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Old 1st September 2012, 10:11 AM   #1
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Default Preamp for ceramic piezo pickup

Hi,

I need some help in designing a preamp using a single 5v power source &
opamp with high input impedance (very small input current).
I want to transfer the audio signal to an ADC, do some DSP to it and
then transfer it back to an amplifier using DAC.

1. What are the considerations when designing preamp for ceramic piezo
pickup using opamps? what is the right opamp for this?

2. How much resolution is considered good quality for audio A/D and D/A
when dealing with acoustic guitar audio?

Thanks,
Michael
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Old 2nd September 2012, 06:02 PM   #2
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I wouldnt use an opamp for amplifying, a single transistor may be better. any decent audio transistor should be ok. as for resolution, i think generally twice the highest expected frequency, so 44.1Khz is a fairly standard sampling rate for most things.and you probably want at least a 16 bit ADC/DAC/DSP.

I'm not too up on DSP, but i hope thats helpful.

I may be better to just use an analogue preamp, what are you hoping to achieve with a DSP?
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Old 2nd September 2012, 06:18 PM   #3
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Thanks razorrick1293 for your response,
I measured the following frequencies from my guitar: 70Hz to 1200Hz, so why would I
need to aim for 22.05KHz ?
I mainly need the opamp for buffering and not to really amplify the signal, because I have a very precise amplifier inside the DSP unit.
I want to make multi-effects generator using the DSP.
Why I need a 16bit resolution for the ADC/DAC ?
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Old 2nd September 2012, 08:47 PM   #4
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Quote:
I measured the following frequencies from my guitar: 70Hz to 1200Hz, so why would I need to aim for 22.05KHz ?
The fundamental frequencies for all the notes on a standard guitar do indeed fall in the 82 to 1200 Hz range. This assumes that there are only pure sine wave tones, which is pretty hard to do ao a guitar. All guitars generate harmonics and it's the level and range of these harmonics that give each guitar its distinctive sound. If your preamp doesn't pass all of the harmonics, then your guitar won't sound like your guitar after it goes through the A/D and then the D/A even without any processing. I would assume at least 10KHz for the range of harmonics, which leads to a 20 KHz sample rate, but as already stated the CD standard is 44.1 KHz at 16 bits.

Quote:
Why I need a 16bit resolution for the ADC/DAC ?
Have you listened to anything sampled with 8 or 12 bits? Those voice recorder chips are 8 bits. Sometimes you can recognize the speaker, sometimes you can't, but you can understand what is said. I got a 12 bit sound card for my PC back in the 80's because that was the best there was then. Voice, and maybe disco was minimally acceptable, but real music sounded rather grainy. It was useless for guitar. I started recording live music when 16 bit cards at 44.1 KHz became available.

Look through a catalog, or go on the Musicians Friend (or SAM ASH, Guitar Center, or.....) web site at what is being sold for guitar interfaces that connect a guitar to a PC. $40 buys you a USB interface that connects to the PC via USB. There are several to choose from, they are all at least 16 bit, most are 24, and they use a 44.1 or 48 KHz sample rate. $99 gets you 24 bits at 96 KHz.

DSP guitar effects boxes use 16 or 24 bit converters at 48 or 96 KHz but they all have a fatter internal bit width. Some are 48 bits internally. Why? When you multiply two 16 bit numbers, you get more than a 16 bit result. Do this many times, with a truncation each time, and you are back to grainy sound.
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Old 3rd September 2012, 02:32 AM   #5
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Thanks, now I understand...
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Old 3rd September 2012, 03:08 PM   #6
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Since the source impedance is high (and output current low)I think you would need a high impedance input stage for a ceramick pickup; so a (J)FET may be necessary there.

YMMV

Last edited by fastfolkert; 3rd September 2012 at 03:09 PM. Reason: typo
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Old 3rd September 2012, 05:42 PM   #7
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Quote:
think you would need a high impedance input stage for a ceramick pickup;
I just purchased a set of Ghost Pickups for a guitar I am building. The Ghost system has a seperate ceramic element for each string located in each string saddle on the bridge. I chose not to buy the matching preamp since it looked like $10 worth of parts for $70. Their preamp board contains 3 dual opamps. I am assuming one opamp stage for each of the 6 pickups.

I plan to make my own preamp. Plan "A" is to use subminiature tubes. If that turns out to be too big, I will use opamps.

I will mount the saddles on one of my Squire Strats for experimenting once I get caught up on all the BS I have to do.
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Old 13th September 2012, 04:44 AM   #8
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Hi Guys

If the instrument has a raw piezo pickup - no volume pot or preamp - then you need at least 10M input-z to capture the full signal especially at low frequencies. A jfet follower with a 10M gate-leak and 1k gate-stop will suffice. Any n-channel will do. Use a 1k source resistor to start and adjust as required. A 10uF coupling cap to the output will also suffice. This could go to a volume control so you don't overload the sound card input.

In this situation you do not need gain. The piezo signal is really a current related to variable capacitance, so the impedance or loading is the most crucial aspect. The jfet may idle at a mA or so, where most ICs like a TL071 need 2.5mA. The jfet opamp can also be fitted with high-value input resistors, and does allow for tone shaping via its feedback network.

This is a project in 'Tonnes of Tone' which was also presented in 'Guitar' magazine in the UK a few years ago.

Have fun
Kevin O'Connor
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