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building an amplifier, need some help

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rasterburn

Member
hello, a friend and I are planning on building a small guitar amp and need some minor instruction, most of it we have figured out cause the schematic for it is drawn out and publicly accessible but we plan to make some minor adjustments, some features of the amplifier we plan to remove and replace with other features, the most notable being the built in overdrive, we plan to remove that and replace it with an effects loop. For me this seems to be an easy task, I already know how to remove the overdrive from the board, that is simple, and I have a vague idea on how to install the effects loop, more or less I am just looking for either a confirmation that my idea is correct or an explanation where i screwed up in my idea....

So for starts I have 3 1/4" mono switched jacks laying around, 2 of which i will be using for the effects loop, on the bottom of these jacks it has 3 pins labeled 1, 2, and 4 according to my test meter pins 2 and 4 are connected while nothing is plugged into the jack while pin 1 is "sleeve" leaving pin 2 as "tip". If something is plugged into the jack then pins 2 and 4 become disconnected.

My idea for this effects loop looks something like this:

Code:
inst. jack           effect send             effect return           pre-amp

   tip ------------------ 2 --------------------- 4
                          4 --------------------- 2 ----------------- signal
   sleeve --------------- 1 --------------------- 1 ----------------- ground

From what i can tell, a person could plug a tuner into the send or return without effecting the circuit between the instrument and the amp but also enable the use of an array of effects pedals without unplugging their instrument from the amp.
 
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ljcox

Well-Known Member
I suspect that the reason why no-one has replied is that they are like me - I can't work out exactly what you're asking.

Obviously, if you plug something into the "send" jack, it will be connected through to the pre-amp.

But I can't see what happens if you plug something into the "return" jack. It goes through to the "send" jack but no further.

Please give us more explanation.
 
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ljcox

Well-Known Member
I think I know what you're wanting to do.

See attachment.
 

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rasterburn

Member
ljcox, you would be correct. The only difference is that i was thinking of putting the send and return before the pre-amp only because the instrument amp I have does not have an effects send and return, instead the effects are placed inline with the signal sent from the instrument to the pre-amp, is there any pros or cons to placing the effects loop between the pre-amp and the power amp instead of before the pre-amp? The only problem I can see with your sketch of the pin-out for the send and return jacks is that either nothing is plugged in or an external loop is created (this rules out the use of a tuner on the send line). I think by crossing pins 2 and 4 the all or nothing issue would be resolved, the only issue to follow up on is the enforcement of the send and return but I dont think thats anything a diode cant fix :D

Edit:
ok so my ascii drawing is slightly crude, and ljcox' sketch is extremely crude so i figured i would upload an image of the ideas about the effects loop i used expressSCH (part of expressPCB) to make it and export it. It should give anyone looking a good idea of what i am trying to do, maybe someone can chime in and point out the pros and cons for it?
 

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ljcox

Well-Known Member
The advantage of having a pre-amp before the Send jack is that is amplifies the signal and therfore improves the signal to noise ratio.

But this may be not essential as the difference in S/N ratio betwen to 2 options may be marginal.

The other possible issue is whether the output from the effects will be too high a level for the pre-amp input.

However, I'm wondering that - if you don't have a pre-amp, why do you need the Send & Return jacks?

In other words, plug the guitar into the effects equip and the output of the effects equip into the pre-amp input.

I don't understand your point about the tuner or the diode. Are you suggesting using a diode as an analogue switch?
 

ljcox

Well-Known Member
I just noticed your edit. It is very hard to read with the red background.

I had to save it and open in Paint to be able to read it.

Comments.

1. You can't use diodes as you have drawn for AC signals,

2. The Inst & Send inputs are in parallel. Normally you use summing resistors or a summing amp for combining analogue signals.

See my mod to your drawing.
 

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rasterburn

Member
I completely forgot that audio is AC current, the diode thought was more to enforce the "return" jack as an "effects in" where as the send is the "effects out" i also intended to place a diode between the pre-amp and "Con2" to block any signal being backfed to the pre-amp, but since audio is AC current diodes do not work.

In regards to the Inst and send being in parallel like i said in a previous post on this topic that pin 2 and 4 are connected but become disconnected from each other when something is plugged into the jack thus the crossing of pins 2 and 4 between con2 and con3 to ensure there is always a circuit across the connectors.

A tuner is just a device for tuning instruments, many digital ones have a 1/4" TS jack on the side of them that you can plug your instrument into but its the end of the line in terms of circuits
 

ljcox

Well-Known Member
In regards to the Inst and send being in parallel like i said in a previous post on this topic that pin 2 and 4 are connected but become disconnected from each other when something is plugged into the jack Which Jack? thus the crossing of pins 2 and 4 between con2 and con3 to ensure there is always a circuit across the connectors. I don't understand this.

A tuner is just a device for tuning instruments, many digital ones have a 1/4" TS jack on the side of them that you can plug your instrument into but its the end of the line in terms of circuits

2 & 4 are always connected unless there is a plug in both Con 2 & Con 3.
 

rasterburn

Member
In a sense yes, pins 2 and 4 are always connected unless something is plugged into both con2 and con3. In another sense when something is plugged into say con2, pins 2 and 4 on that specific jack are disconnected from each other but the signal is still passed to the power amp, this would be handy if a musician was to plug a tuner into the send but still wanted to hear how it sounded coming out of the amp, with your diagram this is not possible. The basic theory behind my diagram is that a musicians signal is un-effected until an external effects circuit is created... The only time the circuit would be broken is if someone were to plug a cable into both con2 and con3 and leave the other end of the cable dis-connected from anything else.
 

ljcox

Well-Known Member
If you want to be able to combine the instrument & tuner signals, then I suggest you re-read point 2 in my poat number 6.
 
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