mrdon46
1 post
May 11, 2009
8:11 PM
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A question for those with experience playing through 2 amps--I've tried this and really like it, but I'm getting a bit of hum which I assume is a ground loop issue. The hum increases as the volume controls are turned up, and while not THAT bad (still quieter than the guitarist's amp) it's annoying. I plug both amps into the same power strip; the splitter I'm using isn't truly passive, it's a Jayphat (JFET impedance matcher) I built with 2 output jacks. I'm wondering if anyone has had this problem and can suggest an easy (read cheap) fix.
Don
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jawbone
26 posts
May 11, 2009
8:39 PM
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Hey mrdon - I've had the same thing - I couldn't find any "Y" connectors so I built one with 1/4" female jacks - basically I used a special 20 box and put 3 jacks in it - I find if I grip one of the cord male jacks, the hum goes away so my thinking is if I ground it the hum should stop but I haven't gone any farther than that. When I got ready to try it the setup wouldn't hum to start with, then the guitar player stepped on the box and sort of squished it and I moved on to trying to solve bigger problems - but I'm hoping to learn a bit more here.
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If it ain't got harp - it ain't really blues!!!!
Last Edited by on May 11, 2009 8:40 PM
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isaacullah
242 posts
May 11, 2009
9:08 PM
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hum is sometimes difficult to diagnose becasue it could be due to one of several faults. Ground loop is when elements in the signal chain have different ground reference points relative to eachother. How can this occur? Well, many circuits use resistive divider networks to creat "artificial" ground references. This is especial common for amplifier designs that require postive/negative power supplies. Instead of using a center tapped transformer to do this correctly, they use a regular transformer and create a "fake" center ground by using a resistive divider network to create a "floating" voltage reference point, which is referenced in that circuit as "ground". This central point actually has a positive voltage, but if it is used as 'ground", any voltages less than it are "seen: as negative by the circuit. Unfortunately if you tie this circuit to any other circuit that uses a different reference point for ground (like 0 volts), then you will have a ground loop. The two devices want to see signals that are referenced to their own specific ground point, and the difference between the two grounds is creates a low frequency "electrical" hum. This hum can then get sent to the amplifier and is amplified along with the sounds you want to hear. This can occur if two amps are put in parallel (what you are doing) because the inputs are electrically connected at ground. This could be why you are getting hum. The only way to make sure that you won't get hum when connecting two devices with different ground references is to use an isolation transformer. This transformer has primary windings that are exactly the same as the secondaries. In other words, the voltage into the transformer equals the voltage out. But since the nature of transformers is that secondaries are electrically isolated from the primaries (secondary voltages are induced by the voltage across the primaries), the false grounds never see each other, and a ground loop cannot occur.
Hum can also occur due to short circuits, faulty components, bad soldering, too high gain at one amplification stage, etc. It's kind of hard to diagnose which one of these things it is that is causing it. If an isolation transformer does not work, then it is one of those other causes.
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The magnificent YouTube channel of the internet user known as "isaacullah"
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isaacullah
243 posts
May 11, 2009
9:10 PM
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I forgot to say, that one thing you might think of is to try connecting two identical amps with your current splitter. If you get no hum, then the issue IS a ground loop. If you still get hum, then it might be something else. If you still get hum but it's drastically quieter, then it could still be ground loop because even identical amps may be biased differently, and have slightly different ground points.
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The magnificent YouTube channel of the internet user known as "isaacullah"
Last Edited by on May 11, 2009 9:21 PM
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MJ
38 posts
May 12, 2009
8:13 AM
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WHAT????
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jawbone
27 posts
May 12, 2009
8:47 AM
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Thanx Issac (I think) - I guess it's not as simple as wetting my fingers and putting them in my ears? Darn - maybe I should thank my guitar guy for stepping on my splitter box!!!
Now I know why some guys get paid way more than me.
