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Leaf Audio PA-03 MKII Review (Tube Preampifier)

mhardy6647

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If you mean the 1950s - 1960s, I'm not aware of any tube gear which wasn't designed to be as transparent as was possible with the available technology. The Fisher, Scott, Marantz, Mcintosh of that era may have had differing ways of achieving that goal, but they weren't the snake oil hucksters we have now, with this preamp being the poster child.
I was referring to '70s and 80s, as @amirm had done in his review. :)
I was thinking of the knob, dial, and meter laden "high end" massmarket receivers of the era which probably had good specs, but not necessarily arrived at by the best possible means.

I was emphatically not referring to the brands you mentioned n the era you mentioned -- it may be recalled that there are shelves full of that stuff here ;)
 

Mike-48

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Luxman up front, Parasound around back.

There is no reason to insult Parasound. While their gear does not always measure the best, they do have a competent engineering staff (including consultants) and don't sell this kind of nonsense. If it really were "Parasound around back," it might be worth considering.

People ask how the product can be sold so cheaply. No advertising, no engineering, and minimal quality control . . . those all reduce prices considerably.
 

egellings

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I added a metal shield plate between the two wafers on my home brew preamp's rotary input selector switch and that cured the x-talk problem.
 

E. Damstra

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JDS

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There is no reason to insult Parasound. While their gear does not always measure the best, they do have a competent engineering staff (including consultants) and don't sell this kind of nonsense. If it really were "Parasound around back," it might be worth considering.

People ask how the product can be sold so cheaply. No advertising, no engineering, and minimal quality control . . . those all reduce prices considerably.
I didn't intend an insult at all. The back panel is solidly functional. I actually have an A23 myself. But the back of this device looks very much like the back of my A23.
 

Mike-48

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I didn't intend an insult at all. The back panel is solidly functional. I actually have an A23 myself. But the back of this device looks very much like the back of my A23.
Ah, apologies, then! I totally misread your comment.
 
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SIY

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I started in on "fixing" this. There's still some work to do, but I thought it would be interesting to see the effects of (literally) a ten cent modification.

Here's the spectrum at 4V out before the modification:

Leaf 4V out before mod.PNG


Here's the spectrum at the same output voltage, but after the first mod:
Leaf 4V out after mod.png

More promising! 2nd order is still too high, but there's a BIG dropoff of higher order harmonics. Over the next few weeks, I'll do a few more things which should knock down distortion significantly further.

edit: NB that my measurements and Amir's are pretty close. Yet another example of the repeatability of measurements when there's transparency.
 
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pma

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More promising! 2nd order is still too high, but there's a BIG dropoff of higher order harmonics. Over the next few weeks, I'll do a few more things which should knock down distortion significantly further.

Looks good. Just my curiosity, would you please extend the X-axis down to 20Hz? Or show spectrum of 60Hz+7kHz SMPTE IMD? That might be interesting as well.
 

SIY

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Looks good. Just my curiosity, would you please extend the X-axis down to 20Hz? Or show spectrum of 60Hz+7kHz SMPTE IMD? That might be interesting as well.
LF noise is reasonably low in both cases. Note that the top is off the case during this measurement and that I did exactly zero fussing with grounding to try to minimize 60 Hz. More later, gotta go work the griddle for breakfast at the café.
leaf LF spectrum.PNG
 

dfuller

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There's still some work to do, but I thought it would be interesting to see the effects of (literally) a ten cent modification.
I'm not familiar with this circuit, but I'm going to guess this was adding local feedback or changing the gain of a stage?
 

SIY

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I'm not familiar with this circuit, but I'm going to guess this was adding local feedback or changing the gain of a stage?
... or tuning the SRPP operating conditions re load used.
Nope. And surprisingly, this turned out not to be a pseudo-SRPP. Once I traced out the circuit and did some baseline measurements, the answers were more obvious.

My next steps are, though, the optimization of operating points. Fortunately, this uses 6DJ8-type tubes, and I've collected a pile of data on them over the years while developing my own preamps which use that tube (or its equivalents), so this should proceed rapidly.
 

El_Arte

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Nope. And surprisingly, this turned out not to be a pseudo-SRPP. Once I traced out the circuit and did some baseline measurements, the answers were more obvious.

My next steps are, though, the optimization of operating points. Fortunately, this uses 6DJ8-type tubes, and I've collected a pile of data on them over the years while developing my own preamps which use that tube (or its equivalents), so this should proceed rapidly.
I find it exciting to follow how far you’ll manage to take this preamp.
 
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El_Arte

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Also, I like the idea of an affordable balanced tube preamp.

I don’t always want what tubes do to my music. But sometimes I do. It’s a thing.
 

SIY

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I find it exciting to follow how far you’ll manage to take this preamp.
Honestly, it won't be that hard. The design errors are glaring and relatively easy to sort out, as my first crack at it demonstrated.
 

tubesguy

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Shouldn't we all be guessing what ten cent modification has been done by SIY? I'm going to guess it's a resistor that improves power supply decoupling and isolation of the amplification stages.
 

MakeMineVinyl

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Honestly, it won't be that hard. The design errors are glaring and relatively easy to sort out, as my first crack at it demonstrated.
Tell us what you did! This is getting like reality TV amplifier modification suspense. :confused:
 
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SIY

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Shouldn't we all be guessing what ten cent modification has been done by SIY? I'm going to guess it's a resistor that improves power supply decoupling and isolation of the amplification stages.
Excellent guess!

Wrong, though. :)
 
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