The ultimate DIY? A Stax SRM-T2!

[Voltron]

I am getting quite anxious for my solid chunk of aluminum from Ayre to show up. Should be soon.

Are you fellas doing both Pro bias or one of each on your T2s?

[kevin gilmore]

Well the power supply was not really designed with low bias in mind. but a rearrangement of

the zeners allows for the right low voltage, and extra wire, and probably a little circuit

board with 2 resistors and 2 caps needs to be added to the power supply.

This is corrected on the kgsshv board and power supply.

The alternative is actually quite easy. The 250 volt power supply can be used as the

bias and is already very low noise and accessable. As there are two different wires

for the two different stax jacks, this is the no modification approach.

So you can have one high bias, and one low bias jack at the same time.

[Craig Sawyers]

Well the power supply was not really designed with low bias in mind. but a rearrangement of

the zeners allows for the right low voltage, and extra wire, and probably a little circuit

board with 2 resistors and 2 caps needs to be added to the power supply.

This is corrected on the kgsshv board and power supply.

The alternative is actually quite easy. The 250 volt power supply can be used as the

bias and is already very low noise and accessable. As there are two different wires

for the two different stax jacks, this is the no modification approach.

So you can have one high bias, and one low bias jack at the same time.

That sounds real neat. So one 5M resistor gets 560V and goes to the high bias jack, and the other resistor gets 250V which feeds both bias pins (shorted together) on the low bias jack?

[kevin gilmore]

some schematics say 230 volts for low bias. Some say 260.

the resistor is indeed 5 meg.

except for one very old schematic that says its 2 meg.

i would go with 5 meg and 250 volts.

Edited July 15, 2010 by kevin gilmore

[Inu]

I added 5P terminal on the PS board and changed R-CH 580V bias line to the 230V low bias line.

A combination of the Zener Diodes on my unit is; GND > 80V+150V(230V from here)+150V+200V(580V).

http://picasaweb.google.com/inuponken/T2#5493931120123168562

[kevin gilmore]

Yep, that works too...

where did you find those white caps ??

Edited July 15, 2010 by kevin gilmore

[nattonrice]

Those jacks look great!

All pro bias for me =)

[Voltron]

I think I will stick with Pro bias.

[struts]

the 4 people getting chassis

this month, let me know whether you want low bias or

high bias jacks.

Looking great KG. One of each for me please!

[Craig Sawyers]

Looking great KG. One of each for me please!

That is what I have gone for too. Getting all hot and sweaty now - just waiting for:

Alumina insulators

Long transistor bushes

2SC3381's

Casework

[spritzer]

The normal bias is actually 200V for the 1967-1975 models (the SR-1 is also 200V but the very first units were 150V). The 1977 and onwards models all used 230V bias but many models still used pre-1977 drivers and worked just fine off the higher bias voltage.

A simple voltage divider off any of the PSU lines would work for a normal bias supply or any of the other bias voltages we are using (180, 360, 500 and 540V).

I am getting quite anxious for my solid chunk of aluminum from Ayre to show up. Should be soon.

Mine is sitting here right now. 16kg of aircraft grade aluminum just for the BH amp boards. I'll post pics when I have some time...

[Craig Sawyers]

The normal bias is actually 200V for the 1967-1975 models (the SR-1 is also 200V but the very first units were 150V). The 1977 and onwards models all used 230V bias but many models still used pre-1977 drivers and worked just fine off the higher bias voltage.

A simple voltage divider off any of the PSU lines would work for a normal bias supply or any of the other bias voltages we are using (180, 360, 500 and 540V).

Mine is sitting here right now. 16kg of aircraft grade aluminum just for the BH amp boards. I'll post pics when I have some time...

That is really useful information - thanks. My Lambda's are 1987, so 250V as per KG's suggestion sounds like the way to go.

[Inu]

Yep, that works too...

where did you find those white caps ??

Local parts store that selling new and surplus parts from local industry.

I can't remember the price but it was very cheap. <$1/pc

[spritzer]

That is really useful information - thanks. My Lambda's are 1987, so 250V as per KG's suggestion sounds like the way to go.

