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Old 25th September 2004, 03:52 AM   #1
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Default What makes an amplifier a musical one?

I'm in highschool, and I just hit some weeks off. During this time I wish to design a solid state guitar amplifier for myself, around the 60-100W power range. I have dabbled in electronics for a while, but I'm highly uneducated in the field.

Firstly, this amplifier is to have no "distortion" effect as found on most guitar amps...I want the best clean sound I can get.

I am wondering what it is that makes, for example, a HI-FI amplifier different from a "clean" guitar amplifier (apart from obvious things such as the tone controls). If you are to respond, could you please include details about whatever it is I should know?



Thankyou very much!
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Old 28th September 2004, 10:00 AM   #2
Enzo is offline Enzo  United States
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ANy system has three elements - power supply, power amp, and preamp. Power supply is a given, of course. It has to be able to provide the power you want to put out, and you want some headroom there for reliability. A power amp is a power amp. A well designed power stage should be clean regardless of purpose. In a solid state guitar amp, the distortion is not generated in the powr amp section. You can clip them, but that is not what the musician wants.

The difference between a guitar amp and a hifi amp is in the preamp.

Before I continue, I would also point out that a hifi amp is trying to REPRODUCE sound, while the guitar amp is trying to PRODUCE sound. The guitar amp is part of the instrument - even if it is "clean." And the speaker is a very important part of that. In hifi you want the speakersz to be neutral - to add nothing. But a guitar speaker is selected for its tone. They all sound very different from one another - one man prefers the Clestion, while another likes the EMinence, adn like that. It is part of the instrument.

Guitar speakers roll off by 5kHz. There is never a tweeter on a guitar amp. The guitar makes no sound up at 10kHz. If you disconnected a speaker from one and recorded the signal there, it would sound tinny, trebly and thin.

Hifi preamps are flat and pass the whole audio spectrum 20-20lHz or something similar. Guitar preamps are not at all flat, and sound awful if they were. Play a guitar through a stereo hifi and it sounds dull and pointless. When you say you want no distortion, I must assume you mean no screaming death metal extreme distortion. But you do not want a flat pure response. Listen to BB King or even jazz cats. Their sound may seem clean, but there is still distortion there. Distortion gives the sound some edge, and other beneficial things.

The guitar preamp is designed to control the tone of the guitar audio signal. it will have bass, treble, and probably middle controls as well as presence. Presence affects high freqs. Many have a BRIGHT switch to enhance the upper range. Ther will always be a volume control, but there is often more than one. The gain control is in the first stage as a rule and can be though of as a drive control. Then the master volume - also called POST - is at the end of the chain. Gain up and master down for more edge and distortion from overdrive. Master up and gain low for cleaner sound. turn up the gain for the desired amount of bite and the master controls the overall level of sound.

In the preamp various methods are used to shape the tone. A little emphasis here, a litle rolloff there.

A musician choses the brand and model of guitar amp just as he would the guitar. "I play a Marshall." or "I play a Fender." tells you something about what the guy sounds like.

That is the basics. Other things like effects loops, internal effects, recording line outs, reverb, are also common.

What you might do is look at some schematics of existing products for ideas. Peavey is very nice about schematics. write to parts@peavey.com and request a couple schematics. Tell them you are in school and working on a tech project. A couple amps come to mind there - the original BANDIT, and the more complex SPECIAL 130.

CArvin is helpfull. COntact them and ask for a fairly simple solid state amp schemtaic. I have no model numbers in mind here.

Remember, you can combine the preamp from any amp with any power amp. You can take the preamp from a 15 watt practice amp and play it through a 400 watt power amp if you wanted. And vice versa.

Do you play? 60 watts is a pretty loud amp. Certainly many players out there doing gigs with 60 watt amps. That is a lot for the bedroom player. How did you decide on that level of power?
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Old 30th September 2004, 06:34 AM   #3
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Enzo, thankyou so much for your extensive reply. Your advice led me to opt for 40 watts instead of greater or equal to 60. I have decided to use a typical 'Gainclone' power amplifier, due to its simplicity (this is my first project of this size). National Semiconductor provide documentation on a 40W amplifier using their LM3875 chip, and I plan to use this.
I have started designing the pre-amp. Because this is my first amplifier design, I'm keeping it minimalistic. I play jazz guitar, and all I'm realy after is a good clean tone that compliments my archtop. At the moment it only has a volume control, and a baxandall tone stack with treble and bass controls. I don't want to make it much more complex than that. I decided to use a baxandall tone control after reading this page http://www.duncanamps.com/technical/tonestack.html on Duncan's Amp Pages.

