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Rewiring 7-band EQ

whiterabbit

Sep 14, 2013
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I can try the dual 6v batteries I used initially to test the EQ initially. Hopefully that will be enough to run this second test; if not, I'll just add one stage of inductors and capacitors and see if that helps.

I ordered 8 of each of the three parts you recommended.

I also recently ordered several kits: capacitors, inductors, and transistors. Nothing fancy, just wanted to have a stock supply on hand for immediate testing purposes for my upcoming projects.

I've been looking into high and low pass filters for a separate project... that's basically what the capacitors and inductors are used for, right? The plans I've seen for passive low pass filters often have inductors in them. And the high pass have been capacitors and resistors alone. Am I on the right path with this line of thought?
 

KrisBlueNZ

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Right, two 6V lantern batteries in series is perfect for testing.

Filters on audio signals are usually implemented using op-amps like the dual op-amp in the buffer circuit, with capacitors only. Inductors have real-world issues that can be problematic - they are susceptible to magnetic fields, they can be physically large, expensive, inaccurate and/or unreliable, and real-world inductors are never perfect, the way resistors almost are, and good quality capacitors almost are.

Look at the schematic diagram of your seven-band EQ and you'll see that it uses only capacitors and resistors. Google op-amp filter and you'll find enough material to keep you busy for years!

The inductors in the power supply path are a bit different; they are not used in a signal processing function and they cannot be replaced with op-amps.
 

whiterabbit

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I will definitely be spending a couple hours looking into op-amp filters soon! I was experimenting tonight with some high/low pass rc filters and really wasn't getting anywhere. One of the things I am hoping to make on the other thread I started about FX boxes was a 3+ band EQ, so that will surely come in handy.

I will see about running that test tomorrow after work. I may have a guest coming over to join me in a little [home] studio time so the evening might go to that. And if not music, then back to these electronic projects! :D
 

whiterabbit

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Oh! Almost forgot. This weekend I noticed that the hum was influenced by my proximity to the control panel with all the pots. Dunno if this is possibly related to the radio components within it, or the magnetic field being generated, or what.... but veeeeery noticeable.
 

KrisBlueNZ

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The hum was just affected by your body's proximity? May be hum picked up by your body and capacitively coupled into a signal that isn't properly screened/shielded... It's not magnetic pick-up because your body is not magnetic (unless you have false limbs :)

The next time you notice it, try moving your hand near some earthed metal (e.g. the chassis of the receiver) and just moving your finger slightly to make contact with it. If the hum goes away, or reduces, when your finger makes contact with the chassis, that is what's happening and it's probably caused by a signal that has not been shielded properly.

What instrument and what kind of music do you play? I'm a musician but not active at the moment; my main instrument is bass guitar.
 

whiterabbit

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Yes it seemed like within 1.5 feet of the control panel the closer I got the stronger the ambient hum would get- until I actually touched the chassis somewhere [like you said]. I just chalked it up to the electromagnetic field I naturally generate from working in I.T. so long... (this field is the reason why when you go to show IT your broken computer/printer/phone it magically starts working again until they leave...ha).

I should be able to pick up some newer shielded RCA cables at the very least, which are currently the connection type between all of the major component panels (including the EQ). I'll check into that after I run the DC test this evening and see what's available. The new body for the record player we've built is mostly wood with some plastic and metal accent pieces- do you expect that installing the components back into a frame will help shield it at all? Maybe I don't understand how shielding works, and I'll look into that as well today on my break if I get a minute.

-----

With regard to music- I actually mostly do electronic music production. I've also recently begun learning the craft of the DJ. But I grew up playing viola in school until high school, and then I switched over to guitar and started singing, and took a piano class in college. After graduation is when I started getting into music production seriously. I still muck around on the guitar and my keyboard every now and again but my strength lies more in digital production. Do you have any of your music up anywhere?
 

whiterabbit

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Alright, I'm scratchin my head on this one.... Tonight the sound was perfectly fine... both with DC and the AC>DC source. There was still some veeery mild hum when the input pot is set to AUX and the volume is turned up really high... so I guess I may as well put the filters on the power source. Can't hurt, right?

I'm thinking maybe one of the RCA cables had a loose connection. I noticed when I moved the control panel around (thereby moving the connecting wires) it had an effect on the overall volume of the song that was playing.
 

KrisBlueNZ

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LOL :)

If the existing cables are shielded, I doubt that using newer ones will help, but you can try.