I remember reading a big squabble on a guitar site about taking the hum out of guitar pick ups - I notice when I touch the strings or the pick up the hum goes away - I was hoping it was a "simple" grounding solution.
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If it ain't got harp - it ain't really blues!!!!
Last Edited by on May 12, 2009 8:52 AM
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mrdon46
2 posts
May 12, 2009
9:28 AM
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Thanks Isaac for your thoughts. Both of the amps I'm connecting have center tapped PTs so the ground reference should be the same. I think the simplest thing is just to make up a simple isolator and try it out--I'm assuming I'll just need one on the signal path to one of the amps in order to decouple them. Mouser has a number of inexpensive options, I'm assuming (I seem to be assuming a lot here) anything with a 1:1 ratio and impedances that fairly closely match the impedance of my amp input will work? You seem to know what you're talking about way more than I, would you have any specific recommendations?
Thanx
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Jim Rumbaugh
39 posts
May 12, 2009
9:41 AM
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This might give you some insight.
Take a regular guitar chord, 1/4" male both ends. Plug it from input of one amp into the input of the other. If you get hum, suspect the ground loop thing. If not , then...... well... I don't know if you dont't. It could of been that your junction box was plastic and not a solid pice of metal. The hum could of entered via the unshielded wire in the box. A solid metal box shields the wires.
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jawbone
28 posts
May 12, 2009
10:21 AM
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Thanx Jim - I never thought of a metal box - I'll give that a try.
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If it ain't got harp - it ain't really blues!!!!
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isaacullah
245 posts
May 12, 2009
12:08 PM
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jawbone: Yes, Jim is right. Hum can also be introduced by rf interference if your box and/or wires are not properly shielded. A layer of aluminum foil on the inside of a plastic or wooden box is all it takes to shield (make sure it is electically connected with the ground of the circuit. Usually, letting it touch the ring of the input or output jack is all that is necessary).
Mrdon46: Here is a link to a well known DIY "hum free" (groundloop lifting) signal splitter circuit. You can't go wrong if you follow it. The circuit as shown in the diagram is active, and requires two 9-volts for a bi-polar powersupply for a signal boosting opamp stage before the split. You can make it passive by removing the opamp stage (and hence the battery powersupply stage), but you will need to make sure your input signal is hot enough to pass through without significant loss. Any preamp or distortion stompbox should be able to get your signal hot enough to pass through the isolation tranny, but I have not built or tried this out, so you might do a bit of googling to verify this.
Cheers,
Isaac
PS. Even though your two amps both have CT trannies, there still may be some ground reference offset. If the positive and negative secondary windings of a CT tranny are not EXACTLY the same length, the CT will not be at 0, and therefore will likely be offset with regards to the ground point of the second amp. Ground loops are a real bitch to track down. In fact, many times you may be getting a ground loop simply because you plugged the amps into different outlets. Outlets further along the houshold electical circuit have long paths to hard ground (usually the buildings metal plumbing pipes), and therefore will have different ground reference (usually only by a little bit) than other outlets in the house. Try using a powerstrip, and plugging them both into it.
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The magnificent YouTube channel of the internet user known as "isaacullah"
Last Edited by on May 12, 2009 12:19 PM
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mrdon46
3 posts
May 12, 2009
2:34 PM
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Isaac, thanks again--the link answered my question, the 42TM108 audio transformer in the schematic was the one I was looking at from Mouser (10K/10K). I'll just add the splitter output stage to my Jayphat, it gives a boost to the signal so it should be plenty hot.
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mrdon46
7 posts
Aug 09, 2009
6:23 PM
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A follow-up note here. I finally got around to dealing with this--I'd gotten one of the Xicon 42TM018 tranformers from Mouser, but hadn't gotten around to installing it (its frequency response isn't that great, 300-3.4K Hz, so I was sort of keeping my eye open for a better option). I recently ordered an OT from Edcor as I've heard good things about them, and noticed they have a line of signal transformers, so I added their WSM10K/10K to my order for a bit over $10--its freq response is 20-20K Hz. I put it in the Jayphat leading to one of the output jacks, and problem solved, with no apparent loss of signal strength or tone. Can't wait to try the OT I bought for my next build, their stuff seems good.
