Chorus Pedal with PA weirdness - uh-gain

Bob-Dubery

OK... I have had another bout of pedal weirdness at TJs.

They have a PA that looks fairly standard (mostly Yamaha kit, including the desk and the amp). They run their microphones into a snake. The snake has the 3-connector balanced line female sockets.

The leads they use for guitars have a jack plug at the guitar end but the other end goes into the snake.

Now my set up (which is maybe too grand a term).

The guitar has an LR Baggs Dual Source pre-amp/pickup system. As per the manual this system has an output impedance of 1200 ohms and an output level of -10 db.

From the guitar I run a lead into a Carl Martin Classic Chorus pedal. This is described as a true bypass pedal. The manual says input impedance is 1Meg ohms and output impedance 100 ohms.

The output from the pedal goes into one of TJ's guitar leads (Ie jack on one end, XLR on the other).

Now with the pedal in bypass everything is fine. I kick the pedal in and the signal level drops substantially.

I have tested the pedal at home into my PC's sound card. OK... not an exact replication of the problematic setup, but it doesn't show the same substantial drop off.

I have used the pedal with Keira's rig. No problems there.

So why into a PA is there a problem? Is this an impedance issue? After all the stomp box is really designed to feed the input of a guitar amplifier. Is a DI box between the pedal and the PA going to improve matters?

AlanRatcliffe

[EDIT - I just looked at the Carl Martin site and it shows the Classic Chorus as having a volume control - can't you set that to keep the volumes even between the two? That's what it's for...]

One big damn puzzler... but my money would on an impedance difference. If the pedal is true bypass, then the impedance will change when it is on (100 Ohm) or off (1.2K Ohm). Driving a normal high impedance 1/4" input (optimised for 500K Ohm or lower) you will be fine with either, but when you are plugged into a lowZ balanced input (optimised for 600 Ohm or less), this can make a difference. One of the reasons true bypass isn't always a good thing and also why I would never plug an unbalanced source into a balanced line without a DI.

So try a DI - If the problem is as I suspect, that should sort it so the desk sees a constant impedance and you won't get the volume drop.

Bob-Dubery

Alan Ratcliffe wrote: [EDIT - I just looked at the Carl Martin site and it shows the Classic Chorus as having a volume control - can't you set that to keep the volumes even between the two? That's what it's for...]

Well it's REALLY there so you can overdrive an amp when you kick the chorus effect in, so you can have an overdriven chorus sound. The control doesn't add any additional shaping to the signal, it just ups the output signal - so it's still clean.

Yeah, I could do that. Should have set it in the sound check.

One big damn puzzler... but my money would on an impedance difference. If the pedal is true bypass, then the impedance will change when it is on (100 Ohm) or off (1.2K Ohm). Driving a normal high impedance 1/4" input (optimised for 500K Ohm or lower) you will be fine with either, but when you are plugged into a lowZ balanced input (optimised for 600 Ohm or less), this can make a difference. One of the reasons true bypass isn't always a good thing and also why I would never plug an unbalanced source into a balanced line without a DI.

Yeah. It's been a while since I sat on an electronics workbench, and I've forgotten a lot of stuff. It did seem to me though that the change in impedance could be causing problems. And as I said, working with Keira's rig - where the signal path was Chorus -> DI -> mixer - I had zero problems.

Hmmm... I have a Baggs gig-pro pre-amp. That has i/p impedance 10 meg ohms, o/p 800 ohms. Maybe if I put that after the pedal...

AlanRatcliffe

X-rated Bob wrote: Well it's REALLY there so you can overdrive an amp when you kick the chorus effect in, so you can have an overdriven chorus sound. The control doesn't add any additional shaping to the signal, it just ups the output signal - so it's still clean.

Hmm... Unusual. Chorii don't usually have volume at all, but with any pedal that has an output level you should always try and set the level so it remains the same when bypassed. And it's possible that it will need a different setting depending on what you are plugged into. So I'd guess you just had it set too low.

Gearhead

X-rated Bob wrote: Should have set it in the sound check.

Yep. Sounds like you tend to leave things to chance that you shouldn't. Once you know something can go wrong, you will adapt. Next time you will expect to have to adjust the volume knobbie, but that is besides the point. The whole thing with sound check is to find the things you're not expecting. Try everything you need. No more bouts....

