Author Topic: Cont. Converting Stompbox to Line Level  (Read 2981 times)

audiolife

Cont. Converting Stompbox to Line Level
« on: February 28, 2013, 08:48:07 PM »
Not sure why I can't respond to this previous post...

http://www.diystompboxes.com/smfforum/index.php?topic=101615.0

So, RG, what you're saying is that in order to work effectively the entire circuit needs to be reformatted for line level functionality?

R.G.

Re: Cont. Converting Stompbox to Line Level
« Reply #1 on: March 01, 2013, 12:20:32 AM »
What I was saying is that the line level input needs attenuated in a (resistive divider perhaps) by about 10:1 to work properly. It's good if the stompbox has some gain at the end to boost the signal back up to line levels.

Line levels are nominally about 1Vpeak into 600 ohms. Guitar levels are about 100mV, 1/10 of the same size. This is a very crude approximation, as there are not good standards for guitar outputs.

A 9.1K and a 1K in series makes a good 10:1 attenuator. The high signal goes into the 9.1K, the low signal comes out of the junction of the two. The other end of the 1K is grounded. A more flexible arrangement uses a 4.7K to 6.8K into a 5K pot. This lets you adjust the levels somewhat.

Most distortion pedals have large output levels, and can be set to do the proper 1Vpeak output.

These are gross generalizations, but will get you in the ball park, especially the one with the pot.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

PRR

Re: Cont. Converting Stompbox to Line Level
« Reply #2 on: March 01, 2013, 01:43:48 AM »
ARON!  
Don't move this on-going thread into FAQ section until done.... folks can't reply in FAQ.

> why I can't respond to this previous post...

The FAQ section is locked to replies. I agree this thread ought to be archived as a FAQ; but not yet.

__________________________________________________

R.G. has the outline. Myself, I'd consider (perhaps after a test) a gain-up stage after the pedal. While R.G. is right that many pedals can output high voltage, some can't; and some can't cook into the low (10K-22K) impedances of many modern line inputs.

OTOH a gain-up needs power, battery or something.

Here's a sketch. It has to be fleshed-out with power and bias details. In a +9V world, the opamp inputs would instead go to V/2 supply. But 9V is small for Line Level work. Most line inputs will accept the few volts this can muster; however it is customary to run line-stuff on +/-12V or more.



You did not say *what kind* of "line level signal". Unbalanced is far easier. Most modern "balanced" line gear will take unbalanced signals just fine. Adverse conditions (long audio lines around hashy power systems) will favor true balanced. There are many ways to dress that puppy.

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R.G.

Re: Cont. Converting Stompbox to Line Level
« Reply #3 on: March 01, 2013, 10:12:05 AM »
It's worth noting here that the difference between guitar level and line level is the problem at the bottom of using guitar pedals in amplifier effects loops.

While the construction and levels on amplifier effects loops are anything but standard, they tend to be about line level. Doing a combined attenuate-down/amplify-up get the levels to what floor pedals expect and put out. Some amps have variable output levels and switchable sensitivities on the return, but by no means all.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.