Stereo Amp setup causing loud "BRRRVVVV" noise

[acobberson]

Hi all, I live in Germany where the power runs at crica 230V. Most of my amplifiers are setup for 220-240V except for my Silvertone 1483. It still runs on 120V for which I use a step-down converter (240V to 120V)

I often run a two amp stereo (or dual mono) setup. I have no problems running two amps together EXCEPT when I try to run the Silvertone with the step-down converter with another amp. As soon as I turn the Silvertone on I hear a VERY loud "BRRRVVVVVVVVVVVV" coming from the other amp. I doesn't matter which other amp I use, they all do this as soon as I turn the Silvertone on.


If I play the Silvertone on its own (with the converter of course) it works as it should and sounds great.

For info, both amps and the converter are all plugged in to the same power source. So it shouldn't be a ground loop issue, and the sound is different than a typical ground loop hum. It sounds more like a loud, revved up motorboat.

Does anyone have any idea what could be causing this? My guess is that the converter is somehow messing things up.

Second question: Does anyone know if the power transformer on these old Silvertones can be rewired to run at 220-240V? I'm guessing probably not, since they were budget model amps, but I figured I would ask if anyone had experience with the PTs. I'd rather not replace the PT with a new one unless I know it will fix the issue.

Any help or thoughts are welcome!

[Mick Bailey]

If you unplug the connecting signal lead (with both amps switched on) does the noise stop? Am I right in thinking the step down converter is a non-isolated auto transformer type, and have you checked the earth continuity?

Also, do you still have the 'death cap' present in the Silvertone?

Connecting any two pieces of equipment can cause ground loop noise, and that noise can vary - not necessarily just a low level hum. You may need to isolate the signal lead using a 1:1 transformer (loop breaker) or DI box.

[acobberson]

Hi Mick, thanks for the response. Yes, I forgot to mention that the Silvertone has already been updated to a 3 prong US cable and the death cap has been professionally removed.

I haven't tried unplugging the signal cable, but I can give that a shot and see what happens. As for the transformer, I'll have to check if it has earth continuity. It's an older piece, so I'm not sure at all if it is isolated. How would one test this?

One thing that just occurred to me is the polarity of the two prong EU plug from the transformer into the outlet. Normally it shouldn't matter how it is plugged into the wall outlet, but if I'm not mistaken, when powering something with 3 prongs the polarity of the plug into the outlet now becomes a factor. Is this correct?

[mozz]

What model? Is it a widow maker or does it have a power or isolation transformer?

EDIT found the model number in your first post, need to learn to read.

[Mick Bailey]

To check if the transformer is isolated measure the resistance between the input and output. With a non-isolated transformer the live and neutral will give a reading between input and output.

You say the transformer has a 2 prong connection. This means that the earth on your Silvertone 3-prong plug isn't connected to the mains earth, which poses a shock risk should the chassis become live for any reason. The only connection to earth would be via the signal lead connection to the second amp (but only if that amp itself is earthed). That could be part of the reason for the noise issue.

If the step down transformer is isolated then the polarity of the 2 prong plug is less of an issue. If not, then the amp's live and neutral can be reversed according to how the plug is inserted. This means that the amp's mains fuse could be unpredictably on the neutral side, with the consequence that if the fuse has blown the live side of the wiring inside the amp is still hot.

[acobberson]

mozz haha, yeah it has had the death cap removed and the 2 prong converted to 3 prong US cable.

Mick Bailey sorry, I didn't explain the EU plug well. It is what's called a Schuko plug. It has 2 prongs, but earth in the middle.

It seems as though it is isolated. There is continuity from earth input to earth output and I get no reading from neutral and live input to output.
Here is a photo of the transformer.

[Mick Bailey]

Agreed - it looks to be an isolation transformer, so the input orientation doesn't matter and it's a much safer arrangement than if it was an auto-transformer style, non-isolated unit.

Mostly, older USA made amps were manufactured with a specific voltage mains transformer for domestic use. Some schematics list an alternative export model mains transformer for some brands, but this seems to be a later trend.

I still suspect you have a ground loop issue. Despite sharing a common mains power source you still have two connected pieces of equipment where ground currents circulate and because the signal ground connects at some point to each amp's chassis ground (which is connected to mains earth) these circulating currents result in noise. The best way to investigate this is to use a transformer in your signal lead, or a loop-breaker box. A simple check to start with is if unplugging the interconnecting signal lead kills the noise it's likely to be a ground loop.

You may read that lifting the mains earth of one amp 'fixes' the problem. It may do, but I condemn this method as it means the safety earthing is compromised and in a fault situation could present a serious shock risk.

[acobberson]

Mick Bailey I think you might be correct.

