Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: joecool85 on March 31, 2006, 01:20:51 PM

Title: Lets design a preamp!
Post by: joecool85 on March 31, 2006, 01:20:51 PM
Ok, so we are going to base this thing off from the K-20X preamp because its simple and has great tone.  But, we are going to ditch the OD for simplicity's sake, plus then people can add their own OD later, and we are going to add a buffer to the front.  Anyone want to throw up a quick schematic of their best idea for it?  If we get this thing looking good, we can get some PCBs printed up with ssguitar.com on them and sell them for cheap.
Title: Re: Lets design a preamp!
Post by: teemuk on March 31, 2006, 02:20:15 PM
The OD channel seems to be pretty solidly implemented into the design, i wonder if it can be ditched without affecting the tone. ...And i really doubt the need of a buffer, what use would it be?

Instead of using a buffer i would replace the first opamp stage with a jfet gain stage that has a variable gain and bigger input impedance, something as simple as the schematic attached. I would leave the protection diodes and RF filter intact and maybe alter the coupling capacitor values if needed. Might need a NFB cap too, (10pf to 100pf). With correct resistor values this simple circuit should give a nice variable gain from 2 to 20 easily: Lot more gain than from the 5x of the first opamp stage (if you happen to need it). Also provides a suitable high impedance input for a guitar. Works either as booster or "transparent" gain stage. You could also make the bypass capacitor switchable ie. for pedal use. The unused half of the TL072 can be used to buffer either the output or the tonestack: Lot more use for a buffer there.
Title: Re: Lets design a preamp!
Post by: joecool85 on March 31, 2006, 02:38:09 PM
I was going by what someone suggested to me.  Good to see some ideas, lets see some more!  I'd love to see this thing actually happen.  Brian told me he would do the design and get 5 boards printed for $100, or if we can get 10 people to do the lm3886 group buy he will do it for free!
Title: Re: Lets design a preamp!
Post by: RDV on March 31, 2006, 02:38:24 PM
An Error Has Occurred!
It seems that you are not allowed to download or view attachments on this board. :(

RDV
Title: Re: Lets design a preamp!
Post by: joecool85 on March 31, 2006, 03:01:48 PM
You'd have to ask crystallas about that...I haven't tried the upload stuff.  If I remember right he has it shut off.
Title: Re: Lets design a preamp!
Post by: joecool85 on April 01, 2006, 05:02:49 PM
Hey RDV, how do you like this one:
http://aronnelson.com/gallery/Fatboy-Effects/NotSoSuperSimplePre_Amp5?full=1
?

I know you used it a while ago, are you still?  Does it work well?  If so, maybe we should do this one.

Also, anyone good at PCB design and/or making PCBs?  If we could do all of this "in house" we could save money and then if it kicks ass we could get some professional ones done up later.
Title: Re: Lets design a preamp!
Post by: RDV on April 01, 2006, 05:28:50 PM
I never built it, but it should work cause I more or less copied it from an existing design. I'm having some issues so I've not been building for better than a month(don't ask). Switchable diodes would make for a fake 2nd channel.

RDV
Title: Re: Lets design a preamp!
Post by: joecool85 on April 01, 2006, 06:15:10 PM
Hmm.  Well, we'd want to test it before we plan too much around it.  I don't have time to perf one out right this second.  Maybe in a few weeks, but its hard to tell.  The semester is coming to an end, about a month left, so it's lots of school work for me  :grr
Title: Re: Lets design a preamp!
Post by: RDV on April 01, 2006, 09:06:27 PM
This one wouldn't really qualify as "my design" either. I have designed one though(or two). I'll try to clean them up and present them later.

RDV
Title: Re: Lets design a preamp!
Post by: RDV on April 01, 2006, 09:45:57 PM
*Here's one I did design with some improvements (http://aronnelson.com/gallery/Fatboy-Effects/Not_So_SuperSimplePre_Amp6?full=1)*. I'll build this one tomorrow perhaps.

RDV
Title: Re: Lets design a preamp!
Post by: Christian G on April 04, 2006, 04:28:25 AM
Sounds like an awsome project...I might be interested in buying a PCB if it turns out nicely.
Title: Re: Lets design a preamp!
Post by: joecool85 on April 04, 2006, 08:26:15 AM
Cool.  Feel free to post your ideas for it up here.  I want as much input as possible.
Title: Re: Lets design a preamp!
Post by: teemuk on April 04, 2006, 04:42:18 PM
*Here's one I did design with some improvements (http://aronnelson.com/gallery/Fatboy-Effects/Not_So_SuperSimplePre_Amp6?full=1)*.

