SRX-Plus Build Thread

[Blueman2]

This thread is a follow on to the original SRX Revisited thread and the SRX-Plus Group Buy thread.

As of 2024, I count at least 12 members with working SRX-Plus Amplifiers:

• JimL
• congo5
• MLA
• Sorrodje
• mwl168
• luca
• jose
• blueman2 
• Juggernaut1101
• tomislavkufrin
• simmconn
• oldie

=============================================================

Here is an initial Bill of Materials for the Amp.  Please provide input on errors and omissions and I will update.  This is my first ever electronics build, so bear with me as I learn the ropes.   I am initially building a KGBH psu, so do not have the BOM for the JimL shunt psu.  I have linked to a post for the PSU BOM below.

SRX-Plus Amp by JimL (SRX6 board by KG)

Caps 	
7	 0.1 µf/450 V
2	 100 µf/6 V, NON-POLAR!!
4	.22uf / 1000v

Resistors	
18	 100 O, 0.25 W gate stoppers for DN2540, and sense resistors
2	 102 O, 0.25 W (source resistor for output cathode sink) ***Congo5 advises using 75 O instead
4	 250 O, 0.25 W (source resistor for output current load) ***Congo5 advises using 200 O instead
4	 500 O. 0.5 W
6	 1.0 kO, 0.25 W gate stopper for 10M90S
2	 1.43 kO, 0.25 W (source resistor for input current sink) ***Congo5 advises using 1.2 kO instead
4	 2.0 kO, 0.5 W
4	 5.1 kO, 1 W/500 V
8	 124 kO, 0.5 W, 350 V rated
8	 220 kO, 2 W, 500 V rated
4	 300 kO, 0.5 W, 350 V rated
4	 500 kO, 0.5 W, 350 V rated

Silicon	
10	 Supertex DN2540 N-Channel depletion mode MOSFET (TO220)
6	 IXYS IXCP10M90S current source (TO220)

Trim Pots	
2	 20 O multiturn trimmer for output current sink—set current to 17 mA
4	 100 O multiturn trimmer for output current load—set current to 7 mA
2	 500 O multiturn trimmer for input stage current sink—set current to 1.33 mA
2	 5 kO, 0.75 W multi-turn trimmer for input stage balance

Tubes	
4	 12AT7, may also use 12AX7 or 5751 for top tube only
2	 6SN7GTA or 6SN7GTB

Connectors	
3	3 pin
3	2 pin
1	4 pin

Heatsinks	
2	25 x 16.5 x 16.1mm Aluminum Heat Sink
4	42 x 35 x 25mm Aluminum Heat Sink

This list is based on the BOM from the December AudioXpress article by JimL.  I have adjusted it for the SRX6 board designed by Kevin Gilmore.  Changes I noticed were:

1. Use of a bipolar 100uf capacitor across the 5K input pot, which removed the need for 2 100uf caps and 1 100K resistor per channel.     
2. Change from 510K to 500K resistors going into the output tubes (6SN7's)
3. Change from 250K to 300K resistors at upper input stage
4. As noted above, changed values of the resistors in series with the trim pot of all the cascoded CCS as advised by Congo5 and MLA (Output current source: 200R in place of 250R, Output current sink: 75R in place of 102R, Input current sink: 1.2K in place of 1.43K)

 

I recommend against trying to build the Shunt PSU as I am not sure JimL ever got this working to his satisfaction.  I recommend you try another PSU.  For the Shunt PSU BOM, refer to this post by jose.  Note that the original board showed use of a TIL731 which is incorrect.  It should use SPX431.    

 

 

Edited April 25 by Blueman2

[MLA]

Well, point to point is cool but board is simpler, at least in my experience. I would recommend the SRX-plus edition, it's a fantastic amp built with relatively few components, so limited things can go wrong, and figuring out how to trim all the current sources to balance everything out is a great learning experience (or at least was for me :)).

Since there's only like 13 components per channel for the amp and a similar amount for the PSU, making a BOM is not so complex, and Mouser is your friend in this :). However, If you'd prefer a listing, JimL has a complete component list in his articles on the SRX-plus in Audio Express, the Oct and Nov issues, along with a lot of advice on how to grasp and trim the circuit. Very good read!

As for boards, last time I checked making a board run of the SRX plus from Dr Gilmore's gerber files the cost was around 70-80 USD from one the cheaper PCB firms for a run of 5. If more people are interested and if you'd like help on this, I could make a small run for 5-10 boards and ship out, no problem!

[gepardcv]

I am in for a couple of boards if you guys do a run.