As an update, I have tried a few different setups with a different transformer, a different power outlet, a different signal splitter.... pretty much anything I could change I did change, one at a time.

What I have found is this:

1: Turning on the transformer adds no noise on its own
2: Plugging in the Silvertone (turned off) to the transformer (turned on) adds no noise
3: Turning on the Silvertone (but not connecting the input from guitar) adds NO noise
4: As soon as the Silvertone is turned "on" and has the guitar input plugged in (but still in Standby) there is a bit of noise
5: When Standby is turned off and the guitar input is plugged in, the full amount of noise is heard
6: When the guitar input cable to the Silvertone is removed the noise stops, even if the amp is on and standby is off

This to me says that there is a ground-loop issue at the amplifer.

I tried a ground-lift on the signal send (using a Radial SGI active Guitar Interface) and it didn't change.

The Silvertone 1483 also has a "Ground" switch, but when the amp was updated to the 3 prong chord this switch was obviously removed from the circuit.

I will investigate the 3 prong wiring update to make sure it was done correctly.

Any thoughts on this update?

[Mick Bailey]

The Radial SGI does not appear to be an isolated device, so the results from that are misleading - it looks to me to be a line driver with a ground lift for the XLR output. You need a transformer-coupled isolation box, but note that the passive ones have a relatively low input impedance that's too low for guitar use unless you use a buffer up front. Look for an active, buffered unit. The only true isolation is by means of a either transformer or optical coupling.

If you fancy a DIY approach this is a good start and can be simplified to eliminate the battery switching (circled) with a DPDT switch http://www.geofex.com/FX_images/splitter.gif. Also, you probably only need the isolation in one amp feed.

[acobberson]

Thanks Mick Bailey !!! So you're saying I can build that as a passive splitter? No power necessary?

[Mick Bailey]

No. A passive device will not have the necessary high input impedance for guitar use. Typically these are less than 1K Ohm, so will kill the high frequencies. A buffer before the transformer increases this impedance to prevent this. With the link I posted the socket operated switching circled can be replaced with a DPDT switch - it will still be a buffered circuit. This simplifies the component count. An alternative if your guitar output is low-level and you don't need the headroom is to use a single 9v battery, or regular DC power supply. There's a slight modification needed to the circuit to do this but it's fairly straightforward. Yet another approach is to use a passive transformer loop breaker/isolator and use a buffer pedal or simple DIY buffer circuit in front of this. It doesn't need to be a splitter, though. I'm thinking you only need the isolation in the cable that links the amps. Ideally it would be best to borrow something first to fully establish that you have a loop problem. Even a passive box with the associated tonal loss will help to diagnose the issue. Or even buy a 42TM018 transformer as an experiment and solder it up a socket to each side with some short hookup wire. It will tell you once and for all if there's a loop problem.

Are you currently using a Y cable or ABY box? You can get isolated ABY boxes such as the Orange Amp Detonator; https://orangeamps.com/products/fx-p...amp-detonator/

EDIT: Here's another DIY schematic that simplifies things. You can replace the output XLR with a1/4" socket (needs to be isolated from the case) and omit the ground lift;

[Jazz P Bass]

The Orange Amp Detonator seems to be the ticket!

[Mick Bailey]

Here's a picture (not mine) of the insides of an Amp Detonator. It looks to be a fairly standard circuit with a dual opamp for buffering and a transformer to isolate one of the outputs - similar to what's already been mentioned. The interesting part is the power supply board that produces a +/- voltage from a single supply, (looks to be an 8 pin IC from the traces - ICL7660, maybe?) which increases headroom;

[acobberson]

Mick Bailey Thanks for all your help in this. I checked the 3 prong cable wiring. Looks good to me. Green is going to ground, black is going to the fuse and white is going to power transformer.

I unearthed one end of the cable going to the Silvertone and that fixed it! So yes, it was a grounding issue. I'd rather than have to rely on this setup though, so I'll most likely just add an isolation/split box like you mentioned and call it a day. I can get a used one from Lehle for around 40-50 Euros. Even though I'd love to break out the soldering iron again and whip something up, I'm not sure I'll have the time.


Thanks for everyone's help! Just goes to show you, sometimes you can't trust your ears to tell the whole story.

[Mick Bailey]

Not wishing to offend anyone, but for the avoidance of any doubt, I'll repeat what I posted earlier;

You may read that lifting the mains earth of one amp 'fixes' the problem. It may do, but I condemn this method as it means the safety earthing is compromised and in a fault situation could present a serious shock risk.

Operating the setup with an earth removed is a really bad idea. I don't even offer this up as a method of testing in case someone does this and then gets complacent with the situation, forgets about it, or passes the equipment on.