I do not wish to sound like i'm nagging, but isn't the gain of 22 quite high for the first stage regarding the fact that it has no gain control? Allthough guitar signal is usually around 20mV - 200mV, a humbucker pickup might give an output of even 2 volts. 2 x 22 = 44! Sounds like serious distortion to me. I know the circuit's benefit lays in it's simplicity but consider this option: Lower the resistance of either 4.7k or 100k resistor and connect it in series with a suitable potentiometer or trimmer to have a gain control. With it you should be able to get rid of any unintentional preamp distortion.

I didn't too any calculations but the 100n capacitor in the gain set of the first stage seems rather small. There might be some issues in producing the full bandwidth. Then again, maybe not. As i said, i didn't do any calculations.

As more practical additions, i'd leave PCB a place for a high frequency NFB capacitor in the second stage (in order to avoid any disappointments considering oscillation), add an RF filter in front of the first stage and protect the opamp inputs from static charges with diodes. These little additions only should lift the circuit into a way more "professional" and reliable level.

The stages might also benefit from a mild diode clipping. In my opinion it's always a better option than driving the opamps to clip. (Unless you find an opamp with fast recovery or nice clipping characters of course). Connect diodes, leds or zeners in series to the feedback loop so that their voltage drop will go just below the opamp's voltage swing per rail. (Which unfortunately depends on the opamp model. 13V is a quite safe bet, 14V might be a good value too). The diodes could be made switchable for an "extra clean" output. An effect loop or reverb would be a nice addition also. ;)
Title: Re: Lets design a preamp!
Post by: RDV on April 04, 2006, 05:07:38 PM
*Here's one I did design with some improvements (http://aronnelson.com/gallery/Fatboy-Effects/Not_So_SuperSimplePre_Amp6?full=1)*.

I do not wish to sound like i'm nagging, but isn't the gain of 22 quite high for the first stage regarding the fact that it has no gain control? Allthough guitar signal is usually around 20mV - 200mV, a humbucker pickup might give an output of even 2 volts. 2 x 22 = 44! Sounds like serious distortion to me. I know the circuit's benefit lays in it's simplicity but consider this option: Lower the resistance of either 4.7k or 100k resistor and connect it in series with a suitable potentiometer or trimmer to have a gain control. With it you should be able to get rid of any unintentional preamp distortion.

I didn't too any calculations but the 100n capacitor in the gain set of the first stage seems rather small. There might be some issues in producing the full bandwidth. Then again, maybe not. As i said, i didn't do any calculations.

As more practical additions, i'd leave PCB a place for a high frequency NFB capacitor in the second stage (in order to avoid any disappointments considering oscillation), add an RF filter in front of the first stage and protect the opamp inputs from static charges with diodes. These little additions only should lift the circuit into a way more "professional" and reliable level.

The stages might also benefit from a mild diode clipping. In my opinion it's always a better option than driving the opamps to clip. (Unless you find an opamp with fast recovery or nice clipping characters of course). Connect diodes, leds or zeners in series to the feedback loop so that their voltage drop will go just below the opamp's voltage swing per rail. (Which unfortunately depends on the opamp model. 13V is a quite safe bet, 14V might be a good value too). The diodes could be made switchable for an "extra clean" output. An effect loop or reverb would be a nice addition also. ;)
Nag away, that's what we're here for. I built this before and it did distort too much but I had the stages directly in line(feeding each other) with the tonestack afterward. That didn't work so well. I'm thinking that perhaps with the tonestack between the 2 stages perhaps the 1st stage won't really OD the 2nd stage except for extreme tone settings. I could be wrong of course. It seems to me that trimpots in both FB loops would allow fine tuning of response. I built the 1st version all bunched up on a small piece of perf so it was hard to mod. I shant make that mistake this time. I'm going to spread things out a bit.

I'm hoping for some clean headroom this time actually, the last version had none. I'm also thinking of adding gyrator stages to make certain freqs stick out, but haven't really worked on that as yet. I'd like to see you draw something up for us yourself as you seem to have good ideas.

