DIY Pre-Amp - Subwoofer summing circuit. Comments requested

I'm designing an op-amp based input-selector/preamp with subwoofer output(s). The input switching side is a set of relays feeding a remote-control volume pot directly and that in turn feeds both a preamp stage (classic simple op-amp design) in parallel with the subwoofer summing/buffer circuit below.

The preamp buffer stage has 4k7/2k2 feedback resistors, giving a gain of 3.14 (1+4.7/2.2). The circuit below (my understanding) has an inverting/summing stage of unity gain, followed by another inverting buffer with a gain of 4.7/1.5 = 3.13, matching the preamp close enough. I also included a larger feedback bypass cap, C10, on the second stage to give a roll-off point of 300'ish Hz. I may omit the first stage bypass cap, C9, as I'm not sure it's needed - but will keep the PCB space in case.

So specific questions I have are as follows:

1) Does this design seem OK in general? :)
2) Does the large feedback cap (C10) do what I think it will do and/or could it be a stability (or other) problem?
3) Do my gain calculations make sense? And should this make the sub volume track the main speaker volume as I hope?
4) The output of this is paralleled directly to two RCAs, in case I move to dual subwoofers in future. Is that OK or do I need to put the split point further downstream and double any or all of R20, C11/R22?
5) I was going to use an NE5534 OpAmp for this, but have read it's not unity stable so was worried about using it in the first stage (hence the TL072 marked on the diagram). Is that a correct assumption?

Any and all answers and/or constructive criticism welcomed! Thanks!
 

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1/ Functionally it is fine.

2/ The 0.1uF will do as you think. Absolutely no stability issues what so ever with that circuit.

3/ Your gain calculations are correct although of course the gain will depend on the frequency tested at. The input stage has a gain of -1 and the following a gain of 3.13

4/ In practice there should be no issue paralleling the output to another socket although adding another R20/C11/R22 network would be a technically better solution.

5/ The 5534 will work at unity gain but needs external tweaking, the 5532 (dual opamp) is unity gain stable. The TL072 is actually a first rate device, so don't be told otherwise ;)
 
That 1.3 k-ohm load that the 10 k ohm feedback resistor of the first stage along with the 1.5 k ohm input resistor of the 2nd stage is putting on the output of the 1st stage is too much, the TL07x op amps are not able to cleanly drive loads under 2 k-ohms or thereabouts. You should either scale up the impedance level of the 2nd stage, or switch to an op amp the can drive heavier loads like a NE5532 or the modern TI offerings like the LME49720 that are rated for 600 ohm loads.
 
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THANK YOU guys for the inputs! It's quite a relief to know I'm heading down the right path...



I guess I'll switch back to 5532 for this then. I was under the misunderstanding that the 5534 was the dual pack of the 5532, but now I realise it's the other way 'round (Doh!).
 
A bit of a thread revival - but I have the same issue with my setup. I've got myself a small active sub, 50L (1.75 ft3) with a 12" peerless long throw woofer, Q designed to be 0.7. I use a set of active monitors, they already have high pass filters built-in. So a subwoofer summer and buffer will do the job nicely for this system.

Any changes to your original circuit? Did you use a NE5534?
 
Yes, I did make a couple of component changes... I ended up using the LME49720 op.amp (as recommended by Michael) and changed some resistor values. Attached is the current (!) circuit and a pic of the PCB layout & built device. I added a ground loop track around the summed signal input, too (FWIW).
 

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I could share the Gerbers, but it doesn't seem to make much sense, since you only want the subwoofer part, right?



I'm not really sure you'd want to do everything I did, as this was a full active/passive preamp, with multiple custom (my) PCBs, incl. Transformer/Frontpanel/Pre-amp. Added to that was the ZeroZone Power-02 dual rail audio PSU bought as a kit off fleabay - though I replaced most/all of the component parts with better ones. I also bought the 5V power supply kit and remote volume off eBay. The input and active/passive switching is handled by relays via pushbuttons and custom MCU code. Pics will give you a better idea.
 

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