Problem with the SWR Marcus Miller Pre-amp

[Geester]

I bought an SWR Marcus Miller pre-amp a few weeks ago and, while the tone is great, it has a particular problem and I wondered if anyone else has experienced the same issue.

Basically, when any mains powered (9v adapter style) or battery powered effects are plugged into the effects loop it makes a dreadful hum. I've had my effects board checked out and there are no problems there, I have several other amps and it works with them no problem. I went through every possible configuration to make sure it wasn't a cable or connection of any kind.

The unit has been sent back to SWR/Fender and they don't appear to be able to fix the problem but they now know it exists.

Has anyone else seen this issue?

Cheers

Mike G.

[The Funk]

Put the effects pedals before the input - not in the effects loop.

It sounds like you have a ground loop and they're a total bitch to sort out.

[Geester]

There's definitely some kind of problem. I had an electrical engineer check it out and he reckoned there was an issue with the design of the pre-amp, it may be an issue with the 115/230v migration or just the fact that the effects loop is incorrectly wired.

As for putting the effects before the pre-amp, for me it defeats the object of having the pre-amp. I wanted the compressor in the chain first and then use the 'Wet/Dry' mix facility to give just a hint of effect (like chorus/phaser/distortion) mixed with the dry sound.

If you are thinking about buying one of these I would definitely check out your effects on the loop first. One interesting thing is that I talked with an SWR endorsee in the US and he mentioned that SWR have seen this problem before on the SWR900 amp, maybe they've used a similar design and the problem has re-occured.

Another key point is that Fender/SWR UK have been fairly useless at sorting this out. They didn't believe me at first, I had to physically show them the problem and even then they have not got a clue how to solve it. They also are unwilling to give me my money back or even offer an exchange for another product.


Just thought I better let everyone know.

[RhysP]

Do you get the hum problem with other pedals but WITHOUT the compressor?
An effects loop isn't the best placement for a compressor IMO. It sounds like the compressor is increasing the volume of an otherwise inaudible hum.

Personally I'd use compression & any distortion/overdrive effects in front of the amp and only run the delay & modulation effects through the effects loop.

Edited May 16, 2009 by RhysP

[alexclaber]

I remember the SWR Grand Prix was star grounded so IIRC a tech could lift the ground to deal with these problems.

However, I would not recommend putting any 9V powered effects in the effects loop. Effects loops should be ignored unless you have rackmount effects, the signal levels and impedances are all wrong for pedals. I understand your logic behind your compression and wet/dry blending but I think you'd get better results with dedicated line level rack gear.

Alex

[Geester]

Appreciate the feedback and thanks for the advice. The SWR MM pre has a 'built-in' compressor so it's not really in the effects loop, it's in the signal path before the eq section. This means the compressor doesn't see the hum from the effects loop since the effects loop is after the eg section.

I can see the theory behind not having 9v pedals in the effects loop but I can't understand (and neither can SWR) why everything works fine with my Eden WT800, GB750, Aguilar DB750 and Markbass SD800. It does also rather defeat the object of having an effects loop and a 'wet/dry' blend feature. I have also tried it with my rack mounted mains powered Line6 X3 in the effects loop and the problem is still there.

I guess folks are thinking that it must be a classic 'humming pedal' problem which is exactly what Fender/SWR thought till I physically showed them the issue. Believe me, I tried everything and it is most definitely a fault with the pre-amp.

The MD of Fender UK&I is now involved and still no resolution. It looks more and more like a design fault.

[BassBod]

I know my old Grand Prix would hum like a big fuzzzzy bee when racked with the Stereo 800 power amp it was designed to work with (!) but it did have an internal mod to allow a fix (the star earthing that Alex mentioned). I could never work out why..particularly as the older Studio 220 is completley silent.

Hassle them..and email MM to let him know!

[dr.funk]

[url="http://www.dv247.com/invt/61016/"]Not sure if one of these would be any help?[/url]

[BassBod]

There is a box called an "ebtech" hum eliminator (or something very similar) which has a reputation for fixing persistent hums. But, I think its more important for SWR themselves to sort out a very expensive bit of equipment...it just shouldn't be a problem?

[jazzbassist]

I have a similar problem except when I engage the EQ section I get the dreadful hum and when I disengage it the unit is almost completely noiseless... I sent it to the licenced fender/swr repairer they just told me I had to over drive the input gain stage and turn down the volume to get rid of the noise... Anyone else have this issue?