Older Amplifier Clipping

[AlanG]

Hello all -

 

I am responsible for the sound in our Church. We are currently in the process (with a decent consultant) of developing a new sound system in our late 19th century designed church.

 

In the mean time - I am having a problem with distortion coming out of the system. At first, I attributed it to the speakers falling apart - but now, I'm convinced that the amp (an early 60's Dekane - 70 volt system) is clipping . . . or dying.

 

What I am doing right now, is using a mixing board (provides phantom power to the podium mic - and an increased number of wireless mic inputs) - and running the mixer line out to a mic input.

 

I have tried various input/output level combinations, but still get distortion.

 

Any ideas as to what I can do - or what I'm doing wrong. . . or, is the darn thing dead

 

THANKS - Alan

[Brad Harris]

whats this mic input your refering to?

 

If its a xlr input on the amp, you should be fine (afaik, I've never used a dc system or know how they work really). If the input is 'mic level' opposed to line level, put a di inbetween the mixer and the amp and see if that does any good.

 

 

Brad Harris

[AlanG]

Wow - I hadn't considered a DI box...

 

The setup uses an XLR input on the Stage/alter area - but the stupid amp uses screw terminals to connect the wires to the amp

 

This amp is a commercial PA system type - that's built into a wall.

DuKane is the name (spelling error in original post).

 

edit - additions:

Also - just to make it clear - this is a mixer/amp unit. It has 4 mic level inputs, an aux input - for tape (hmmm - should I be using that? That connection is made via screw terminals as well), a Master Output and an Aux output (tape recorder).

 

I can't wait for the Church moneybags to let loose the $$$'s to pay the consultant and buy the equipment (I'm agitating to have the computer analysis done to the space - it's only about 5 grand - it's a large vaulted ceiling sanctuary).

 

Thanks for tip, Brian.

Anyone else have an idea? Would the DI realy help?

 

Alan

[agedhorse]

Sounds like you are overdriving the mic input on the DuKane. Do you have the volume for that at a VERY low position? If so, you are probably overdriving the input stage in front of the volume control.

 

If the output of your mixer is balanced XLR, then you will need a balanced XLR pad device (at least 20dB, but probably 40dB would be better. Rapco makes them in their "BLOX" series. Otherwise, if you can take a 1/4" out from your mixer into a passive DI, the level stepdown inherent in a passive DI (by design) will accomplish much the same effect.

 

Beware of connecting your mixer directly to the aux input. This may work ok, but there's always a good chance of increased hum due to a ground loop between the equipment. If you have hum, but ok signal levels and no distortion, then a 1:1 audio isolation transformer would be the proper solution.

[AlanG]

Thanks, agedhorse -

 

Nope - I have the input at 10 o'clock position (off is at 6:00). I have tried higher input levels, with the master pulled down....or with the master up, and the mixer pulled down...all to no effect.

 

BTW - we are using a Allen Heath 16:2 mixwizard.

 

The Dukane is a solid state amp - and old. If the amp is dying - would distortion be a result? If that isn't a typical death mode - then obviously it is an over drive situation. . . . I 'll just have to be smarter than the gosh dang amp (can't use bad language - it is a Church Amp, after all )

 

Ah well - a quick trip to Guitar Center to find a DI is next.

 

 

Thanks again - ALAN

[agedhorse]

Be SURE that you get a passive DI (Rapco DB-100 or Horizon) and NOT and active Behringer or the like.

[RickJ]

That Dukane is a basic mixer/amp. It's not designed for anything more than simple paging and background music. Screw terminals are standard on those systems (to this day).

 

The mic inputs have preamps but no pad and no readily available gain or trim adjustment, so you're definitely overloading the mic input.

 

The aux input is probably -10db to connect to consumer tape decks. I would try that connection before using a DI on the mic inputs and run the board's masters no hotter than -10 or so. If it causes a hum, try a DI w/ground lift on that input. (As Agedhorse said, a passive DI.)

 

Since it's a 70-volt system, it is very limited in the bass freqs it can deliver. It might be high-passed at 150Hz or so to prevent the transformers from loading the entire system. (Bass frequencies through those xformers can reduce the load impedance to near zero, which looks like a direct short to the amp.)

 

Don't expect it to reproduce full-range concert sound. Remember, it's designed primarily for voice. Even boosting the bass on a mic channel strip of the console will cause distortion.

 

Why not ask the consultant to have a look at your present system?

[AlanG]

Thanks for all the replys-

 

We haven't "signed de paperz" with the consultant -and the system is so horribly installed that I didn't want to waste his time on nasty system that needs to be replaced.

 

The system is only used for voice, so concert quality sound is not an issue (new system will be designed for this in mind) .

 

In trying to kill the distortion - I have the bass rolloff pads engaged on the mixer - as well as the trim pots turned down.

 

Hmmm - the distortion is worse now, than when we were using a cheesy little berhinger mixer - perhaps the output of the A-H mixer is hotter than the Berhinger?

 

Thanks again - I'll try hooking in to the aux input this Sunday.

 

ALAN