The ultimate DIY? A Stax SRM-T2!

[iwik]

I am adding KG's digital attenuator to my K2 build. Undecided on adding a Balance control. Good idea or not really necessary ?.

If so suggestions on Dual Concentric or two separate pots. Dual Concentric then raise the question of suitable knobs.

Any comments appreciated.

Thanks

Les

[Kerry]

Hi Les,

 

Since the boards are all digital control, you could use one digital encoder with a momentary push switch.  You could then program it to be a balance / volume control.  If you wanted to use traditional pots you could, but would need to interpret the resistance and convert that to a digital value.  This is easy enough with an Arduino board.

 

Regards,

Kerry

[iwik]

Thanks Kerry,

My brain seems slow now, using the same pot/encoder definitely the cleaner answer. Ill have to try and do a little

programming. Only touched on Arduino code a wee while ago so will have to get up to speed. Lest I make work

with just volume and then redo code later for balance.

Regards

Les

[luvdunhill]

I'm playing around with an optimization to the circuit and was wondering if I could post a survey question. All the measurements I'm asking for are available via the test points, so it should be easy

 

voltage to ground from each of the battery test points

voltage to ground from each of the outputs to ground

 

I remember seeing a few of the folks not be able to get the output to ground measurement to zero, which impacts the maximum swing on the amplifier. For example, Inu's build. I think I might see the problem here (a small problem, granted 50V here isn't a huge deal, but for us that are perfectionists...), which is due to the fixed base biasing method of the darlington servo pair. Thus the hfe of the transistors might very likely have been hand-selected by Stax with a certain beta that has been lost to posterity. Also, the fact that the original was a furnace would have an impact on this aspect of the circuit.

 

For extra credit, I'd love at least one person who can zero things out to measure voltage across R73 (6.2K) and R64 (510R) for one channel (I can send beer as a thank you!)

[spritzer]

The stock amp didn't have zero offset either.  I would have to dig through my email archive for the exact figures but it got a lot better once I redid the PSU

[luvdunhill]

That's very useful to know, thanks!

Do you think the absolute values of the PSU helped or do you have another theory?

[spritzer]

The caps were shot so replacing them helped.  Didn't cure the ever present hum though... 

[luvdunhill]

Even if you can zero the offset, I want to see where the batteries end up. I'm guessing in some of the builds they are quite lower than they should be.

[GeorgeP]

Thought I would would check for fun to see how things were doing. The servo on this thing is simply amazing - just rock solid with barely any up and down movement.

 

Does it have to be zero, or pretty darned close to zero?

 

My measurements are the same as in this earliler post with the batteries (re-measured a few months ago) sitting at around -200 and +544 on each side. Not sure how keen I am on earning extra credit though...

[kevin gilmore]

you mean +200 and -544. About 750 across the battery which is close

to what mine runs.

[GeorgeP]

yes, switched them... went to bed at 1am and was up at 5:30am so a bit baked at the moment.

[blubliss]

that is what my batteries measure as well.

[iwik]

Guys,

Tubes for this beast. Thoughts on JJ El34 and new Mullards(Russian). Thinking of buying from Thetubestore. Really like XF2 but finances a bit

stretched presently. Don't want to buy something that will do this amp an injustice. All comments appreciated.

Thanks

Les

[Voltron]

Get xf2s anyway!  

[nopants]

NOS RFTs can be found for around 40 a pop, as national electric I believe

Edited May 20, 2014 by nopants

[iwik]

Thanks,

Re Chassis for T2, was thinking of putting the transformers on a bracket so access to the bottom of chassis for service would be easier.

Worried I would then lose heat sinking offered by the large bottom cover. Good or bad idea?. I know the p/s gives little trouble.

Birgir

 Your Front Panels look really swish. I presume they are Silk Screened?. Are they done commercially and if so what do you pay

for say the front of your KGSSHV.

Les

[luvdunhill]

A bracket connected to the front panel?

[iwik]

Heat sink to Heat sink. I have L shaped extrusions around the edge of my chassis to secure top and bottom panels.

I was going to mount a piece of 6mm aluminum and screw it to these extrusions.

[spritzer]

My front panels are laser cut but for the silver they put some material on it that the laser burns into the front panel.  They are done by a place that normally makes small signs and the cost varies on a number of factors.  Around 100$ or more for a panel is the norm. 

[iwik]

Thanks Birgir,

Any comments re mounting of Transformers on a bracket?

Les

[luvdunhill]

My concern with the bracket as you propose it would be if the transformers have any buzzing it would get amplified due to the arrangement...

[spritzer]

Assuming the transformers behave then a bracket isn't an issue.  Supporting the weight and all that goes without saying and it would be a good idea to allow for some holes to cool the transformers. 

[luvdunhill]

I hope he isnt using HumR if expects them to behave.

[spritzer]

Hehe....   

[iwik]

Comments noted and thanks,

Yes I hope I wont end up with HumR but at least I will be in good company eh.

Richard will be getting a few strong words if my ones hum.

Surprised you guys haven't got stuck into him.  Someone needs to if he prides'

himself in so called Audio Products.