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Author Topic: [help] guitar preamp  (Read 10728 times)

abe

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[help] guitar preamp
« on: June 27, 2012, 06:05:35 AM »
greeting Mr. Moderator...
hi all... After join this forum read here read there but I don't find something what I make a reference, maybe because I'm dumy about reading the schematics, :)
Does any one want to help me build or design a guitar preamp with 2 chanel (clean&drive) basicly jfet (mpf102 or similiar) why I choose jfet cause it has a tube taste, tone control using baxandall using 15v to 24v,has gain,drive,hi,midd,low,master, and I use Ocl power amp with transistor final was sanken,
help are really needed.
thanks

Roly

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Re: [help] guitar preamp
« Reply #1 on: June 27, 2012, 09:02:27 AM »
Here you go @abe.

This stage from the input to the "gain" control, follow up with 3-band Baxandall as shown here;

http://www.ssguitar.com/index.php?topic=2635.0

Except I'd put a blocking cap on the input and output, and bias the op-amp to half the supply so you can use a single supply rail, a TL071 or TL081 would do.

Then I'd follow the Baxandall with another stage like the first, except I'd omit R1 and R2, and the second U1 would become the "Master".

I'd make the "Drive" channel identical but I'd add two clipper diodes in parallel from the wiper of the "Gain" pot to ground, and if possible I'd use two different diodes, or one in one direction and two in series in the other direction. A red and a green LED (or any two LED's with different turn-on voltages) would be worth trying.

In each stage the source resistor R5 might need some tweeking if the idle voltage across R7 is too far away from half the supply.

If you say theory and practice don't agree you haven't applied enough theory.

abe

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Re: [help] guitar preamp
« Reply #2 on: June 27, 2012, 09:33:50 AM »
thanks for your input @roly... Maybe my question was not specific, in many schematic,baxandall tone was using opamp,what I mean in here is baxandal tones using transistor [MPF102 or similiar] I try this... From output your attachment then go to input in  to my attacment then how? I can't do that....
« Last Edit: June 27, 2012, 09:50:51 AM by abe »

Roly

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Re: [help] guitar preamp
« Reply #3 on: June 28, 2012, 01:19:13 AM »
Option 1; stick with the op-amp, but take "-ve" to ground, "Ground" (op-amp non-inverting input) to a half voltage divider, and +ve to the supply.

Option 2; use the same basic FET+BJT gain cell instead of the op-amp.

Option 1 will give better results because of the much higher available gain.
If you say theory and practice don't agree you haven't applied enough theory.

abe

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Re: [help] guitar preamp
« Reply #4 on: June 29, 2012, 02:25:09 PM »
@roly... Thanks for your input... Step by step finally I can design my guitar preamp... Thanks a lot....

Roly

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Re: [help] guitar preamp
« Reply #5 on: June 30, 2012, 07:08:49 AM »
As you can see, a lot of what passes for "design" is not much more than sticking established modules together, a bit like Lego bricks.

The core of the FET+BJT gain cell I sketched out has a very high input impedance (meaning it won't load just about any source in front of it) a very low output impedance (meaning it can drive just about anything following it), a gain of about 26dB, and a pass characteristic of a FET, meaning it's a bit triode-like.

About all you have to then take care of is making sure you have DC blocking between stages where required, then it's just the old matters of decoupling the supplies, star grounding, and layout to avoid crosstalk and instability.

You may have to move your "gain" and/or "master" volume controls forward or backward a stage to get the desired operation, say depending on the tonestack you finally settle on and its insertion loss (Baxandall low, Fender &c high).

Where it will get interesting is around the clipping stage of the dirt channel - you may spend more time tweeking this to get the desired result than you spend on the whole of the rest of the preamp.

Keep us posted on how you get on, and what you find as you go.  :tu:
If you say theory and practice don't agree you haven't applied enough theory.

abe

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Re: [help] guitar preamp
« Reply #6 on: June 30, 2012, 08:21:33 AM »
@roly... I lied to you... Actually I didn't get what I want, as a nubie or beginer I'm just make simple guitar preamp like I hope,I'm not designer or electro mechanic,I build what I can do,even you guide me but actually I dont understand at all, maybe I'm just looking instant like pcb layout on the web,but all of them not even one succed, I always failed... But I alyas try n try until I got one, I'm just learn alone no body guide me,even the are my friend,he is Audio Video mechanic who blind about guitar equipment...

J M Fahey

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Re: [help] guitar preamp
« Reply #7 on: June 30, 2012, 11:12:05 AM »
Don't worry, my first projects never worked either, that's part of the learning curve.
Start by building the simplest amplifier: an LM386 .
Google its datasheet, it suggests a simple one.
After you build it, enjoy and feel proud.
You can then build, say, a TDA2003 powered amp and use the LM386 as a preamp to drive (and overdrive) it.
Neat, huh?
The LM386 is happy with a 9V battery; the TDA2003 will ask for 12V , either from an 8 AA battery pack or some cheap wall wart.

