Effects with SWR-400

[Paul Lindemans (palembic)]

Hi friends,
is there anybody who can give me some advice on this one.
I play an SII 5 string with the DS5 box through an old (but very fine) SWR SM-400.
About a year ago I bouhgt a Lexicon MPX 100 Dual Channel processor as a effect unit for my rig. It was not very expensive and the guy in the store here loved the quality of the effects.
Normally you should take the signal send of the effect loop from the SWR (on the back of the amp) to the input of the MPX 100 (on the back of the device) and then come out the MPX and go back to the SWR (stiil on the back - signal receive)(all of this in mono). You have pots for input, output and signal mix (effect pure/guitar pure) on the front of the MPX. The problem is when playing on loud volumes and opening the mastervolume on the SWR the MPX start to clip as hell, red warning light for overload are flickering on the MPX but not at the SWR pre-amp side.
Also, there is a constant hum on the speakers and sometimes (playing loud) I seem to hear an effect from the MPX even when he is in the bypass mode.
I tried to solve it by coming out (mono) from the DS5 box, going direct into the MPX 100 (on the back) with a 1 meter jack-jack cable and com ing from the output of the MPX from the back and plug into the SWR-front input (again with a 1 meter jack-jack).
This solution gives no hum and the overload on the MPX never clips. It depends only on how high I put the input on the MPX.
Beware: the sound and the effects are really good.
But the way I do this is rather unusual. I don't understand why the effect send loop on the SWR is not doing the good thing. Are there some things I forget or is there a procedure that I need to follow using the effect on the SWR?
It's a long story but thanx for taking the time to read and maybe give some advice.
Paul

[Valentino Villevieille (valvil)]

This is just a guess, Paul,
 
but it may be a problem of mismatched impedance between the FX send of your SWR and the input  of the MPX100 , which I believe is a high z input rated to take instrument signals ( it is on the MPX 110 , its successor); your SWR has, I believe both a mono and a stereo FX loop, I take it you are using the mono one; it may be that it's a low z one and that might lead to the clipping/ hum etc...solution in that case would be an impedance adaptor...but, this is just a guess, I could be totally wrong. A bad jack on the SWR could explain the hum, but I don't know that it would account for  the clipping. Definitely the output from it seems to be too hot for the MPX.
 
Good luck
 
Valentino

[Mark DuFresne (markus)]

My 2 pfennigs:
 
I think Valentino's guess may be right. I had the same problem as you with my Aguilar DB-680/MPX-1/power amp setup. I ended up measuring the impedances of all the inputs and outputs of each device. The measurements from the DB-680 turned out to be a bit different than what was stated. I ended up taking the Lexicon out of the effects loop and putting everything in series (Aguilar --> MPX-1 --> amp). Works fine now -- plus I get great STEREO effects! There's nothing like an Alembic sustaining a big bright long note and hearing it move all over the room.
 
Remember, you can plug a low impedance output into a high impedandce input and everything will work fine. If you do the opposite you'll get a weak, distorted signal. That was my problem and it sounds like it might be yours too.
 
Mark

[Paul Lindemans (palembic)]

Hi Mark and Valentino,
these advices are very valuable to me.
Yesterdayevening I took the manual who came with the Lexicon (it's about time) and there they're giving a sketch where the Lexicon comes BEFORE the amp. They also talk about using the Lexicon in a Send-receive situation but I did what they ask to do and I get these situations already described.
I don't have rehearsal this week but I will try some of the things you suggested (and were in the manual) next week and keep you posted.
If I'm not coming out of it I'll contact a friend of mine who is a amp-doctor and we look at it together.
Anyway, thanx and you'll hear from me soon!
 
Paul