA's, the hum switched sides.  More to come once I install a new set of Tung Sol's tomorrow when they arrive.  

Now, as to SRD-7 vs SRX-Plus, the winner is the SRX-Plus pretty clearly.  Even when driven by a very high quality big $$$ power amp, the SRD-7 has a rather hollow and metalic tone to it compared to the richer, fuller, warmer SRX-Plus.  Interestingly, when I put my 507's on the SRD-7 and my son's 407's on the SRX-Plus, the 2 compared pretty well.  So the 507s are a similar step up from the 407s as the SRX-Plus is to the SRD-7.  I will keep the SRD-7 as a backup, but the SRX-Plus is now my daily driver.  

One other modification I would like to make is to add a delay relay to the headspeaker output, which keeps open circuit for 10 seconds and then connects the L+ and R+ output lines to the jack.  Any idea of where I could source such an item? 

 

Edited October 29, 2017 by Blueman2

[jose]

I wouldn't put anything on the out put.

To have a soft start I put a relay in the trafo or HV part (between amp and PSU).

With 10 seconds is sufficient for the heating filaments to have some temperature.

[JimL]

11 hours ago, Blueman2 said:

One other modification I would like to make is to add a delay relay to the headspeaker output, which keeps open circuit for 10 seconds and then connects the L+ and R+ output lines to the jack.  Any idea of where I could source such an item? 

 

I agree with Jose.  The output voltage can jump around when the amp first starts up, BUT, the output voltages track - in other words, if the R+ output is +50 volts, the R- output is also close to 50 volts, and the same with the L+ and L- outputs..  This is because the output stage is differential.  Since it is the difference in voltage that produces sound, ss a result, the headphones see very little difference between the two stator plates, so there is no appreciable turn-on thump.

[Blueman2]

6 hours ago, jose said:

I wouldn't put anything on the out put.
To have a soft start I put a relay in the trafo or HV part (between amp and PSU).
With 10 seconds is sufficient for the heating filaments to have some temperature.

 

1 hour ago, JimL said:

I agree with Jose.  The output voltage can jump around when the amp first starts up, BUT, the output voltages track - in other words, if the R+ output is +50 volts, the R- output is also close to 50 volts, and the same with the L+ and L- outputs..  This is because the output stage is differential.  Since it is the difference in voltage that produces sound, ss a result, the headphones see very little difference between the two stator plates, so there is no appreciable turn-on thump.

Makes sense.  And I agree, I do not get any pops or harmful signals coming from the headphones.  I do like the idea of the filaments heating up before the HV kicks in. Jose, would it be better to put a relay on the HV lines from the transformer to the PSU, which are AC rather than the DC from PSU to amp?  I tend to dislike switching DC due to arcing.   

Another issue I continue to have is the volume control. I have a ALPS RK27 50KA Audio Taper Potentiometer. I mounted it to a PCB that was supplied that gives a 3-wire in and out.  The 3 lines are L,R,Ground.  I am using unbalanced RCA inputs.  I tied the 2 RCA grounds together and run shielded 3-wire cable from RCA to volume control.  Then another shielding 3-wire cable from volume control to inputs.  On the board, I have tied GND to I- for both channels.  I connect the ground from the volume control to both channels (Ground and I-) and connect the hot wires to I+ of each channel.  The result is a lot of noise and rhythmic clicking. Any idea what I am doing wrong?  When I bypass this setup and connect the headphone jack of my phone (again, 3 wires with R+, L+ and GND) to the I+ of each channel and common ground to both inputs (each tied to I- on the board), it works just fine.  

BTW, I ran the amp for 6 hours yesterday.  After 15 minutes it was dead silent with no hum.  Temperature of the case maxed out at 38C, which I feel is reasonable.  Sound quality seems to reach maximum level at about 15 minutes and does not vary after that.  Really sweet sound compared to the SRD-7 I have been using for the past 2 years.  

Edited October 11, 2017 by Blueman2

[mwl168]

Here is an excellent wiring scheme johnwmclean created some time ago. I've followed it with my every build with excellent outcome.

Thanks again John!

 

KGSSHV WIRING SCHEME.pdf

[Blueman2]

Thanks, mwl168!  I just took all the input wiring out and re-did it using info from your link and other posts, and magically it works!!  Volume is smooth and quiet.  Yes!!

