Amp design ideas from M3/PPA/Pimeta?
Sep 16, 2009 at 9:26 PM Thread Starter Post #1 of 4

hiker101

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Hi,

I'm trying to come up with a list of features to include in my next amp. I have Grado headphones so I'm targeting my choices toward that direction. I've spent some time studying the PPA, M3 and Pimeta designs and discussions and I'm wondering how people would rank the various features of those amps in terms of the way they affect performance.

0) Layout.
I would like to build this on breadboard. I have done two breadboard amps so far (CMoy and Apheared 47) vs. a Mini3. I had a lot more fun doing the breadboard amps, even though the Mini3 sounds better. I think I can also make the breadboard very modular so that I can just snip out individual elements if I want to try something different. Like discrete buffers vs. op amps, for example.
Are there SQ issues in using breadboard vs. printed circuit boards?

1) Topology.
I really like the pimeta v2 design. All opamps with global feedback. Part count seems reasonable compared to circuits with discrete segments. The buffers used in the pimeta2 are supposed to sound very good.

2) Power supply.
I'm thinking 24V wallwart with a TREAD-like regulator circuit in the amp case. A capacitance multiplier might be a simple way to improve the performance of the regulator circuit. I got this idea from Morsel's M3 web links- maybe it's overkill, but it looks easy. I would like to end up with very quiet 20V- plenty for most opamps.

3) Buffer PS isolation.
This just seems like a good idea. Both M3 and PPA do it. It doesn't seem hard.

4) Running the opamps in Class A.
Every non-beginner amp seems to do this, it must be a good thing. I don't know if I'll implement this from day one... or maybe I should?

5) Single vs. dual opamps.
I think single opamps make the layout more symmetric and less cluttered. I can make 3 identical miniboards. How big a performance issue is this?

6) Bass boost.
Real easy to do. Good examples in PPA and M3. I definitely want this.

7) Molex connectors for the hardware.
I hate casework. This way I can make a crummy first attempt at a case and then if I decide to improve things later it will be easier to move the "guts" to a new case.

8) Boutique components.
Starting from the default components you might see in a Pimeta, which components might produce the best bang-for-the-buck?


I'm interested in any comments you might have. Which of these things do you think are important, which aren't. Have I left anything out?

Thanks!
 
Sep 16, 2009 at 10:24 PM Post #2 of 4

FallenAngel

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0) Would be important, ground planes are nice for noise.
1) LMH6321 does sound nice, shouldn't be too hard to breadboard.
2) LM317 is an easy way to get a nice regulated supply.
3) Thought that the opamps are isolated from PS, not buffers.
4) Class-A output stage is nice
5) Doesn't matter
6) Meh, I never use it.
7) "Shouldn't" matter
8) Bang/Buck, not "Audiophile", look at specs.
 
Sep 18, 2009 at 10:45 PM Post #4 of 4

hiker101

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Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif
0) Would be important, ground planes are nice for noise.



Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif
^
Ground planes are harder to make right, it's not an easy application.

A star ground is usually better.



After reading around a bit I got the impression that a star ground was actually superior to a ground plane. Breadboarding is the one feature I'm stuck with (and no ground plane) if I want to take a Pimeta schematic and add too many PPA/M3 features to it. I'm hoping that if I have a grounded metal case then that will at least shield from external noise.

I know that a ground plane is very important for high speed digital applications, where you want the signal traces to behave like transmission lines with a characteristic impedance.

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif

3) Thought that the opamps are isolated from PS, not buffers.



You're right! I stated this backwards.

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif
4) Class-A output stage is nice


Ha ha. You don't mean op-amps here, you mean running the buffers in Class A, right. I wonder if the LMH6321's can be biased to 40 or 50 ma? I noticed that a thread recently popped up about biasing the Sijosae discrete buffers. Certainly this is something I could try adding to my amp- I'll have to leave space for it.

Thanks for the comments.
 

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