Is an "effects loop" possible?

Submitted by dimipana on Thu, 01/17/2013 - 16:21

Hello -

I recently ordered the kit and can't wait 'till it gets delivered to start putting it together. I'm very excited about this project. I too have build some basic small amps based on the LM386 and I agree, they all sound -more or less- the same.

I read through all posts in the forum and could n't find a similar thread so I hope it's OK to start a new one.

My question is how would you go about "cutting" into the existing circuit in order to create a "effects loop" out/in points. I want to use that to feed an effects unit I have and -if I am correct- the correct placement would be in the "middle" after the pre-amp and before the final stage of the unit.

I would appreciate any comments you may have on that.

Thank you very much!

Member for

11 years 5 months

John

Thu, 01/17/2013 - 22:36

When you get your kit, take a look at the board. See how R12 goes to pin 3 of the Trim pot? I think pin 3 is the most natural place to put your effects loop. You can use the trim pot to turn down the signal coming from your effects if need be, which can be quite helpful.

So in this case, pin three on the board is effects out, and the wire leading to pin three on the pot is effects return.

Member for

9 years 10 months

dimipana

Fri, 01/18/2013 - 16:57

... for the quick response.

Yes, I see how R12 connects to pin 3 of the pot trim. So, if I understood correctly, what I have to do is: 1st) Not solder pin 3 (on the PCB) but use it as the OUT (i.e. feed) point and 2) Use that point on the PCB (where pin 3 would otherwise connect) as the IN (i.e. return) point. Am I right?

Now this assumes of course that all connections on the IN/OUT phone jacks have been done correctly, especially making sure the polarities are not switched around and that I use the right type of 1/4" phone jacks (e.g. the ones that do not alter the signal path when nothing is plugged in). I know there is a proper name for that, but I forget it now (lol).

One more thing that comes to my mind is the use of the "trim" pot. What effect does it have on the circuitry? Is it a tone control or it's more than that. I am asking because, I really do not want to complicate this mod by adding a level control, my fx unit has both IN and OUT level controls and a "clipping" led so I can control my levels from there.

From the "Noisy Cricket" I built some time ago (my first real DIY electronics project) my understanding is that there is a "gain" that controls the pre-amp level, a "volume" that controls the final output and a "tone" that acts as an apparent treble boost. Is the "Stella" designed along the same idea or it functions in a different way?

Finally, I asked the same question at Petey Twofinger's YouTube channel (http://www.youtube.com/watch?v=e_n-GVSh_As) and his response was that even though an fx loop is feasible, I could simply run the effects unit in front of the amp without the sound getting too muddy, etc. Now, you John, being the designer of this amp, what is your opinion on that? Is an fx loop worth it? I am definitely a noob but anyone will tell you that the proper placement of an effects unit would be in between the pre and and the final amp stages, right?

Sorry for the long msg and the questions, I really appreciate your time!

Best Regards,

--
Dimi P.

Dimi P. (not verified)

Sat, 01/19/2013 - 10:38

I received the package yesterday, fast delivery, the included resistor chart is so useful. Thank you for everything, I'll start building next week, hopefully it will be a straight forward process. Thank you again!

Member for

11 years 5 months

John

Sun, 01/20/2013 - 00:04

You're welcome!

If I was you, I would build the entire amp as designed first. Try your effects in front of the amp. Turn the gain all the way down, that will pass the signal through the first stage of the amp without amplifying it.

Then turn the trim pot all the way up.

The level pot will then be your overall amplifier level control.

Depending on your power supply and input signal level, as you increase the level control, the Stella Amp may start breaking up and distorting in a non-musical, dissonant fashion. If that is the case, you can turn up the level control all the way, and turn the trim control down until you reach the maximum volume and distortion characteristics you are looking for.

Then leave the trim control set, and turn the level control up and down for maximum or minimum volume.

Or, you know, you can wing-wang those pots around all over the place and do what you like. :)

Anyway, the whole point of having an effects loop is if you wanted the effects to take place after you get the preamp to distort for you. To know whether or not you want to go through the hassle with this amp, I would suggest you listen to the preamp and how it distorts first.

Increasing the gain of the preamp stage significantly is going to affect how the trim and level controls interact with the preamp, and with each other.

It's hard to give concrete advice about the trim and level settings, because it really depends on what voltage you use to power the amp with, and whether you use batteries or a power supply to power your amp.

I tried 3.3 volts, 4.5 volts, 5 volts, 6 volts, 9 volts, 12 volts, and 15 volts (after swapping out the opamp) when I was designing it. They all sound good, but they distort in a different way. Settings that will get you mild overdrive on 9 volts may give heavy thrash distortion on 3.3 volts.

I could have designed this thing to run on 9 volts and that is it, but I wanted something more flexible. That is why the trim pot is there. At higher supply voltages the amp sends too much voltage to the TBA820M and it starts feeding back, and distorting in very bizzare ways. You can turn the trim pot down to get rid of a bit of that crazyness. OR you can just crank the trim pot all the way up and never crank the level pot so high.

I guess I'm rambling. Let me go look at your question to see what you were asking.

No, the trim pot does not affect the tone. It affects the distortion characteristics of the level pot. If you crank the trim pot all the way up you can just ignore it. WIth the trim pot 100 percent cranked, the Stella Amp is nearly exactly how I designed it the first time. But you may find you don't like the wonky distortion at 100 percent.

A note on the gain control. If you crank it all the way you will nearly certainly start the opamp oscillating. Crank the trim pot WAY down, way way down, maybe 20 percent or even less. Crank it down to stop it oscillating, basically. You can get a really nice smooth fuzz from cranking the distortion way up and the trim pot way down, but it's a real narrow slice of usable space.

I'm rambling again and getting off topic. Let me finish up.

Build it as designed. Try it out. See what you like.

Now, if you like the distortion of the preamp sound all by itself, AND you want to put another effect on the signal after the preamp, then put in your effects loop.

OR you can put in an effects loop if you want to use the preamp as a buffer for your entire effects chain.

If you decide to put in an effects loop, if you want to keep the interaction between the trim and level pots, cut the wire that just goes to terminal 3 on the trim pot. The wire coming up out of the board goes OUT to your effects loop. The wire going to the trim pot comes back from the effects loop. You will have to solder your own ground wire.

If you want to eliminate the trim pot entirely, cut the wires and set the actual pot aside. Then, pin 3 is the signal going to your effects loop, pin 2 comes back from your effects loop, and pin 1 is a handy ground wire.

You will have to figure out the mechanics of the switched jacks for the effects loop yourself. I've got a pretty good idea of what I would do if it was my amp but I'm not entirely sure, and I don't want to lead you astray with any speculation on my part. I would start with one of these:

http://www.mouser.com/ProductDetail/REAN-Neutrik/NMJ6HCD2/?qs=sGAEpiMZZ…

Those are the switched jacks you are looking for I think.

Member for

9 years 10 months

dimipana

Sun, 01/20/2013 - 23:40

John -

Your "rambling" basically saved me hours and hours of unguided reading and aimless testing. I now have a better idea what to expect in what I am about to do! You gave me a lot of insight that will prove very useful. Like I said I am new with all this and the more I think about it (and after reading your last post) the best thing to do is build it as is and then "play" with it to become familiar with what the pots can do to the sound. Also, the more I think about it, the more I like the idea of the "Trim" pot altering the distortion characteristics and NOT being a "tone" control. I can't wait to build it and listen to it. By the way any luck on fixing that BRS40 shipping cost software glitch?

Thank you very much for the time and thought you put in your messages!