Isolated ground references for op amps? - Page 1
Forum section: electronics / beginners

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 04:20:40 pm
content: Hello you lovely nerds! I'm not a complete n00b, but I need some advice for a project I'm working on.Preamble: Years ago, I was getting into building simple analog distortion pedals (RunOff Groove stuff mostly, nothing terribly interesting), then went through a few years of hell, and now I'm back on my feet. "Goodbye. The end. Any questions?"Anyway, before the dark times, I heard about a particular American-made practice amp that gained notoriety for sounding "tubey" and is now quite hard to come by, so I found its service manual and decided to make a near-clone, with a few mods (not relevant to this post, but one will hopefully be a subject for a later thread) and some lily gilding like bypass caps on all the op amps. I've been drawing it up and laying it out one op amp at a time to keep the rat's nest from getting too tangled, and was about to start the "drive" section when it occurred to me that distortion transients can make their way through the ground plane and cause oscillations that the bypass capacitors would miss, so I thought "Why not really gild the lily and give each op amp its own isolated ground reference?" What I have in mind is mocked up in the attachment. I intend for each op amp and related passives to have its own separate, isolated ground pour on the PCB.Will the resistive dividers be able to provide suitable ground references as drawn? (EDIT: My best Google-fu didn't turn up anything helpful, and I was planning on sharing the completed project here in the near future.)

author: wraper
author_rank: Unknown
timestamp: November 18, 2020, 04:37:31 pm
content: This circuit makes no sense. And arrangement of capacitors on virtual ground in particular. LM3886 needs a real dual supply with a real ground to work. Or they use a large capacitor on it's output in series with speaker as a workaround.

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 04:44:46 pm
content: Yes, the LM3886 will get the real dual supply; I intend the op amps to see just the 24VDC with a ground reference created across the resistive dividers, as is done in practically any single chip distortion pedal with a 9V battery. What's shown is just a mock-up so far; the power amp section will be completed later.The 220nF capacitors are the initial "lily gilding" I mentioned, placed directly at the chip's power supply pins to shunt whatever AC might still be on the supply rails.

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 05:18:29 pm
content: I withdraw the question. The original amp used Zener shunts to provide a +/- supply for the op amps; I'll just replace them with a 7809 and 7909 (more than enough headroom for the circuit in question) and call that "gilded enough." CBS obviously didn't need to worry about transients on the ground plane, so I guess I shouldn't either.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 18, 2020, 05:22:17 pm
content: the question is not as wrong as the schematic you postedmaybe get a look at the Big Muff schematics; they work without any opamps and have a great sustained distortionhttp://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part1.html

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 05:30:45 pm
content: I appreciate the suggestion and am a fan of the Big Muff and its clones, but the amp in question has a curiosity in its preamp stage (subject for another discussion) that might be exploitable. I'll continue with a more traditional power supply.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 18, 2020, 05:57:49 pm
content: Maybe have a look at the datasheet of the good old XR2206.(or maybe other sine generators from that era).They used several diodes in series to add "knee points" together with resistors to shape a triangle waveform into a sine. This is very similar to a "soft clipping" feature. You can integrate some diodes in the feedback path of the opamp to create non-linearity.LM3886 is also a quite powerful beast.It probably makes more sense to do the circuit in separate sections. Make some soft clipping or wave shaping section with low power opamps, and use the LM3886 only for power amplification.

author: pvsage
author_rank: Contributor
timestamp: November 18, 2020, 06:07:55 pm
content: I'll risk the wrath of CBS and mention that I'm attempting a not-quite-clone of a Bronco practice amp. ("Gain" section is typical LED crap; one of my changes will be to make it more Zen-like.)This should be much better. It turns out one of the few transformers I was able to hold onto over the years has a center-tapped 24V secondary; after filtering and diode losses, we'll call that close enough to +/-24V since Kicad has those as stock power rails. (I'm showing the rectified power coming in through a connector because I'll be putting that on a separate board.) Power amp omitted for clarity.

author: dmills
author_rank: Super Contributor
timestamp: November 19, 2020, 08:30:34 pm
content: 24V up the snout of a 7812, at anything more then 100mA it will want a heatsink.When looking at opamps and ground in audio, the way to think is to look at the current loops, all currents flow in them, and defining where they are has some value. In particular an opamp in class AB (Which is to say most of 'em) sources current from the positive rail but sinks it into the negative rail, so the loop switches depending on the quadrant in play, and this switching introduces a mess of harmonics to the power distribution traces.For this reason I often use a 22uF MLCC at each port pin connected to a common point and then to ground, with a 22 ohm resistor in each rail feeding those caps. That common point is also the return point for the reference used by the NEXT stage input side doings.This minimises the area of the loop carrying the harmonically rich mess from the output stage switching and less loop area means less chance for it to contaminate surrounding doings, yea a bit tricky on the layout, but 4 layer boards are cheap these days.

author: pvsage
author_rank: Contributor
timestamp: November 20, 2020, 06:11:50 pm
content: Thank you dmills! As mentioned earlier, the original amp used a couple 12V/5W Zeners and 240 ohm/2W resistors to drop the power for the op amps. Do you think whacking in a couple 18V resistor/Zener shunts before the regulators would be a good idea?The op amps really aren't drawing much power in this circuit; it's mostly voltage amplification. I still plan to give the regulators some sinking though.The feed resistors in the op amp supplies sounds like a great idea! It will also help with a layout issue I was running into with the supply; now the supply rails can go back to the top copper.What about star grounding, again with 22 ohm resistors lifting each op amp from the return path?

author: dmills
author_rank: Super Contributor
timestamp: November 21, 2020, 03:24:59 pm
content: Star grounding is mostly lazy thinking in my view, it CAN be the right thing to do, but really think about loops, think about common impedances and think about what is measuring relative to what. There is a lot of rather ridiculous treatment of this stuff as being magic in the audio fraternity.Usually you don't want any more impedance in the reference connections then you can help.Regards, Dan.

author: pvsage
author_rank: Contributor
timestamp: November 21, 2020, 04:25:46 pm
content: Thanks again!I remember one vintage stereo amp in particular where the phono preamp was lifted from system ground by a 10 ohm resistor, but as you say, that was probably a product of "magical thinking".I'll "bludgeon on with brute force and ignorance" as Clive would say, using 22uF on the power supply pins and giving myself the option of adding 22 ohm feed resistors if power amp current manages to get past the voltage regulators into the preamp.What about the power issue you mentioned with the regulators and adding in the RZ shunts in front of them?Regarding ground loop area, there are a couple points where there is a small chance of that being a problem with just a bottom copper pour, but I'll have a ground pour on top as well. Close inspection of my layout to date shows short paths back to the bypass caps.

author: pvsage
author_rank: Contributor
timestamp: November 25, 2020, 04:18:59 pm
content: Just needed to post a correction: the transformer in question has a 12V-0V-12V secondary, so allowing for diode losses in a full-wave center-tapped bridge it should give +/-16-17V. This makes sense, as I pulled it from an ultra-shaffordable practice amp with a TDA2030 power amp (which itself isn't a bad chip for what it does; weak link was most likely the speaker). The upshots of this are that this appears to be right in the sweet spot for the LM7812 and LM7912 regulators (assuming I'm reading the datasheets correctly) and it's still well above the minimum operating voltage for the LM3886. Output will be shy of 20W, but with a reasonably efficient instrument speaker, that should be adequate.