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kudzurunner
651 posts
Aug 09, 2009
7:40 PM
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Interesting thread. I, too, use two amps in tandem.
I use a Kay 703 with a 2-prong plug and a HarpGear with a 3-prong grounded plug. I also use a Boss DD-3 box and it has a power adapter--2-prong.
When I get hum, usually all I have to do is unplug the Kay and turn the plug around. Ground problem is solved. Sometimes I also have to turn around the Boss power adapter plug.
But once or twice that hasn't solved things. Sometimes the problem is neon lights and/or lights on dimmers. Clubs sometimes have these. Ask the club to unplug the lights.
Recently, NONE of these fixes worked. We were playing in a tent on an asphalt playground in Harlem across from the Apollo Theatre. I guess the somewhat portable nature of the electricity was problematic. Bad grounding. You know what worked? The sound man took my 3-prong Harpgear plug and plugged it into an ungrounded 2-prong plug extension, then reversed it. Bingo! UNgrounding the plug and reversing it did away with the hum.
File that little fix away. It's counter intuitive. Shouldn't the problem have been eliminated simply by turning the Kay 2-prong plug around (and/or the power adapter plug)? But no: one step more was required.
And remember the neon/dimmer bit of wisdom.
There's one more occasion in which I've experienced hum: when I was in England, playing through a power adapter. I ended up having to play through the vocal mic. Couldn't use the amp.
Last Edited by on Aug 09, 2009 7:41 PM
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mrdon46
8 posts
Aug 09, 2009
8:32 PM
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Lifting the ground from the power cord of one of your amps by using a 2 prong adapter should eliminate a ground loop problem (that's how I initially diagnosed my problem as being a ground loop), but I take seriously the cautions against doing so as a long-term solution--theoretically, I guess, the "ungrounded" amp would be grounded through the signal ground wire connecting the 2 amps, but I don't know as though I'd want to rely on this, especially playing outside or on a cement floor. (But then the old-timers used 2-prong amps all the time without a second thought....)
Another "fix" that I've heard suggested is connecting one of the amps with an instrument cable that has the ground (shield) disconnected at one end--sometimes reconnecting the shield with a .1 cap and/or 100ohm resistor is recommended--this supposedly blocks all or most of a 60 cycle hum.
But your point is well taken--often we're presented with problems and just have to grope our way towards a solution, and sometimes when we find one we still don't understand why it worked. Sounds like life....
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Ray
57 posts
Aug 10, 2009
8:25 AM
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Radial "Big Shot" are designed to handle those problems.
Go to ebay and type in, guitar aby
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MrVerylongusername
453 posts
Aug 10, 2009
10:52 AM
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OMG... never ever disconnect the earth from a valve amp!
Not unless you want to end up like Keith Relf of the Yarbirds anway.
Plenty of good advice here that won't kill you here. Behringer do a cheap active DI/splitter box that will break a ground loop. The earth wire is there for a reason leave it connected.
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Jim Rumbaugh
76 posts
Aug 10, 2009
11:31 AM
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tid bits about grounding.
All 2 wire 120 volt lines are a combination of 1 hot wire and 1 ground wire. In a 3 wire system the ground wire is a redundant ground.
In those old amps, especially with a switch you could throw left or right to turn it on, you would typically find a small capacitor run from 1 leg of the 120 line to the amp's chassis ground to "ground it".
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mrdon46
9 posts
Aug 10, 2009
11:59 PM
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I suspect there's a reason they called that the "death cap"
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walterharp
21 posts
Aug 11, 2009
7:26 PM
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looks like the radial big shot is not truly isolation, cause it makes direct mechanical links, but it does allow for that polarity flip, and doing it that way seems safer than loosing the ground.
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