KurtW META42 Portable Amp
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puppyslugg

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Quote:

Originally posted by charlesb
I don't know if this an absolutely "maxed out" meta42, but it sure sounds good!


Not quite 'the maxed out meta'. I think kfaiman's Meta with el2009 buffers and Blackgate ps bypasses is hard to beat. Though it's not good for portable use.
 
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KurtW

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Unfortunatly the term "maxed out" META is confusing and probably overused. To me the META42 was designed as a portable amp since it was small and optimised for battery use (ie, single supply voltage), so this would limit what components were to used for a portable application. I've build META42s with EL2002 and EL2008 buffers and they do sound better, but I don't consider them to be good portable units since they take a lot more power. Blackgates and Cerafine's have limited portable use as puppyslugg points out, as they aren't small enough for the Serpac SRH65 case, for example. And Blackgates are said to need power for a long time to sound their best, again not something you want in a portable amp.

The EL2001 buffer is a great sounding buffer for a reasonable cost and power consumption. Given this, it can be doubled or even tripled up for better sound and still have lower power consumption and even lower costs than some other single buffers.

In the next day or two I plan to run some tests with double vs triple EL2001 buffers with some low and high impedance headphones, and I'll report on the results.
 
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KurtW

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Okay, here are the results of my double vs triple EL2001 buffer tests. It was conducted with two identical portable META42 amps, built within a day of each other using parts from the same batch, each with slightly less than 1 hour burn-in, and both having the AD8620 op amp and using fresh Plainview batteries. The source was a Panasonic SL-CT570 pcdp using the line out to a Markertek cable to the amp.

The expected and actual benefit was in the impact of the bass. To evaluate this properly you must match the compared levels carefully, as typically the louder unit will sound like it has the bigger bass impact. I adjusted each amp to have as close to the same gain as possible. This is harder to do than say, as the volume control only has a finite resolution and there is also some channel inbalance in each unit. I adjusted the outputs of each amp to be on the average of within 0.5 dB of each other, and later changed the slight error in favor of the other amp and repeated the listening tests. Here are my findings with both low and high impedance headphones:

Low Impedance phones, Grado SR-80 and Audio Technica W2002:
There was a noticable increase in bass impact, depth and tightness with the triple buffers. When the volume slightly favored the unit with the triple buffers I thought the impact was quite impressive, but when the double buffer unit had the slight (within 0.5 dB) volume advantage the triple buffer's advantage was still there but more modest.

High impedance headphones, Sennheiser HD580 with blue Clou cable:
I heard the same thing as with the low impedance headphones but to a lesser degree.

In conclusion: going from double to triple EL2001 buffers does increase the bass impact, although low impedance headphones will show more of a difference than high impedance headphones according to both theory and my somewhat limited experiment. There is a price and battery longivitity penality to pay for the extra buffer per channel but is is fairly modest. For me it's probably worth it (although not a must) for low impedance headphones, and for high impedance headphones it would depend on the value of longer lasting batteries.

Some people have gone much higher than just triple buffers, but three is the maximum that will fit into the Serpac H65 case.
 
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puppyslugg

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kurtw, thanks for taking the time to do the comparison. I'm sure a lot of folks out there wanted to know. I think using multiple buffers lowers the output impedence, which in turn helps 'control' the hp drivers better. Resulting in the tightening of the overall sound. Sounds reasonable?
 
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morsel

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You got it, PS. Thanks for doing the test, Kurt.
 
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KurtW

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Yep, two buffers have half the output impedance as one, etc. When I built my first META42 it was actually morsel that listened to it and suggested more than once that I should put two buffers per channel in instead of just one, even though I was pretty impressed with even one. Since then two buffers per channel has become my standard and three are a little bit better yet. The question I had to answer was how much better is it, and what is the affect on headphones with high impedance where the source impedance should matter less .
 
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kelly

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Kurt
In other solid state amps I've compared, the difference in bass impact was often more easily discerned with low impedance headphones. Nonetheless, the bass is somewhat important to me and even when using the HD600, I want every last drop. I'm very curious about the triple buffered META.

Have you compared it with your Corda Blue at all?
 
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KurtW

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My META42 has a different op amp and different buffers than my Corda Blue, so the sound is also different. The double EL2001 buffered META42 I had compared eariler to the Corda Blue (with double BUF634 buffers, I don't have the triple ones in yet) didn't sound as good as the CB, but then a different META42 with different buffers (EL2008) sounded like more of a fair comparison, although still a bit different sound. I haven't compared them enough yet to know which I prefer.

By the way I noticed that Jan stated that adding more buffers to the HA-2 wouldn't change the sound because "the buffers of the HA-2 are placed inside the feedback loop of opamps. These opamps make the sound, not the buffers."

The META42 also has the buffers in the op amp feedback loop, but multiple buffers or different buffers do indeed influence the sound.
 
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puppyslugg

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The Corda Blue uses a regulated linear supply as oppose to the Meta42 using a vgd, rail splitter, etc. I believe the results would be different if the Meta used a regulated linear ps, bypassing the vgd, railspliter, etc.
 
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tangent

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Quote:

I noticed that Jan stated that adding more buffers to the HA-2 wouldn't change the sound because "the buffers of the HA-2 are placed inside the feedback loop of opamps. These opamps make the sound, not the buffers."

The META42 also has the buffers in the op amp feedback loop, but multiple buffers or different buffers do indeed influence the sound.


I passed over this comment when Jan first made it, but it's starting to become ingrained in the culture so it's time to nip this mythlet in the bud.

I think what Jan is thinking about has to do with the buffer's output impedance. When you have buffer outside the op-amp's feedback loop, its output impedance will cause the output voltage to drop when driving a high load. Paralleling buffers in this situation helps mitigate the voltage drop since it lowers output impedance.

Now, when you put the buffers inside an op-amp feedback loop, the op-amp works to correct any output voltage drop. One way to think about it is that putting buffers inside the loop drops the amp's effective output impedance greatly, so theoretically you only need as many buffers as required by your load -- if you've got a load that demands only about 10 mA, like HD600s, you might think that going beyond one 100 mA buffer doesn't help. But it does! Why?

One reason it helps is that even with a feedback loop around it, the output impedance doesn't hit 0. So, you can still benefit from lowering the output impedance still further. Also, paralleling buffers lets the buffers share the load, which increases their fidelity. Another reason may have to do with speed -- a 100 mA buffer can put out 100 mA, yes, but how quickly can it go from +10mA to -10mA to track the complex waveforms we call music? If you have four buffers sharing the load, each one only has to swing from +2.5mA to -2.5mA to track that signal.
 
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