The DIY Electrostatic amp thread

[nopants]

http://www.ebay.com/itm/1-Military-Gold-10-Pin-Twist-Male-Female-Connector-LF10-/180475627186?pt=LH_DefaultDomain_0&hash=item2a052f86b2

[justin]

might check them out. too bad no black/silver

[spritzer]

They are nice for the price but nowhere near the US/Euro milspec stuff. 

[chiguy]

Can someone tell me the diameter of a Stax plug?  I'm trying to determine how big of a hole I need to make in a front panel using teflon Stax jacks from Justin/Headamp.  I'm seeing in pictures that the hole is slightly larger than the diameter of the jack.  I would just measure it but I don't have a pair of Stax headphones and I'm still waiting on my SR-407s to ship.  Thanks!

[spritzer]

18.5mm or so for the larger style plug. 

[eggil]

The umbilical connector can be something like this:

http://www.ebay.com/itm/1-Military-Gold-10-Pin-Twist-Male-Female-Connector-M10P-/300562711241?pt=LH_DefaultDomain_0&hash=item45faef02c9

Been using them for years and there is also a female version. You can also just generate the normal bias from the B+ inside the amp chassis via a voltage divider.

I used those for the Megatron. Went to the Miramar airshow 3 weeks ago and saw Cobra helicopters using similar connectors

The Russian (and I assume Chinese also?) versions are larger, bulkier and not as nice as the Amphenols, but they work just the same.

Edited October 14, 2014 by eggil

[spritzer]

These are rated to 500VAC and have worked perfectly for me for years.  I have one set on the BATE I'm listening to right now. 

[spritzer]

Something "new" for people to build. 

 

 

Fully balanced GES Mk2. 

[Laowei]

Oh my!

[Remolon]

Is there any link to the GES Mk2 schematic?
Thanks

[spritzer]

No schematic but it's just the one off Headwize with the phase splitter now a LTP setup. 

[Kerry]

I'm guessing that will change the sound signature.

Kind of like Marshall vs Fender.

Have you built one yet?

[spritzer]

Yup, built one in 2012 fully point to point.  Sounds much better than the old version plus lower distortion. 

Edited October 17, 2014 by spritzer

[Kerry]

Interesting:). That's what I was hoping you'd say.

I may need to pull mine out and rewire as well.

[Remolon]

Thanks for the information. I am planning to substitute the 5687 tube output of my Stax amplifier and try the 6S4A, increasing the voltage to 600VDC, with this schematic.

The input tubes could be 6DJ8, 12A?7 or a combination of 12A?7, depending of the voltage amplification needed.

[oogabooga]

Trying to fix my old SRA-3S (distortion on one of the channels, scratchy pot). Going to replace all caps and clean the pot. I did find the schematic Spritzer provided elsewhere, but I don't know enough EE to reconcile it and changes the previous owner made. Much easier is looking at Spritzer's rebuilds and seeing where mine is different

 

1. On each channel, two giant caps are connected in parallel to a triangle of mounting holes (red circle in the photo). One cap is on each side of the PCB - am I right to assume that the bottom, tape covered caps shouldn't be there? Would it be best to just remove both caps and replace them with a single Philips 100 uF cap at 500V or higher on each channel?  (http://www.head-fi.org/t/677809/the-stax-thread-iii/360#post_9796944)

 

(also, why three mounting holes?? I thought caps only had two posts).

 

2. A blue wire was added to the bottom of the PCB between the 50 K resistor and the tube socket... but only on one channel (yellow circle in the photo). Does this make sense to anyone? 

 

I've attached Spritzer's rebuild and the schematic. From what I can tell, the upper left portion of the schematic is for the phono pickup which I don't much care about.

 

[DefQon]

There is a trace going from that pin on the tube socket to the bottom on the left most 50k 5watt Riken resistor, so if a blue wire is added there then it could be the previous owner attempted some repair or modification and damaged the solder pad or trace and bypassed it with a blue wire.

There are two 100uf 250/350v capacitors for filtering paralleled on the opposite side where you have circled in red. There are additionally two other axial capacitors (22uf 500v on mine) that are separate one on each side, you can't bypass them into a single 100uf capacitor as it's best to have the energy storage separate for the two sections. Judging from the picture the previous owner fitted some very large sized film? based capacitors and fit them onto the outside of the PCB. I would recommend removing them and go with 2 22uf 500v axial electrolytics and fit in them in the original place, make sure they are of appropriate size as the standoffs holding the PCB and the clearance between it and the centre chassis quite limited in measurement.

Also every Stax schematic has errors in it, including that SRA-3S schematic.

Edited April 15, 2016 by DefQon

[spritzer]

I'd remove all the non stock stuff and start over.  The main PSU caps were just dual section units so you can swap in a normal single section cap. 

[oogabooga]

Thanks Defqon and spritzer!

I know enough to keep the same capacitance and voltage (or moar V), but that's about all I know. Digikey seems stock lots of Nichicon, Illinois, and Vishay BC - are there any brands to prefer or stay away from? Any other parameters to keep in mind when choosing a decent cap? I'm not looking to break the bank on the SRA-3S but I don't want crap either. 

If anyone's got the time, I think I've found some replacements and would appreciate any advice from those who've done this before! 

