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Discussion Starter #1
Hi folks. I've had an ongoing problem with my Koch Studiotone since I got it about a year ago. Whenever I try to use the outlet on the back which is supposed to go to the "clean" input of another guitar amp, I get a huge hum. I've tried it into the normal channel of a Crate V30, the Low input (and effects return) of my OR80 and the clean channel of a Fender SS amp with every cable in my bag, including some new "George L"s. Any Koch users out there experience similar problems? I contacted the distributer and they sent me a new pre amp tube, but the hum didn't go away. Other than this, the Studiotone is an incredible amp.

Any suggestions?
 

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Ground loop?

washburned said:
Hi folks. I've had an ongoing problem with my Koch Studiotone since I got it about a year ago. Whenever I try to use the outlet on the back which is supposed to go to the "clean" input of another guitar amp, I get a huge hum. I've tried it into the normal channel of a Crate V30, the Low input (and effects return) of my OR80 and the clean channel of a Fender SS amp with every cable in my bag, including some new "George L"s. Any Koch users out there experience similar problems? I contacted the distributer and they sent me a new pre amp tube, but the hum didn't go away. Other than this, the Studiotone is an incredible amp.

Any suggestions?
Dunno why they thought a new preamp tube would help. If the tube were faulty I would think you'd have the hum without plugging into another amp!

I can think of two possible problems. One is that you're getting a ground loop, where both amps are grounded through their power cords but also "looped" by the grounded shield on the connecting signal cord connecting them.

When 3-wire wiring first came on the scene we saw "adapters" being sold that would accept a 3-wire appliance cord and then plug into an old 2-wire wall receptacle. The adapter had a flying lead with a "fork" terminal that you were supposed to connect to the screw that held the plastic plate that covered the receptacle. The thinking was that even though there was a 2-wire receptacle there was a ground wire in the house wiring that grounded all the metal receptacle boxes. Putting the lead on the screw would connect the 3-wire ground prong to ground.

This at least let you use the new appliance in your old home. Dunno about the ground to the screw, though. Most times if your wiring was that old there wasn't any ground wire in the house wiring anyway.

Anyhow, if you're ever digging through your granddad's garage or basement and you find one of these adapters you should grab it! It is a great way to test these ground loop situations. You just don't use the flying lead and thus have only the 2-wire connection. This will break any such ground loop and stop the hum.

Another trick is to take one of those cheap plastic extension cords that are only "2-wire". They usually offer 3 "plugins" at one end, two on one side and one on the other. You can often plug a 3-wire cord into the top 2-wire receptacle with the ground prong sliding free across the top of the cord's head. Sometimes there's a bump in the plastic/rubber that prevents this but an Xacto blade fixes that really quickly!

Number two is that the signal out circuit in your amp is either bunned or not outputting a compatible type of signal for the way you think it should be connected. If it is a "line out" that is designed to feed a power amp without any preamp circuitry in the way then feeding it into a guitar jack is likely to be a problem. Line signal voltages are much lower in impedance and far stronger than a guitar signal.

Is this a new amp with warranty? I've never worked on one of these so I don't know any specifics about it. Surely they have a factory techie somewhere who can advise you. You may have to get past the sales guys at the store that sold it. Sometimes they think they know more than they actually do and give you a quick but wrong answer instead of actually contacting the factory techical department.

If I had a schematic I could tell you something more useful but this is the best I can do...
 

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Discussion Starter #3
Silent Night

Yep, it was ground loop; Now how do I sort this out without having the ground lifted on one of my amps? I was thinking a two conductor shielded cable with the shield only connected at the source end, but would that open up the RF can of worms? How much does the cable length contribute to the hum?

Thanks Wild Bill.
 

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"Details, always details!"

washburned said:
Yep, it was ground loop; Now how do I sort this out without having the ground lifted on one of my amps? I was thinking a two conductor shielded cable with the shield only connected at the source end, but would that open up the RF can of worms? How much does the cable length contribute to the hum?

Thanks Wild Bill.
The longer the cable the more hum picked up, WB!

Actually, I'm not sure if breaking the shielded cable at one end would do it or not. You wouldn't have to go 2 conductor. Just don't connect the shield at one end. The power cords would provide the return path. We do that with things like the cables connecting spring reverb tanks. Worth a try anyhow.

The usual method is to get inside the amps and lift the green ground wire. You then ground it back again through a pair of back-to-back power diodes like 1n4007s or 1n5408s. This lifts any ground currents unless they exceed the voltage drop of the diodes, which at 1.2 volts typical is far greater than the level of any hum voltage. You also bypass the diodes with a .047 or so cap, rated for 600 vdc or whatever makes you feel safe.

Any line power ground problem that would leave a high enough current on the chassis or knobs to hurt you would be totally bypassed by the diodes and cap, so you are not defeating the 3-wire safety situation in any way. You are just making the "loop" connection between the amps too high a resistance path for ground hum currents and they then flow down their own amp's power supply ground return, like they're supposed to do.

I've built a number of stand-alone reverb units, a la the old Fender 6G15 unit. I had the same ground loop problems until I heard about the back-to-back diode trick. Works great!
 

