Anderton Practice Play Along question

[Ben N]

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I'm trying to put a DeadAstronaut Astrosim (https://www.diystompboxes.com/smfforum/index.php?topic=121846.0) in a 1590BB along with a stereo TDA2822M headphone amp board from Parts Express and a mini-mixer for stereo program content. That's where the PPA comes in, but I have a few questions.
1) Since the guitar input is actually from the Astrosim line out, can I ditch the buffer and booster stages (IC1D and IC1C) and go straight into the level pots, or do I still need to buffer there, and would a discrete device do (allowing me to use one dual opamp for the mixer stage)? If so, any suggestions?
2) What's with all the 2uf coupling caps? I mean, ok, I guess for the program inputs I get it, but for guitar? Can they be reduced?
3) Can this be safely converted to a single +9-12v supply? Both the Astrosim and the headphone amp run on single supplies, although I suspect that +9v is probably marginal for the TDA2822. Alternatively, any reason to anticipate a problem with supplying just this leg with bipolar juice from a charge pump?
Many thanks,
Ben

[Ben N]

And, BTW, I tried to post a schematic, but failed. Any advice on how to get a link from imgbb to work?
Ed: OK, trying Google Photos.

Well, ok, at least there's a link in the first post.

[duck_arse]

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[Ben N]

Thanks, Duck. 

[duck_arse]

no worries. why the frowny face?

[Ben N]

Its an embarrassed face -- at my old guy tech ineptitude.

[Ben N]

So, no thoughts? C'mon, help a brother out.

[ElectricDruid]

1) Since the guitar input is actually from the Astrosim line out, can I ditch the buffer and booster stages (IC1D and IC1C) and go straight into the level pots, or do I still need to buffer there, and would a discrete device do (allowing me to use one dual opamp for the mixer stage)? If so, any suggestions?

I'd say no buffer required. Go straight from the Astrosim line out to the line ins or the level controls.

2) What's with all the 2uf coupling caps? I mean, ok, I guess for the program inputs I get it, but for guitar? Can they be reduced?

The design seems to be aiming to keep bass all the way down to <20Hz. Lowest note on even a Bass guitar is 80Hz, so you could definitely go to 1u or even 470n without suffering much except in extreme cases.

3) Can this be safely converted to a single +9-12v supply? Both the Astrosim and the headphone amp run on single supplies, although I suspect that +9v is probably marginal for the TDA2822. Alternatively, any reason to anticipate a problem with supplying just this leg with bipolar juice from a charge pump?

So long as you make sure you've got a decent virtual ground to connect all those ground points to, I'd guess it should work. As you say, 9V looks marginal, so go for 12V supply throughout.

And since you're just using the PPA stage as a mixer, you can probably use any common-or-garden dual op-amp too. TL072, whatever.

HTH,
Tom

[Ben N]

Thanks, Tom. By "a decent virtual ground," you mean 1/2 Vcc, correct? I figure I may have to upgrade a few puny electrolytics to 16v, but a +12v supply throughout seems like a good approach.

[ElectricDruid]

Thanks, Tom. By "a decent virtual ground," you mean 1/2 Vcc, correct?

Yes, exactly. With the typical electrolytic cap from the 1/2 Vcc to ground.

I figure I may have to upgrade a few puny electrolytics to 16v, but a +12v supply throughout seems like a good approach.

16V?! What are these dizzy heights you're scaling?! Splashing out on expensive caps then?!

T.

[Kevin Mitchell]

Doesn't the Astrosim already have a headphone amp on the board?

-KM

[ElectricDruid]

Doesn't the Astrosim already have a headphone amp on the board?

Yes, it does, but it's mono-only. OP is looking to feed in a stereo line signal as well and have a stereo output, I think. That's a bit above and beyond what the Astrosim does on its own.

[Ben N]

Correct. Plus, the headphone amp chip has enough power (I think 1w/ch) to hook up a couple of small speakers should I choose to do so. I thought about just adding a pair of 5532s to handle mixing and the stereo headphone output, but then one of these TDA2822M boards from Parts Express came my way, so I figured to try and cobble the whole thing together. If I wasn't lazy, I'd have done my own layout for the mixer and headphone amp, since the PA board wastes a fair amount of space, but WTH, I like to use what I have.

[Ben N]

Here's what I came up with for a mixer with panpot. Any comments? The values are pretty much guesswork/hodgpodge-cut&paste, so I'd welcome any input. BTW, "9v" is a mistake--I still intend to run the whole mess on +12v (if not higher).

[ElectricDruid]

The "classic" values for the panning circuit with a 10K Lin pot like you have are:

R1, R2, R3, R4: 15K
R7, R8: 51K

The gain from the 51K compensates for the loss in the panning circuit itself, and gives unity overall. You'd have to adjust R5 and R6 to taste. It'll depend on exactly how loud your stereo Line In signal needs to be, and how loud it is relative to the cab sim.

You may find that with there being two signals and with the panned signal being the output from the Cab Sim ( and therefore potentially a bit louder itself) that you need less than unity to maintain headroom, and the 51K could come down a bit.

For the theory, check RG's paper on the topic:

http://www.geofex.com/Article_Folders/panner.pdf

[Ben N]

Wow, thanks! I'd completely forgotten that RG had put that out. Much better to actually have some understanding. OK, time to get the breadboard out.

[PRR]

> Here's what I came up with

You fumbled the DC conditions. Try this.

[Ben N]

Paul, for the line from the Astrosim I was relying on the 100n on its line out. Do I need more capacitance on that line, and if so, wouldn't it be best to put it before the pan pot circuit?

[PRR]

> from the Astrosim I was relying on the 100n on its line out.

If permanently connected, that's fine.

If jack connected, someday you may connect something else.