I have included the schematic so far. I'm using a mosfet input buffer to get a high input impedance, to conserve whatever tone the pickups are outputting. This stage does not actually amplify the signal, because the input voltage for the 40W gainclone stage is not to exceed 1V, according to the datasheet. With my humbucking pickups, I can get over 2V out of them when i strum hard, and this would cause clipping. I designed the pre-amp for a maximum input voltage of 3V, incase some effects unit before it amplified the guitar further. With 3V on the input to the pre-amp, the output to the power section is about 1V.

Now, as you know, i'm a student, and I don't have experience in this field. The reason I am posting the schematic is so that I can learn what is wrong with it and how to improve upon it, so any comments are definately welcome!

N.B. The sets of resistors (Bass1, Bass2) and (Treble1, Treble2) are realy potentiometers; my program does not have pots so i used these instead.

Once again, Thankyou so much for your reply!
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Old 30th September 2004, 08:40 AM   #4
Enzo is offline Enzo  United States
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Remember, if you use American circuits in Australia, they must be operated upside down.


Just kidding, that's my favorite down under joke.

The LM3875 is a good chip. Many if not most of the small amps I see use the TDA2040. The same family also includes the TDA 2030 and 2050. Last two digits reflecting potential power out. These come in a 5 leg TO-220 package, which is easy to mount and wire in. Look up the TDA2040 and check it out.

More than you need, but interesting anyway is the TDA7293.

AS a student, none of this has to be permanent. With separate preamps and power amps you can upgrade either end when you feel like it.

Your design looks good to me, I must admit I am not real experienced with MOSFETs. HAve you considered doing a similar design with JFETs, just to be scientific? FETs will emulate tubes better than bipolars. Most amps use op amps rathre than transistors anyway - in the commercial marketplace. Daily I mostly see op amps and tubes.

If you write to Peavey and get the schematic for the Rage/Blazer you will see a basic little amp that sounds pretty good done with bipolars in Darlington. This is meant to emulate tubes somewhat. If you ask for ir, make sure to specify the "Transtube" version. They have made several versions of amops with those names. It might also be informative since hte r=two amps are identical except the Blazer has a reverb. Something you may want to add at some point.

A lot of the small practice amps sound crappy because they have a tiny crappy speaker. it is amazing how much better these little things sound through a nice full size speaker. The speaker is an important part of the sound, as I mentioned.

SOmething to consider. Whatever the output signal voltage winds up being, you can always pad it down before you send it to the power amp. (Does you rpower amp actually clip at 1V? Or can you peak over that?) Though the guitar can output a strong signal when strummed, much of the gentle picking results in a small signal. The smaller signal is that much closer to the noise floor. Your amp will be amplifying the noise along with the signal.

Generally we expect a pretty good level of gain at the first stage to elevate the signal over the noise floor. The EQ is passive, so it is cut only, there is no boost. Setting the controls at mid point does not result in boost as you turn them up, it just cuts less. SO the tone stack represents a large level loss, and the stage after it is ther to recover the signal level. The noise will always be there, but if the signal is many db louder, the noise will be masked. SO in general it is good to get some initial gain to give you enough level to work with.

Is that circuit running on 18VDC? if so, you have the room for almost 9V peak of signal before clip. Or do I misunderstand the design? DOn't worry about the output level, you can adjust it afterwards with a simple pot. Or is R12 already a pot?

I notice you bypassed the R7. Bypassing R5 would have an effect on gain and low end response. COnsider bypassing with a couple different cap values as an experiment. I am not suggesting it needs them, but it might be instructive to watch the effect. Again, I am not a FET guy, but in tubes and other devices this is the expected result.

