Need ideas for Another amp building project.

[jogina111]

hi, here I'm to build(again) another solid state amp. But this time, I have a jrc4558 to use for the preamp section. I want the amp clean sounding yet can get around a little overdrive. I also want a tonestack, a scaled down baxandall maybe... Now the questions, can a simple marshall guvnor with clipping diodes removed and tonestack between gain stages do? I also got the idea of gain stage-tonestack-gainstage. But I don't know how to design something like a vanilla gain stage using opamps. Help please.. c:

[Minion]

Well , doing a gain stage is pretty easy ... In a non-inverting configuration the gain is derived using the ratio between the feedback resistor and the resistor going from the feedback loop to ground ....

by putting a pot in place of R1 you can adjust the gain ..... if you use say a 100k pot for R1 and use a 1K resistor for R2 you will have an adjustable gain up to about 100 Times ...

You could do an adjustable gain in the first opamp stage and a lower fixed gain after the tonestack to make up for losses in the tone stack , could even put in some clipping diodes on switches so you could switch in some distortion ....

Cheers

[jogina111]

how loud would 100x gain be? I dont want the amp to distort a lot.   whats the difference bdtween inverting and the non inverting inputs? Do you have a schematic? Thanks.

[GibsonGM]

Well....I think you should spend some time learning about opamps, and how they work, Jogina!!  You'll be very glad you did.  You can find TONS of instructional sites by searching "opamp tutorial" on the web.   The major differences (besides signal inversion) are in how you set up the gain network, and how you would bias them for use with single-sided supplies.   

"100x gain" is kind of irrelevant when you're talking about a preamp.  If your signal input was 1V, then you would have control over zero volts up to your supply voltage (pretend supply voltage is 12V).   When you crank up the gain so that the output comes close to the supply, 12V, your signal will begin to clip off since you're asking for 100X gain, but only have 12X to give.    So you will get distortion, not really "loudness".  Turn the pot more, you get more distortion (of the device, which may or may not sound good).    You deal with volume and actual output at the point where you're designing and feeding the power section (PA), which will determine how much POWER you can deliver to your speakers.  First you need to get your preamp working right!

You can easily limit the gain control to not allow much distortion, by keeping that pot a bit lower than the level that would cause distortion. You can use fixed resistors with the pot to achieve this, as well. 

Start reading up on inverting and non-inverting opamps, my friend!  ) 

[jogina111]

Gladly, I will. But I'm on a hurry to make one since moving day is close.. Has anyone already made one or know some schematics? 
Maybe a klon buffer to a bax stack to a simple opamp booster would that be ok?

[GibsonGM]

Every stomp box you make IS a preamp.   

I would take a generic buffer, use it to feed your tone control (not Baxandall! Either FMV or James or something similar).    Then a recovery/gain stage, followed by a section like a Tube Screamer, with clipping diodes in the feedback loop (since you don't want distortion, but overdrive).  You'll have to tweak some cap values to get it sounding the way you want.   The TS section will give you plenty of gain (voltage-wise) to feed a power amp.   This is your preamp. 

Throw this chain into a small capacitively-coupled power amp section, maybe a simple IRF510 power amp to a small speaker (3 parts, ha ha)...you'll be cranking in little time, and can always remove the output section and make a BIGGER, LOUDER one later!   

It's just about connecting circuit blocks, at first....do you have a suitable power supply, and know what voltage you're running at?  Does it put out some power, 1A to 2A?

Look at "Mosfet mini amp" down the page:  http://www.beavisaudio.com/index2.htm

[jogina111]

Yes, I do have a 12v 2A power supply...I just need a clean sounding preamp schematic using a jrc4558...Also, what do you mean by "capacitively coupled"?

[jogina111]

Yes, I do have a 12v 2A power supply...I just need a clean sounding preamp schematic using a jrc4558...Also, what do you mean by "capacitively coupled"?

[GibsonGM]

If you just make a simple buffer with your 1st stage of the 4558, and then a gain stage after that, you'll achieve what you are looking for, at least to get going.

You can put a tone control (or stack) in between the buffer and gain stage.   A VERY basic guide to opamps will show you how the buffer and the gain stage are set up.

