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Old 4th November 2015, 07:20 PM   #1
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Default Preamp schema modification

Hi fellow dyiers,
I'm working on my first project to build preamp based around PGA2320 and I was looking around for some existing schemas but I did not find one, which would fit my needs.

So I have taken one, which was very similar and modified it a little bit to get desired components and outputs in place. Thanks for the original design goes to member "taz" from polish dyiaudio, but as I don't speak polish I will ask here.

I have attached screenshot of the modification to the schema, mine edits are made in red, so you can easily spot them but basically:
1. Replace NE5532D with OPA2134
2. Include Tape OUT
3. Include Fixed OUT

Click the image to open in full size.

Main question is, do you see any issues with mine modifications?

- I'm not sure whether I need to include also those 100k resistors in parallel for fixed out, as they are used in variable out path?
- Can i replace existing opamp with OPA2134 without changing the values of resistors and capacitors around it?
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Old 6th November 2015, 09:02 AM   #2
AndrewT is offline AndrewT  Scotland
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Buffer tape out.
It's a very common mistake (to save money) made by commercial manufacturers to do unbuffered tape out.
we often see tape out output impedance >>1k0 and yet pre-amp output impedance is stated as<<500r

Make C12/13 match opa datasheet. or larger but still virtually on the opamp pins and very short leads.

RF attenuation is set to 0.1us (f-3dB ~1.6MHz)
Is that an effective interference attenuator?

OPA output offset is larger than need be. 22k vs 101k on the -IN and +IN pins.
Is using an opamp with an open loop gain approaching a half million the best way to implement a +0dB Buffer?
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Old 6th November 2015, 09:46 AM   #3
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R7 and 8 are not required as this op amp has null gain.
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Old 7th November 2015, 08:41 AM   #4
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Thank you guys for feedback, seems like I will have to do more reading on this topic, as it seems more complex than I original thought. I'll start with OPA datasheet.
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Old 7th November 2015, 11:14 AM   #5
AndrewT is offline AndrewT  Scotland
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Now here is a Member after my own heart.
He finds replies asking questions, or giving answers, he did not expect, nor even think of asking.
So he proposes to go and do some homework to presumably allow him to formulate more questions.

Whereas we have many Members who come here with the spoon fed attitude of
"Don't bother me with all the details just tell me what to do" !

I do have to admit that in my Thread on slow PC I fall between these. I hope I'm not too close to the spoon fed variety.

Lake,
well done.
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Old 22nd December 2015, 07:55 PM   #6
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I want to thank you all, who helped me either here or via private messages.
Finally I had more time to make some progress and wanted to share first pictures of the "raw" preamp.

Works and sounds great, at least from initial hearing test. I was afraid of hum or some distortion when changing volume or switching sources.

Next steps will be to buy some case so it starts to look like a preamp and not like a mess
And of course finish the software, add infrared receiver, etc.

But so far I'm satisfied with the outcome.
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File Type: jpg 20151222_201727.jpg (208.0 KB, 115 views)
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Old 24th December 2015, 07:58 AM   #7
AndrewT is offline AndrewT  Scotland
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Find and sort the Hum before you start to assemble inside a casing.
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