Optimizing preamp design

Thread Starter


Joined Jan 31, 2016
Some time ago, I designed a little battery powered preamplifier, to connect dynamic microphones, balanced line signals and electric guitars to a mobile phone or a computer.
The prototype works fine, but I'd like some professional advice before putting it in a real enclosure.

The circuit runs of a single Li-Ion cell (3.7V~4.2V)
I used a UA741 to split the supply in two (~ +/-2V) and to create a virtual ground. Is this overkill? Would a simple resistive voltage divider suffice?
When I added smaller bypass capacitors (100n~1µF) in parallel with C1 and C2, it started oscillating, so I didn't include them.
X1 is a XLR/TRS combo Jack. I added R3 and R4 to prevent the unused connector from picking up noise. Should they be pull-down resistors instead? And is 1MΩ too much?
R5 and R6 increase the input impedance for using electric guitars.
R11 adjusts the differential amplifier (to make sure that the inverting and non-inverting input have the same gain).
I've seen some schematics that add a ~30KΩ resistor between the + and - input of the first op-amp (U1A). Why is this? And should I add one too?
Would an instrumentation justify the extra cost?
R13 sets the low-cut frequency.
Op-amp U1B is the second stage, its gain is set by R15.
X2 is the output to a TRRS connector for connecting to a smart phone/tablet. R20 lets the phone detect that a microphone is connected, and together with R19, it creates a voltage divider, to lower the level.
X3 and X4 are line outputs.
R18 is for trimming the VU meter.

The most "important" problems are:
  1. The battery has a 5V USB charge port. Because the ground level of the audio is different from the "ground" of the battery/USB, connecting the charger (via USB) to the same device (e.g. a computer) as the audio output therefore shorts the output of IC1. Can I just connect the grounds and shielding of the audio connectors to the negative terminal of the battery (since they are DC coupled using C3,4,5,6,9,10,11) or will this create too much DC "pops" when turning it on? Can this damage the microphone/guitar?
  2. Same problem with the VU meter: its ground is connected to the negative battery terminal, while the audio ground level is higher. When turning on the device, the audio ground level rises, and while C8 charges through R18, the VU meter fires. This is not really a problem with a digital VU meter at a low voltage, but what would happen to an analog meter at, say, +/- 15V?
  3. If I turn the gain (R15) up (50% or more), the circuit starts receiving AM/MW broadcast. Is this inevitable? Or should I add some shielding?
  4. How should I determine the polarity of the electrolytic decoupling capacitors (C5,6,9,10,11)? Does this matter at such low voltages? Should I use non-polarized caps? (Those are much more expensive though.)

Here's the schematic:
And the component values:
R1 33K
R2 33K
R3 1M
R4 1M
R5 270K
R6 270K
R7 22K
R8 22K
R9 180K
R10 27K
R11 50K (trim)
R12 180K
R13 50K (pot)
R14 5K6
R15 50K (pot)
R16 1K
R17 1K8
R18 50K (trim)
R19 1K5
R20 4K7
R21 150K
R22 150K

C1 220µ
C2 220µ
C3 100n
C4 100n
C5 1µ
C6 1µ
C7 100n
C8 47n
C9 4µ7
C10 4µ7
C11 4µ7

Thanks in advance,

Thread Starter


Joined Jan 31, 2016
Thank you for the replies!

Are the op-amps you have chosen specified to run from such a low supply voltage?
No, I just happened to have a few of these laying around. I'm aware of the fact that +/-2V is on the low side, but it works just fine.
Could you recommend some low-voltage audio op-amp(s)?

Just use two batteries and center tap them for ground. Remove the 741 virtual ground portion of the circuit.
I've considered this before, but my conclusion was that it would only make things harder: I'd need a 2 cell charger, a balancing circuit, it would take up more space ... And since the output levels are more than high enough (the microphone input of the recording device starts distorting before the preamp does) with a single battery, I didn't change it.


blocco a spirale

Joined Jun 18, 2008
Thank you for the replies!

No, I just happened to have a few of these laying around. I'm aware of the fact that +/-2V is on the low side, but it works just fine.
Could you recommend some low-voltage audio op-amp(s)?

Since your question was about "optimizing" the design I thought it important to point out that running the op-amps way below their minimum design voltage is unlikely to produce optimal performance. IIRC both the 741 and 5532 need around 10V minimum so it's surprising that your design works at all.

I don't have a recommendation for a low-voltage op-amp but the good news is that, due to the proliferation of portable battery operated audio equipment, there are plenty to choose from. Take a look at the offerings from Analog Devices, Burr Brown and Texas etc.

blocco a spirale

Joined Jun 18, 2008
It looks like fairly standard stuff.
  • C9 appears to be in the wrong place; shouldn't it be in series with R19 only?
  • I wouldn't use electrolytic capacitors for coupling the audio but if you must, look at the DC conditions to figure out which way round they should go.
  • I don't see any problem with the the VU meter being AC coupled. It won't matter whether the display is analogue or digital; they will both have similar circuitry to rectify and filter the input signal.
  • An isolating DC-DC converter in front of the battery charging circuitry will solve the short-circuiting problem you get when audio and power are connected on the same device.
  • The correct op-amps might fix the RF and instability issues