[Blueman2]

MLA, thanks for the fast and helpful reply.  I am pretty new to all of this (retired mechanical engineer who always wished he had gone EE).  Yes, I prefer the SRX plus version but was scared off by the concept of getting boards made since I have no experience with that.  If you were to be so kind as to do a board run, I would be in for  2!  Let me know and I can paypal you money up front to help get this started.  

I just looked at subscribing to Audio Express, but did not see any way to gain access to past issues.  Is there another way to buy the articles?  Or are past articles available to anyone who is a current subscriber?  I was going to buy both electronic and paper sub.  

thanks again!

[kevin gilmore]

the board is 9.35 x 6.85 inches

the heatsinks prevent the board from changing that 6.85 inch number more than maybe .1 or .2 inch

the 9.35 I may be able to reduce .5 inch but things are going to be tight.

neither of those changes is going to influence the board price much.

373 holes

 

Edited December 18, 2015 by kevin gilmore

[mwl168]

I agree Kevin. After looking at the SRX Plus board I have on hand (thanks MLA). Plus, this is a proven board so I vote to leave it alone.

No promise, but if there is enough interest, I'll consider organizing a GB and use PCBNET. I and many others have been very happy with the quality of the boards from the Carbon GB. 

[Blueman2]

Thanks Kevin and mwl168.  I am certainly in for 1 or 2 boards should anyone want to order a batch.  I used pcbway.com to get a quote for 10, and it was about $130 shipped.  So I am certainly happy paying $20-$25 per board from someone who takes the effort to order a batch of them.  

[DefQon]

$20-25 per board sounds great if a GB for it ever does happen. Was very impressed with Mikes (mwl168) GB run of KGSSHV boards half a year ago.

[UFN]

Could be interested in a board as well if someone places an order.

 

//UFN

[MLA]

And I'm interested in 2-3 boards as well; have some beautiful 6SN7 GTB awaiting use.

[sorenb]

if the PCBs are made by PCBNet I'm in for two sets. 

[MLA]

As for previous issues of Audio Express, I believe the two issues prior to the current one are always available for download if you make a digital subscription, so October and November will be downloadable for a few weeks more (i.e. before the January issue comes out).

[JimL]

Hi,

My modem died last week so I'm just getting back on the net and saw this thread.

A couple corrections.  The articles are in the November and December 2015 issues of AudioXpress.

 

The original SRX circuit is indeed quite spare:  3 tubes, 1 trimmer, 4 caps and 13 resistors per channel.

The SRX Plus is more complex: 3 tubes, 8 MOSFETS, 5 trimmers, 4 caps and 35 resistors, but I think the performance takes a substantial jump.  See the SRX revised thread and also the Current Requirements in Electrostatic Headphone amplifiers elsewhere in the DIY section for all the gory technical rationale.

 

I am also in the final stages of working up a PCB for the shunt regulator PS.  It includes input caps but not the rectifiers, as I intend it to be used with either a cap input (in which case you just need the transformer and full bridge rectifier, or choke input (in which case you need two chokes also), which would be mounted off board.  It's a slow process since I am learning to use KiCAD while I'm also doing the board.  I've lost count of the number of times I've used Exit without Save when I've screwed up and had to go back to an earlier iteration.  I still have to figure out how to do the mounting screw holes and the component mask layer.  Currently the board measures about 6.5" x 3.75" and one edge is intended to attach to a heatsink so it won't blow up if all the current has to run through the shunt.

 

BTW if there is a group buy I would be interested in a board.

Edited December 19, 2015 by JimL

[Blueman2]

On 12/17/2015 at 3:59 PM, mwl168 said:

No promise, but if there is enough interest, I'll consider organizing a GB and use PCBNET. I and many others have been very happy with the quality of the boards from the Carbon GB. 

 

On 12/17/2015 at 10:07 AM, MLA said:

As for boards, last time I checked making a board run of the SRX plus from Dr Gilmore's gerber files the cost was around 70-80 USD from one the cheaper PCB firms for a run of 5. If more people are interested and if you'd like help on this, I could make a small run for 5-10 boards and ship out, no problem!

mwl168 and MLA, sounds like both of you have had experience doing this.  Would one of you be willing to lead a group buy?  Alternatively, if one of you could set up the order on PCBNET on my account, I will be happy to make the buy and send out boards.  I just do not feel comfortable knowing all the options to correctly set when ordering boards.  Let me know.  Sounds like enough interest that 10 or even 20 might be justified.  Again, I will willing to front the costs and do the re-shipping to everyone if that helps.  Otherwise, count me in for 2 srx6 boards!

 

2 hours ago, JimL said:

I am also in the final stages of working up a PCB for the shunt regulator PS.  ....

ooo!  That is my next to-do.  Though I figured the power supply would be simple enough for point to point.  But if there is a board, I would definitely want to go that route.  I recommend we not wait on it and go ahead and order the srx6 boards, and them come back later and do another group buy for the PS boards.  