Another possibility is that the chassis of the record player is NOT earthed. Try the same experiment, but instead of touching the chassis of the record player, touch something else, which you know is earthed. If that reduces the hum too, then the record player is earthed and everything is OK. If that makes the hum louder, then there's a problem with the earth connection to the chassis of the record player, which you should fix.

I've also done computer-based music production in the past, using Sonar. Before that, I had a Fostex G16. I've recorded tracks for several bands that I've been in. I haven't done anything in the last few years though.

Most of the music I have recorded was written by others and I don't have permission to post it, but there is one original that I wrote and recorded a long time ago that I have uploaded to my YouTube channel:

Hmm. That's odd that the noise disappeared. Is it possible that it's frame buzz from a TV transmitter that's not transmitting at the moment?

Right, it can't hurt to put the filters in there.
 

philgritsr2

Oct 21, 2013
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wow Kriss

watched you tube vid very impressed loved the harmony would have liked more, that has to be difficult to do:)
 

KrisBlueNZ

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Thanks Phil :)

No, the harmony vocals weren't hard to do. If I had done them properly - that is, all starting and ending at the same time - they would have been more difficult :)
 

whiterabbit

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Bravo sir! Wasn't sure what to expect when I clicked the link... but you are definitely far more talented than I, heh. Even with my **** ear buds destroying the low end it was still quite enjoyable :p

If curious, my shtuffs are posted here: https://soundcloud.com/whiterabbitmusic with this [ https://soundcloud.com/whiterabbitmusic/throughburst/s-5d5oH ] privately shared demo being the most recent thing I've posted. Keep in mind though that I've done quite a few hours of work since posting it cleaning it up and working on the mixdown and mastering aspects and it sounds much better. //end preface rant

I still have to check out that magnavox site that Phil has kindly linked to see if I can find a proper schematic for the beast we've been working on.

Currently, while I've been testing everything with the EQ and buffer circuit the entirety of the internal components are just laid out on a board in my living room up off the floor supported by a few glass blocks. So there is no grounding to speak of. Hopefully this weekend I'll be able to wrangle my friends back over for one final push and get the chassis ready to install everything into. Once it IS back in the chassis, how should I go about grounding it? Just run a wire from the panel(s) to a resistor somewhere on the wood body? Also, with the buffer circuit, since I'm installing that into the EQ body, should I just run a wire from 0v to the metal case of that? Or separately out of that and into the wood chassis of the player?

When I had that loud hum last weekend I was testing the system by plugging my phone into the record-in RCA jacks and had it laying within 2ft of the rest of the panels- maybe that was causing the hum? Similarly to how some speakers click/pop when a text message is received nearby? (Is that what shielding counteracts?)

Lots of questions, heh :D
 

whiterabbit

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Just using the breadboard to get the layout right before I put it on perfboard and solder that all together (so just ignore the conductivity of the breadboard).

Leads to wall wart on top through the inductors, outputs to EQ and buffer at bottom after the cap, ground left, pos right. Correct?

If this circuit needs to be cascaded, should it be doubled in its entirety and then put in series? I don't think it will be, I'm just curious.
 

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KrisBlueNZ

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Yes, that looks good. You could duplicate the two capacitors at the other end too. This makes it similar to a "pi" filter (see https://en.wikipedia.org/wiki/Pi_filter).

Then if you need to add another stage, add two more inductors and a pair of capacitors on either end.

Edit: in reply to your earlier questions:

Re grounding. All the metal chassis (chassis is the plural of chassis) should be grounded. The connections between the different modules will connect the metalwork together; you just need to make sure that one of them is connected to the ground pin of the mains plug. Normally this is just done by having the earth wire from the mains cable connected with a screw into one of the chassis somewhere near where it enters.

If there's a large wooden case, it will have some conductivity and should also be grounded; normally just having all of the metal chassis attached to it may be enough. Sometimes a sheet of thick aluminium foil is glued to the large areas of the woodwork and connected to a chassis, for extra shielding.

Earthing for the buffer board and the graphic equaliser should probably be a single wire starting from the chassis of the unit with the controls on it, to the ground of the buffer board, and on to the ground and case of the graphic equaliser. Make sure the case doesn't make direct contact with any chassis, otherwise you'll get an earth loop. You can run the +12V supply (after filtering) alongside it using separate wires or "tru-rip" (aka "figure 8") flex.