Tell us how this works once you build it.
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Old 30th September 2004, 07:21 PM   #5
stroy05 is offline stroy05  United States
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I myself am in the same boat. I am currently building a bass guitar amp. This amp is a class D. I am not sure about the preamp section. I do know that I do not want it to be a tube preamp. I am looking to have two channels with independent controls for each preamp channel.

Is there anyone that has designed a preamp like this. I would assume that this setup is a dual preamp design with a summing output to the power amp section.

The setup I am wanting for each channel:

-pre gain
-bass +/- 12 db @ 40Hz
-Lo mid +/- 12db @ 400Hz
-hi mid +/- 12db @ 800Hz
-treble +/- 12db @ 3.5kHz
-post gain
-send loop
-recieve loop
-di out and send level

I would also like to control the di out with a post/pre/off switch.

If there is anyone out there that can help me out with this that would be great.
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Old 1st October 2004, 08:03 AM   #6
Enzo is offline Enzo  United States
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There are a million bass preamps out there. Some on the front of heads, others stand alone, but there are many to chose from . I don't design these things, I repair and maintain commercial products. But if it were me, I might find a commercial product from a company like Peavey, Carvin, Fender that fit my needs and get the schematic. it would be a place to start and would show you what the circuit would involve.

A few dirt cheap op amps and you have the base for all the features you described.
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Old 1st October 2004, 02:52 PM   #7
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Default Distortion is your friend :-P

Quote:
Before I continue, I would also point out that a hifi amp is trying to REPRODUCE sound, while the guitar amp is trying to PRODUCE sound. The guitar amp is part of the instrument - even if it is "clean."
Enzo, you must have read my mind! I made an almost identical comment to Bob Pease (the other 'one and only') several years ago. http://www.elecdesign.com/Articles/A...5037/5037.html

I like to think of guitar amplifier distortion like this: In a solid state amp at least, when it hits clipping it is as sudden and as unpleasant as a string buzzing on a fret. With a valve/tube amp though, the overload point is rather more gradual, sort of like when you play a piano softly you get one kind of sound but when you really pound the keys it sounds a lot more urgent and exciting maybe. The harmonic structure of the note is different. This kind of thing is anathema to hifi but quite valid for a musical instrument.
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Old 2nd October 2004, 09:02 AM   #8
Enzo is offline Enzo  United States
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Well, yeah. Tubes don't really clip, they compress. The curve just bends over and pinches the signal off. No hard edges.
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Old 2nd October 2004, 03:31 PM   #9
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Default Enzo...

...you obviously know what you're talking about, so I thought I might impose with a couple of quick questions:

1. would it be a good idea to connect a guitar amp to a PA speaker (you know, the kind that has a 12" woofer and a horn tweeter)??

2. the amp is currently connected to a 12" driver through a twisted pair of insulated conductors. Would it be ok to connect the two wires to the outer and inner conductors of a coaxial cable, the kind that terminates to a 1/4" connector (the kind that is also used to plug in a guitar) and then to the speaker input?

Thanks a lot for your time!

George
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Old 3rd October 2004, 10:04 PM   #10
Enzo is offline Enzo  United States
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You can try using any speaker you like, it is your ears you need to please, not mine. In general a guitar amp sounds bad through a PA speaker. The guitar rolls off and there is not really much of interest over about 5kHz. Guitar speakers roll off by then. When you play through the PA it tends to sound thin and weak. It will sound tinny due to the extra empaphasis on highs.

And it is possible some screeching might damage the tweeter.


Plug your guitar into the stereo and listen. That is the sound of the guitar with no help. Pretty lame.

If you play a guitar head through a PA speaker, keep the level a bit low until you know how it reacts.

Never use a guitar cord for speaker lines. If you want to use 1/4" connectors, wire pair wire to them. In the coax, the center conductor has to carry the whole current, and it is tiny wire. I never use smaller than 16ga wire for speakers, but the stuff in coax is 24-30ga or something similar. Very small. You get resistive losses as well a heating. High enough power and it can melt through the insulation. it can affect the sound.

SOme guys like to say that the capacitance is a problem, but I doubt that. And I don't think the speakre signal is a voltage hazard for the center insulator either. SO I don't buy that reason.

Can you get away with it? Probably, but it is unsafe and will often affect performance, so for the price of a few feet of wire and the need to have a special cable, do it right.
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