Project Berocca Bass Preamp Power

[maxamendel]

This started as a question about the TL072...

Its evolved over the evening into a question about voltage regulation.

Theres a Rob Mods project to build a bass preamp. I'm doing a few of them. Since they're onboard, they're gonna need batteries.

On his website, he doesn't make it clear if he uses 2 batteries for 18v or just 9v. Lets assume its 9v. I do have one bass thats already routed for a single 9v and its staying that way, so for this discussion I'm assuming 9v instead of 18v.

At the front of his schematic, he has a voltage divider that brings the Vref down to 4.5v.

This connects to pins 3 and 5 on the tl072.

Is that necessary? Why can't I just omit the divider and tie pins 3 and 5 to the 9v power?

If I can't just use 9v, would it be beneficial to do a voltage regulator like the lm317 instead? I was thinking of doing that to get to 7v instead of 4.5v for the Vref.

I have a feeling this is a question about headroom, and voltage difference between 2/3 and 5/6 determine headroom, but im not really sure if that's right and if it is idk how to optimize that.

thanks for the help. its be going into an altoid tin, so I'll repay help for a build report and minty pics after im done. cheers

 

[Robert]

The opamp output voltage fluctuates around it's input bias voltage (VREF).

Like you guessed, it's set to around half VCC for maximum headroom in both cycles of the signal. If it's too low or too high the opamp output will clip.

You can't simply connect those points to VCC, and 7V wouldn't be an ideal voltage.

You could eliminate the need for VREF if you used a two-battery bi-polar setup, but that's trading two resistors and a capacitor for an additional 9V battery.

 

[maxamendel]

So it is what it is then. Thanks for the help!

I'll leave it as is for the project. but I'll have to experiment in the future with bad voltages for a overdrive or something. The only reason this came up is concern about sound quality and about dying batteries. Is there a noticeable difference between 4.5v and 9v? I'm interested in the two battery solution. I'll give that a go too, but il have to look up how that goes. Im guessing VRef would tie into the -/+ series junction on the two battery clips, and then ground would be - on the first battery and Vcc would be + on the second battery. That would actually save space on the board because the batteries are going into their own cavity.

Again, thanks for the help. what a fun project. now that it's breadboarded, im just experimenting with cap values and this topic came to my attention. trying to get a full understanding of the thing before I box it up and be done with it.

cheers.

 

[maxamendel]

to reiterate, my understanding now is that a resistor voltage divider is better than a voltage regulator, because vref being half of vcc is more important that being a certain value. So even if I had a regulator set to 4.5v, when the battery gets low, itd be bad for the regulator to put out 4.5 if the battery is only sending 7 or 8v