Regards

RDV
Title: Re: Lets design a preamp!
Post by: teemuk on April 04, 2006, 06:17:12 PM
This is what i had in mind:
http://img427.imageshack.us/img427/5465/preampproject2dz.gif
Title: Re: Lets design a preamp!
Post by: RDV on April 04, 2006, 06:35:09 PM
We're awfully close to being on the same page. That's cool! What do D1 & D2 do?

RDV
Title: Re: Lets design a preamp!
Post by: joecool85 on April 04, 2006, 07:33:20 PM
Wow, this is great to see you guys work out things like this!  I'm excited to see what comes of it all.  So far no one has expressed interest in the lm3886 GB, so it looks like we are going to have to shell out a bit of cash to get some prototype boards printed up.
Title: Re: Lets design a preamp!
Post by: teemuk on April 04, 2006, 07:35:54 PM
The diodes D1 and D2 shunt voltage peaks exceeding +/- 15,65 V to the supply: This is a common way to protect high input opamps from fatal transients.

Edit:
This article concerning opamp failure modes is very educative reading:
http://www.geofex.com/circuits/when_good_opamps_go_bad.htm

I drawed the circuit in quite a hurry and already find some places to tweak: The first gain stage might benefit from a bit more gain, as could the second. As is, the poweramp will require perhaps too much gain in order to compensate pickups that have a low output signal. Adding a trimmer (in a test purpose only) in series with the R3 might help in determing the right amount of gain/headroom required. I wouldn't exceed a gain of 10 in the first stage though. After the voltage drop caused by the tonestack there should be much more headroom for higher gains. The point is this: It's hard to say how much the right amount of gain should be since a) I don't know the maximum voltage swing of the power amplifier and b) I don't know the maximum output swing of the instrument plugged in nor the guitarist's preference for headroom. Using trimmers to set the gain might be the easiest solution that pleases everyone. Reliability aspect would require that they have series resistors so that the opamps do not operate on a gain level that causes them instability issues.

The potentiometer R12 could also be eliminated by replacing R14 with a potentiometer that is in series with the decoupling capacitor C8, (although this solution might cause instability issues too). Unless the parts count is an issue i do not recommend this.

Title: Re: Lets design a preamp!
Post by: Stompin_Tom on April 06, 2006, 05:59:50 PM
hummm... I'd be interested in a pcb of this preamp if it all works out in the end... I think i'm interested in the same performance as you guys... something that will breakup a little if pushed, but is basically clean. outboard distorto.

joecool, what's the group buy all about? just pcb or kit? any discount or just free preamp pcb prototype?
Title: Re: Lets design a preamp!
Post by: joecool85 on April 06, 2006, 06:07:46 PM
The group buy was for a mono lm3886 kit, this includes one amp PCB and components, along with one power board PCB and components for $50 shipped.

http://www.ssguitar.com/index.php?topic=32
Title: Re: Lets design a preamp!
Post by: RDV on April 06, 2006, 06:17:07 PM
The preamp project may take a while as it's only in the planning stage right now.  :loco :duh xP

RDV
Title: Re: Lets design a preamp!
Post by: joecool85 on April 06, 2006, 06:21:30 PM
Exactly.  And what the group buy has to do with it is, if we can get 10 people to GB the lm3886 kit from brian for $50 shipped, then he will do the PCB design off the schematic we create, and we won't have to eat any of the prototyping costs, but we will have to still buy the PCBs.  This isn't necessarily how we have to do it.  We could do this all on our own.  However, if we design the PCB, then print 5 as prototypes, thats $100.  Until you get past 50 or so boards it doesn't get cheap.  And you don't want to print 50 boards and find out they have something wrong with them.
Title: Re: Lets design a preamp!
Post by: teemuk on April 06, 2006, 08:04:45 PM
We could do this all on our own. However, if we design the PCB, then print 5 as prototypes, thats $100.

I don't see any reason for such a high cost of prototypes - assuming we speak about the preamp here with very cheap parts etc. Almost anyone who owns, or has access to, etching equipment can make five prototype boards that cost only a few bucks. If those boards are ok then he/she can share the design here. Maybe i'm just missing the point here...?
Title: Re: Lets design a preamp!
Post by: joecool85 on April 06, 2006, 08:41:22 PM
This was for getting them printed from a company.  If someone on here can make them, thats way better.  I for one, don't make my own PCBs.  But yeah, that was basically the $100.