Now that the Krell FPB 300 shuts down in 10-90 minutes of operation or even idle with shorted xlr's and open outputs, I'm using the Hafler 9300 almost continuously. The Hafler a great amplifier, I believe, built with one of the world's greatest designs: the trans-nova. I'm also lucky to have a minty and perfectly operating copy. I think the Krell amplifier might be a tad cleaner but after going back and rechecking, I've never been sure I could hear a difference, and many times I've mistaken one for the other, using thinking it's the Krell when it's really the Hafler. This pertains to power as well, I've never had a case where conclusively the Hafler sounded less powerful. Now I find that recent experiments were marred by volume adjustment creep on the DACs. The correct adjustment is -7dB for the Krell and -4dB for the Hafler, a -3dB level difference. I'm reminded of a friend who didn't believe levels could or should be matched. My matching is within about 0.13dB, the best I can do and lucky it's that good, with the 0.25dB settings on my oldest Stealth DC.
Looking at the 9300 schematic, this is what stands out.
1) It's very simple for a reasonable power amplifier, though perhaps not as simple as I had been thinking, not simplicity uber alles.
2) It'd direct coupled with DC feedback and no servo. The best when you can do it, and simple. NO capacitors in the signal path to worry about. No inductors neither, and no inductors or anything at the output.
3) The schematic is drawn wierdly around the supply capacitors. There is nothing weird about the supply caps, in fact they are nicely bypassed with 4.75uF caps (probably film). I had been thinking they formed a surrogate cap coupling, but not in this circuit.
4) What is a bit weird is that the - terminal is driven, and the + terminal is really the signal ground (explaining why the capacitors are drawn as they are...to show the ultimate signal ground). The amplifier is internally inverting, but this is corrected simply by labeling the terminals in the correct polarity, therefore it appears to the user as a correctly polarized amplifier (just don't connect the grounds...which many amps warn you not to do when it wouldn't have mattered).
5) Essentially standard "the best" layout: input dual jfet diff amps with bipolar current mirror feeding bipolar drivers driving MOSFET output banks.
6) MOSFETs don't need no stinking current limiting, no stinking shutdown, no stinking anything except there are rail fuses, which have never blown.
7)The novel feature is the MOSFETs are in gain mode, making the amplifier inverting, so feedback is taken from their input. I'm not sure why it is done this way, but I suspect ultimate "peak" power, which is actually the way it seemed on the bench. Distortion began slowly rising above the rated level (150W at 0.003% distortion) however it seemed it might put out as much as 500 watts peak. This power availability also translates to more than negligible output at 2 ohms or less. Basically, there is minimal resistance (I don't see any!) in the output circuit, so it's wide open.
8) Feedback is therefore isolated from swings in power supply, and well as ultimate device linearity, meaning the amplifier doesn't "double down" when the stored power is running down. That probably a GOOD thing. It gives the best effort and moves on, rather than getting stuck in the mud. Power supply isolation comes from the MOSFETs themselves, which are excellently so isolated.
9) Despite that lack of feedback around the output devices themselves, the distortion is incredibly low for a power amplifier.
I only think it would be slightly better with what the Krell has: a regulated power supply. That where I imagine the differences I probably imagine the amplifier sounds as having comes from.
AND, looking at the 9303/9505 schematic, it's almost entirely different, far more complicated with many more bipolars in the front end and the jfet diff amp buried within all these bipolars. Also there's a servo which the 9300/9500 don't need, and even...gasp...what looks like a protection cutoff circuit. Not a jewel like the 9300 IMO--which doesn't need servo or protection circuit. The Nelson Pass designed Adcom 5500 is somewhere in between, more complicated than the Hafler 9300 but less complicated than the 9303/9505, but, characteristically, uses almost entirely jfets and mosfets with just a bipolar driver stage (like the 9300). While the Adcom looks like a fine amplifier, the Hafler 9300 is just the cat's meow of simple yet effective designs, I've never seen one done more nicely save the First Watt F5, which is power limited by comparison because there are just limits as to what's possible when leaving out the bipolar drivers.
Looking at the 9300 schematic, this is what stands out.
1) It's very simple for a reasonable power amplifier, though perhaps not as simple as I had been thinking, not simplicity uber alles.
2) It'd direct coupled with DC feedback and no servo. The best when you can do it, and simple. NO capacitors in the signal path to worry about. No inductors neither, and no inductors or anything at the output.
3) The schematic is drawn wierdly around the supply capacitors. There is nothing weird about the supply caps, in fact they are nicely bypassed with 4.75uF caps (probably film). I had been thinking they formed a surrogate cap coupling, but not in this circuit.
4) What is a bit weird is that the - terminal is driven, and the + terminal is really the signal ground (explaining why the capacitors are drawn as they are...to show the ultimate signal ground). The amplifier is internally inverting, but this is corrected simply by labeling the terminals in the correct polarity, therefore it appears to the user as a correctly polarized amplifier (just don't connect the grounds...which many amps warn you not to do when it wouldn't have mattered).
5) Essentially standard "the best" layout: input dual jfet diff amps with bipolar current mirror feeding bipolar drivers driving MOSFET output banks.
6) MOSFETs don't need no stinking current limiting, no stinking shutdown, no stinking anything except there are rail fuses, which have never blown.
7)The novel feature is the MOSFETs are in gain mode, making the amplifier inverting, so feedback is taken from their input. I'm not sure why it is done this way, but I suspect ultimate "peak" power, which is actually the way it seemed on the bench. Distortion began slowly rising above the rated level (150W at 0.003% distortion) however it seemed it might put out as much as 500 watts peak. This power availability also translates to more than negligible output at 2 ohms or less. Basically, there is minimal resistance (I don't see any!) in the output circuit, so it's wide open.
8) Feedback is therefore isolated from swings in power supply, and well as ultimate device linearity, meaning the amplifier doesn't "double down" when the stored power is running down. That probably a GOOD thing. It gives the best effort and moves on, rather than getting stuck in the mud. Power supply isolation comes from the MOSFETs themselves, which are excellently so isolated.
9) Despite that lack of feedback around the output devices themselves, the distortion is incredibly low for a power amplifier.
I only think it would be slightly better with what the Krell has: a regulated power supply. That where I imagine the differences I probably imagine the amplifier sounds as having comes from.
AND, looking at the 9303/9505 schematic, it's almost entirely different, far more complicated with many more bipolars in the front end and the jfet diff amp buried within all these bipolars. Also there's a servo which the 9300/9500 don't need, and even...gasp...what looks like a protection cutoff circuit. Not a jewel like the 9300 IMO--which doesn't need servo or protection circuit. The Nelson Pass designed Adcom 5500 is somewhere in between, more complicated than the Hafler 9300 but less complicated than the 9303/9505, but, characteristically, uses almost entirely jfets and mosfets with just a bipolar driver stage (like the 9300). While the Adcom looks like a fine amplifier, the Hafler 9300 is just the cat's meow of simple yet effective designs, I've never seen one done more nicely save the First Watt F5, which is power limited by comparison because there are just limits as to what's possible when leaving out the bipolar drivers.
No comments:
Post a Comment