[tomb]

The Millett guys claim OPA551PA may actually sound better than the BUF634, and works in the same socket if you jumper pins 2 and 6. Jumpering these pins do not affect the BUF634 if you plug them in, instead. They also state the OPA551P1 can't be stacked and doesn't need it.

Would this option (OPA551PA) work on the PIMETA?

(assuming it can be found - looks like it's as scarce as the BUF634)

 

[SubRosa]

I've used it successfully in a Pimeta with an OPA2107, with AD843s it was absolutely impossible. I think it'd be fine with AD8610/20 though. If you really want a nice replacement get some Intersil HA-5002, not pin compatible but I'm no expert and I easily adapted the pinout with no extra adapter, just a bit of lead bending and some careful soldering, they sound better than the OPA551 in my opinion.

Edit: Avoid the OPA227 in ground with OPA551, while I had no equipment to measure such things, it is said to oscillate, and I did notice decreased stability with this combo, try the OPA277 if you're going the the low current OPAG route.

Ack, and the Intersil HA-5002 has no output protection so be careful not to pull the headphones out while the amp is on as it will short in the jack and cause the buffer to send its maximum current output, frying it.

 

[n_maher]

[edit]See amb's post below regarding the OPA551[/edit]

I'd suggest toying around with some sort of drop in diamond buffer. There are a couple of designs around and I think Sijosae (sp?) had designed one to fit in the same psyical space as a DIP8 chip, pretty slick if you ask me. I've experimented briefly with DB's in a PIMETA (see here) and found them to sound quite good. I did not have a chance to compare it to a stock PIMETA unfortunately.

Nate

 

[Polaris111688]

I've currently got plans to etch a board that'll give my PIMETA the PPA's discrete diamond buffer stage and PPA's class-A biasing stage. Anxious to actually order parts, build it, and test it. I plan on biasing the ground channel too.

 

[tomb]

SubRosa,
Do you have a pic of that pinout conversion? Nate's thread seemed to indicate it's not so simple.

Nate,
Neat stuff, but I was hoping for some easier solutions for a noob like me. I'm just worried about trying to stack the BUF634, much less make a pair of DB's.

I ordered some sample OPA551PA's from TI, but they won't have them until the end of May. I couldn't find them in stock from any of our normal suppliers - even from the secret BUF634P source: Jameco.

 

[SubRosa]

Quote:

Originally Posted by tomb SubRosa, Do you have a pic of that pinout conversion? Nate's thread seemed to indicate it's not so simple.

No, I don't. I don't own a digital camera but I'll try to borrow one. It wasn't hard, I experimented on some salvaged DIPs beforehand until I was satisfied with the "manufacturing process". If you look for my Pimeta thread you'll see I attached a diagram that might give you some insight. Anyways, as I said, I will borrow a digicam.

 

[n_maher]

Quote:

Originally Posted by tomb Neat stuff, but I was hoping for some easier solutions for a noob like me. I'm just worried about trying to stack the BUF634, much less make a pair of DB's.

Well, it sucks that the supply chain for these parts is still so screwy so you may be stuck trying to figure out the HA-5002s or buiding your own buffer. I can only imagine what these supply problems are doing to major manufacturers that use these parts

.

Best of luck,

Nate

 

[amb]

I would advice not to use OPA551 to replace the BUF634 in a Pimeta. Such a substitution is ok on the Millett Hybrid because the Millett has no global negative feedback; but on the Pimeta, the buffer is wrapped within the global feedback loop, and the limited bandwidth of the OPA551 might make the whole thing unstable. The Pimeta uses Jung multiloop without compensation caps, which might make such instability even more likely.

 

[thedoctor]

Quote:

Originally Posted by n_maher [edit]See amb's post below regarding the OPA551[/edit] I'd suggest toying around with some sort of drop in diamond buffer. There are a couple of designs around and I think Sijosae (sp?) had designed one to fit in the same psyical space as a DIP8 chip, pretty slick if you ask me. I've experimented briefly with DB's in a PIMETA (see here) and found them to sound quite good. I did not have a chance to compare it to a stock PIMETA unfortunately. Nate

diamond buffers are hard to beat for the transparency they hold. but your mod is using the DBs for the Millet Hybrid, which is based on the PPA v.2 buffers. IMO, they should be close sounding to a PPA v.2 than to a PIMETA.

Quote:

Originally Posted by Polaris111688 I've currently got plans to etch a board that'll give my PIMETA the PPA's discrete diamond buffer stage and PPA's class-A biasing stage. Anxious to actually order parts, build it, and test it. I plan on biasing the ground channel too.

by then it's closer to a PPA than to a PIMETA. might just skip the hassles and build yourself a PPA v.2
your suggestion is viable (IMHO) if you're build something less complicated like the sijosae simplified diamond buffer.

 

[cetoole]

Here is what I plan to try rolling into my Pimeta.

The reason for the odd layout is because of the lack of space around the ground buffer, because of the cap bank right there.

 

[thedoctor]

^ unprotected output. might get burned if the output is shorted while unplugging the cans.
this might be safer :

though i've never tried the newer ones. so can't comment how it stacks up to the version you've put up.