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Old 25th January 2013, 09:47 AM   #2621
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Quote:
Originally Posted by RNMarsh View Post
First one to offer SW-OPA pcb and/or BOM/parts gets my by order. Thx-RNMarsh
Ummm . . . just out of curiosity . . . how big is the order? What features and capabilities are you looking for?

Dale
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Old 25th January 2013, 09:58 AM   #2622
bcarso is offline bcarso  United States
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Originally Posted by dchisholm View Post
- The selected components are surface-mount devices in 0805 or SOT23 case sizes. This was judged to be a reasonable choice for home construction.
- The specified resistors are 1% thick film parts. Going to 5% tolerance could save roughly US$1.00 in parts cost.
One immediate comment: although 0805 are not as bad as still smaller ones, I've seen horrendous excess (or so-called "current") noise in thick film parts. Rohm ones were among the worst. Although their availability is limited to 0.1% tolerance which is not of any particular benefit here for the most part, thin film parts tend to be very much better. But they are a good deal more expensive.

MELF parts are also fine except for their tendency to roll away
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Old 25th January 2013, 10:30 AM   #2623
qusp is offline qusp  Australia
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yeah I have to concur, using thick film in a design of this caliber is a waste and you would not expect to get anything like what its capable of.
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Old 25th January 2013, 03:50 PM   #2624
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OK, back on track. My HP triple supply decided to become a 200kHZ power oscillator on the minus side. At first I thought it took out my breadboard but it is OK. On the downside the input stage had several layout mistakes that I had to cut and patch to get it working (and I forgot the offset trim).

The input stage seems to work as expected powers right up at 15mA, I did take the time to match the current source FET's so untrimmed offset is only 5mV. I made it into a closed loop gain of 10 inverter with 50k and 5k and the distortion is close to sims (scary). Now I can put both input and output together.

I did use an LSK389 on the input and I was going to try one with BF862's soldered on the same pads but I will need to spin another board (the cutting and jumpering was a pain).
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Last edited by scott wurcer; 25th January 2013 at 03:56 PM.
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Old 25th January 2013, 03:53 PM   #2625
EUVL is offline EUVL  Europe
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5mV is pretty good for offset without servo, by any standard.




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Old 25th January 2013, 04:24 PM   #2626
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I thought one of the objectives was access inside?
Otherwise we could use an off the shelf op amp.
With the tight spacing of the parts as described, how will DIYer be able to access internal points to experiment?
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Old 25th January 2013, 04:42 PM   #2627
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by dchisholm View Post
Ummm . . . just out of curiosity . . . how big is the order? What features and capabilities are you looking for?

Dale
zero, if all surface mount parts. MF resistors only, pls. jFET input.
Or, make 2 pcb versions.... sm and thru hole.

Last edited by RNMarsh; 25th January 2013 at 04:48 PM.
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Old 25th January 2013, 04:48 PM   #2628
bcarso is offline bcarso  United States
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yeah I have to concur, using thick film in a design of this caliber is a waste and you would not expect to get anything like what its capable of.
There are a few circuit positions where thick film noise probably wouldn't be noticed, close to the output for example, but only a few. That noise source bit me badly on a front end of a subwoofer amp once and I did a fair amount of study resolving the issue, and "published" the results on the old prodigy-pro forum some years back. As I recall Panasonic parts were better than some. Susumu seems to be the go-to brand for thin film in readily-accessible distribution.

Although you need current in this case to produce the excess over thermal noise, it's also important to realize that can be signal current, not just "standing" bias current. So your results for dynamic range based on peak signal versus zero signal noise can get quite overly optimistic with excess noise a function of signal.
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Old 25th January 2013, 04:51 PM   #2629
RNMarsh is offline RNMarsh  United States
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Originally Posted by bcarso View Post
Although you need current in this case to produce the excess over thermal noise, it's also important to realize that can be signal current, not just "standing" bias current. So your results for dynamic range based on peak signal versus zero signal noise can get quite overly optimistic with excess noise a function of signal.
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Old 25th January 2013, 05:35 PM   #2630
jcx is offline jcx  United States
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I suppose if big enough it could be annoying (in far critical bands, with low tonal compelxity signal) but isn't positive correlation of noise amplitude with signal the best case for psychoacoutic masking of it
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