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Author Topic: Effects Loop Questions  (Read 3024 times)

aoresteen

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Effects Loop Questions
« on: November 09, 2011, 04:12:20 PM »
I've never used an effect loop so please bear with me.  I've always just put my effects infront of the pre-amp.   As I understand it, the FX loop:

1. Sits between the pre-amp and the power amp.  When used the signal from the preamp is routed to the FX SEND jack.

2.  The signal then goes to the IN of some 'effect' unit (say a flanger stomp box) and then the OUT of the stomp box is connected to the FX RETURN jack.  The FX RETURN jack then routes the signal to the power amp.

3.  The effect unit is constantly on; no need to hit the foot switch.

Sidenote: Years ago we used to have 'pre-amp out' jacks on some amps (balanced or unbalanced - XLR or 1/4") that would send a COPY of the pre-amp signal out so it could feed a larger power amp or be used for recording.  I guess this was the begining of an effect loop.

Questions:
========

1.  When using an FX loop is the *entire* signal routed out the FX SEND jack with nothing going to the power amp?  Or is a portion of the signal still sent to the power amp?   So if I plug a 1/4" cable in the SEND jack will that mute the power amp section unilt a cable is connected to the RETURN jack?

2.  If I have a 2nd signal source (say another preamp) and connect it to the the FX RETURN jack, will that mute the internal pre-amp signal (assuming nothing is plugged into the FX SEND jack?


Practical Problem:
=============

Here's what I want to be able to do.  I want to have an FX RETURN jack on my amp so I can use an external preamp INSTEAD of the existing pre-amp and have the power amp section run the signal to the existing speakers.

I also want to control it by using a foot switch in front of the amp to change from the internal pre-amp and the external pre-amp.

My amp does not have an effects loop so I would have to mod the amp to install the RETURN jack and cut into the tracce that connectes the internal pre-amp out signal to the power amp.

Is this doable?  Any pitfalls I might encounter?

Thanks!
Tony Oresteen
Peavey Bandit Silver Strip, Revolution 112
Marshall MOSFET Lead 100 3210
Squier SP10
Newnan, GA

Enzo

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Re: Effects Loop Questions
« Reply #1 on: November 09, 2011, 08:07:30 PM »
Most FX loops are simple breaks in the signal path through the jacks.  SOme fancier ones have a little drive circuit for the send.

You can verify this in a 10 second test, but plugging anything into the return jack generally cuts out the path from the preamp.  The send jack doesn't cut out anything, so your description of the two jacks' function was kinda backwards.  SO you can use the FX send as a preamp out line without affecting the amp's operation.  Plugging into the FX return cuts off the preamp feed, otherwise whatever was coming into the return would be trying to force-mix itself into the preamp signal.

Plugging into the FX return should cut off the feed from the preamp to the power amp regardless of what is happening at the send jack

Most FX loops are what people like to call "serial" FX loops.  Those are as described, simply a break on the path.  Less common are "parallel" FX loops which are what you described as allowing some dry signal through.  Most of those will have some sort of balance control so you can adjust the mix between returning FX signal and dry signal.

I am sure there are exceptions to any of these descriptions.


Why don't you tell us what the make and model of the amp is so we can consult schematics.


You want to have switchable using the internal or external preamp with the existing powr amp, correct?   A standard FX return jack would break the path from the preamp.  So you would need a way to get the preamp back into the power amp when selected.  The way I would think to do this would be to add the FX return, assuming there already is a preamp out, then have some sort of mixing circuit connected to the return.  On input of the mixer would be the preamp, and the other input would be the external preamp.  SO now whichever preamp gets a signal, it will feed the power amp.

Then put an A/B/Y switch out in front of the amp.  That would then let you send the guitar signal to A (internal preamp), or B (external preamp) or Y (both preamps, like a Y cord).

The mixer could even be passive and amount to a few resistors, or you could use a small active mixer.  Most makers like Peavey and Behringer and others make small inexpensive 2-4 channel mixers

aoresteen

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Re: Effects Loop Questions
« Reply #2 on: November 09, 2011, 11:19:18 PM »
Thanks Enzo!

I don't have a guitar amp that has an effect loop in it so I was trying to figure it out 'on paper'.

I guess more info would help :)

I'm building/modding a Squier Strat to be a MIDI guitar.  I'm using the Ghost system.  The guitar will have a humbucking pickup at the neck and the pizo acoustic pickups at the bridge.  It will also have the HEX kit for MIDI out via a standard 13 pin MIDI cable.

What I want to do  (in a live situation) is to run a regular cord out out to my pedal board for the humbucker/pizo acoustic sounds and a 13 pin cable to my Yamaha G50 controller 1U rack size.  The G50 will drive an Alesis QSR 1U rack MIDI sound module.  I want to feed the outputs of the QSR (mono mode) into the power amp stage when I'm doing a sax/horn/flute solo etc via MIDI.  So I need the pre-amp stage to be muted and just have the QSR sounds come out the power amp stage.

I have used a 1U PA power amp driving a set of speakers but that is a pain to set up.  By going throuh the existing amp I eliminate the need for a seperate power amp & speakers.  Also when the amp is miced it's miced for both regular AND MIDI sounds.