On the top of the main PCB:
1. two giant radials at 50 uF 350V (standing up on the PCB). Does upgrading to 100 uF here make sense - if so, would either of these work:
http://www.digikey.ca/product-detail/en/nichicon/LKX2G101MESY35/493-14387-ND/2541222
http://www.digikey.ca/product-detail/en/vishay-bc-components/MAL215937101E3/4814PHBK-ND/5637711
2. four orange caps (metalized PP?) labelled "473K 600V". The one in the photo measures 49 nF on the Fluke  - would one of these work:
http://www.digikey.ca/product-detail/en/epcos-tdk/B32642B0473J/495-7144-ND/5414294
http://www.digikey.com/product-detail/en/kemet/PHE450PD5470JR06L2/399-7724-ND/3465772

On the pickup card:
3. four ceramic caps labelled "5-D". Are these 5 pF, and would this work?
http://www.digikey.com/product-detail/en/murata-electronics-north-america/RCE5C2A5R0C0DBH03A/490-7480-1-ND/4277397
4. two film caps (metalized PP?) labelled 50WV .22. The one in the photo measures 270 nF on my Fluke. Would either of these work?
http://www.digikey.ca/product-detail/en/vishay-bc-components/BFC241642204/BC2062-ND/502842
http://www.digikey.ca/product-detail/en/rubycon/250MPH224J/1189-1804-ND/3720026
5. two caps I can't ID labelled CS13B/F334K/7924A. Best guess is this is a 0.33 uF tantalum cap, 6V, 10% tolerance. The one in the bottom of the photo measures ~450 nF on the Fluke. Is this about right: http://www.digikey.ca/product-detail/en/kemet/T322A334K035AT/399-4596-ND/946614
6. four radial ELNA 100 uF 6.3 V: http://www.digikey.ca/product-detail/en/RFS-6V101MG3%235/604-1134-ND/2171185
7. two axial ELNA 33 uF 50 V
http://www.digikey.ca/product-detail/en/elna-america/RFS-100V330MI5%235/604-1105-ND/2171156 (this is radial, so I'd have to jerry-rig it).
http://www.digikey.ca/product-detail/en/vishay-bc-components/MAL202138339E3/4069PHCT-ND/263272

Thanks!

 

[spritzer]

Don't have the time to check all the links but in the main amp section (back half of the schematic) you can upgrade the caps.  Back in the day capacitors were much bigger so feel free to upgrade within sensible limits.  Double or even triple will be fine. 

On the EQ you can't do that except for the PSU stuff.  The small caps need to be the same value or you will mess up the RIAA curve. 

[oogabooga]

I've desoldered the wiring to the front and rear panels, which helped me get a feel for the circuit, and realize that the entire upper half of the circuit diagram is duplicated for the other channel . Since I'm not touching the Mag PU section (in red), the shaded blue audio region is where I'll keep the same capacitance.

For the audio section, I've found Elna SILMIC caps to replace the existing Elna electrolytic, but I have no idea on replacements for the 0.22 uF 50V and 0.05 uF 630V film caps. I've found options from Panasonic, Kemet, TDK... general purpose, high frequency switching, etc. Does it really matter for an SRA-3S? I really don't want to get the 0.05 uF 630V caps wrong b/c they'll be a bitch to replace once everything is back together.  

 

[Craig Sawyers]

The mag PU design was in a Mullard apps note from the mid 1960's, published by Dinsdale in Wireless world Jan 1965.  You're right to leave it alone - the performance was marginal at best.  In fairness to Stax they are running it from 30V, which improves linearity as compared with the usual 12V, and the emitter bypass on the second transistor of 100uF kills of capacitor nonlinearity - so they thought about squeezing every ounce of performance out of mediocrity.  It's noise performance is either dominated by the 2.2k resistor in series with the input, or the poor noise performance of the first 2SC458 transistor (16dB noise figure at 30Hz).

The red line is the correct treatment!

[spritzer]

Well you can just unplug it so that that would be recommended.  It is a pretty crappy circuit indeed... 

As for the actual amp, the main HV coupling caps can be upsized with no issues.  I always used 0.22uf/630V polypropylene caps here as I had them in stock.  There are no bad effects from increasing the value on these at all.  What I'd do is measure the pin spacing and then go into mouser and find the right caps.  I perfer the molded caps (i.e. not the box ones) as they are easier to place.  Just go to film caps, polypropylene and the correct pin spacing. 

Plenty of 50V film caps as well.  I like Panasonic for these as  they are often quite small. 

A couple of other things, the 10pf feedback caps I'd also change.  I suppose 500V units would suffice here but higher is better.  I'd use silver mica because they are awesome. 

The two back to back100uf/6.3V caps could be swapped out for a single bipolar unit or even a ceramic unit.  I would also swap out the 1K pot too as it will be pretty crusty by now. 

[Craig Sawyers]

In fairness to the 2 transistor circuit, the original 1965 circuit used germanium transistors, Mullard OC44.  Price in 1965? 8s3d from the Mullard price list, or in today's money 41p. Correct that for price inflation and you get £7.61 or $11, times 2 or $22, times 2 for stereo - $44.   - for which you can now buy 4 AD797 or 16 LM4562 or 68 NE5532.

The 1967 Quad 33 used the same circuit, but moved across to silicon BC109 - which were equally expensive back then.  The 12 transistors in the 33 would have been the determining factor in the unit's cost.

You can make that 1965/7 performance much better by adding a transistor (to up the open loop gain, and give enough drive current for the RIAA), something they probably knew - but that would have added a lot to the semiconductor cost.

Which is all horribly irrelevant, since it is not going to be used!

Edited April 20, 2016 by Craig Sawyers

[spritzer]

I do love the markings on old equipment stating the number of semiconductors inside.  They should do that on computers these days...