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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
WSG Filter troubleshooting...
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Thu May 31, 2012 8:26 pm    Post subject: WSG Filter troubleshooting... Reply with quote  Mark this post and the followings unread

I'm in some down time waiting for parts on various things (how quickly I've managed to get too many irons into this fire Smile ), so I thought I would work on a WSG for my boys to play with. I found a singleton Magnavox bookshelf type speaker at a thrift store that is shaped more like a spacecraft than a speaker, and I think I can get everything to fit inside it and be awesome; it has the bonus of the speaker being right there (I may need to put in a small amp, and I plan to make sure I have the volume control mod...my wife may insist on a remote control power switch too Laughing).

Of course one of the parts challenges is that I'm flat out of 1M linear pots (probably going to try and fix that at RS tomorrow and see if that helps). So for building the filter I used an audio taper instead. I've also only built Voice A and the filter, again, because of shortage of the appropriate pot sizes. I have a ton of 100Ks, go figure.

I have two things that I don't get.

First, if I have R10 (1K) in place going to ground at the point between the mixing and the input of the filter, I get NOTHING out. If I pull it out, I get the expected output over at the downside of C6. I'm *guessing* that it's supposed to operate with C1 as an input filter, but not sure why, and baffled as to why it would just cut me out completely.

Second, my filter does not sound at all like anything I've heard that other folks have posted about it. I get the tiniest bit of high end cut if I crank it all the way up. I thought perhaps my audio/log taper was causing me trouble and might be backwards, but if I flip that pot around I get the same results. Altering the resonance does zip. I've verified and reverified the wiring repeatedly, tested the values of the various components, etc. The 741 is clearly working aside from that.

Like I said, I'm going to try to get to the shack tomorrow and get a couple more 1M linear pots, and maybe that will fix it, who knows. But in the mean time if anyone could shed light on the theory of operation here that would help me understand what I might be doing wrong, either with that 1K or with the filter, that would be great. (And yah, I'm going to go poke google for info on active filters in general too, but advice specific to THIS filter would be excellent).

Thanks...
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Fri Jun 01, 2012 7:20 am    Post subject: Reply with quote  Mark this post and the followings unread

So having done some reading I think I have a better grasp of the filter, but I'm still not sure what I could be doing wrong.

So it's a single-supply application of the op amp, so we have a virtual ground created with R16, R17, and C8. The feedback loop contains the 10M R4 and in parallel to that, we have two 470pF caps with a resistor and pot between them, and one side of that resistance goes to the virtual ground through the coarse and fine pots and a 1K limiting resistor.

The resistor and the pot act in parallel, so that setting the pot at a low value cuts the total resistance between the caps, and setting at a high value approaches the 470K, give or take tolerances.

This topology doesn't match any of the filters I've found, although it somewhat resembles a typical low-pass application (which has a single cap instead of the cap/resistor/cap branch).

But that's as far as I've been able to get. Any pointers to similar circuits with explanations/equations would be awesome.
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Fri Jun 01, 2012 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's a slow Friday....

I was able to identify the key problem with the filter; I had the coarse/fine pots going to pin 2 rather than pin3 of the op amp, so they weren't going to the virtual ground. Corrected that, and I get much better response to the filter. Go figure...

I still need to try putting that 1K back and seeing if that was somehow related, and I want go through the oscillators some more to see why it seems that when I increase the amount of zaniness it changes the pitch of the main oscillator (even when the period is very slow, during times when I'd expect Q1 to be "off" and the main oscillator to be operating the same as when there's no zaniness at all....).
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Fri Jun 08, 2012 9:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

SO I was able to figure out my filter problem--it's actually the amp I'm trying to use, it's an antique Realistic SA-150 (cheap, cheezy, lying around, gave me enough sound to hear things but not so much as to wake up the 9 year old sleeping in the room above me...). I don't know exactly what it's doing that's causing a problem; I get crap levels from the WSG using it, but I can plug into my guitar amp under my desk and it's fine--just hard to keep at a low but audible volume. That, and putting back the 1K resistor did the trick and the filter sounds great.

