Subwoofer and PC audio questions - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: January 24, 2024, 08:48:05 am
content: I bought and fixed/modded a receiver, mainly for analog 5.1ch output from a PC, from a good soundcard. And I ordered some better speakers.But now for the subwoofer, the receiver only has a pre-amp for the SW. My other system is a 5.1CH Logitech x-540. So I was going to use just the SW of it.However, as the Logitech is built right now (I rebuilt it ~2yrs ago), the only bass signals that make it to the SW, are from the FrontL/R channels. None at all seems to make it from the Center/SW input. IDK if I missed a circuit or connection, but I never noticed any problem with it. In games and music, I could turn the bass up too high on the Logitech remote, and it all seemed like I remembered from when it was the original circuit/PCB. I made an almost complete schematic, but maybe I did miss something.Right now looking at the Cen/SW output from my soundcard, if I do a test of 5.1CH in the soundcard's control panel, I had 140mVrms max from Center, and 607mVrms max on SW, thats open circuit voltages. Playing a 100Hz tone from youtube is no good, it's probably only stereo, and I don't get a thing on the scope from the SW channel.And looking at the op-amp section of this AVR-2802 receiver, the one for the SW, has no low pass filtering at all, 10kHz passes just as well from it as it does any other channel. And actually at any frequency, the SW output from the op-amp was ~1.8x bigger than the other channels. IDK if that's typical, but I'm sure I'll need to reduce it (the Logitech doesn't need a pre-amp).And so far I don't see any circuits passing LF from the other channels, to the SW op-amp. Also, the op-amp circuit's on this AVR, are way simpler than the cheap Logitech. At least it has a decent LP response from it's SW circuit. But again, the only place I see LF getting to the SW circuit of the Logitech, is from the FL/R op-amp circuits, and not from the SW channel input, yet it sounds normal.Anyone here know for sure how SW's and PC audio works ? I've lately heard of crossovers, and that's related to getting LF's to pass to the SW. Maybe I have to make some extra circuits for that, and add to the receiver ?

author: MathWizard
author_rank: Super Contributor
timestamp: January 24, 2024, 08:14:07 pm
content: I just read else where about my logitech X-540, and indeed the only bass signal that get used, comes from the FL/R channels, and none from the SW input itself. According to the schematic I made, the Cen/SW are mixed with resistors, right after the jack, that signal goes to 1 volume controller, and then just to the center op-amp section. And that circuit and/or the center speaker, can barely play anything under 200Hz.However, the FL/R inputs also go into that chip, and then both L/R outputs are buffered, then a small portion of each is summed and fed to an amplifier, then off to the 2nd volume controller, that handles this bass signal, and SurrL/R.So IDK if some of the bass signal is added to FL/R in that volume IC, it can also simulate surround sound from stereo sources, the chips have no datasheet, and no one knows about them.But I will try sending the pre-amp FL/R and SW signals from the AVR, to the logitech, and see if that works. Aside from the AVR having a little bit more power, and discrete output transistors, it's almost getting pointless.

author: madires
author_rank: Super Contributor
timestamp: January 24, 2024, 08:47:01 pm
content: Disable the Logitech's matrix mode (pseudo 5.1 from stereo input) and connect the receiver's linelevel subwoofer output to the Logitech's subwoofer input. When you feed true 5.1 from the PC to the receiver set the receiver to true 5.1 mode. Most 5.1 systems have two input modes, i.e. true 5.1 and stereo. For stereo input there's usually an option for pseudo 5.1 audio output (some magic sound processing to generate 5.1 from stereo). And there might be also support for the old analog encoded Dolby Surround (stereo signal).

author: MathWizard
author_rank: Super Contributor
timestamp: January 26, 2024, 11:56:42 pm
content: Well I'm using the 5.1CH analog input's, I'm not sure all the digital stuff works, for now I don't have an optical cable either.For now though, on the AVR, I used the op-amp circuit of the phono input, altered it to a buffer, and copied the FL/R over to the CD L/R inputs, to reuse those connectors. So I had to change some resistors, and cut a few traces. So that passes to the FL/R input over to the logitech, at the correct strength. I can see it working.Then I added a 1 stage high-pass filter to the op-amp circuits of the AVR FL/R channels.For now, the SW output from the computer, goes nowhere. With the logitech powered up, I'm not getting any bass I can hear tho, besides some 60Hz hum.With a sig.gen, when I applied a strong enough signal to the FL/R input of the logitech, I could get bass out of the SW.I also applied a bass signal directly to the SW input of one volume controller, on the Logitech, and it only took 20-30mV to be too loud out of the SW.So maybe it is working now, and I just have to turn it up higher, I just hope nothing blows up.

author: madires
author_rank: Super Contributor
timestamp: January 27, 2024, 03:17:44 pm
content: Yep, in your case it's probably better to use the analog inputs. Digital (S/PDIF) is stereo PCM, optionally compressed 5.1 (DDL or DTS).

author: MathWizard
author_rank: Super Contributor
timestamp: January 29, 2024, 04:45:46 pm
content: OK a correction about the logitech x-540 for anyone else working on 1, then Center and SW are summed across resistors, then fed to a volume IC. But then that output, and FL/R and SL/R channels are summed across resistors, then fed to the U5B op-amp, then passed on to the other volume IC. Then it goes to the main bass op-amp and power amp.So I never did re-build the bass section right, but I guess turning it up made me not notice, or I forgot about it.I'm still using some of the original PCB, so I had to un-solder it and go over the traces around the analog switch IC's.If anyone really wants the schematic or a section of it, PM me.So now I'll wire in 3 more resistors, and I should have all the signal that should be summed before the active LP filter/ bass op-amp. And then I'll have to buffer the SL/R over to the logitech aswell. Or just get splitter cables, it's not like I need full power.