On Board Instrument Preamp Headroom before Wireless Transmitter

[YouAre]

I'm designing an on-board mixing preamp for a buddy's upright bass, which blends 3 different input elements. (Bass players can be such finicky divas... )

I initially was going to run the unit at 9v, but I then realized that the output of this mixer would be feeding a wireless unit. The wireless runs off 2AA batteries. That's what, 3V of headroom available? That makes sense in normal situations, where a guitar is 1-2Volts max. Put a preamp in between, and I foresee some issues if we're not careful.

Assuming my Preamp is running at 9v, that means my reference voltage is 4.5 volts. Using a TL072, the max output swing should be 0-6V? (Someone please correct me if I'm wrong on that). So if my understanding of op-amp biasing is correct, then the output signal of this will be between 0-6v, centered at 4.5 volts. Is this correct?

My assumption is that this signal centered at around 4.5V will create issues with the wireless, which wants to see a signal between 0-3V. Can anyone comment on that?

My immediate thought was to run at such a voltage that the output would be 3v (~5V on the rails). But is attenuating the 9V battery down to 5V killing the battery life?

Alternatively, I did read the AMZ article on Low-Voltage Boosters, which details using AA/AAA batteries to power transistor effects. Would anyone advocate for/against the use of AA batteries in an op-amp based effect?

If anyone else has any other solutions, I'd be extremely appreciative and receptive to your advice. Thank you.



Other design notes:
I'm providing each input with a buffer (non-inverting op amp stage), each with a gentle amount of boost to compensate for some of the weaker pickups. This will give them more usable range in the mixer and also will be to make up for the filtering/tonestack that follows directly after.


Thank you.

[R.G.]

Have you read my recent posting here about an onboard preamp/buffer? The article is at geofex.

The power issues were the principle issue to be resolved. I wound up designing in two 3V lithium coin cells in series.

[YouAre]

Have you read my recent posting here about an onboard preamp/buffer? The article is at geofex.

The power issues were the principle issue to be resolved. I wound up designing in two 3V lithium coin cells in series.

R.G.,

I did not see it before, but I just read it right now. My initial impression is that 6v may be still an issue. But a single 3V Lithium coin cell could be perfect. Thank you.

But how will this behave with a tl072 op amp? Biasing that seems like it presents different conditions/challenges.

Sidenote: Just finished reading PCB Layout for Musical Effects. It was extremely insightful, and an invaluable resource.

[PRR]

You need much more than 7V to life-up a TL072.

Your signal can be ANYthing you want (up to supply).... you can always play softer or turn-down.

If it becomes convenient to run higher signals in the TL072 than the wireless likes, you pad-down between mix and mike.

[YouAre]

You need much more than 7V to life-up a TL072.

Can you please clarify this?

Your signal can be ANYthing you want (up to supply).... you can always play softer or turn-down.

If it becomes convenient to run higher signals in the TL072 than the wireless likes, you pad-down between mix and mike.

Since the output stage is an inverting op-amp mixer, would you say it's "better" to fix the gain <1 with the feedback resistor or to use a typical voltage divider volume control at the output? "Better" in this case meaning finer control (less touchy), less noisy (i'm sure boosting gain then cutting it has it's issues), etc.


Thank you for the advice. I greatly appreciate it.

[kodiakklub]

i think the easiest option would be to pad the output of the mixer/buffer, as PRR said, down to a level that wont blow the transmitter. even stick an attenuation knob/toggle switch in there.

[YouAre]

i think the easiest option would be to pad the output of the mixer/buffer, as PRR said, down to a level that wont blow the transmitter. even stick an attenuation knob/toggle switch in there.

I think you're right. I'm guessing that I will end up clipping the op-amps if I run them at 3v.

Effectively, I'll be making a "reverse" compander. Boosting signal at each input to compensate for tonestack losses and to have the pickups be at a more even level. This is followed by the tonestack. Then we have an inverting op-amp mixing stage to the output.

Question is, Is it better to attenuate with gain less than 1 with the inverting opamp (R input > R feedback) or with a volume control? This will likely be a set and forget internal trimpot.