Preamp as an Amplitube front-end - diyAudio
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Old 23rd April 2004, 02:19 PM   #1
mattw is offline mattw  United Kingdom
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Default Preamp as an Amplitube front-end

For those not familiar with Amplitube, it is a fantastic piece of software for the PC that allows digital modelling of a complete guitar recording signal chain (preamp, power amp, speaker cab, microphone, stomp boxes and send/loop effects). The user can independently set preamp, power amp, speaker cab and microphone models, giving a large set of sounds that doesn't involve miking a large amp and disturbing the neighbours. It is also great for people like me who very occasionally play the guitar on a wet Sunday afternoon and don't want the clutter (or cost) of all the equipment Amplitube models.

I have just sold my mixing desk, since I was only using it for two purposes - (i) A large volume knob for my monitor speakers (ii) A DI/Preamp for my guitar.

I now need something new to fill the need of the guitar preamp/DI role. I cannot find anything anywhere that meets the following simple objectives:

* Low cost
* One channel is enough!
* High input impedance suitable for guitar (1 MOhm?)
* Low output impedance (to go to my soundcard)
* A gain knob
* Simple. low component count construction
* Preferably op-amp based
* Preferably possible to power from a cheap off-the-shelf regulated wall-wart PSU. (I really need to find out why some audio equipment requires dual-rail PSUs and others can survive with single-rail supplies)

I haven't done any electronics for ages! I am starting to read up again but would be very grateful for any ideas or help on this subject.

If the AudioBuddy was single channel and minus the mic input it would be perfect. Although it is quite expensive for what it is (60)

I was wondering whether many changes would be needed to a simple non-inverting op-amp circuit (image below from ESP Amplifier Basics )

Click the image to open in full size.

Sorry for the long ramblings, I have been searching all over the net and can find nice JFET based fixed gain (but simple) schematics, and horribly complex multi-stage preamps with tubes or distortion circuits. It is really difficult to find a dead-simple preamp with a gain control.
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Old 27th April 2004, 03:08 PM   #2
mattw is offline mattw  United Kingdom
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Can nobody help? Please!!!
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Old 28th April 2004, 10:58 AM   #3
dimitri is offline dimitri  United States
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please follow the link, if it will be hard to redraw it for single supply, I can help

http://www.diyaudio.com/forums/showt...192#post367192
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Old 28th April 2004, 11:18 AM   #4
mattw is offline mattw  United Kingdom
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Thanks for the link!

I could use two 9v cells to get a -9, 0, +9 configuration of PSU - is that right? Preferably I would like to use a single 9v supply (cheap PSU) - is it a simple conversion? Thanks for your help - I am reading Horowitz and Hill as fast as I can!
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Old 28th April 2004, 01:09 PM   #5
dimitri is offline dimitri  United States
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Default for the single supply

you will need two extra resistors and three caps

1. put a decoupling cap 0.1u in series with input 2.7k resistor
2. put a decoupling elco 100u in series with output 2.7k resistor, plus to the opamp output
3. disconnect input resistor 1M from the ground and connect it to the common point of two additional 10k resistors. One of this resistors is connected to groud, other to +9V, so the common point will be +4.5V Bypass this point to ground by 100u elco, plus to the coomon point
4. +E op amp terminal connect to +9V, -E op amp terminal connect to ground
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Old 3rd May 2004, 10:59 AM   #6
Optical is offline Optical  New Zealand
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Out of interest, what soundcard/computer/sequencer do you run amplitube with?
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Old 4th May 2004, 08:04 AM   #7
mattw is offline mattw  United Kingdom
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Hi!

I am running Cubase SX 2 as the VST host with an RME 96/8 PST. My PC is a home-built 2.6 GHz P4 with 512MB RAM which seems to swoosh along quite nicely!
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Old 5th May 2004, 10:55 AM   #8
Optical is offline Optical  New Zealand
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hmm, wouldnt mind one of those RME cards
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Old 25th May 2004, 10:19 AM   #9
mattw is offline mattw  United Kingdom
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I finally plunked for a design which can be found in the following digest:

http://www.harmony-central.com/Guita...uildPreamp.txt

(search on nm1@ukc.ac.uk)

I built it on a breadboard yesterday, and to my delight it worked first time and sounded great! I used an NE5534AN opamp, metal-film resistors throughout and experimented with both ceramic disc caps and monres caps for the non-electrolytic of the circuit. I also tried various values for the input capacitor and couldn't hear any audible difference! In the end I went for a 0.22uF monores (I couldn't hear any difference between that and a 0.22uF ceramic disc)

I used a 10uF general purpose electrolytic for C2 as I couldn't find a 15uF elect. Is it just me or is it really difficult to find the right part values when shopping around for one of these projects? I did manage to get a 22uF for C3.

I tried using R1 = R2 = 220k, but also tried R1 = R2 = 1M since, I believed this would give a more lively guitar sound. Actually, both values proved excellent but since I had about 1 or 2 dB extra noise when using 1M resistors I went back to using 220k.

For R4, I ended up using a 15k metal film resistor - as 8k2 gave me a little distortion. I currently have my soundcard switched to +4 dB line level, so I am going to switch it to -10 dB as I can then get away with less gain in my preamp.

I do however have more questions! I am going to make a permanent version of this preamp on stripboard, and want to add a variable gain control. Having little-to-no electronics knowledge I need some help. I can see that I could swap R4 for a pot configured as a variable resistor, or I could swap R3 and R4 for a pot being used as a potential divider. Can anyone please tell me:

1. Variable resistor or pot as above?
2. Log or linear?
3. What value to use
4. Should I use a non-carbon pot (plastic conductor?)

Thanks very much for any help.
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