If the phone you were testing with was a cellphone, and it wasn't plugged into the charger, then it would not have caused the hum. GSM cellphones can cause the "galloping horses" sound - "dig-a-dig... dig-a-dig...") when they transmit, but not mains hum. Yes, shielding should help somewhat with the galloping horses interference. Iron is better than aluminium for shielding, but expensive and heavy.
 
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whiterabbit

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Thanks for the link! I actually felt like I understood at least part of that this time :D

I did end up doubling up the pair of caps so the inductors are in the middle. When tested, it did an amazing job!! Reduced about 95% of the hum when the rest of the equipment was set to default on a comfortable volume. When the EQ was turned up to full on all bands it was again quite noticeable- but I don't think I'll ever need to boost the volume that way. I may as well add another filter in there while I'm at it just to be on the safe side, while I'm working on it now.

You've actually brought up something that I didn't consider earlier- this unit is so old that the power cord doesn't actually have the 3rd grounding pin on it. That being the case, it seems I ought to replace it. It probably wouldn't have hurt to do that anyways since the old plug is very flimsy. From what you've said it sounds like in order to do this I'd need to buy a new cord with the 3rd grounding rail, swap it out with the existing cord, and then simply attach the wire of the 3rd pin to the metal chassis of the panel that the power cord connects to? That sounds simple enough. Then separately ground the EQ and buffer board with a wire from the control panel chassis to the ground of the buffer board, and then on to the ground AND case/chassis of the EQ- right?

The EQ will be set into the wooden body alongside the actual record player itself, so the only thing it will touch is wood- we should be good there.

Yesterday we got a coat of polyurethane onto the body for protection and finish, and I think today we will be able to put the lid on and maybe even start installing some of the electrical components and figuring out the lighting!
 

KrisBlueNZ

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Good!

Yes you should replace the cable. Connect the earth wire to the chassis using a screw through a hole into a nut, or a self-tapping screw if the chassis is strong, that is not used for any other purpose, with a solder tag for the wire and at least one lock-washer. The cable must enter through a rubber grommet or similar and must be clamped nearby. Make the earth wire long and put the screw near the entry or the clamp, so that if the wire is pulled out, the earth connection breaks last.

Earthing to the buffer board and the graphic equaliser can be done with a pair of wires carrying earth and +12V. I suggest you bring a pair of wires from the power supply, through the pi filter, and into the chassis with the controls. Earth the earth wire to the chassis somewhere near the volume control. Then continue the wire pair on to the buffer board, then on to the graphic equaliser.
 

whiterabbit

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Okay I can certainly replace the original power cable.

Also- do you think it would work to strip the female end off of an old computer power cable to use here? I could also just go by an extension cord somewhere and strip the end off of that.

Just want to make sure I'm on the same page as you with regard to the grounding/earthing and everything else.

There will be the main power cord going to the original components of the player (which is the one I'm replacing). I will connect the 3rd pin to the body/chassis of the metal panel with the tubes on it (or rather, the larger of the two panels with tubes) that the power cord connects to. From there, that ground/earth connection will be shared with the remainder of the original components through the existing wires.

Separately, there is the power cord that comes from the wall wart which then runs through the pi filter and splits off to power the buffer circuit and the EQ.

When you say:
I suggest you bring a pair of wires from the power supply, through the pi filter, and into the chassis with the controls.
Do you mean add an additional branch of wires coming off after the pi filter? With - attaching to the volume control panel?

Or do you mean have one set of wires coming off the pi filter that go past the volume control panel and have - soldered onto it that then continue to the EQ/buffer and then branch out to power both of those components?
 

KrisBlueNZ

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Right. I guess the power cord currently comes into the power amp chassis. That's where you should connect the earth wire, and the shields of all the signal cables will propagate the earth to the chassis of the other modules.

I guess the ideal connection for the +12V and 0V wires from the wall wart would be:

+12V and 0V from the wall wart go into the pi filter.
+12V and 0V outputs from the pi filter go into the chassis with the controls on it; the 0V wire connects to the chassis somewhere near the volume potentiometer.
The two wires continue past the potentiometer and go to the buffer board, then daisy-chained off that, to the graphic equaliser. So, your second option.
 

whiterabbit

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Okay, great! Thank you for clarifying that for me. I can probably get that done tonight after work if I don't get distracted with any of my various other projects :D

And yeah, the mains power connects to the amp module chassis.

Do you think an old computer power cable do the trick for a replacement? I have a few old ones laying around that I don't mind sacrificing to this project.
 

KrisBlueNZ

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Yes that should be fine, as long as the cable is long enough.
 
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