Hi,

I built this circuit today, $20 VU kit. It has a low and a high input. I want to use it on my acoustic guitar, but the LOW input is still too low. With the pot all the way, I get it barely registering on the first LED.

Looking at the schematic, should I replace R1 with a 4k7? Or possibly change the gain on the op amp by changing R4 to say 5k or 2k, etc.?

Also, this only has an INPUT, no output. So with it in my signal path, the signal basically gets cut in half. Is there a way around this?

Thanks!

 

possibly change the gain on the op amp

That may be the best choice.
But R4 has no direct effect on the gain.
Try adding a resistor in series with the output (right side of schematic) of pot RV1.
Start with about 200kΩ, and go from there.

 

What kind of microphone are you using? Dynamic (coil and magnet) or electret? An electret mic needs to be powered.
The very old kit uses a 54 years old uA741 opamp and an obsolete LM3916.
Its "low in" gain is only 4.68 but must be 100 to 200 for a microphone.

If you reduce the resistance of R1 then it might decrease the output of a microphone and will decrease low frequencies much more, unless you increase the capacitance of C1 the same amount.
Its R4 does not increase gain.

 

No not a mic, a passive guitar pickup.

 

Oh wait this is an inverting op amp so gain is RV1:R2/R1, right? So lowering R1 should increase gain? Or even increasing R2 since it’s in parallel with rv1.

maybe changing R2 to 100k and R1 to 22k, that would give max gain of 3.1, currently max gain is 0.8.

 

this is an inverting op amp so gain is RV1:R2/R1, right?

Not quite.
It's an inverting amplifier with two diodes in the feedback loop that make it a precision, positive-output, half-wave rectifier.

The positive output gain (for the negative portion of the input waveform) is RV1 / R1 (for Low In).

The negative output path through R2 and D1 is just to keep the op amp from saturating and contributes nothing to the output going to IC2, since any negative output is blocked by D2.
Actually R2 could be zero and the circuit effective output would be unchanged.

So you can increase the gain by reducing the value of R1 and/or increasing the value of RV1.
The limit is how low an R1 resistance your guitar pickup can drive.
If the pickup has a high-impedance output, lowering R1 may not affect the gain that much, as the pickup impedance could dominate the gain equation.

An LTspice simulation of the 741 circuit (below) shows a positive half-wave Out (yellow trace) with a gain of 4.7. as determined by RV1 / R1.
Note the op amp output (red trace) has a higher positive output gain as compared to the negative output gain of 1, which is determined by R2 / R1.
The output is offset by the DC voltage of the Zener diode, ZD1.

 

Don't reduce R1. Increase the gain by increasing RV1 (or adding resistance in series). At some point you will need a better op-amp than a 741 because of the bias current - a 741 has a bias current that could be as much as 500nA. That will create a 0.5V error with a 1M resistor.
R1 should be a minimum of 100k. The output impedance of a guitar pickup is 6k+6H. The gain is RV1/(R1+pickup impedance) so it is almost equal to 47k at 1200Hz, so the gain will be less than you think it should be.

 

It will be difficult to add resistance in series with RV1 since it’s soldered to the board, would take some modification. Would be much easier to change R1 or R2. Or swap op amps, TL071 has same pinout, would that work better? Or replacing RV1 with something bigger.

what about adding a simple op amp circuit before the VU meter to add gain and bolster impedance so it doesn’t affect my guitar signal?

 

FWIW, most acoustic guitars passive piezo based pickups have an impedance between 100k-1Mohm.

 

It will be difficult to add resistance in series with RV1 since it’s soldered to the board, would take some modification. Would be much easier to change R1 or R2. Or swap op amps, TL071 has same pinout, would that work better? Or replacing RV1 with something bigger.

what about a adding a simple op amp circuit before the VU meter to add gain and bolster impedance so it doesn’t affect my guitar signal?

TL071 would do a much better job. You could increase RV1 to 4.7M if you can get one.
R2 is not usually present in a half-wave-rectifier circuit, so changing it will make no difference.
Reducing R1 won't work (as stated earlier)
A buffer would help, with or without gain.
How about taking the signal from elsewhere - is there an effects loop output on the amplifier?

 

FWIW, most acoustic guitars passive piezo based pickups have an impedance between 100k-1Mohm.

I was assuming an electromagnetic pickup - piezo ones require even higher input impedance for the amplifier.

 

It will be difficult to add resistance in series with RV1 since it’s soldered to the board

You should be able to cut one of the traces with a sharp knife going to RV1, and solder an added resistor across the cut.

Or replacing RV1 with something bigger.

If you can find a replacement RV1 of perhaps 1-2MΩ that fits,

what about adding a simple op amp circuit before the VU meter

Adding a TL071 as a non-inverting amp at the input to the circuit should also work.
The TL071 has a JFET high-impedance input, thus the input bias resistor can be 1MΩ or more, so it shouldn't significantly load down your pickup.

 

I swapped out the 741 for a TL071, it might have helped. But I placed it after a compressor pedal that allows me to adjust the level so I could gain it up. That worked.

I also found a 3M trim resistor on ebay, Piher PT10 size, should be a drop in replacement when it gets here. But I also may try doing @crutschow 's trick.

 

Is your "passive pickup" a magnetic one that senses the strings or is it a piezo one that senses the wood vibrations?

 

Is your "passive pickup" a magnetic one that senses the strings or is it a piezo one that senses the wood vibrations?

piezo, either SBT (sound board transducer) or UST (under saddle transducer). But honestly this VU pedal won't "see" the pickup since it will be after either a tuner or compressor, both buffered.

 

A 330k in series with RV1 seems to put me in the right range. Cool, thanks.

This was just a prototype, proof of principle. Ultimately I'd like to build an optical compressor and have this circuit show either the signal or the reduced signal (part being subtracted via compression).

Could a TL072 be used to duplicate a single signal? Can 1 signal be common to both input1 and input 2? Then separate gains for each?

 

Could a TL072 be used to duplicate a single signal? Can 1 signal be common to both input1 and input 2? Then separate gains for each?

Yes.

 

From the TL072 datasheet

If I use a TL072 instead, first stage distributes to the VU meter and output A, since VU meter already had an op amp, and then output A could be the output?

 

Why do you need 4 opamps? The first opamp in the TL074 has a gain of 11 times and a fairly low input resistance of 100k that can easily be increased. This opamp can drive up to fifty 100k volume controls.

 

No, a TL072 before the VU circuit