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Old 8th February 2014, 08:48 AM   #1
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Default Balanced/unbalanced line receiver

Hello everyone, I hope this isn't too much of a newbie question, but I need some advice on an input stage I am trying to build.

Basically I am trying to build an input stage that can accept either an unbalanced input (from a guitar) using a mono TS cable or a balanced input from my effects processor using a stereo 1/4 inch TRS cable.

The single output of the receiver will drive various different effects pedals depending on how I set up my looper.

I have built a very basic circuit using a to ina134 based on figure 1 of the dats sheet

http://www.ti.com/lit/ds/symlink/ina134.pdf

So far I have tested only with an unbalanced input ie my guitar tip connected to the hot input and the cold input tied to ground.

My problem is that the circuit is giving me some attenuation and is colouring the guitar signal, making it sound a little muddy or muffled.

I have built a very nice sounding buffer which on its own works great, but even if a put this after the receiver CCT, the problem exists.

I wondered if the basic receiver CCT I am using is not suited to the hi-z guitar output or if I am simply using too basic a receiver CCT.

Does anyone have any thoughts or experience with a similar problem, as you can probably tell, my audio electronics expertise could fit in a match box without taking the matches out first so apologies if this is a dumb question, be gentle with me
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Old 8th February 2014, 01:54 PM   #2
Zero D is offline Zero D  United Kingdom
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Hi, the buffer needs an input resistor of about 1 Meg from the OpAmp Positive input to Ground. Also it probably requires some Gain to match the output of the ina134, as suggested below.

I would include a Switch to select between the Output of this & the ina134.
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Old 8th February 2014, 04:10 PM   #3
AndrewT is offline AndrewT  Scotland
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the balanced input has two signal pins (inputs) and one Chassis pin.
A standard balanced interconnect matches this perfectly.
An unbalanced interconnect will attach to the two signal pins. pin 2 is signal Hot. Pin 3 is signal ground/return. You need to add a manual switch that shorts pin3 to audio ground inside the balanced receiver.

That makes your bal/unbal receiver compatible with both types of Source.

But you have TS and TRS cables. The ring is pin3 and the TS does not have the ring. Problem ! Do you modify your TS source to make it a TRS? Your decision. I'd suggest you DON'T modify your guitar TS plug.

The guitar needs is own receiver circuit. Use what you have, but fit a trs output to that circuit. It won't now be compatible with TR inputs. Label it as such.
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Old 9th February 2014, 01:45 PM   #4
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Hi guys thanks for the replays, I will definitely try the additional resistor, but Andrew, are you saying what I am trying to do won't work? Ie the balanced and unbalanced inputs need to be separated?
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Old 9th February 2014, 02:46 PM   #5
tinitus is offline tinitus  Europe
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your guitar connected directly needs a hi-z input
your effects may work ok with less

only your processor or other PA gear needs the bal/se converter, and this will not need the guitar hi-z input at all

but isn't it most common now to use the effects as pre stage between guitar and amp ?
you want the pedals on the floor where you stand, and makes little sense to use long loop cables
you can get small pre/gain/booster 'effect pedals', and they are often basicly just a small hi-z preamp
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Old 9th February 2014, 02:56 PM   #6
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Thanks tinitus that makes sense, I just tried the circuit with a hi-z buffer in front of it and definitely the receiver was being loaded down by the guitar pickups.

I am going to try building the instrument amplifier version shown on the ti data sheet, do you think this will work? My goal is to make an input stage that allows me to plug in either the guitar or a balanced output from an effects unit without having to use separate inputs or a switch.
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Old 9th February 2014, 03:05 PM   #7
tinitus is offline tinitus  Europe
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I'm not sure
depends on your setup

I do not use any effects for my bass, so I need amp with hi-z input

but if you use effect pedals with hi-z input, then your amp wont need a very hi-z input
but most pedals are still designed for relatively high input z
maybe 100k will do, and your BAL/SE receiver may just do that perfectly, or maybe less will do

and you can give you amp a second hi'z input jack for direct connect

but when it comes to BAL/SE converting you better listen to Andrew

I have been tempted to try some PA gear, but everything is mostly balanced, and I gave up on it, so far ...
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Old 9th February 2014, 03:14 PM   #8
AndrewT is offline AndrewT  Scotland
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I am NOT anything like an expert.
I read Whitlock and Rane and Self and Jensen and etc .....
I regurgitate what I read/learn.
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Old 9th February 2014, 07:06 PM   #9
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I added the 1 meg resistor from above but it didn't seem to help. Have some parts on order to build the instrumentation amp CCT from the data sheet, hoping this will work.
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Old 10th February 2014, 04:27 AM   #10
Zero D is offline Zero D  United Kingdom
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@ Bigtim1966

The 1 Meg resistor should have worked ? If you could draw a circuit of what you've done so far, & what you're proposing, it would assist us better in helping you
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