TomCat

Hey Bob....I remember you going down this road before with you and TJ's. I still recon the answer is a proper Direct box.

By using one you are telling the mixer / snake cable that your setup is just another microphone and all will be well.

I've just got some of these......http://www.dbxpro.com/db10/index.php.....after some dingbat at church stole three of my prised BSS AR133 direct boxes. I tested them over the weekend on a few acoustic guitars and they work very well.

You feed the last output from your effects into the input and the output is XLR (3-pin), the same as a microphone.

Bob-Dubery

TomCat wrote: Hey Bob....I remember you going down this road before with you and TJ's. I still recon the answer is a proper Direct box.

By using one you are telling the mixer / snake cable that your setup is just another microphone and all will be well.

I've just got some of these......http://www.dbxpro.com/db10/index.php.....after some dingbat at church stole three of my prised BSS AR133 direct boxes. I tested them over the weekend on a few acoustic guitars and they work very well.

You feed the last output from your effects into the input and the output is XLR (3-pin), the same as a microphone.

Won't work - because they typically have several acts and need to turn them around quickly. I'd need something with a jack on the output side. Otherwise they have to start yanking things out of the snake. The sound guy at TJs works as a volunteer, often unassisted (or assisted on an ad hoc basis by people who think they know what they're doing) and I don't want to complicate his life.

AlanRatcliffe

Many DI boxes also have an unbalanced but lowZ jack output.

So if someone shows up with a guitar that has an XLR output, the engineer still uses a 1/4" unbalanced conversion cable to hook them up? Weird.

Otherwise smack the engineer around a little until he starts keeping at least one decent DI box onstage. To be honest, I find it strange he doesn't have one there as a matter of course already. Also means if someone turns up with a guitar that has a piezo but no preamp (which usually have an o/p impedance of 1 - 2 Megohm), they can plug in and get a decent sound too.

Bob-Dubery

Alan Ratcliffe wrote: Many DI boxes also have an unbalanced but lowZ jack output.

So if someone shows up with a guitar that has an XLR output, the engineer still uses a 1/4" unbalanced conversion cable to hook them up? Weird.

Otherwise smack the engineer around a little until he starts keeping at least one decent DI box onstage. To be honest, I find it strange he doesn't have one there as a matter of course already. Also means if someone turns up with a guitar that has a piezo but no preamp (which usually have an o/p impedance of 1 - 2 Megohm), they can plug in and get a decent sound too.

They have some kind of pre-amp stomp box (Behringer) that they use of things like stick-on piezo pickups.

Guitars with an XLR output?

It occurs to me that I did not have this problem with my line 6 MM4. That is NOT a true bypass device, so I presume it is presenting a constant impedance to the PA.

I might get away with using my Baggs pre-amp on the output side of the pedal - but then I start having lots of bits and pieces to plug in, and that leads to a pedal board and a complicated life. Maybe I should just abandon dreams of being John Martyn and just eschew all the pedals.

AlanRatcliffe

Guitars with an XLR output?

Many Yamahas, Washburns, etc. have them built in. Plus obviously anyone using a DI or a processor (Boss AD-series, etc.) with an XLR out.

Bob-Dubery

Alan Ratcliffe wrote: Many DI boxes also have an unbalanced but lowZ jack output.

Yes. The ones I've looked at have had a very high input impedance and a low output impedance. So presumably if I use one of these then when the pedal kicks in and the impedance it presents on the output side changes the DI box will be OK because of it's high input impedance AND will still present the same impedance on it's output side and thus buffer the PA from whatever changes are going on between DI and instrument.

AlanRatcliffe

Exactly. ?

TomCat

RE the XLR out on Acoustics.....starting to become quite common....Even Ibanez... ?.. has them.

I have a few musicians at church with XLR outs on their acoustics. Just plug straight into the mic input on stage and away we go.
The output is usually quite hot so I pad the desk 20dB to give more control of the desks input gain.

BTW these pre-amps also have a jack out so you can plug through pedals etc. Never tried out whether the jack mutes the XLR out though....Hmmmmmm