[MLA]

8 hours ago, JimL said:

A couple corrections.  The articles are in the November and December 2015 issues of AudioXpress.

Indeed, my mistake. I think maybe the original publication intent was october? and then it was pushed back (grasping at straws to excuse my bad memory here :)).

As for a GB, mwl168 and I had a quick chat on this and given that it may work out for him to take the lead and the very good quality of the boards from PCBnet, I think that would be option A. If the cards fall differently for some reason, I can be plan B (no, not a literal quote from the spouse).

As for payment, a common way of doing this that has been successful in the past is to have people commit and then paypal their order's worth to the GB lead, who then places the order and ship things out, so each participant fronts his/her own cost.

[JimL]

As a matter of fact, MLA, your memory is fine.  The original publication date was indeed October, and it did get pushed back a month.

[Blueman2]

7 hours ago, MLA said:

...I can be plan B (no, not a literal quote from the spouse).

Yeah, I am pretty sure we are all Plan B or C from our spouse's point of view.  

7 hours ago, MLA said:

As for payment, a common way of doing this that has been successful in the past is to have people commit and then paypal their order's worth to the GB lead, who then places the order and ship things out, so each participant fronts his/her own cost.

So I am in for 2 boards.  Let me know how much and where to send the paypal.  

Thanks!   

JimL, what wattage rating to you have for the resistors?  Are they all the same?  Also, for the .22uf 1000V caps, are those Axial or the rectangular type?  Or does that matter?  I have ordered a subscription to AudioXpress, but it takes 24 hours before I can have access to electronic copies.  Can't wait to read your article!

[mwl168]

Apologize for my absence. Have been busy working on a project that's in trouble.

I will contact PCBNET Monday and get some quotes. Giving the tubes are mounted on the board, I suggest we go with 2mm thick and 3oz copper PCB. It'll add to the cost but I think it's worth it.

Edited December 20, 2015 by mwl168

[Blueman2]

 

1 hour ago, mwl168 said:

Apologize for my absence. Have been busy working on a project that's in trouble.

I will contact PCBNET Monday and get some quotes. Giving the tubes are mounted on the board, I suggest we go with 2mm thick and 3oz copper PCB. It'll add to the coast but I think it's worth it.

Good Idea.  Worth the extra cost.  

[JimL]

Most of the resistors are 1/2 watt, although I used 1/4 watt for the current source resistors.  The 220k resistor chain in the output stage uses 2 watt/500 volt resistors, and I used 1 watt/500 volt resistors for the 5.1k output safety resistors.

 

As for the .22 uf/1000v caps, I think the board is designed for rectangular caps.  I built mine point-to-point and I used 0.1 uf/630 volt axial caps.  Basically whatever fits.

 

I think a 2 mm board is good.  It's not that the tubes weigh that much but pushing them into the sockets will take some force.

Edited December 23, 2015 by JimL
correction

[MLA]

Definitely 2mm. Spent some time swapping tubed of different age and make into alternate positions to find a low inital offset to start adjusting from, and 1,6 mm flexes a bit too much for mental comfort...

Also, as the dn2540 show quite a bit of variability, make sure you order a couple of extra values for the current source/sink set resistors. I had to go about 20% lower than specified on the board to get the currents right.

[Blueman2]

On 12/20/2015 at 7:10 PM, mwl168 said:

I will contact PCBNET Monday and get some quotes. ...

Were you able to send them the files for a quote yet?  Excited about getting parts together for the project.  

[mwl168]

My bad, did not send them the Gerber file till later today. Got busy yesterday and did not send it as promised. Hope to hear back tomorrow if people have not already taken off for the Holidays break.

Will report back soon as I receive quote.

[mwl168]

I got quotes back from PCBNET. 

Basically, we can get per board cost to $25 US plus shipping to each participant if we can get the number up to at least 15. This is with 2mm thick, 3oz copper board.

I will start another thread to gauge interest in a day or two. Before I do that we first need to decide if to include PSU board in the GB and, if so, which PSU to use. By looking at the board, we need regulated +/- 340VDC and - 20VDC. I am thinking the GoldenReference HV board with one of the LV configured for - 20VDC is a possible option. 

What are your thoughts?

Edited December 24, 2015 by mwl168

[Blueman2]

I would love to have a PSU board as part of the GB.  I was hoping Kevin Gilmore might be able to turn JimL's PSU design into a board, but I do not think he can get to that very soon.  In any case, I am in for a PSU board as well, whatever the group decides.   

 

As for the Amp board, I am in even if it's cost is above $25.  Probably anything up to $35 each if that is what it takes.