Solo Box

mictester · Post by **mictester** » 03 Mar 2010, 00:15

Last week, my nephew wanted a simple effect - on tromp, he wanted some extra level. Just a simple boost. Then I thought about his stage rig, and remembered that he was always a long way in front of his backline, so some buffering would probably be a good idea. The first prototype used just one dual op-amp, but gave inverted output from the "gain" side. Knowing how anal some players can be about phase relationships, (not my nephew, but many others) and having a glut of TL074s, I thought I'd re-invert the gain side, so that the straight and "gain" outputs are in phase with each other, and I'd use the last op-amp in the package for the biasing. The circuit is pretty simple:

: SoloBox1.1.GIF (8.38 KiB) Viewed 1480 times

The LED switching is filtered to prevent clicks - I used a bi-colour red/green LED (common anode). The range of level available is musically useful, any my nephew is happy with the way it works. It's designed to have no effect on the signal passing through it other than the gain - there's no tonal colouration or distortion, but it does also provide buffering. There's also RF bypas, so that it doesn't receive the local AM station!

PCB and Vero to follow!

analogguru · Post by **analogguru** » 03 Mar 2010, 09:53

Both 10k resistors at the non-inverting inputs of stage 2 + 3 can ne ommited (jumpered) when using FET-input opamps.

The bias-divider can be made with two 100k-resistors, thereby saving a small amount of power-consumption. I can´t see a reason to parallel a 100µF and a 10µF at the bias-divider.

The two 470 ohm resistors are better placed within the feedback loop (between op-amp output and feedback resistor) of the relating op-amp-stages for better compensation of capacitive loads.

analogguru

mictester · Post by **mictester** » 03 Mar 2010, 12:53

analogguru wrote:Both 10k resistors at the non-inverting inputs of stage 2 + 3 can ne ommited (jumpered) when using FET-input opamps.

The bias-divider can be made with two 100k-resistors, thereby saving a small amount of power-consumption. I can´t see a reason to parallel a 100µF and a 10µF at the bias-divider.

The two 470 ohm resistors are better placed within the feedback loop (between op-amp output and feedback resistor) of the relating op-amp-stages for better compensation of capacitive loads.

analogguru

Thanks. The 100uF should be across the 9V supply - I'll fix and replace the diagram. The 10k midrail bias resistors could be whatever resistor you have on hand, really, and the saving in current consumption is really negligible - the LEDs take most of the current!. I used 10k because I've got a reel of about 50000 of them! The 10k resistors from the midrail to the non-inverting inputs actually just makes the board layout simpler - but will allow the use of other, "better" op-amps without other modification if required! The 470 resistors aren't strictly necessary at all - they're there just for protection against shorts to ground. I find that the 072 / 074 drives cables really well.

: SoloBox1_1.GIF (8.38 KiB) Viewed 1416 times

Further notes - the latest one of these I built had two 9V batteries in it, and was configured for split rail (+/-9V) to get more headroom. I used 35V rated electrolytic capacitors throughout, so single or split rail doesn't matter -the headroom increase at 18V is useful.

livingnote · Post by **livingnote** » 03 Mar 2010, 16:55

Hey Analag, what a surprise! Didn't think I'd see you here. So you're an old stompbox nut too?

bool · Post by **bool** » 03 Mar 2010, 19:42

Analogguru isn't Analag.

Btw. Livingnote, what in the world brought you here?
Aren't you ever-too-busy tweaking the MegaBrutalEnlightenedGsslForever?

Regarding the schem - Wouldn't it be possible to use one 072 with circa half the parts to get the same gain result - and phase coherence.

Even more, the same result could be obtained with just a 071 if the switch were used to bypass the gain-set potentiometer.

But hey, if that 074 wanted to find a new, musical "home", I guess it's all good...

livingnote · Post by **livingnote** » 03 Mar 2010, 19:58

Ah ok...that sig threw me off good.

Yeah - it's quite a thing. Prob is that I still have to get all my metering I/O setup together with all the metal work and wire soldering or I'm just in for it when I debug it. So I end up hanging back a little and playing guitar, and missing all the cool stompboxes I ended up giving my friends because I wanna share the fun

But it's pretty nice for design - sometimes a hiatus gives the ideas time to set. When the project is that big you're sometimes best advised to stand waaaaay back and let some time pass, I've seen the opposite and it's total S&M

Plus all the etching work enjoys reducing whatever time's left. But it's getting time to at least finish Rev4, everybody keeps asking...

And I guess it's comforting with stompboxes, you kinda do the layouts for breakfast and a laugh

Ahhh...instant gratification for a change.

mictester · Post by **mictester** » 03 Mar 2010, 20:03

bool wrote:Analogguru isn't Analag.

Btw. Livingnote, what in the world brought you here?
Aren't you ever-too-busy tweaking the MegaBrutalEnlightenedGsslForever?

Regarding the schem - Wouldn't it be possible to use one 072 with circa half the parts to get the same gain result - and phase coherence.

Even more, the same result could be obtained with just a 071 if the switch were used to bypass the gain-set potentiometer.

But hey, if that 074 wanted to find a new, musical "home", I guess it's all good...

You could, but you'd sacrifice the active bias (I like it). You could easily split the incoming audio into two paths, feeding two op-amps equally - one with gain and one without. I've got a stupid amount of TL074s, and I'm using them up - this seemed like a good place for one!

bool · Post by **bool** » 03 Mar 2010, 20:07

mictester wrote:
You could, but you'd sacrifice the active bias (I like it).

Not with 072...

mictester · Post by **mictester** » 30 Apr 2010, 11:56

Slightly modified - a bit more gain at maximum (this was found to be good on stage), and it now mostly uses 10k resistors.
I built one last night, using a pair of TL072s and low current / high efficiency LEDs. It draws 3.8mA with the red LED on and 4 mA with the green on!

: SoloBox.GIF (8.54 KiB) Viewed 1241 times