connecting input directly to ucd, no input buffer

[mazurek]

If instructions for this exist, please point me to them, the only information I have seen has been regarding removing decoupling caps and changing the op amp power supply.

I am gearing up for fully active loudspeakers. Shortly, I will be posting crossover pcb, +/-12 v power supply pcb, project writeup, etc.

The next step will be to put the crossover circuit board and ucd amp in one box. I have a spot for adjustable gain on the crossover, so I can jack it up to 5, and hopefully connect the output directly to the UCD, bypassing the op amps. The OPA2134 in my board are low dc offset, I don't believe they will be a problem except for maybe the bass due to an LT circuit potentially amlifying any interstage DC emitted (in that case I'll use decoupling caps).

Any directions on connecting the circuit boards directly, bypassing the op amps. Without any further instruction, I may just try connecting the signal ground to amp ground and soldering input directly to the existing opamp output. I anticipate dealing correctly with the grounds will be a problem due to my unbalanced inputs.

I will report back in two to three weeks if everything goes well.

Lee

[classd4sure]

Hi Lee,

If it helps, the AC coupling caps are _after_ the op amp buffers. So if you remove them you can solder your inputs directly to the comparator, or output side, of the coupling cap via, and you're good to go.

Use the signal ground to connect your supply to, not the power/amp ground.

Trace the inputs back to the op amp and you'll then know which coupling cap was acting as inverting and non inverting, then you should know all you need to make a go of it.

[mazurek]

So I can just remove the ac coupling caps and connect the input to the non-inverting cap, and the ground to the the inverting cap, as shown in the picture? I located the caps, and circled their output pins.

[classd4sure]

Yeap, I'm zombie brain at this hour but it looks right

You can also try being super slick and tweaky (....ahah..... sigh) and trace those pins over to the header pins of the daughter card, solder them direct to that, all for the sake of saving ~1cm of signal length and totally avoiding those wimpy lil traces.

I'd make a real effort to get them soldered fast though, and heatsink the header pins you're soldering to on the other side of the board (probably don't want too much heat getting into the daughter card!).

Whatever you feel most comfortable doing, but I think that will work.

Disclaimer time.... I haven't tried any of this myself

Let me know how it turns out.

[mazurek]

Thanks, sounds like a bit of a warning about the daughter card. I think I'll be safer soldering to the main pcb. The other option sounds pretty extreme, and I'm out of money.

I should get around to doing this early next week, my girlfriend is going on vacation with her family, and I will be playing electronics a lot. I've got circuit boards lined up for the day she leaves, and they have an adjustable gain so I'll be able to mod one channel only and compare. Though I hesitate to make any sweeping comments other than works/not works, good/bad, I'll see if I can get it done in time for the columbus diy meet so more experienced people can give their equally subjective but more reputable opinion.

[classd4sure]

Yeah it's a little bit of a warning, they never hurt. Proceed with caution and all. I don't think you'd want enough heat getting into the daughter card to reflow anything, or even create a poor joint with the header pins going into it. You can visually inspect that and touch up if need be though. Some forceps clamped to the pin on the top side of the board would make an excellent heatsink while you solder it.

Probably the real advantage of doing it this way, far more so than avoiding that little bit of trace length, is that you can keep your wires tightly twisted right up to where you're soldering. If you do it the other way, at the cap solder pads, you're going to have a nice little loop of greatly increased area, be a great way to pick up noise.

If you trace it out, you'll see the pins you need are the top 2. I think that's how I'll do it when I try it.. eventually.

[mazurek]

I see you've given this some thought, I suppose I'll try the daughter board. It will be my payment for free advise, which I will inevitably nag for again in the future.

You think radiated noise is more important as the connection is closer to the switching components? If you think so I may go get some proper shielded microphone cable, I was just going to try twisted pair for experimentation and sub in better stuff if necessary.

At least if my goal is to remove the op amp, and I break it, I can buy one of the cheaper versions with no worries about op amp flavor.

[classd4sure]

Hm, I think experimenting with normal twisted pair first would be the best idea, why complicate it if you don't need to?

The HF carriere wave will appear at these inputs since you're skipping the buffer stage, that's likely to be more of a concern than proximaty to the switching node.

What good a shield would be since you're driving it single ended I'm uncertain of, probably twisting will do the job nicely.

Another thing you could try would be a ferrite choke or clamp on the twisted pair, close to the module, I'd probably opt for that before trying to add a shield.

If you do decide to try it with a shield, I think you'll also conveniantly find the signal ground maybe just one pin down

Pop the coupling caps off, like you were going to do anyway, and you should be able to leave the op amps in place for all your testing, can't break em that way! It might be a good idea to tie both their inputs to ground though (at the usual connector) to ensure they don't their outputs flying all over the place.

[mazurek]

I'm going to try this tonight, or tomorrow night. Does the ucd need to be connected to a low impedence output.

i.e, my crossover has a 200 ohm output resistor. Is it better to remove this or leave it in? There is approx 1.5 feet of cabling between crossover and amp boxes.

[lucpes]

Originally posted by mazurek
I'm going to try this tonight, or tomorrow night. Does the ucd need to be connected to a low impedence output.

i.e, my crossover has a 200 ohm output resistor. Is it better to remove this or leave it in? There is approx 1.5 feet of cabling between crossover and amp boxes.

I'd replace the 200 ohm with a lower value, say 22R or 47R.

The direct input impedance on the + input is 8.2k afaik, so it's a safe bet to have an as low as possible impedance on the source side.