Roly

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Re: [help] guitar preamp
« Reply #8 on: July 02, 2012, 12:35:38 PM »
abe,

[response to PM]

I'm sorry but I don't have a PCB layout for these circuits, and I don't intend to do one because It's a lot of work and I don't have a personal need; there are two more stages of circuit development before you get to anything like a final PCB layout; and you obviously intend to make commercial use of this design.

I don't have a problem with giving you a bit of help with developing something without reward that you are going to sell, but there must be a serious limit, and if I do it for you then you learn nothing.

So professional advice for free;

* you need to obtain some Veroboard, stripboard, or dabboard, some components, and build a first prototype stage by stage to prove the circuit works and to change any component values as required to get it to work as you wish, particularly the drive channel clipper.

This means drawing a physical layout of the components that takes into account their size and which lead is which on the transistors. 

This also means you will also have to build the prototype power output stage and cabinet/speaker box so you can hear what it does, and prove that circuit as well.

* the next stage is transferring the proven circuits and layouts to a PCB design (on paper) and then etching a second prototype onto actual PCB laminate, say using a waterproof felt tip pen for resist.

PCB Tutorial
http://www.guitargear.net.au/discussion/index.php?topic=32366.0

Once this stage is proven;

* you then need to create a proper PCB artwork so that a PCB manufacturer can make proper PCB's for your amp manufacturing.

If you are operating on your own and only intend to build a few amps a month then you may decide to stay with hand-drawn, home etched boards.
If you say theory and practice don't agree you haven't applied enough theory.

Roly

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Re: [help] guitar preamp
« Reply #9 on: July 03, 2012, 11:35:10 PM »
Well abe, look what has turned up!  Circuit, FET's, AND a PCB layout.  :tu:

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

HTH
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: [help] guitar preamp
« Reply #10 on: July 05, 2012, 09:53:39 PM »
Excellent example of (some) of the steps between an idea in your head (or drawn on a napkin) and a working product.
The steps are (roughly):
Original idea > drawing a sketch on paper > calculating some values > correcting and redesigning (iterate , say, 5 times) > designing a first prototype > building it on Protoboard > tweaking/redesigning/shattering it against a wall (repeat as needed) > rough layout and building on perfboard > tweaking > if happy with results (or too tired/anxious to keep tweaking) design PCB > build one and populate it > correct design errors > test "almost ready" preamp > tweak > test (repeat as needed) > relax > show to friends/post in Forum/Post MP3s in My Space or some Video in You tube.
I am not exagerating at all; just look at my late friend Albert Kreuzer's pictures .

Roly

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Re: [help] guitar preamp
« Reply #11 on: July 06, 2012, 04:12:51 AM »
{I think we've lost @abe}

"shattering it against a wall" brought back some memories.   ;)

Some time ago I was in a forum trying to explain why kits cost as much as the built item, that the designer has no control over the construction and testing, and making that bit idiot-proof can take a great deal of work.  Even done all in-house it is amazing what mistakes can sometimes slip all the way through production and testing and out the door to the unsuspecting client.

And what is one to make of a specification that says a conveyor brake energiser must have a "linear logarithmic ramp" output?  Of course it turned out that the young engineer who wrote the specification didn't have a clue  what sort of output was required.   ::)
If you say theory and practice don't agree you haven't applied enough theory.

francolok

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Re: [help] guitar preamp
« Reply #12 on: December 31, 2012, 03:35:54 AM »
Hi all... check here: http://www.aresaudio.com/uniamp.html  ;) ;) ;) ;) ;)

Roly

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Re: [help] guitar preamp
« Reply #13 on: December 31, 2012, 06:29:56 AM »
Thanks very much for that @francolok.

{Hopefully this mess will give you a translated version;

http://translate.google.com.au/translate?hl=en&sl=it&u=http://www.aresaudio.com/uniamp.html&prev=/search%3Fq%3Dhttp://www.aresaudio.com/uniamp.html%26hl%3Den%26client%3Dfirefox-a%26tbo%3Dd%26rls%3Dorg.mozilla:en-US:official%26biw%3D1280%26bih%3D870&sa=X&ei=YXLhULXkCqikigf0xYHIDw&ved=0CDQQ7gEwAA

...or stick the link into Google and click on "translate".
}

This is a very complete DIY offering and I take my hat off to the (unnamed) author; yours Franco?

I should make the comment somewhere that the profile of the Baxandall tone control is not generally favoured by guitarists.  I used it in my Twin-50 keyboard amp where it works quite well with keys/synth, but (putting on my guitarist hat) I would be personally more inclined to use a Jones or Fender style tonestack in an amp intended for guitar.

The other projects on that site are also well worth a look, equally well presented.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: [help] guitar preamp
« Reply #14 on: December 31, 2012, 07:47:28 AM »
Quote
...or stick the link into Google and click on "translate".
Non ho' bisogno nessuno di traduttore, tutti gli Argentini siamo mezzo Italiani  <3)

 

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