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Old 9th October 2012, 08:51 PM   #11
Bigun is offline Bigun  Canada
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Yes, it's strange

so - current source, how about this concept:
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Old 9th October 2012, 09:32 PM   #12
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You don't need R3, R4. In fact, they may cause instability as they form a HF pole with the input cap of the op-amp.

The op-amp controls the base voltage on Q2 such that the voltage difference from the non-inverting to the inverting inputs of the op-amp become zero. Hence, the current in Q2 will be V(+)/Rsense.

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Old 9th October 2012, 10:12 PM   #13
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Thanks Tom, I may play around with this concept a bit further. What I like is that the sense resistor can be of a low value, with less self-heating it will perform better and drop relatively little voltage. All made possible by the high gain of an op-amp. And who said you shouldn't use op-amps in hi end audio !
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Old 9th October 2012, 10:21 PM   #14
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Quote:
Originally Posted by Bigun View Post
Thanks Tom, I may play around with this concept a bit further. What I like is that the sense resistor can be of a low value, with less self-heating it will perform better and drop relatively little voltage.
I like that about it as well. You will have to drop some voltage across Q2, however. One thought that has crossed my mind is to have a high-efficiency switcher up front. This switcher would deliver Vout = Vfilament + V(Q2) + V(Rsense). You'd probably have to drop some 500 mV across Q2. Still, with a 300B filament (my example) this would lead to 750 mW dissipated in Q2 and 225 mW dissipated in Rsense. That's pretty manageable.

You could also let the input voltage be unregulated - as in your example. Then you'd have to design for 500-ish mV across Q2 under worst case input voltage and ripple conditions and size the heat sink for the other worst case (high input voltage, low ripple).

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All made possible by the high gain of an op-amp. And who said you shouldn't use op-amps in hi end audio !
Well... You only have control over the playback chain. Who says the recording chain hasn't had a few hundred op-amps in it? But that's a topic for another day.

~Tom
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Old 9th October 2012, 10:53 PM   #15
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A regulator will cost voltage headroom and consequently more power dissipation. Given that we already have a current regulator I don't see a need for two, so maybe leave more headroom across the current regulator transistor instead - overall fewer parts. The bottom line is having lots of capacitance to minimize voltage ripple - and the associated need to be careful about the initial current demands when these caps charge up. I haven't looked into the use of some inductance yet.
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Old 9th October 2012, 10:55 PM   #16
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Yes, it is easier to dissipate heat from transistor than from resistor. But resistor can run hotter.
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Old 9th October 2012, 11:29 PM   #17
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Better if we can dissipate very little in either.

The real trick would be a dc heater supply for a 2A3 that can hook up to an existing 2.5V heater transformer. Not a lot of spare voltage to play with there.
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Old 10th October 2012, 12:11 AM   #18
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A regulator will cost voltage headroom and consequently more power dissipation.
That's why I said "high efficiency". A switching supply doesn't dissipate much power and some switchers can boost the voltage so you can design around the headroom issues. But whatever floats your boat.

~Tom
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Old 10th October 2012, 12:13 AM   #19
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I see this trick in modern flashlights made in China. A single 1.5V source powers a LED keeping average current stable.
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Old 10th October 2012, 12:34 AM   #20
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I remember there's a thread somewhere on the solid state forum where a chap makes a switching current source as a load for a Class A amplifier. I thought it was really clever. Perhaps that is the 'next generation' dc heater method.
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