need help with resistor value

[aragorn723]

I have been thinking about tweaking the matching between the Quickie and my amp (Inter-M R500).  The output impedance of the amp is 10K, and the Quickie has the PJCCS.  What value resistor do I need for this, and where does it go on the Quickie?  The amp is turned way down because the gain of the Quickie is so high.  I would like to be able to turn the volume on the Quickie up more.  Thanks,

Dave

[Paul Joppa]

By output impedance, I assume you mean input impedance?  :^)

The pad goes at the input of the power amp.

[aragorn723]

Yes, input impedance   I'm guessing it goes on the signal (not ground) part of the rca?  How do you physically wire it, with a little box with rca jacks, and solder the resistors in the middle?  Also, what would be the best value for the resistor?  Thanks,

Dave

[Grainger49]

Dave,

What are you trying to do?  If you want to use the full range of the volume control on the Quickie, you need an attenuator at the amp's input.  The best place to do this is to use a pair of resistors for each input.

But there is no reason to match the Quickie's output impedance to the amp's input impedance.  The amp's input impedance should be at least 10 times greater than the output impedance of the Quickie to get the best sound.

[aragorn723]

by matching impedance, i mean getting the input impedance of the amp to be 10 times greater than the output impedance of the Quickie.  How would you suggest hooking it up? 

[Paul Birkeland]

by matching impedance, i mean getting the input impedance of the amp to be 10 times greater than the output impedance of the Quickie.  How would you suggest hooking it up?

This is a separate issue from not being able to turn the Quickie up very much because the amplifier is so sensitive.

To decrease the sensitivity of an amplifier, you can add a pair of resistors at the input.  In your case, try a 47K resistor in series with the input and a 10K resistor to ground.  Here's a page with some decent photos: Amplifier Padding

Doing this will also increase the input impedance of your power amplifier. 

[Grainger49]

You need to look inside your amplifier to see where the input resistance is now.  Many amplifiers have a resistor from the center conductor of the input RCA jack to the ground, or outer conductor of the input RCA jack.  Removing it and changing it to the needed resistance would work.

Others have the input resistor on a circuit board where the first stage of the amplifier is.  You can trace it through the input wires.

[Paul Joppa]

On the web I could only find specs for a version of this amp with balanced inputs. That makes the attenuator question more difficult, especially without a circuit diagram of the input.

[aragorn723]

There are 2 versions of the amp out there- an R500, and R500 plus.  The one I have is the R500.  Not sure this will help, but here is the manual (the one at the bottom of the page).

http://www.manualslib.com/products/Inter-M-R-500-584310.html

I'm using the 1/4" jacks, but based on the manual it looks like both the 1/4" jacks and XLR are balanced??

[aragorn723]

Better yet, here's the service manual:

http://www.eserviceinfo.com/index.php?what=search2&searchstring=R150%2C+300+%26+500

[Paul Joppa]

I read the manual the same way - there is only a balanced input, of nominal 10K impedance.

According to the manual, there are input level controls on the amp. Turn them down, then you can turn up the Quickie controls.

[aragorn723]

that's what I wound up doing, it improved the sound of the Quickie!  I get a more detailed sound, and better high end response as a result.  Is there a drawback to doing it this way vs. changing the input resistors?  The only one I can think of is volume, but i'm already getting enough of that (and my wife is too lol).  Thanks!

Dave

[Paul Joppa]

Looking at the block diagram, the only way it can be a problem is if the Quickie would overdrive the balanced input stage. If it's a balanced input for pro applications (as it is) then it should be able to handle at least +20dBu peaks, which is more signal than the Quickie is capable of generating. No problems!