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Old 19th April 2007, 11:59 AM   #1
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Default Audio Research SP12 heater arrangement

My ARC SP 12 has just expired and it looks as though the 24 volt reg for the heater supply has failed.

There doesn't seem to be much info on this model but it uses 3 x 6DJ8 in the phono stage and 2 x 6DJ8 in the line stage and it was a budget model.

I can't find a 24V T03 reg locally so I started to make a little add on board with an LM350T and then the upgrade bug got hold of me and I was wondering if I could improve the heater supply. I was thinking of trying a constant current type supply for series heaters but then it clicked: 24V and 5 tubes doesn't add up.

I checked the circuit and 4 of the tubes are in series while the output tube gets its own ac supply from a separate transformer winding. There are also 2 x TL071 being powered from the 24V supply.

Has ARC done this for a reason or are they being cheap?

Should I try improve or just leave it alone?
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Old 10th May 2007, 11:47 AM   #2
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anyone?
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Old 10th May 2007, 11:56 AM   #3
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Not having a schematic at hand, it's tough to comment, but my inclination would be to not mess around with it too much. ARC is many things, but "cheap" is not among them. Build a nice 24V reg (LM350 is a good choice).

If I were to guess, the last tube has a separate winding because of Vhk issues, but that's just a guess.
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Old 10th May 2007, 12:28 PM   #4
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Thanks SY.

I think I will fix the regulator first, but I have another question.

The standard supply uses a 4700 uF cap before the 24 V reg and no cap after it. Since I am making a new reg is it worth improving this supply even just by using a cap after the reg?

Would it be worth making a separate supply for the TL071 which are part of the regulated B+ supply?

Thanks!
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Old 10th May 2007, 12:55 PM   #5
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Chances are it won't make much difference. If heater supplies have a failing, it's usually common mode noise; differential mode noise is cleaned up pretty thoroughly by the reg and by the thermal inertia of the heater.
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