SR-Lambda (6-pin) is 230V but 250 is close enough. You can also shave off a few volts since the danger of arcing with this beast can't be underestimated.

Edited July 15, 2010 by spritzer

[Craig Sawyers]

Hokay - remaining outstanding items have arrived.

2SC3381 - check

4171G - check

7721-3PPSG -check

Now - a question for Inu. How did you get the 7721-3PPSG to fit in the holes of the TO220 devices? Faced with the fact that they don't (for me anyway) I looked up the spec. Holes in devices = 2.6mm, bush diameter 2.85mm. Did you ream out the holes in your devices, or am I missing something?

[Craig Sawyers]

Now - a question for Inu. How did you get the 7721-3PPSG to fit in the holes of the TO220 devices? Faced with the fact that they don't (for me anyway) I looked up the spec. Holes in devices = 2.6mm, bush diameter 2.85mm. Did you ream out the holes in your devices, or am I missing something?

Um - the sketch I had was misdimensioned. The official bush diameter of the 7721-3PPSG is 3.56mm and so should fit in a 3.6mm hole. Mine actually measure a tad over 3.6mm - so it takes a very firm push to get them home in the device tab. But they *do* go in.

[Inu]

Um - the sketch I had was misdimensioned. The official bush diameter of the 7721-3PPSG is 3.56mm and so should fit in a 3.6mm hole. Mine actually measure a tad over 3.6mm - so it takes a very firm push to get them home in the device tab. But they *do* go in.

Craig,

7721-3PPSG is only fit to the 2SC3675.

I am replacing all to the polyamide (they call RENY) screw now.

Plastic Screw Characteristics

Picasa Web Albums - inuponken - T2

[swt61]

a stax of staxjacks

Just catching up in this thread. Those are fantastic Kevin! The single milled piece of Teflon is really awesome!

[nattonrice]

Hehehe Nabeya... pot store =P

[Craig Sawyers]

Craig,

7721-3PPSG is only fit to the 2SC3675.

I am replacing all to the polyamide (they call RENY) screw now.

Plastic Screw Characteristics

Picasa Web Albums - inuponken - T2

OK Inu - I'll bite. Where did you get those screws? What an amazing material.

[Inu]

Hehehe Nabeya... pot store =P

Thank you for translated a Japanese word into English.

[Inu]

OK Inu - I'll bite. Where did you get those screws? What an amazing material.

I ordered NBK with my home address in JPN.

SPA-M3x15 100pcs　￥1,500

SPA-M3 Nut 100pcs ￥1,500

Shipping ￥500 + From JPN to US ￥1,300 (included hundred of Duel Masters cards for my son)

Total ￥4,800

I used same way for all semiconductors and Takman. My son has over two thousand cards now.

[justin]

The Alps RK50 are still available, i confirmed this today

[Craig Sawyers]

some schematics say 230 volts for low bias. Some say 260.

the resistor is indeed 5 meg.

except for one very old schematic that says its 2 meg.

i would go with 5 meg and 250 volts.

I remembered that my 1987 Lambdas came with a step-up transformer based driver. It connected to the output of a regular loudspeaker amplifier. The bias came from voltage multiplying and smoothing one of the output drive signals. Called an SRD6SB.

I pulled it out the attic and took the lid off to see how it limited the bias, and what the voltage was.

One of the headphone drive signals connects to a 100k resistor with a 100nF in series, with a 100V Zener to ground. So that limits the bias at that point. It then goes to a voltage tripler with 100nF caps, and a final 1uF across the output. Then a 4M7 to the diaphragms. So I reckon this unit provides a bias of 300V. The tolerance of the Zener is 10%, so the voltage could be between 270V and 330V. Seems a bit high as compared with the above, and circuit losses and leakage might limit the voltage to a bit lower.

Might shove it on the sig gen and a random power amp and see what I get at the bias output.

[nattonrice]

WOOT future elec and mouser orders are in. Tomorrow night I'll do an Allied and Digikey run.

To top it off the wank-factor-9000 resistors have been ordered too