Clean boost with lm741 or lm386????

[wsgarner]

 :icon_question:I have 2 lm386's (from sterio Little Gem I built, but didn't like) and a lm741 (from Blend project that I could never get working correctly). I need a clean boost and have these opamps availible. Can any one give an idea or a schematic using either one of these for a clean boost??

Thanks,
Scott

[slacker]

You could build an MXR Microamp with the LM741. There's a schematic and stuff here http://www.generalguitargadgets.com/index.php?option=content&task=view&id=92&Itemid=120

I don't know if there is a 386 based booster out there. You could try playing around with Aron's Smash Drive you might be able to come up with something.

[The Tone God]

The 386 is really a power amp intended to drive speakers and not be a booster. The 741 is an opamp more suited to be a booster. You can look for any standard non-inverting or inverting opamp circuit to perform this function.

Andrew

[MikeH]

Yeah.  I built a booster with a 386 and it didn't sound great.  Although it did work.  If you want to try it, basically you leave the gain "loop" (pins 1 and unused and use something in the area of 0.01-0.1 for input and output caps (pins 2 and 5).  A little filtering on the input and power supply, ground pins 3 and 4, 9V to pin 6 (I think?) and bam.  That will give you the basic idea.   It give's plenty of gain but it's not exactly clean and the breakup doesn't sound natural the way a FET does.

[markm]

I don't as if there is anything real "Clean" with a 741 however, It's a favorite of mine! 

[wsgarner]

OK, somebody tell me if this will work:


Scott

[doug deeper]

ive messed around with a slightly dirty boost using a 386, tacking on a jfet buffer makes it pretty ok.
clears things up a bit.
but the 741 is probably a better choice.

[Steben]

OK, somebody tell me if this will work:


Scott

it will work, though I would suggest different values.
That output cap of 47 is a very big choice.
1M/500k duo also very big. 100k/50k will work as good. I don't like big values since they induce more noise...
With same 47pf you'll have less treble cut however (more presence), it could be wanted, maybe not.
If you lower a resistor, you should raise a cap. So 470pf will have same treble cut, it is 3.5kHz however, a bit low.

[wsgarner]

Steben,
OK, so let change some values and see what you think.

1. 500K pot to 50K
2. 47uF to 0.1uF
3. 47pF and 1M?? What if I eliminate them all together. I don't want to color the sound at all. I just want a straight up boost for leads.

Let me give some details on my rig and why I need a boost. This may give you some info on how to point me the correct direction.
I have stomp boxes (first to last: boss me30 multi[using wah/valume--flanger--compressor: this goes away once I get the needed stomps to replace it]), digital delay, effects loop[destortion and noise gate in loop], chorus, EQ and am running into a Crate gx1200h head with celestion 4x12 cab. Rhythm is fine (mixes in well), but leads don't pop out at all. I have a hard time hearing myself even, so leads tend to come off as very mechanical (no feeling).

I want to build this with parts I currently have (this way all I have to buy for this project is an enclosure, LED, and knob).

Thanks for the help,
Scott

[d95err]

3. 47pF and 1M?? What if I eliminate them all together. I don't want to color the sound at all. I just want a straight up boost for leads.

You could eliminate the cap, but not the resistor. The resistor makes up the feedback loop that constrains the gain of the circuit. An opamp has (for all practical purposes) infinite gain, so the result would be just noice or a horrible square wave (would sound awful).

What was suggested was using a 50k pot (instead of 500k) and a 100k resistor (instead of the 1M).

Another small note: The negative pole of the battery should be 0V, not -9V (unless you're using an 18V battery...)

[wsgarner]

So, does this look like it would work "more better":


Thanks for the help,
Scott

[d95err]

So, does this look like it would work "more better":

You need to add a resistor from the bias voltage divider (10k + 10k) to the input. (You had an 1M resistor in the previous schematics , so I'm sure you just forgot to add it to the new one).

There seem to be some mistakes in the LED and switch circuit.