When you move from stage to stage, you capacitively couple them (just, run the signal thru a capacitor).  This removes DC bias voltage from the previous stage, and also allows you to trim out low end so the next stage isn't muddied from too much bass (or worse, blocking distortion with tubes, but that is off topic).

Start looking at stuff like this: http://www.sentex.ca/~mec1995/gadgets/741/741.html  and also search "opamp tutorial" online, to get a better understanding of how this all works.  Also search "opamp buffer" and "opamp gain stage".   Just adapt pin numbers on other opamps to the corresponding pins on your DUAL opamp package. 

Now, you WON'T be driving a speaker with your 4558 preamp!  You can get an LM386 to do that easily (low volume, small output) or and IRF510 (a bit better, but still not stage volume, that comes later)...this would be 'adding the power amp'.    For now, try to get something you can run into a regular amp, like a stompbox (which IS a preamp).   

Glad you're interested, and if I get time I'll mock up something to put up so you can see what I'm getting at!

[jogina111]

thanks Mike, I gonna use it in front of a tda2003 amp. 

[psychedelicfish]

There was the fire power booster which was posted recently, it might have used two dual opamps though.

[psychedelicfish]

You should also take a look at the preamp of the 100W guitar amp at sound.westhost.com

[head_spaz]

Solid State Guitar Amplifiers - Teemu

[GibsonGM]

YW, J.  The TDA will do you well, I think.

Yeah, with all these things, you have to be flexible and be willing to get a shipment for a few dollars.  The 4558 is a nice chip, but it's certainly no 'Cadillac'....I would prefer to use TL072 or '082, which would give you lower noise and better performance (higher input impedance etc).    Good news is, you can swap them in later if you use a socket for the 4558!    They are very inexpensive and worth grabbing at least a few.    I personally hate to be confined in what I can build by the things in my parts box, and will order up what I need so I am happy with the results.

Later this afternoon, I'll post up something you can mess around with.  Whatever you use, you will have to tweak to your own taste - and that is the most fun part! 

[jogina111]

thanks again mike. I can pretty sure adjust that to taste by altering caps and resistors but living here in the philippines, I cant afford to order other IC's so I'm stuck with 4558's and lm358 which pretty much sucks..

[GibsonGM]

I understand, Jogina.  By using the photo below as something of a 'guide', you can get a nice preamp together. 
The power block at the bottom shows how you need to set up your power supply to use the opamps with a single power source.  Unless you plan on providing negative voltages, just do this 


The first opamp section is a buffer, so that when you add a tone control (or more) after it, you can source the current needed to drive it.

The second opamp section is the 'recovery stage', just a gain stage.   The amount of gain is set by the pot ("U3" here) in relationship with the input resistor (R4).    The pot, which is really the feedback resistor, divided by the input resistor gives you your total possible gain.  100K / 10K, here, = gain of about 10x.     You need to play with these values to get what you want.   You can also add a volume control to the output after C4, at the "to amplifier" point. 

This is by no means complete or perfect, but it will get you building!  Next one, we'll work with a higher supply so you can do more without clipping, and all that...   Now, start putting something together, and let us know how you make out!!   

[jogina111]

thanks..Im gonna try this...got 2 questions.. If im going to put a tonestack in, should i remove the 1uf cap? And about the buffer input impedance, should i put a resistor in the input parallel to ground?

[GibsonGM]

The buffer should have good input impedance as-is, I would think.  Try it out!  If needed, you could increase the resistor already there (Vref).  Probably OK to increase it to even 100k! 

If you insert a tone stack, you can omit the cap.   That is there now mainly to 'separate' the circuit blocks (in my mind, ha ha).   If no tone stack is there, that cap will control how much bass goes to the next stage...something smaller, like .22u, would roll bass off.   So just put in your tone stack.  It would be an issue if there were DC in that area, but here it is not a problem.

On the same point, the input cap (1u here, you can play with that value - bigger = more bass) MUST be there, to keep the bias DC voltage from flowing backwards into your guitar.  Wouldn't hurt you, but might not be good for the circuit.      You can put a volume pot right after C4, which also can be a larger value if you like.  100k, Audio taper would be nice there. 

This whole thing is just blocks from other easy to make pedals, like the TS-9, Fulldrive, Distortion +, etc.  Check all of those pedal schematics out and modify as you like!   You can put caps in the feedback loops to change frequency response - this is only a basic starting point!  )