Now I'm on to other questions Smile

I have (what I think is) a pretty cool enclosure idea, see the photo below. That's the case from a Phillips Magnavox surround speaker of some sort, I was able to pick up at a thrift for cheap. The switches are from American Science & Surplus if anyone wants some like them, unfortunately they're not lit.

As you can see, it has an integrated speaker (6 Ohms). The output from the WSG is not sufficient to drive a speaker. I want something very simple I can add to drive it reasonably, and I'll hook it up so that if I plug into the jack the speaker is disconnected, easy enough with an NC jack. The idea being that it will bypass both the speaker and the mini-amp.

Teh Googles tell me to use an LM386, but I don't have one. I've spent the evening trying out various transistor amp circuits, but while I'm getting almost enough volume, they sound like crap. I was incautious on one of them and burned my thumb--not TOO badly--after running way too much current through a 50 Ohm resistor...live and learn Smile.

I do plan to check for the LM386 at my local Radio Shack...one LM386 circuit that seems well thought of is the Ruby amp (http://www.runoffgroove.com/ruby.html)...I really only want volume to the speaker, not a lot of control over gain (plan to set it so it doesn't clip)...does it seem reasonable to use a 1K trimmer instead of a pot, set it and forget it?

Does anyone have any other alternatives that would actually drive a speaker nicely? Or am I doomed to have to wait until I can lay hands on an LM386?

Thanks....


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Uncle Krunkus
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Joined: Jul 11, 2005
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Location: Sydney, Australia
Audio files: 52
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PostPosted: Sat Jun 09, 2012 5:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Beautiful enclosure! Cool
The LM386 ruby type amp is the only hand rolled one I've tried. Mainly just for situations that need to be portable (9V battery stuff) Any other time, I'd go through the mixer or the Fender guitar amp.
There are quite a few power-amp-in-a-chip options out there though. As they get bigger, they tend to become SIL chips, up to Power Amp Modules which are a slab of plastic half a cigarette pack size with a big heatsink on one side and connections along one edge. Check out the TA7xx chips, they are around the 5-20W size.
Yeah, chuck a trimmer on it, set and forget. I think you can get a very clean (ie no "gain" type overdrive distortion) signal by removing the pot across the 386 completely. Or do you short those pins? It's one or the other.
Also, for the signal coming from the WSG, you wouldn't need the front end of the ruby amp. The section with the FET. Just connect direct to the top of the Volume pot and take the wiper to the 386 as per normal. The FET pre-stage does work well with guitar level inputs though, so if you want that option, leave it in place, still bring in Synth levels straight to the top of the Vol though.

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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Sat Jun 09, 2012 9:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the complement and the information, Uncle K...

Went ahead and picked up a pair of 386's at Radio Shark, hooked it up, and that does the job great.

I do seem to have a problem with loading/grounds somewhere in my breadboard circuit; I'm assuming it's been moved around enough that something is touching that shouldn't / wasn't before, or something is loose in the breadboard. My 5 year old and I were finding that we could change the behavior of the circuit by running our fingers around the edge of the breadboard plate and by bapping the fine tune pot so that our fingers briefly shorted either side to the wiper. Also touching some of the knobs would change sound/behavior too. It's a 9V battery, so i wasn't worried about dangerous currents, and he thought it was hilarious.

I mention it because i also had loading effects when I'd increase the volume of the amp and it would change the frequency. The output is as shown on the circuit diagram, a 1K resistor and a 1uF cap, but I'm wondering if I ought to consider increasing that 1K or increasing the pot I'm using for volume.

I'm planning to wire things up sort of modularly; the voices on one protoboard and the filter and now amp (assuming I can get my honking huge 220uF caps to fit) on another. I did finally get 2/3/4 pin MTA-100 headers and plugs so I can do that pretty easily, I think.
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Sat Jun 23, 2012 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

The 386 worked great, at least on the breadboard.

Newbie lesson for the day: if you are inside an enclosure (as opposed to behind a panel), make your wires 2 to 3 times longer than you thought you needed. Very Happy

I can get it connected and closed, but I cannot troubleshoot. I will probably unscrew sections of the controls to do the ts on the bench, but we'll see....
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