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DIP8 sized pcb for bootstrapped opamp
DIP8 sized pcb for bootstrapped opamp
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Old 22nd August 2018, 11:30 PM   #1
00940 is offline 00940  Belgium
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Default DIP8 sized pcb for bootstrapped opamp

There was a lot of discussion about bootstrapped opamps in a recent headphones amp thread, as a way to reduce common mode distortion.

I tried to see how compact I could make a pcb for an opamp operating into pretty much ideal conditions: it has both bootstrapped supplies and a small class A buffer isolating the opamp from the load. With only 7.5ma max in a SE class A output stage, it's not made to drive heavy loads obviously.

The feedback resistors aren't onboard as the board is made to serve as a direct replacement for a single opamp. Still, the ratio of the dividers driving the transistors has to be matched to the overall gain, as explained here: http://www.analog.com/media/en/techn...22599AN232.pdf It's not a plug and play solution. Minimum suggested gain is 2.

Comments, suggestions for improvements ?
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Old 23rd August 2018, 12:00 AM   #2
DPH is offline DPH  United States
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Suggestion would be to find a way to get local PSU bypass onto this board so that the inductance from the DIP8 interconnect doesn't dominate at high frequencies. Will stave off a few worries about stability.
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Old 23rd August 2018, 01:37 AM   #3
xrk971 is online now xrk971  United States
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DIP8 sized pcb for bootstrapped opamp
Finally, a schematic!
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Old 23rd August 2018, 08:39 AM   #4
00940 is offline 00940  Belgium
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@DPH: I've tried and that's not quite working out. Soldering a 7.5mm pitch cap over the + and - pins would be the best solution. That and restricting the board to slowish opamps like the opa134.

Well, not quite. The best would be to switch to a API sized format, which gives a gnd pin too and would allow for a better layout.
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Old 23rd August 2018, 04:03 PM   #5
xrk971 is online now xrk971  United States
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DIP8 sized pcb for bootstrapped opamp
You can also make boards stand on edge like a flag and that will give you more real-estate.
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Old 24th August 2018, 02:27 PM   #6
00940 is offline 00940  Belgium
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Indeed, that would be a possibility. Otoh, the API format enables stuff like this. Yes, it's bigger but there's a lot more onboard. A clipping circuit at the input, proper decoupling, a powerful opamp as output.

Disregard the opa134 label for the second opamp, it should be a lm6171. Btw, this one favors low value resistors for its feedback.
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Old 24th August 2018, 03:00 PM   #7
jackinnj is offline jackinnj  United States
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DIP8 sized pcb for bootstrapped opamp
Here's mine --although shown here in a Crown IC-150 preamp, it will drive any headphone you can throw at it:
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