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howto: replacing the ALWSR pre-regulator by a VBE

Moral of story: supply impedance probably matters, even if it's only a handful of ohms...
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Billy-O. Try putting a 2 ohm resistor in the psu supply lines. Yeugh.
cheers
ced
 
Teddy
One last question
Does R2 on the vbe mod replace R2 on the ALW board or is it an additional resistor.
Thanks
 
The output impedance of the MOSFET is frequency independent. At low enough frequencies, it is better than darlington.

The best output impedance of the darlington is less (by factor 4 or so) than the MOSFET, but is frequency dependent.

Depending on choice of transistors, MOSFET has better HF rejection than darlington like TIP121 as it is much faster. You can make an ultra-fast darlington which would do as well, or possibly even better than a MOSFET, by using say BC550+BD139 (or other fast medium power device).
 
PigletsDad said:
You can make an ultra-fast darlington which would do as well, or possibly even better than a MOSFET, by using say BC550+BD139 (or other fast medium power device).
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PD - Do you see any advantage in this combination over BC547c+D44H11?

The second is very fast and has a higher hfe than the first, allowing higher value resistors.
 
i've tried the IRF610 again, personally still much prefer the Darlington.

In the Arcam the mosfet seemed to add too much coloration for me especially in the bass, it also added this coloration with the SB3 although not as much as it did with the Arcam, I tried it running the dac supplies and the I/V setion.
I'm going to try the transistor combo next
 
I merely mentioned BD139 as medium power device that I could remember the number of, and have a few in my bits box!

D44H11obviously handles more power, but BD139 is really fast - 100MHz, vs 50MHz for D44H11. It has a smaller chip area, so the interelectrode capacitances should be less as well.

I think the differences are probably marginal, but if the BD139 can handle the power, and you have some lying about, it certainly should be a valid alternative.
 
I tried replacing the VBE with the various FET's,and darlingtons(IRF610,TIP32 and 37,BDX33 and 34) and the output measures ~ 10.5 volts with no load and drops to 2 or so with a load to SR.What's wrong?Do I need to change R1 or R4 perhaps or something else?Input is 12v battery.The homemade Darlington works fine.

Mike
 
I measured the output impedance of a MOSFET VBE and it's about 16 ohm. So with high currents you'll have a significant dropout. With a darlington, the output impedance is equal to the resistors being used divided by the hfe. The monolithic darlington's hfe depends very much on the current (because of the internal resistor), with bc547c+d44H11 you can achieve a much higher hfe.
 
I understand some of your response.I was trying to hear the differences in the FET's.That being so-is it possible to make adjustments to get the IRF610's to work?I thought it was a drop in replacement for direct comparison.
 
It depends on the application, and the current. For a pre-amp it's a drop in replacement, for high current applications I'm not sure you can do much. Try reducing the resistor values and see if it helps, but I doubt.
 
Changing the resistor values will not help with the FET.

The FET can do reasonably high currents, but then drops a lot of voltage. If you need high current and low voltage drop, the FET is not for you; use the darlington in that case instead.
 
I tried the TIP132 and the voltage dropped as well.Only the homemade Darlington worked.In the case of TIP132 ,which resistors should I change?R1 and R4 seem to be the ones that control dropout.R1 is 64k and R4 is 1.5m.Would decreasing R1 further and increasing R4 also lower dropout even more?
 
The TIP132 has a lower HFE than the home made darlington. Decreasing R1 and R2 will lower the dropout. Increasing R4 will help too, but less.

If you decrease R1 or R2, you should increase C2 and C1 accordingly, to keep the same filtering effect.
 
I'm now using the Sziklai pair BC547/MJE15031 , recommended by Martin and it works a treat:)
 
With the TIP132-I decreased R1 to 22k,R2 to 10k,increased R4 to 4.5m.Dropout is now ~4volts.C1 is 10uf.How much should I change C2 from .1mf to maintain effective filtering?I don't mean to be a pest-just trying to try something different that may be better.

Mike
 
With above resistors-I'm getting ~230 ma of current.Sufficient for what in need in CD player.Therefore-which resistors are the most critical for increasing output current if I want to use in another application?
Mike
 
None of them, the output current isn't set by the resistors but by the output voltage dropping until (the required current) is supplied, which in turn depends on the total current-gain of the darlington arrangement you have.

The base current required to deliver the output current needed shows up as more or less 'drop out' voltage (= sum of base-emitter voltage drops +(volts lost across R1+R2)). This, in turn, depends on the Hfe of the transistors used to make up the darlington and is a very non-linear function. I fyou have enough raw voltage overhead - don;t worry about it, the gyrator circuit will deliver current even into a dead short given enough heatsinking(!)

Incidentally, at 230mA output, the ouput impedance will be less than 0.1ohm; a Fet-based approach will not get close...
 
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