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Old 26th March 2007, 01:41 AM   #1
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Join Date: Mar 2003
Location: Windsor
Smile DIY guitar amplifier suggestions?

I am currently in the progress of putting together a guitar amplifier and am looking for a few suggestions. The plan is as follows:

Driver: 12" Eminence Texas Heat

Power Amp: Parallel LM4780 (simialr to the kit at http://www.audiosector.com/lm4780.shtml but my own design)

Preamp: Ok here´s where I need the help...I am currently thinking of using the AMZ overdrive (sober drive) www.tonepad.com followed by a RealMcTube II. My other options are the Rod Elliot Guitar Preamp http://sound.westhost.com/project27.htm or the SansAmp GT 2 also from Tonepad

I rather like the overdrive tone I get through the McTube with my current setup, but have not heard either the Rod Elliot design or the SansAmp to compare. This will be used mostly for blues/rock/metal. If anyone has any suggestions please chime in. Bear in mind I am trying to keep this a strictly DIY effort.

G.
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Old 26th March 2007, 05:33 PM   #2
Albertb is offline Albertb  United Kingdom
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As no one else has come in on this yet......

My own personal preference would be for a FET based version rather than an opamp based one, you may differ. Making something a little gentler on the distortion front for your blues licks and butchering the harder metal tones out of it when you want is easy. Doing the reverse is often impossible.

Incidentally, you will not see too many FETs in commercial units. This is purely because of the setting up which is necessary and nothing to do with opamps or BJTs being better! The only problem with FET based designs is that they generally need a fair bit of tweaking to optimise stages. If you feel you are up to this then I have a couple of suggestions you might like to look at to get you going.

For a general idea of what a FET preamp looks like someone else recently posted this link:

http://www.albertkreuzer.com/preamp.htm

This is for bass as it stands but would take only a change of a few values to make it into an excellent guitar version. Most of the additional complexity like the switched caps in the mid range can go, the 2 opamp blocks for tuner and clipping indicator can go, and you are left with a very buildable unit in 3 FET and 2 opamp stages.

I would change the tone controls for a tonestack type around another FET, Baxendall style circuits are not really very good for guitar preamps, and there are a million Fender/Gibson/Vox values available for that. This is about the closest I think you can get to a classic valve preamp without smelling scorched flesh and getting thrown across the room a couple of times while building it!

The other link I personally like is this one:

http://www.runoffgroove.com/thor.html

Not so much a simple project as a tutorial series in aspects of FET design for guitars! Do visit the few links in the text to see other aspects of this circuit. If theory isn't your thing then just skim but if you persevere with this site you will get a good grounding in what gives a nice sounding preamp using FETs. There is also a terrific link here to a good number of sound clips for a range of preamp and distortion circuits. Most are very harsh to my ears but maybe some will give you an idea of adding in a stage to take care of that too.

What you eventually plump for will depend on your level of skill in building of course, but don't underestimate the help you will be given if you just ask nicely.
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Old 27th March 2007, 01:41 AM   #3
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AlbertB , thanks so much for the suggestions. I'll take a look at some of those links. Do you see any real advantage of going with FETs vs tubes, other than the cost? I rather like the concept of using a tube for the overdrive at least, but perhaps a FET based gain stage followed by a tone stack and a buffer might work well.

Keep the suggestions coming.
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Old 27th March 2007, 02:25 AM   #4
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I've built the Runoffgroove Thor box as a stomp box and it is a nice sounding distortion box.
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Old 29th March 2007, 06:48 PM   #5
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
I have Built a LM3886 Based Guitar amp which Puts out about 50w into 8ohms......

I decided to build my Amp so all different Stages of the amps were on different Curcuit boards, Like the Basic Preamps with the Tone controlls were on one PCB and the Power amp stage was on a different PCB and the Overdrive stages were on a Different PCB....
I did it this way so i could easilly integrate other effects (Overdrive, Compression ect) into the amp without haveing to rebuild the whole amp.....

The clean Preamp stage was a very simple Opamp Buffer then into a Bandaxal Tone stack.....The First Overdrive stage used another Opamp with a few Diodes inside the feedback loop and can be enabled or disabled pushing a Button on the Front of the amp....It has a Very Nice Crunchy almost Tubey overdrive sound....

The next Overdrive stage used a 4 transistor Gain stage with Clipping Diodes but this didn"t sound as good and have since tried replaceing it with several other Curcuits but haven"t found one that sounds as good as the first Overdrive stage, I have tried several Fet and Discrete overdrive designs and even a mosfet overdrive but none of them sounded very good compared to what I have in there now......I"m working on Building a Compressor to add to it which should give the amp enough features to make it very versitile, but knowing me I"l probably end up changeing so much of it that it will be constantly evolveing into a new Piece of gear as my DIY skills improve.....

I am also in the process of building a 100w Bass amp useing Two LM3886 in Bridged/Paralell config.....
I have allready designed and built the PSU which provides Unregulated DC for the Power amp stage and Regulated +/-15v for the preamp and other low level curcuitryand has over 40,000uF of filtering.....
I have also finnished designing & building the Power amp stage and just have the Preamp and other curcuits to build....

I have allready designed the Curcuit and PCB for the Preamp and it has first a Buffer and then a Salen/Key Filter to filter out frequencies that my Bass speaker can"t handle (the very high and Ultra Low), Then the signal goes through a Unity Gain Ballanceing stage before it goes into a Gain Stage useing a INA217 Mic preamp Chip (Because I wanted a very clean Bass sound), The signal then goes into a Bandaxal tone stack and then through a Buffer and then it has a switchable Clipping stage incase I want a Dirty bass sound and then to the output.....I am thinking of also adding a compressor to the output before the poweramp......

I now just need a Good Bass speaker a Chassis and Maybe a Transformer.....I have a 160Va EI Transformer but it is a 28v,0V,28v (40v,0v,40v DC) which I think is a Bit high for the LM3886 In Bridged/Paralell config......

Anyways , If there is something I can help you with useing my limited experience then let me know......


Cheers
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Old 30th March 2007, 01:32 AM   #6
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Minion, sounds like a very interesting couple of projects. I agree with your idea of keeping everything seperate, that is essentially what I was planning. The LM3886 is a good choice as well, the LM4780 is nothing more than 2 of these chips in a single die. I opted to go parallel in case I eventually connect to a low impedance 4X12 cabinet...you never know

I have listened to the clips of the Thor, it does not sound too bad. I would like to have heard it untouched (no PC generated reverb etc.) I am still not certain that it will be any improvement over my original plan of the AMZ overdrive followed by the McTube. The Thor does look to be a rather simple circut, so I will likely put together a board and try it out. I still have lots of building/testing to do.

G.
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Old 5th April 2007, 09:50 PM   #7
xov is offline xov  England
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- for what it's worth, there were some interesting vox amps in the 60s, which had transistor pre-amp, hooked up to various valve outputs.....seems like a better idea to me than the other way around, just because everyone does something a different way doesn't always mean it's better..... vox themselves recently made a transistor amp with a valve in the pre-amp, I don't think it was much of a success though...
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