You should not switch the battery off when you go into bypass mode. It would probably cause wierd noices and stuff when you switch it back on again. The battery should be on all the time, even when in bypass mode. Most stompboxes use a stereo input jack that is wired to automatically switch the battery on when a plug is inserted and off when the plug is removed (so to save the battery, you need to unplug the input when you're not using it).

Second, the LED is wired backwards. Also, you are feeding the LED only 4.5V with a 10k series resistor. The typical solution is to use 9V and a 4.7k series resistor. Use a separate series resistor for the LED, don't use the bias voltage divider resistor.
Here's a layout from GGG on how to wire true bypass with an LED indicator and input jack battery switching:
http://www.generalguitargadgets.com/diagrams/switch_lo_3pdt_tb_battery.gif

Another tip: You should put "pop-stopping" resistors from input/output to ground between the input and output caps and the switch. A value of 1M should work for the input and 100k or so for the output. This prevents popping noices when you hit the bypass switch.
http://www.geofex.com/Article_Folders/box_pop.htm

[GibsonGM]

Neat project...
Be prepared to swap in/output caps once you get it together, I think.  You might find the 741 to be a little tinny as you go up in gain, without as much "body" as say a FET-based booster.  Look at the MXR Distortion +, I'm assuming you got your idea from that?   Your 4.7uF cap in the feedback loop is a good idea to counter that tinny-ness.   With d95's suggestions, it'll work, though!   

[Steben]

Still I would place the cap across the 100k back in it. It goves stability at high gain setting. Don't forget guys, at full gain (pot= zero!), it goes almost to open loop gain. It means the booster will surely clip severely. Minimum gain is (100+50)/50= 3. Max gain before clipping is  - according to what guitar you use - 35 (sc) to 15 (HB). You see that this is much lower than what this thing will have as maximum. At open loop gain, it will oscillate (wheeeeeeeeee!...). So I would place that 47pf back in.

[wsgarner]

I knew I get all the answers I needed here!!!
I also thought I put that dang LED in backwards too ("Hello, McFly").
So, I think I have all the suggestions in place now. Let me know:


Thanks again,
Scott

[d95err]

A final adjustment would be to add a small resistor in series with the level pot.

This is to limit the maximum gain available, to avoid oscillations and unnecessarily hard clipping. The gain of the circuit is 1 + (100k/level pot) , so as you can see, it gets infinite when the level pot is zero. Decide what you want as maximum gain, and add the appropriate series resistor. E.g. if max gain should be 50, you'd need a minimum series resistance of about 2k.

[wsgarner]

d95err,
I don't want to gain too much. I just want this for a lead boost. I don't want to over-drive the amp input at all. Knowing this what would you recommend?

Scott

[GibsonGM]

Ideally, you'll breadboard this first, and at that time you can try different series resistors to limit your max gain.  Start with the 2k d95 suggested, and you can always increase it     Changes like that are what makes a circuit your custom thing - only you know what you'll like in the end!   

That's why everyone breadboards; what if it's really really shrill, and you need to change some cap values?  You can do it on the board, but not once it's all soldered together (very easily, anyway). 

[wsgarner]

GibsonGm,
Your right. I'm with you, but I don't have a breadboard to work with. That's why I'm putting this out there for the experienced DIY'ers to keep me going in the right direction. I'd go out and buy a breadboard, but I'm not sure I will keep this up. We'll see. So far it's been fun though.
Don't worry, I'll "screw-it-up" before its over. I did my effects loop with Blend. I'm sure I'll do it here too.

Scott

[tonefreak]

GibsonGm,
Your right. I'm with you, but I don't have a breadboard to work with. That's why I'm putting this out there for the experienced DIY'ers to keep me going in the right direction. I'd go out and buy a breadboard, but I'm not sure I will keep this up. We'll see. So far it's been fun though.
Don't worry, I'll "screw-it-up" before its over. I did my effects loop with Blend. I'm sure I'll do it here too.

Scott

I too was hesitant about breadboarding.  But having the ability to change out components and tweak the circuit becomes very addictive and easy with the breadboard.  You start tagging things on, changing out component values, etc... a regular Frankenstein!!