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jabeavers 4th February 2012 08:02 PM

Help with circuit for preamp with eq and balanced output powered by phantom power
I'm fairly intermediate with electronics, but feel confident to be able to build circuits from a schematic.

I want to build an onboard preamp for my acoustic guitar, whose electronics have died. The electronics on it have a piezzo pickup and microphone that are mixed together. There is a three band eq and a phase switch.

I would like to build a similar circuit. I'd like a preamp with eq, a mixer to mix the two sources (piezzo and mic), and a balanced output that can be powered from a battery or phantom power. (Similar to the HT-MC here: ARTEC) I also want a 1/4" hi-z output and a 1/4" channel insert for an effects side-chain. I don't need the phase switch.

I have found several schematics that would be suitable, but in my research, it seems that phantom power has limited current available, so I am confused as to how to continue. I don't know if the circuits I've seen will draw too much power, or what. Therefore, I would like assistance from the group as to a circuit that may be suitable for my application. I mainly need help with the preamp part. Should it be discrete so as to draw less power, or are there opamps that are low power enough to do the job?

Also, I'm confused as to how to get the power out of the xlr (phantom power) and use it to power the circuit.

Thanks for any assistance you can give.

jabeavers 6th February 2012 08:09 PM

Here's a link to a circuit similar to the DI I want to use, I just need to add a preamp with 3 or 4 band EQ to it.

ESP - Direct Injection Box for Recording and PA Systems


jabeavers 8th February 2012 07:32 PM

So, I think I've decided on an AD820 as the opamps I will use in my circuit. If I understand the data sheet correctly, I can use it with single supply power and not need to create a virtual ground. If someone could check my thoughts on this, that would be great.

Here's the link:

I'm working on a schematic, so I'll post when I'm done with it.


WSJ 8th February 2012 08:02 PM

For single supply operation, the op amp can be biased with two 2.2M resistors to a voltage 1/2 of V+.

jabeavers 8th February 2012 08:30 PM

Hey, thanks. I know this, already. I wanted to know if this particular opamp would work without the biasing from 1/2V+.

The data sheet says "The AD820 is a precision, low power FET input op amp that can operate from a single supply of 5 V to 36 V, or dual supplies of 2.5 V to 18 V. It has true single-supply capability, with an input voltage range extending below the negative rail, allowing the AD820 to accommodate input signals below ground in the single-supply mode."

Does that mean it will work without the biasing from 1/2 V+. Also, is the biasing the same as a virtual ground (floating gound), or am I confused?


WSJ 8th February 2012 09:40 PM

The DC gain of the op amp is one, with the + input at 4.5 V the output will be at 4.5 V. This allows the output to swing equally in both directions before clipping. The AD820 output can swing very close to the supply limits, others may not do as well. Keep in mind the input will see the bias voltage and signal +/- peak voltage. The op amp will work well when biased at 4.5 V, but will work within the Common Mode Range. See note 1 in the data sheet.

ddietz 8th February 2012 11:04 PM

I recently installed a coax Piezo UTS pickup in my acoustic. I build a charge-amp buffer that works fine plugged into HI-Z or line level. Tried a few different buffer ideas and that had the best, most realistic acoustic sound that could be had from a piezo. Any filtering would only color the sound, though I sized the input cap and resistors to be -3db down at 40hz to remove some of the string/pump noise. The charge amp is not good for a electret pickup though. A different buffer/voltage amp is needed there.

Here is a good, simple charge amp and basically what I followed....

I see no need to worry about limiting yourself to "single supply" and not biasing to V+/2.

Think about your phantom source supply capability. You will likely be using a few opamps for this so check their current draw and ensure the phantom can handle it all of it. Large/pro supplies will not have issue but if you have a small hobby-level phantom supply might be under powered.

A 3 band baxandall tone might be nice. Lots of designs around the web, here is one to get you started

I'd suggest have separate buffers for each mic type, followed by summing with blend using an opamp, then to a tone control similar to the last link, followed by the balanced output buffer.

I'd question the need for the balanced out however, and would save you work and battery/current. If you skip the balanced output, you could do this with one quad opamp, or 2 duals.

jabeavers 11th February 2012 04:44 AM

1 Attachment(s)
Ok, I've got most of my circuit planned. The top part of the schematic came, mostly, from G. Randy Slone's book "The Audiophile's Project Sourcebook". I chose those circuits for their simplicity and few opamps, thereby, reducing power consumption.

The bottom part is from the circuit at the bottom of the link in my second post (ESP - Direct Injection Box for Recording and PA Systems), but with the same AD822's that I talked about earlier. The only part I don't have done yet is the "panning" from the mic to the piezo. Any help there would be appreciated!

Now, designing/theory are not concepts that I have a good knowledge in yet, so I can't tell you if I'm overloading some part of a circuit or what. I'm just going off what I found in bits and pieces. Therefore, please feel free to tell me if I'm doing something wrong here. Also, I am unable to simulate the circuit because it keeps telling me that gmin step failed, etc.

I've attached a pic of the file (sorry if the stuff is crowded or hard to read, it's a drawback of the program I use).

jabeavers 11th February 2012 04:53 AM

1 Attachment(s)
Here's a much better version of the circuit. It's the same, just in pdf, so you can blow it up and keep the detail. :)

teleman 31st July 2012 03:38 PM

4 Attachment(s)
I went completely the other way! I wanted to design a pre which would have (almost) all the bells & whistles found on a half/full 1U unit, yet would be able to be fitted on the side of an acoustic guitar.

Thinking on similar lines as you have done, I basically designed 2 seperate preamps for;
1. An electret mic -Panasonic WM61A 3wire Linkwitz modified with bass/treble + parametric mid
2. A Piezo Disc with Baxandhall 3 way tone control
3. An Inverter Switch with Simple mechanical Mute
4. A Summing Mixer with both unbalanced/balanced & Passive 3dB balanced output.

"Phantom" power was discarded at a very early stage since I decided to use commonly found DIP IC's like TL072,NE5532,LM6542 etc.I used 2x 1200mA smoke detector PP3 battaries in the begining,but soon realized that these too wouldn't last long,due to the fairly high current demands of some of the ICs I had chosen.

So I opted for a full +/- 15V DC & built my own PSU for it. This required an additional DC power cable from the PSU to the onboard pre. This is only a minor inconvenience as far as I am concerned.

The biggest problem I encounted however was a finding a suitable, light weight casing for the unit! I couldn't find a single unit depite fine combing almost all the manufacturers sites & catalogues!,so ended up modifying an aluminum case only to find out that it was heavy enough to dampen the volume & timbre of the guitar! So, now I'm in the process of making a similar casing in ABS plastic!

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