[jose]

4 hours ago, Blueman2 said:

 

Makes sense.  And I agree, I do not get any pops or harmful signals coming from the headphones.  I do like the idea of the filaments heating up before the HV kicks in. Jose, would it be better to put a relay on the HV lines from the transformer to the PSU, which are AC rather than the DC from PSU to amp?  I tend to dislike switching DC due to arcing.   

I always use this on trafo lines, from EIC to trafo. Usually I use a solid state relay and you can use T2 soft start design for do it.

[Blueman2]

After swapping out the tubes with better ones (Penta Labs for 12AT7s and Tung-Sol for the 6sn7s, not pictured below) I was able to wipe every single bit of hum.  There is still an issue when the volume control is in the 11:00 to 3:00 position where hum comes in, but for now I am just leaving volume at 100% and adjusting from the source.   But I now consider this a successful build!  The sound is a big leap up from the SRD-7 I was using.  

Thanks to everyone in the community who helped on this.  

• James Lin for the design, the great article on AudioXpress, and overall thought leadership
• Kevin Gilmore for the board layout and guidance
• gepardcv for providing KGBH PSU boards and being so patient as I resolved the exploding zener diode issue!  I owe you, man!
• mwl168 for doing the group buy and for providing endless guidance.
• congo5 for doing the early prototype build and showing just how good this design is.  
• jose for doing one of the early builds and giving me guidance
• and most of all, to my son who came up with this project and did most of the soldering after my exploding zener experience!

 

Edited November 3, 2017 by Blueman2

[Pars]

Your hum problem sounds awfully familiar. I posted about this a few weeks ago, see https://www.head-case.org/forums/topic/12840-technical-assistanceadvice-thread/?do=findComment&comment=772776.
 
A few things I have found helpful in eliminating noise:
1. Do not attach volume pot grounds to anything. Makes things horrible.

A bit curious about this one (agree with the rest). You're talking about the ground pins on the pot (1 per gang)?
Pots work basically as a voltage divider with a resistance from the input to the wiper, and another from the wiper to ground. Not connecting the ground leaves this resistance floating.
I always run these to amp ground (Not chassis).


Sent from my iPhone using Tapatalk

[Blueman2]

Well, my son and I just completed the build for our second SRX-Plus.  Went more quickly and more smoothly than the first build using what we had learned already.  I am finding that getting good tubes is essential.  With the right tubes, it is a breeze to get everything dialed in per the specs.  With questionable tubes, is can be impossible.  Oddly though, I cannot tell the difference in sounds when the amp is not dailed in per spec with the trim pots.   For example, I had a upper input plate to plate voltage difference of 120V using some poor tubes!!!  But it still sounded just fine.  With a different set of tubes, I was able to dial it down to 0V delta, but again, no real difference in sound.  Just perhaps a bit less hum.  

So by my counting, there are at least 9 SRX-Plus amps that are built and running:

• JimL
• congo5
• MLA
• Sorrodje
• mwl168
• luca
• jose
• blueman2 (my amp)
• Juggernaut1101 (my son's amp)

I am sure there are others, but I went by who posted pictures or said they completed the build on this forum.  I find the SRX-Plus to be the perfect mate for SR-407 (my son's) or SR-507 (mine).  I would love to see how it sounds with SR-007 or -009, but don't feel the need for that $$$ upgrade yet given how great my SR-507s sound with the SRX-Plus!

Edited October 22, 2017 by Blueman2

[Blueman2]

On 10/9/2017 at 1:15 PM, jose said:

Yes, I guess that on eBay you can find some PCB, but I only use a KBPC5010 bridge diode with a R10 as Nelson Pass recomends.

My ground way :

Psu Ground to Star grounding (before Bridge diode)

Amp Ground (Amp has G on LV and HV, really is the same plane so I only use one of them) to Star grounding (before Bridge diode) or PSU Ground.

Input rca (insulate)/XLR to Volume Pot

Volume Pot (insulate) to Amp.

Star grounding (diode/resistor/cap) to chasis

Transformer shield wire to chasis (On the same place where you put the star Grounding)

Earth ground (plug) to chasis. (On the same place where you put the star Grounding)

Very important:

If you are using an unbalanced input, you must bring the "cold" channel to ground. 

Thanks again, jose.  I just wanted to document here the ground loop breaker circuit that I think you were referring to.  It seems to have worked well for me.