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While I'm sure Wild Bill's suggestions are excellent, why not contact the guys that designed the amp? They are the most likely folks to figure out a solution and would probably appreciate the feedback. According to their website the email address is [email protected].
 

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washburned said:
Yep, it was ground loop; Now how do I sort this out without having the ground lifted on one of my amps? I was thinking a two conductor shielded cable with the shield only connected at the source end, but would that open up the RF can of worms? How much does the cable length contribute to the hum?

Thanks Wild Bill.
Aren't there small isolation transformers that you can buy to cure this problem? I've seen a couple of bass players feed their line outs into sound boards using this method. The London Power Studio amp that I used to own had these transformers build in and I never had a problem feeding the line out to an external device.
 

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Follow the money...

Scottone said:
Aren't there small isolation transformers that you can buy to cure this problem? I've seen a couple of bass players feed their line outs into sound boards using this method. The London Power Studio amp that I used to own had these transformers build in and I never had a problem feeding the line out to an external device.
Yep! They work great too!

How much money you got? They're far more expensive than a pair of power diodes and a coupling cap.

I mean, don't we all need extra money for beer? :)
 

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Wild Bill said:
Yep! They work great too!

How much money you got? They're far more expensive than a pair of power diodes and a coupling cap.

I mean, don't we all need extra money for beer? :)
I missed your post on the on that simple circuit...have to print that one for my files:D

I was just checking the price of the direct boxes, and the good one's (with the Jensen transformers) are a tad spendy, but would be good for interfacing with recording gear or a live board.

I can see why the amp companies don't generally tranformer couple their direct outs...the transformer itself cost about $75.00.
 

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Caveat emptor...

Scottone said:
I missed your post on the on that simple circuit...have to print that one for my files:D

I was just checking the price of the direct boxes, and the good one's (with the Jensen transformers) are a tad spendy, but would be good for interfacing with recording gear or a live board.

I can see why the amp companies don't generally tranformer couple their direct outs...the transformer itself cost about $75.00.
Yippur!

On a related note, I sometimes get a smile watching folks bid on such transformers on Ebay. Some old interface transformer from an amp built in the 40's will go for a huge price! They seem blissfully ignorant of how poor specs on such trannies were in those early years. Or you'll see matching transformers for low-impedance microphones that came out of a old PA amp. The prices seem to indicate that few folks realise that such trannies were NOT all the same! There were high end and low end trannies just like anything else and the reason some brands were cheaper was that they were sold for less "hifi" applications. Why pay for a premium matching transformer designed for studio use if you were going to use it in a PA amp for a banquet hall? How hifi do you need to make wedding toasts?

Conversely, if you understand what you're bidding on better than others you often can snatch up a good deal that others ignore because it has a less "mojo" brand name...
 

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Discussion Starter #11
No luck with plan B

I tried lifting the shield on the connection between the amps, but the hum is still there; the only thing that seems to work is to use the little gray three prong to two prong adaptor on the second amp power cable. I guess I'll be shelling out for a fancy adaptor of some kind, or going inside to try the above mentioned modifications, because I don't like the idea of having no ground on one of my amps. Thanks to all for the help.
 

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Ah, but there IS a ground!

washburned said:
I tried lifting the shield on the connection between the amps, but the hum is still there; the only thing that seems to work is to use the little gray three prong to two prong adaptor on the second amp power cable. I guess I'll be shelling out for a fancy adaptor of some kind, or going inside to try the above mentioned modifications, because I don't like the idea of having no ground on one of my amps. Thanks to all for the help.

You'll still have a ground, WB. One side of the power line is also the white neutral wire, which is ground. The patch cords between amps will also make the entire lashup chassis grounded through the remaining 3-wire power cord on one of the amps.

Still, if you change units in your setup you may change the situation so something more might be a good idea. You could install a ground lift switch in one amp or as I suggested before you could hardwire in the backtobackdiodes trick for a more permanent and safe solution.
 

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Discussion Starter #13
Agreed, but with the two amps ganged, if the white/neutral breaks, the line current is going to be feeding through the ground side of all the wiring, including my guitar strings...not a pleasant thought!
 

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"Well, if it were my amp..."

Have you tried the diode trick? Far cheaper than a transformer and absolutely preserves the protection of a 3-wire ground!

If you're not comfortable working inside the amp it would still likely be much cheaper to pay a tech than buy the transformer. Plus, the transformer will need some kind of cabinet and connection jacks... more expense in parts and labour.

Whatever, as long as the hum goes away.
 

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Discussion Starter #16
:banana:

So I'm walking thru this warehouse clearance center and there is a wall of electronic stuff, mostly computer and video related, but there is a small section of audio cables, adapters etc. and right in the middle is a pile of stereo isolation transformers! So I take one to the check out and ask "how much?", noting the suggested retail was $25: the girl says "$4.95". So I buy it and stop at my local electronic supplier for a couple of RCA to 1/4" adapters ($3.50), go home and connect the KOCH to the ORANGE and ....YESSSS!!! NO MORE HUM! :smile:
 
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