[Teerawit]

Quote:

Originally Posted by ATAT Wait. which pins.. 1 and 3 should be zero (since they're shorted.. duh) The others should be around 4.5 volts (at least thats what I measured on non-new 9vs)

Interesting, thanks!

BTW, what voltages are your opamps seeing? (with an ~18V supply)

 

[ATAT]

AD8058s confirmed working, culprit was BUF634s, oscillating.

LMH buffers fix the problem.

 

[Filburt]

Yay! I'm glad at least someone listens to my op-amp recommendations...and that it works out >_>

I'm also glad the 8058 is working in the PPAS, as that's what I plan to use with mine. I may go all out and use the AD9631 for ground ^_^

 

[ozshadow]

Seems the 2107 is fast becoming the chip of choice for this amp. And tweaking it becomes selecting the ground opamp and buffer of your choice. The 8610 in ground sure did make the 2107 bright, I'd hate to hear it with a 8620 main.

 

[jbloudg20]

What is the deal with the Jfets? Do the four of them go in Q1 and Q2?

 

[ozshadow]

Takes 4 - go in Q1, Q2, Q3, and Q4

 

[jbloudg20]

Quote:

Originally Posted by ozshadow /img/forum/go_quote.gif Takes 4 - go in Q1, Q2, Q3, and Q4

Thought so.

ATAT... maybe put that designator on your BOM?

 

[ATAT]

Good point,, I've been working on modifying the BoM to work with bipolars. I'll upload the new BoM in a week or two.

 

[pddjsteve]

Quote:

Originally Posted by ATAT /img/forum/go_quote.gif Good point,, I've been working on modifying the BoM to work with bipolars. I'll upload the new BoM in a week or two.

Any updates on this? I've got all the parts for my 2nd board ready to go, and I'm thinking of giving the AD8058/????/LMH6321 a try and doing something like you did with the 5AAA and the C801. I need to order a case for my pimeta anyways, so I thought I'd pick up anything else I might need to try this out.

 

[ATAT]

pddj - Yes, I got it working, but I havent typed up the part #s for those, its just tweaking the feedback resistor values.. I'm working on my oversampling usb dac right now so I havent the time to finish the Bom

I'm sure I can find my old mouser BoM and send it to you if you'd like

 

[pddjsteve]

Quote:

Originally Posted by ATAT /img/forum/go_quote.gif pddj - Yes, I got it working, but I havent typed up the part #s for those, its just tweaking the feedback resistor values.. I'm working on my oversampling usb dac right now so I havent the time to finish the Bom I'm sure I can find my old mouser BoM and send it to you if you'd like

Ah, ok. So nothing really drastic then? I've got pretty much everything ordered from back when I made my original (ordered enough to make both), I followed the high-end configuration from the site/thread BOM except stayed with 1% resistors instead of .1%.

So essentially, you made changes to R3, R4, R5, and R6? I read Tangent's cranky opamp article, and his suggestions seemed to be lower the bandwidth and balance R3/R4 with R2 and the pot.

I'm not really interested so much in part numbers as just some guidelines or rough values. If you'd like, I could even work on taking the old BOM and looking up part numbers and adding to it.

 

[ATAT]

R1, R2, R3, R6, R5 are changed, R4 is held constant.

buffers must be LMH6321s for very cranky opamps, there is also need for bypassing capacitors. (basically put tiny 0603 capacitor underneath the big electrolytic caps 100uF ones)

 

[pddjsteve]

Quote:

Originally Posted by ATAT /img/forum/go_quote.gif R1, R2, R3, R6, R5 are changed, R4 is held constant. buffers must be LMH6321s for very cranky opamps, there is also need for bypassing capacitors. (basically put tiny 0603 capacitor underneath the big electrolytic caps 100uF ones)

Things I have on hand:
LMH6321
R2 = 100K
R4 = 10K
R6 = 475K

Calculated based on my limited understanding on Tangent's cranky opamp article and electronics calculator page:
R1 = 33k
R3 = 3.32K
R5 = 30k

Need to get:
~.1uF 0603 bypass (NP0?)

Am I way off base here? You said you lessened the values, did you drop them all? I tried following the equations and it seems like raising R3 lessened offset but led to bigger R5 and R1

 

[ATAT]

Are you balancing for halfway in between the pot value? thats what you'd ideally do.. also smaller resistor values would allow you to fine tune your values...

 

[pddjsteve]

Ah, you replied before my edit went through. Hmm. Base my calculations on the pot being 5K, since that is half of its 10k value? Lemme go through this again.


POT = 5k (half of the 10k rating)
R1 = 4k
R2 = 100k
R3 = 500
R4 = 1.5k
R5 = 3.32k
R6 = 330k

Plugging those into the calculators on Tangent's site, that gives me a gain of 3.87 and an estimated offset of 8.77mV when I plug in the values from the AD8058.

I probably should go a little lower in the values, though. Am I on the right track? I'm pretty much in between projects at work and have the time tomorrow to sit and crunch numbers all day.

Edit: In addition to Tangent's multiloop gain calculator and DC offset calculator, I used the following equations from the pimeta and cranky opamp writeups:

R1 ~= R3 + R5
R5 ~= [5 to 10] * R3
R6 ~= 100 * R5