The ultimate DIY? A Stax SRM-T2!

[luvdunhill]

Nice work! Did you remember to teat the LEDs and what not before installing them? I found 2 duds out of the bag of one I had.

[blubliss]

We did teat the LEDs, yes and all the resistors too. We found one mislabeled resistor.

Edited November 27, 2010 by blubliss

[justin]

Nice work! Did you remember to teat the LEDs and what not before installing them? I found 2 duds out of the bag of one I had.

i've used about 4000 5mm red LEDs and never had 1 dud

[nattonrice]

Well you obviously haven't been teating them properly =P

Nice blubliss!

Good to see everyone is getting in on the high wank factor resistors hehe.

[n3rdling]

Andy, who are you having build the T2 for you if you don't mind me asking? I bet it'll sound glorious Did you ever get your O2s back from that one guy?

[pedefede]

Highly thought of and expensive resistors like Shinkoh and Audio Note tantalum, and RC55 precision film are all magnetic.

Hmm .. i tested yesterday with a magnet.

- Audionote 2w = magnetic

- shinkoh

[Craig Sawyers]

Hmm .. i tested yesterday with a magnet.

- Audionote 2w = magnetic

- shinkoh

[kevin gilmore]

Can anyone find suitable teflon tubing for the power supply modification at mouser?

My mind is drawing a blank... I've been using the stuff for 40 years and just can't find

the right keywords.

[spritzer]

They are all in 100ft lengths so quite pricey. Take a look at this link but be warned, it will change your region settings at Mouser:

Tubing & Sleeving - (Non-shrink)

[mrarroyo]

How about? For only $5.54 each when QTY 50+ purchased - Cable Sleeve kit - Blue | Package Kits

[spritzer]

That won't work as it is techflex, not solid teflon tubing.

[blubliss]

Andy, who are you having build the T2 for you if you don't mind me asking? I bet it'll sound glorious Did you ever get your O2s back from that one guy?

My friend and I are building it together. I ordered all the parts, we are stuffing the boards together, and he does all the soldering. KG is the brains behind the scenes, since this is the first time I have built anything and it's quite a jump from what my friend has done before.

[luvdunhill]

Andy should have some tubing I can send as well. I also have pins for Kevin's Stax jacks. Just send me a list of ancillary items you need and I can send what I have on hand.

[nattonrice]

I guess that the ultimate audio signal resistor today is the Vishay Z-foil - 0.2ppm/C tempco, 0.1% tolerance, and a 1ns rise time without ringing.

Unfortunately the naked version only comes in a 5mm ls and you have to sell no less than 3 vital internal organs.

I used 12 in my b24 using max absolute and set-wise tolerances and yeah... ouch.

[Craig Sawyers]

Unfortunately the naked version only comes in a 5mm ls and you have to sell no less than 3 vital internal organs.

I used 12 in my b24 using max absolute and set-wise tolerances and yeah... ouch.

Yeah - cheap those resistors aren't. But very, very good. And the 5mm pitch means you need to lay out the board to suit.

[nattonrice]

KGITSOJC...

[deepak]

Where are the most critical resistor locations in the T2?

[kevin gilmore]

That is a rough question, and if you ask 3 different people you should get at least 5 answers.

Here is my short list of resistors that are NOT critical.

R99,100,101,102

R88,89,90,91

R80,81,82,83

R84,85

The multi-watt resistors in the amp section definitely are important.

Some may consider the other parts of the servo's to be unimportant, but

i think that everything that touches any of the audio signal would be important.

[luvdunhill]

speaking of the multi-watt resistors, are the ratings on the schematic okay to follow? I notice that in many cases the BOM has triple the rating. It seems that in most cases 1W is sufficient, and even in the case of the series output resistors, 1W might be more appropriate than a 3W resistor for safety reasons.

[justin]

speaking of the multi-watt resistors, are the ratings on the schematic okay to follow? I notice that in many cases the BOM has triple the rating. It seems that in most cases 1W is sufficient, and even in the case of the series output resistors, 1W might be more appropriate than a 3W resistor for safety reasons.

are you talking about 3x the actual power dissipation? or 3x what is already a good 3x margin?

[luvdunhill]

the later. I'd assume the schematic is correct, unless by some chance the BOM was updated to reflect some change after the amp was built and that wasn't propagated back to the schematic.

What's R78 and R79 on the actual T2 circuit?

[Craig Sawyers]

OK, having been listening to the BH for a month or so, I'm girding my loins for the repair of the T2. Since the death mechanism for me seems to be second breakdown in Q23 (the 2SC3675 in the battery) I was trying to get to grips with the design of the battery. Now no-one else has had problems with Q23 in this location, so it is probably a batch issue - the batch that I bought was just too marginal on SOA.

I realise however that I have an imperfect understanding of the operation. I'll go through what I've worked out, so anyone can chime in.

First consider Q16/17, which is a long tailed pair. Nominally the base voltages should be very close to each other. The base of Q16 is fed by constant current source Q22 and resistor R42. The base of Q17 is fed via a resistor chain with RV1 in the middle.

Looking at the resistor values, with RV1 set at the R32 end, Q17 base voltage (WRT the top rail) is 6.2V.

Similarly, with RV2 set to mid track, the equation for an FET constant current source with Idss about 6mA is a drain current of 0.28mA. Flowing through 22k resistor R42 this produces 6.2V at the base of Q16 - precisely the same as that at Q17. So all well so far.

The current through R39 is (6.2V - Vbe)/62k = 5.6V/62k = 90uA, which is shared between Q16 and Q17 - so 45uA through each transistor.

Q18 and Q19 are simply voltage sharing - the bases are connected to the halfway point of the battery, so Q16/17 and Q18/19 only see half the battery voltage - important since these are 600V parts.

Q20 is a perfectly standard Widlar current mirror load, which boosts the gain of the long tailed pair Q16/17.

Problem is that Q23, the 2SC3675 has an hFE of around 50. Its collector current is determined by 5mA constant current source Q30. So the base current of Q23 is 100uA. And here is nub - there does not appear to be enough current in the collectors of Q16/17 (45uA through each) to drive 100uA into the base of Q23.

So what am I missing?

[kevin gilmore]

The test circuit at 740 volts and 5 ma shows a base current of 49.9 microamps. So my guess is that

at the operating point the 2sc3675 is running at an hfe of about 80.

So its likely that the minimum hfe of 30 at 10ma is likely 60 or more at 5ma, and that typical parts

are in the 80 to 100 range.

[Craig Sawyers]

The test circuit at 740 volts and 5 ma shows a base current of 49.9 microamps. So my guess is that

at the operating point the 2sc3675 is running at an hfe of about 80.

So its likely that the minimum hfe of 30 at 10ma is likely 60 or more at 5ma, and that typical parts

are in the 80 to 100 range.

Thanks Kevin. Kind of confirms my ball park numbers. I guess if I were designing this from scratch I'd put quite a bit more current down the long tailed pair to ensure enough margin (power dissipation in the 2SA1486's is not a problem), or make the output transistor a darlington to reduce base current.

[Kerry]

Craig - I modeled the active battery in LTSpice to confirm some numbers. If you adjust RV2 so that the voltage on the base of Q16 is higher (i used 7.489V relative to the collector). You should still be in range with RV1 to get the 740V, but you need to set it towards R33 (see schematic). That will put the current through R39 at about 112uA, only marginally better.

http://img696.imageshack.us/img696/3682/activebatteryspice.gif

If you want the model, let me know.