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Old 25th November 2009, 09:38 AM   #521
Ian444 is offline Ian444  Australia
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Quote:
Originally Posted by AndrewT View Post
BTW. the B1 and the shunt regulator do not need matched pairs. The B1 just needs selected Idss. No matching is required.
So we need to "select" the Idss for the B1 buffer stage. So if we select two devices for each channel of the B1, should we select them to be similar Idss, if so, would not one say that we are "matching" the Idss of each device? Or could I "select" one at 7.6mA and "select" another at 8.9mA Idss and that would be OK? Because there is no "matching" required, then that would be fine, is that what you are saying?

Also, if I may ask another question, what is "Idss", current from drain to source, what is the extra "s" for, source grounded?

Ian.
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Old 25th November 2009, 10:42 AM   #522
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DQ: I'm not an engineer, so I can't do the maths for you, but I "reckon" you should be alright to do that. The lightspeed is a tiny load, so represents a very small extra current draw on that winding. I have often used such transformers for say, 2 positive supplies, and each will have a different load. An example would be to supply the analogue and digital parts of a DAC separately.

In any case, what you are proposing, thats exactly what I will be doing with mine.


Ian, I believe what you are saying is correct - once the values are roughly the same, you should be ok - AFAIK, all you really need are all to be similar for the B1. On the other hand, while you don;t need to match at all for the shunt reg, I think it was designed for sk170 in the 7-10mA range, rather than lower than this. Having said all that, many people do use matched pairs (say a few percent) for the B1.


******************************

I fired mine up last night and it works perfectly, no hum or buzz, just clean sounds. Need to do a few more bits of tidying up before bringing it into the main system.


**********************************************

Salas: re the 10R rather than 68R - I have some 3W 10R here - would I get away with using those rather than the 5W you mentioned?


Fran

Last edited by woodturner-fran; 25th November 2009 at 10:45 AM.
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Old 25th November 2009, 12:56 PM   #523
AndrewT is offline AndrewT  Scotland
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48VA 12+12Vac can supply +-500mA dc continuously and way over that intermittently.

A pair of 100mA shunt regulators, for stereo, will draw 200mA per polarity, that is well below the 500mA capability of the 48VA transformer. i.e. +-100mA times two channels = +-200mA << +-500mA
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Old 25th November 2009, 01:03 PM   #524
AndrewT is offline AndrewT  Scotland
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select Idss to be similar.
The B1 is designed to run the active device at Idss. That requires the CCS device to pass the same or similar Idss.
If you have a pair of 8.15mA and 8.23mA then that is a selected pair. they are within 1% Idss. You could use <2% tolerance Idss. I suspect you could even use <5% tolerance Idss, but I do not know which should be the higher and which should be lower. There are science reasons for NOT running the active device at well above it's Idss but some builders are reporting that they prefer the sound when the active device is running at >100% of Idss. I will ALWAYS run <=100% Idss. I will NOT try >100% Idss.

But, that selection MUST BE DONE at the SAME temperature. This must be checked when both jFETs are supplied and measured at the same time and they are thermally coupled together.
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Old 25th November 2009, 01:51 PM   #525
udailey is offline udailey  United States
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DQ what you have described will work. I am not sure if you thought you needed anything else like the bridges and cap OFF of the DCB1 board but you dont and you shouldnt. Simply connect the transformer to the DCB1 board.
48VA is far more than you need and will work well.
Uriah
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Old 25th November 2009, 03:02 PM   #526
billyk is offline billyk  United States
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DQ, are you asking if you can share the transformer between the DCB1 and the LightSpeed?
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Old 25th November 2009, 03:09 PM   #527
DQ is offline DQ  Romania
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Thanks a lot guys.

Uriah, do you mean I should use one pair of bridges and one cap from the DCB1 PSU to feed both the LS's 7805 LDO as well as half of the DCB1? My intention was to use separate bridges + cap for the LS, not those in the DCB1. Your post answers this but my English is not good enough to be sure I got it well.

Yes, billyk, either sharing the transformer only or sharing both the transformer, 2 bridges and 1 cap. From Uriah I understand the second option is correct.
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Old 25th November 2009, 03:38 PM   #528
billyk is offline billyk  United States
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You can share one of the transformers 12v outs to use for the power supply for the LightSpeed. I chose not to do that, I used a 10v transformer I had in my spare parts bin.
Hope this helps.
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Old 25th November 2009, 04:05 PM   #529
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DQ, yes you can do exactly this - thats what I will be doing as well.

Fran
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Old 25th November 2009, 05:53 PM   #530
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Default Transformers?

What rating should the transformer be for the dcb1?
(was that on the bom, if so I must have missed it.)

I'm got to pick some up soon.
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