I haven't finalized my choice of amp quite yet.  I am really close to getting a Line 6  Spider IV 30.  I have plenty of tube amps but I need a power amp with very little distortion for the MIDI sounds.  A SS amp is the best choice.  Since 'modeling' amps get their change of sounds in the pre-amp stage,  the power stage should have a low THD number.  Of couse Line 6 doesn't give out that info.  I don't need a lot of power - 30 watts is plenty as I will have custom cab built for it.

I know the Spider IV 30 doesn't have an effects loop but since I will be adding custom switching anyway modding it for a FX return loop should be a rather simple task (in theory!).

Does any of this make sense?
Tony Oresteen
Peavey Bandit Silver Strip, Revolution 112
Marshall MOSFET Lead 100 3210
Squier SP10
Newnan, GA

Dimi Pana

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Re: Effects Loop Questions
« Reply #3 on: November 10, 2011, 03:32:59 PM »
Hello -

I do not mean to distract you from this very interesting conversation, I just have a quick question for all but especially for Enzo and what he said here:

Most FX loops are simple breaks in the signal path through the jacks.  Some fancier ones have a little drive circuit for the send.

I have a very cheapo practice amp I am experimenting with and being a novice, destroying basically a toy is not the end of the world. ;)

It appears that this amp is build around a JRC4558 for the preamp and a TDA2030 for the power amp (please show mercy I use the terms loosely. :-[ . There is also a FET at the amp input acting -I think- as a buffer.

The toy-amp has one Volume, one Gain, and one Tone control. 

Assuming that I can identify the point where the signal is handed-off from the preamp to the power amp and given the cheap-o-ness (read: simplicity) of the design, is it safe to assume that simply by cutting into the pcb trace at that point[/u] and inserting two 1/4" audio jacks as SEND/RETURN will provide me with a crude yet functional FX Loop ?

From Enzo's post, I understand that some (more sophisticated) FX loops use a drive stage at the output (i.e. return) to compensate for any signal loss. In my case, this is not applicable, I can't build a drive stage plus I think I do not need one.

Because:

I plan to use an FX unit that has separate stereo in/out level controls, plus a mix pot DRY/WET and -if the unit documentation is not lying- a true bypass foot-switch.

So, what my diy FX loop will be lacking, the FX unit will provide thus compensating for the crude hack.

I tried to put the FX unit first, e.g. after the guitar and before the toy-amp but it sounds, well, just bad...

Again, sorry for the intrusion, aoresteen is doing exactly what I want to do (of course mine is extremely simplistic) so I was hoping to ask if this would work.

Thank you!         

J M Fahey

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Re: Effects Loop Questions
« Reply #4 on: November 10, 2011, 05:32:26 PM »
Yes it will work.
Go backwards, from TDA2030 pin 1 , which will typically have a 22K resistor to ground then go through some DC blocking capacitor (usually 1uF).
You can safely cut the track "on the other side" of said capacitor and add your jacks.

weinstev

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Re: Effects Loop Questions
« Reply #5 on: December 06, 2011, 01:21:32 PM »
Yes it will work.
Go backwards, from TDA2030 pin 1 , which will typically have a 22K resistor to ground then go through some DC blocking capacitor (usually 1uF).
You can safely cut the track "on the other side" of said capacitor and add your jacks.

I'm considering doing this type of crude FX loop cut in to my Vox Pathfinder.  It's also a TD2030 chip amp with 4558's in the preamp.  I'm also a novice...   I bought the amp for $50.  Running a Boss GT3 in front of it is not working out for me.  The Boss GT3 sounds much better through my low quality computer speakers.

One issue to consider is that depending on where you insert your effects send and return you may not be able to use the volume control on your amp.  I (believe I have) located a point in the circuit that is just before the volume pot which is itself just before "pin 1" of the power amp chip, the TD2030.  This would allow me to be able to use the volume control of the amp if I decide to only use the return running my guitar cable into the GT3 and its Left mono output into the return jack; in other words, just use the amp as a powered speaker with volume control.  By the way, there is a little jumper on the Pathfinder's circuit board labeled J6 that I believe is the location to insert the loop as I've described.

The steps that need to be tackled include what looks to me like some very precise soldering on the board (I need to use a fine tipped iron from work, perhaps); figuring out where to tie the jacks to signal ground; drilling holes in the pan that houses the PCB for the jacks; strain relief (?) for the new leads coming out of the board feeding my jacks; and actually finding a place to buy the jacks locally.  I think that I might use two switching jacks one for the send, one for the return, since these seem to be what is available and then just tie the two inputs on the send jack together.

Anyway, it all looks extremely simple on paper, but there are definitely a few details that make a rank beginner such as myself pause before proceeding.  I understand that it is a cost savings to not include these additional jacks on low price amps, but it sure would be a lot easier if they just made them that way...

I could conceive of a manner in which to buffer the send and returns with a couple of opamps on perf board, running 9V from the power supply out to the perfboard, connecting the send and return signal lines off of the PCB and to/from the send return jacks to the opamps in the appropriate manner.  However, as a simplification, the Boss GT3 should be able to properly buffer the signal.



 

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