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PPAS Build Thread. - Page 16

post #226 of 254
The ferrite bead would have to go in series with OPA690's output pin (pin 6), and there is no provision on the PCB for it. I can't tell you what ferrite to use because that has to be tested with appropriate instrumentation, and could vary from design to design. As I said, OPA690 is a wideband, high-speed opamps and the board and amp has to be designed for it. If you do anything but solder the opamp directly on the board, then even a short 5mm piece of wire could have enough inductance to render it unstable.

Just give up on the OPA690 idea and use an opamp that is known to work in that circuit. If you must try OPA690, build a Mini³ v2.

Different opamps are not as "plug and play" as many people make them out to be!
post #227 of 254
anyone care to suggest a good replacement for my opa690 then? using ad8066 as OPALR, and ive got 3 lmh6321 for my buffers. thanks
post #228 of 254
Since you have buffers on all three channels, the opamp choice becomes less critical, especially for the ground channel. What's wrong with using a tried and true, well-behaved and excellent chip like AD8610?
post #229 of 254
sounds good. i ran into a bit of an ugly problem with jumpering BUFG, the pad of pin 8 (i THINK pin8, opposite pin 1), the silver contact on the PCB peeled up and off of the board. from looking at the schematic with what limited knowledge i have IM NOT sure that specific pin is necessary so its not the end of the world..but i could very well be horribly wrong..anyways whats the best course of action now..glue the pad back on or something, to solder onto? or go without...?
post #230 of 254
Heed MisterX's advice and read the LMH6321 datasheet... it will show you plainly what pin 8 is, and whether it matters that its pad had delaminated.
post #231 of 254
hm i was mistaken it was actually pin 5 that became delaminated. seeing as this is the ground...whats the best course of action i should take?
post #232 of 254
You should use a small piece of (very thin gauge) jumper wire to connect the chip's pin 5 to ground. The buffer's internal current limiting circuitry would probably misbehave without the ground connection.
post #233 of 254
whichever ground i connect to should be fine? ie, could i just attach that pin 5 to the GOUT since thats the closest in location, or do i wanna go to VGND?
post #234 of 254
Technically VGND is more correct, but I don't think it really matters. All that is required of that pin is that it be tied to a point where the voltage is halfway between V+ and V-, and GOUT is just as good.
post #235 of 254
Quote:
Originally Posted by el_matt0 View Post
whichever ground i connect to should be fine? ie, could i just attach that pin 5 to the GOUT since thats the closest in location, or do i wanna go to VGND?
GOUT is active ground, you need the "ground" that is relative to the V+ and V-, so you connect it to VGND.

EDIT 1: AMB is correct, as long as the ground is 1/2 of the voltage, it should be good.

EDIT 2: this post seems already covered the LMH6321 ground connection question. In quick summary: the GND pin does not need to be connected if current limiter is not connected.
post #236 of 254
Quote:
Originally Posted by NelsonVandal View Post
Filburt, I've just built a platform to try AD8099 LR and AD8045 ground. I forgot to order resistors for this as well, and I don't know when I have time to finish it. ADA4899 sure looks more interesting. Specwise it seems to be the opamp for headphones. I wish I've heard about it before I ordered AD8099. If there are problems with AD8099/45, I'll go for ADA4899 (I'll probably go for it anyway sometime). Thanks for sharing the information. Have you tried it yourself?
Haven't tried it myself, but I'm curious as to why you think it's preferable to the AD8099; I'd have said the opposite, though the AD8099 *is* more difficult to work with. One of these days I'll get around to using both.
post #237 of 254
Quote:
Originally Posted by Filburt View Post
Haven't tried it myself, but I'm curious as to why you think it's preferable to the AD8099; I'd have said the opposite, though the AD8099 *is* more difficult to work with. One of these days I'll get around to using both.
Even lower distortion, unity gain stability (you can use the same opamp in the ground channel), higher output current, no need for external compensation - less fiddly, claimed to drive low impedance loads, no paddle that needs to be connected to ground, less DC-offset.
post #238 of 254
Now I've finally got my shipment from Digi-Key, and I've finished my PPAS with LME49710/20 and LMH6321. Seems stable. Slightly more noise than Mini3. Only -0.7/-1.7 V DC offset. Even without burning in it sounds very sweet and very very airy. I'm going to burn in this and the Mini3 I just finished. I'll report after some burning in and listening. First impressions are really the impressions of the opamps. PPAS a bit distant smooth and airy just like LM4562 (=LME49720). Mini3 very forward and a bit harsh and grainy like AD8397.

I've put it in a black Hammond C801 (same size as the Mini3), so it's quite small and nice looking. There's 6 AAA's and a trickle charger, and the run time should be good. It's a very tight fit and it's heavy because of the batteries.

Oh, and I forgot I use panasonic FC before and Sanyo OS-CON after the JFET's. I wonder if it would improve by adding a larger cap. I think I could squeeze in a 1000 uF.

I couldn't find SST502, so I use SST505 instead.
post #239 of 254

Just finished my PPAS

picture here
TLE is from Tangent's ADAPTLE kit. BUFs are LMH6321.

just listen to it for a while and have to pull of all the wire for casing. initial impression: definitely sound better than my X-Fi LM4562 mod. and it can be really loud!
post #240 of 254
Some impressions of my (not fully burnt in) PPAS with LME49720/10/LMH6321.

What really strikes you at first are details, refinement and width. Extends very well in both directions. Very smooth, no sibilance but a tiny bit on the bright side, and the treble isn't totally neutral (more energetic than LM4562 used with discrete buffers). Overall it has more slam than LM4562/discrete buffers. My major complaint is lack of mid, a bit hollow sound. Not the same warmth, intimacy and fullness as in the best Analog or TI opamps. I think this has to do with the LME4972/10 (=LM4562) and nothing else. This is how this opamp sound in all the amps I've tried it in. I've never heard it sound better than in this amp.

All in all this is a very good amp, easy to listen to and non-fatiguing.

I'm going to ground the box. LM4562 is the most sensitive opamp to pick up external noise I've ever used (not only in this amp). I think I'll add a 1000 uF electrolytic. Maybe I'll add some ceramic decoupling caps too.
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