Author Topic: Need help with pcb layout and grounding  (Read 491 times)

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Offline hummusdude

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Need help with pcb layout and grounding
« on: April 08, 2021, 03:38:40 am »
This is an audio pre-amp with a 2 band EQ circuit I found online. There are several digital controls for the gain that are surely overkill but I'm just having fun with different things. The main  control is a dpot that controls the gain of the TL072 op amp. the board is 2-sided and the whole bottom is a ground plane.

It's powered by two sources... +- 25 VDC from the main power supply for the power amp section that feeds a couple of linear voltage regulators that supply the 2 TL072 op amps (pre-amp and summing amp), and a 5 VDC source to power the digital components. The design originally used a 5 VDC linear voltage regulator fed by the analog voltage supply but in bread boarding it I noticed it was running quite hot. So I opted instead to use a tiny wall wart SMPS to power the digital components.

This is my first board and I'd appreciate some input about a few things. I'm sure it could be done with a more elegant lay out but I'm mostly interested in addressing noise and any other glaring problems that it might have. So my questions are:

1) Is there enough distance between the analog side on the left and the digital side on the right? Aside from the 4 analog signal lines that go to and from the dpot, the closest gap is around 6 mm. Because it uses a dpot I don't see any way to completely isolate all of the analog signal lines. I've read numerous sources about grounding and I'm a little confused on how to prioritize the placing of analog/digital lines.

One source says separate them by at least 20x the thickness of the board, which according to KiCad is 1.6 mm. If that's correct than that would mean it needs 3.2 cm spacing. I can make the board bigger if that is the best option. But that seems a bit out there. And what does that mean about dealing with analog signal lines that have to connect to the dpot?

One source says that the current return path of high frequency lines (>1 MHz) will tend to follow the digital signal lines whereas low frequency signal return paths tend to spread out and also go more directly to the ground connection point. I put the 5 VDC ground point for the digital components far away from the analog ground. The Arduino runs at 16 MHz so that seemed prudent.

The circuit was pretty quiet on the bread board except for some 60 Hz hum which seems normal for the conditions. But I'm worried now that packing it all in so close on a PCB will lead to unwanted noise.

I appreciate any input/advice about my layout or my circuit!

 

Offline hummusdude

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Re: Need help with pcb layout and grounding
« Reply #1 on: April 10, 2021, 03:19:24 am »
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You might have used some dfm software to analyze your design.

I had to look this up. This is my first board so I'm pretty green. This project is not intended for manufacture or sale...it's for me to learn on and hopefully have a decently functioning pre-amp when I'm done. From what I can see DFM is more about optimizing a design for cost-benefit purposes. I didn't see anything about best design practices, etc. Am I missing something?
 

Offline H713

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Re: Need help with pcb layout and grounding
« Reply #2 on: April 12, 2021, 11:51:53 pm »
For most audio line stages, I have not found it necessary to isolate the analog and digital sections. Usually I just do a ground plane on the back side of the PCB, trying to do as much of the routing on the front side as I can. If I end up with a lot of porosity in the ground plane on the back, then I may be inclined to add a front ground plane as well and stitch the two together with a bunch of vias.

Couple design suggestions you may consider...

The TL072 is an old and venerable chip, but it has high input voltage noise and poor current drive capabilities. For a line stage, the input voltage noise is not likely to be a big deal. It's more of an issue in a mixing console where it does start to add up. Of bigger concern are load driving capabilities. I'm not sure what the output of this is driving, but if it's going to be driving your RCA outputs then I'd strongly suggest replacing the TL072s in the tone control circuit with something that can drive lower impedance loads without high distortion. The OPA1642 and especially the OPA1656 are great choices in this regard, though they are only available in SMD packages.
 

Offline hummusdude

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Re: Need help with pcb layout and grounding
« Reply #3 on: April 13, 2021, 03:47:05 am »
Wow...thanks for the heads up about the TL072. To your point about the load it is driving, it is an LM3886 power amp that I found here https://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/#The-Final-Schematic. I have built it and it is performing to spec. I'm building the pre-amp because my devices do not put out high enough voltage to drive the power amp to it's fullest. The gain for my preamp is about 2.4, so not very much. The summing amp for the tone circuit is unity gain. Max Vout of the whole thing is 820 mV(pk) which is just what is needed to drive the power amp to 35W rms into 6 ohms.

I admit I'm a little fuzzy on the input impedance of the the power amp so I'm not sure how closely the two are matched. With the pre-amp circuit hooked up the LM3886 max power out is very close to the design parameter so I have to assume the impedances are close. I have had it all hooked up and tested with a resistive load and the pre-amp + power amp all seem to be performing well... I don't see any clipping until the calculated max power level and the clipping is even on both channels above max input V.  There's a very small level of 60 Hz hum which I'm guessing is do to the breadboard and the long jumpers.

I will check out the OPA1656 to see about subbing those. I'm already planning to do SMD components so no issue there.
 

Offline H713

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Re: Need help with pcb layout and grounding
« Reply #4 on: April 13, 2021, 12:08:28 pm »
Here's what I'd probably do:

Replace the TL072s on the output with something like OPA1642s or OPA1656s.  While you're at it, you might as well replace the input stage op-amps as well.

I  would then add 47 ohm resistors in series with the output. Without these resistors, you could make the whole thing break into HF oscillation with a capacitive load.

By the way, I cannot recommend Doulgas Self's Small Signal Audio Design book highly enough. It is a fantastic resource for designing everything from a phono preamplifier to a 72 channel inline mixing console. The only thing worth noting is that his "recommended op-amps" section is inevitably outdated- the OPA1656 has only been out for a year or two, for example, and it pretty much blows everything else out of the water in terms of noise and distortion performance in most audio circuits.
 

Offline bson

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Re: Need help with pcb layout and grounding
« Reply #5 on: April 13, 2021, 05:07:33 pm »
Did you test or simulate it with C9 and R26 in place?  Because their cutoff is less than 1Hz, although the load will slightly change this, so it may end up at or around 1 Hz for the -3dB point.  If the goal is to AC couple, then that's not necessary since there is no change in bias between the stages and those two can be removed.  R3 can also be removed, it's a mere 100Ω in series with 22kΩ.  If you hear white noise or hum the first place to check is generally around high impedances.
 

Offline hummusdude

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Re: Need help with pcb layout and grounding
« Reply #6 on: April 14, 2021, 12:12:00 am »
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Did you test or simulate it with C9 and R26 in place?

Yes. The pre-amp and the baxandall tone circuit are from different sources. Likely the designer of the pre-amp expected it would be used to drive a power amp and not be piped into another low-level signal condition part. I tested it again without the high pass filter at the output and I can't see any difference.

 

Offline hummusdude

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Re: Need help with pcb layout and grounding
« Reply #7 on: April 14, 2021, 12:16:12 am »
H713...

I checked the datasheet on the OPA1656 and I can see what you mean. Thanks again for that recommendation....looking forward to getting some of those for my board. And thanks also for the tip about 47 ohm output resistor.

I'm just finishing an EET degree and although I know a little bit about a lot of things this is the biggest project I've attempted. There's so much to know about mixed-signal board layout...been enjoying the deep dive on youTube with Rick Hartly, Eric Bogatin, etc. Who knew that current doesn't actually flow in the traces?!
 


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