I've pegged a TDA2040 before and liked how it sounded.

Hell, JM makes his own speakers.

QuoteAn "off the shelf" Fet gain block that *works* is the Stratoblaster.(Google it)

It's funny but the schematic for it on General Guitar Gadgets says to use either a "j201, 2n5457, or other FET". Wow, FET variation is bad enough using the same type from the same manufacturer.  :duh. But GGG's bad recommendation aside, the circuit looks good to go. If I built one, though, I'd still pick the Rs that most closely suits whatever fet I chose. Then again, being a first gain stage in the chain, one's probably not going to overdrive this, and with the way their doing their gain control I guess a predictable Rd to Rs ratio might be a little more important than a approximate midpoint bias. It looks like they want a minimum of unity gain (I think I got this right), and I guess that might not be so simple with varying Rd to Rs ratios.

QuoteIt uses 9V, 12K source, 22K drain, and doubtlessly (at least on the original ones) a "good" Fet, that's to say one with low Vp (cutoff) and high channel resistance.
I know this is counterintuitive, or "against logic" but it's so.
Unfortunately most cheap and widely available Fets today have the opposite parameters, which makes them good for switching and poor for audio.

I know it's harder to get much gain out of  a FET like a 2n5457 (relatively high Idss, relatively high Vp), but aren't they less noisey? It seems like when playing around with a super simple booster design of mine, 2n5457's were less noisey than j201's. But then again, I really didn't give them the comparison they deserve, so I'm not that sure. I know the first version I built has one j201 and is certainly more noisey than my two stage 2n5457 version. With the first version, I used a couple of mpf102's simply because my 2n5457's had not arrived yet. Of course, even with an 18V supply, one doesn't get much gain from a mpf102, so I added a j201 stage. Of course the noise I'm hearing might not be the j201's fault. Maybe it's the mpf102's causing the problem. Or maybe something else entirely. Since this was just something I wanted to work on while waiting for my 2n5457's to come in, I really didn't investigate. Maybe my impression that low Idss FETs are noisier (noisiness inversely perportional to Idss) is totally bogus? What does your experience with FETs tell you?

Quote
A side note to  Rowdy about his Daughter: "start getting used to it, it's just the beginning".
Mine is 16 and it has been impossible to say her "no" for about, should we say, ¿15 years?

:lmao: Yeah, she started getting her way (well alot of the time) when she was about 1, so I've had 6 years to try to get used to it. She's got a very good "puppydog face" she uses against me if I try to tell her no.

QuoteThanks a lot rowdy_riemer !!

I'm glad to help. To upload an image, click the arrow next to Additional Options and you'll get the UI for attaching a file. Also, you can put a link to a url to a file using the "insert image" toolbutton above.

BTW, my 7 year old daughter is right next to me bugging me about letting her put a smiley face on this reply, so here it is.  :tu:

When the voltage across those diode is high enough for the diodes to start conducting, they increase the feedback current dynamically reducing the gain. The result is soft clipping. Read up on the theory behind the Tube Screamer for a better description. Here's a pretty good description: http://www.bteaudio.com/articles/TSS/TSS.html.

QuoteI suppose since Rd has an effect on output impedance, it makes a bit of difference tone wise. If someone wants a Rd close to a specific value for a....

Opps, I forgot that you were discussing magic Rs values. Still, 100k may be to much to get unity gain for many fets, especially ones with relatively high Idss.

Ok, gotcha. :tu:

I've wondered myself why use a pot for the drain resistor rather than the source. I also prefer using a test rig and using the proper value resistors rather than use a trimpot. It makes building a circuit a bit cheaper too when you can get resistors for a couple cents a piece verses more than a $1.00 (U.S) per trimmer. I've noticed that with a fixed Rd, if Rd is too high, you can't midpoint bias the stage without setting Rs so high that you have less than unity gain, which, of course, proves your point that one really needs to choose resistor values for each individual jfet. When I built my first headphone amp way back when (with mpf102's), I wondered why adding jfet stages made it quieter  :lmao:. I suppose since Rd has an effect on output impedance, it makes a bit of difference tone wise. If someone wants a Rd close to a specific value for a good reason (not just because some plate resistor in a tube amp design used that value), then he or she should probably hand pick a jfet that fits that Rd value.

He probably should follow your advice and check out the AmpCan and Solo schematics, or even the  Rod Elliott project 27 schematic you recommended in the http://www.ssguitar.com/index.php?topic=1426.0;wap2 thread. However, don't be too quick to crap all over jfet designs.

Sure, with op-amps, you don't have the biasing concerns that you have with jfets. But there are some nice sounding jfet designs out there. I think the Dr. Boogey circuit absolutely kicks ass. And while I haven't built any of the ROG circuits (well, other than a few ruby amps), they seem pretty good to go. I think I might try building a BSIAB II one of these days. Most of us aren't going to be mass producing amplifiers, and some of us do not mind spending a little time biasing a FET stage or two every now and then (or maybe we're just gluttons for punishment).  No one should be discouraged from trying their own designs either, whether based on op-amps, jfets, tubes, or whatever. One can learn only so much by copying other designs.

I've got a jfet preamp design that works well for clean. But you may do just as well with a simple textbook op amp gain stage. You might check out this thread: http://www.ssguitar.com/index.php?topic=1426.0;wap2.

That's why I had suggested two lead acid batteries, though bridged tda2005's might be better (and much lighter with just one battery).

The speaker might not be your main concern at the moment, but be sure not to dismiss speaker choice as being trivial. The main purpose of a preamp is to provide at least enough voltage gain for the signal to be useable by the power amp. And yes, preamps often incorporate eq's and other effects. Many preamps are made to be overdriven while others are made to be run clean. Many distortion pedals qualify as preamps. Amps with multiple channels basically have multiple preamps. With preamps, you're not to concerned with power gain. On the other hand, power amps provide significant current gain as well as a little voltage gain to produce power.

You should really read Teemuk's book to become familiar with different guitar amp concepts. His book really kicks lots of ass and it's free. Then, decide what kind of sound you want. Then choose a preamp design that will produce that sound. Letting us know what kind of sound you're looking for will help us recommend a preamp design.

Don't misunderstand my point about the lm3886. I was just saying that lm3886 designs prove that using a hi-fi chip for a power amp is no problem. Of course, there are several tda2040 guitar amp designs that prove the same thing, so maybe mentioning the lm3886 was unnecessary all together. A tda2030 or tda2040 is fine.

You might run into a problem with running the amp from two 9v batteries. I could be wrong, but I bet that a 9V battery will not supply the current you need. If you want a battery-based +- 9V supply, consider using a couple of 12V motorcycle batteries (or any lead-acid batteries with a weight you consider acceptable). That will give you more than +-9V and will supply all the current you need.

And yes, you do need a preamp. There are many suitable preamp designs on this sight and elsewhere on the net. Also, if you get a decent guitar speaker, you do not need to worry about a woofer and a tweeter. If budget is an issue, I suggest one of the Jensen Mod speakers. While using HiFi power amp chips is plenty ok, using Hi Fi speakers for a guitar amp isn't a great idea (Except, I guess, for accoustic guitar amps).

Also, keep in mind that a proper guitar speaker will cut the higher frequencies appropriately. LM3886's are also Hi-Fi chips, but work great as guitar power amplification.

Learn how op-amps work and how you set voltage gain via negative feedback. This will help you understand how to look at the schematic(specifically the resistor ratios in the feedback path) to see what the voltage gain is.

Why not just use any power transformer that supplies the same voltage with the same or higher volt-amp rating?