Thanks. That's all I needed to know I'll just use the pads on the board.
-Matt

You are right, my general electronics background is quite weak at the moment but I'm doing all I can to improve that (Hence my questions here). I have a pretty good understanding of simple circuits like voltage dividers, RC filters (to some extent) and simple volume controls. Basically, the stuff you learn in a simple college intro to electronics class. The various amplifier circuits are where I begin to struggle. What makes this more difficult is that I'm not actually a guitar player I have taken up this hobby because I have a few friends who do play guitar but don't have the time/desire to build amps/effects.

So, to answer your question, here's what I am after in a preamp:
1) Tone: Generally a nice "bluesy" sound... In this case that would be ranging from nearly transparent "clean" to a little dirty and breaking up. So, either a single preamp channel with variable gain or two channels (one clean with fixed gain and one dirty with variable gain to control the distortion)
2) Provides sufficient output to drive my power amp(s). I know this is a tricky one to quantify but the issue I had with the ROG circuit is a prime example. This preamp circuit (if built to spec) would not sufficiently drive my power amp to its full potential. I expected this was easy to resolve (as you indicate) but I'm not sure how to do this while still preserving the character of the preamp's tone.
3)Won't destroy my power amp... As with number 2, this is quite obvious but still a concern to me. One power amp I have should never exceed about 5.5V on the inputs or you risk destroying the chip. Obviously a simple voltage divider can ensure that never happens, but you have to attenuate the signal across the board just to guard against that one possible voltage spike. I would much rather retain the full output of the preamp and simply clip the output before it reaches a dangerous level for the power amp. In one of your other posts you point out that typically a preamp must be capable of much more gain than you would theoretically need to achieve a 1V RMS output. The tone and volume controls along with input/output impedances between the power amp and preamp can attenuate your signal to the point that you don't get enough total gain under normal circumstances.

Sorry for the vague questions but I'm trying to find a good solution to my problem without too much trial and error (which gets expensive ) I would love to be able to just design a circuit that meets my needs but for now it seems my best bet is to try building other people's designs and study how they work.
-Matt

Could you suggest a good op-amp based circuit that I could take a look at? Most of what I have seen so far are distortion pedals. Ideally I would like to have a switchable clean/dirty channel on the amp (which is kind of why I was leaning toward the Ruby as a preamp since you can increase the gain to add distortion... not a switchable solution, but should work).

-Matt

Thank you very much for your reply. When you say the gain on the LM386 is hard to control are you indicating that you don't believe the 1K pot (set up as a variable resistor) technique used by the Ruby design provides very good gain control? I have built a Ruby amp with adjustable gain so I will give that a try once I reassemble my power amp.

-Matt

anyone? 120 views an no opinions?

This may sound odd... but is there a noticable difference in the sound quality of the amp if you mount the 22K feedback resistor in the thru-holes or directly on to the pins of the LM3886? I know early revisions of the board did not have holes for the resistor, but the rev B does... Any thoughts?
-Matt

Great, thanks for the reply! At $1.75 each I really can't go wrong... There is another surplus store close by that sells bulk scrap copper plate. I'll find an appropriate size and go to town attaching heat sinks

On another note, has anyone found a suitable enclosure with integrated heat sink? Something along the lines of a car amp enclosure would be great (I think). Kill two birds with one stone.  Hammond sells one but the heat sink fins are along the top of the enclosure and it uses a t-slot for mounting (slip in a square nut and screw down the component) so applying thermal mastic would be tough.

http://www.hammondmfg.com/sinkbox.htm

-Matt

Great. I may give this circuit a try once my LM3886 kit from chipamps.com arrives. Do you find the tone stack adequate? The values are pretty much identical to the "default" component values listed for fender in Tone Stack Calculator.

Regarding running this off just one side of your bipolar power supply... I'm assuming there are no problems with this approach? I can't imagine what would happen, but I'm no expert!

-Matt

Did you end up running the circuit as-is from the positive rail of your power supply? Or were you able to convert it to use a bipolar supply?

Chassis looks great BTW. Kudos.
-Matt

In anticipation of my LM3886 kit arriving I took a trip down to my local electronics surplus store (amazing place) to see what they have for heat sinks... I picked up a couple medium sized sinks  for $1.75 each. Think they will be large enough? They seem to be on the small side of what I have seen posted here. How about if I mount the chip to larger piece of copper plate and then mount two (or more?) of these heat sinks to the other side? Here are the dimensions:

4 1/2" L
2 3/8" W
7/8" Thick

16 fins.

Your thoughts?

Ok, I'm still trying to find a good preamp circuit for my power amp (maybe amps, depends how this first one goes!). I've built  a small 25W stereo power amp that runs on a single +12V DC supply. So, that pretty much eliminates any bipolar designs (this time). I built a Professor Tweed ROG pedal for a friend and tried it in front of my power amp but found it MUCH too quiet. At anything less than say 70% the volume was way too low. Even at 100% the power amp was not pushing any decent output. I have verified the power amp is good with a mp3 player and hi-fi speakers so I know that's not the issue.

so, I'm at a loss now...
1) is it reasonable to contstuct a preamp running off +12V DC with sufficient gain to "properly" drive my power amp? I would like the most clean headroom as possible as clipping tripath chips is generally not advised. Of course, all volume and tone controls will be passive.

2) How about using a LM386 as the preamp stage? I've read some posts indicating success with a ruby circuit as a preamp. I'm concerned that the low output impedance of the 386 won't be a good match for the power amp though.

here is the datasheet for the power amp chip I'm using. http://www.datasheetcatalog.org/datasheet/Tripath/mXyzxwww.pdf

I would really appreciate any suggestions or advice you guys could give as I'm a bit in the dark here and don't want to waste a lot of time and money building circuits that won't work for my purposes.

FYI, I do have all the parts to build this: http://www.beavisaudio.com/Projects/ValveCaster/MatsuminValveCaster.gif which I will try at 12V, but I don't have high hopes, so I'm looking for back-up plans.

-Matt

anyone?

I'm building an amp head based on a 25W stereo Tripath kit (The AMP6-Basic from 41Hz). I'm trying to figure out which is the best way to split the input into the two channels. If I wire the inputs with a stereo jack then any stereo effects plugged in to the amp will output on discrete channels. Any mono plug jacked in will be split between the two channels evenly. My current plan is to build two preamps (one per channel) and simply tie one to each input of the stereo input jack.

Here's the question.. will I be providing sufficient input impedance to "load" the pickups?

The preamp inputs have a 4n7F input cap and a 1M input impedance. Should I raise the input cap to 1.5M so when "splitting" the input of a mono plug the guitar pickups "see" 750K? If a real stereo plug is inserted (either from a stereo effect like a chorus pedal or from a stereo guitar) then each channel will see 1.5M... Does this seem reasonable? Or should I just build the circuits as-is and see what happens?

Here's a link to the preamp stage I plan to use (times 2) http://www.diystompboxes.com/smfforum/index.php?topic=63479.0 I will be running it at 12V since I already use that to drive the power amp.

Any advice is much appreciated!
-Matt

Thank you for the reply. I decided to buy a Behringer GDI21 to try. At $30 it's much cheaper than any of the complete pedal kits out there. I plan to disassemble the pedal and integrate the electronics into my enclosure. I'll post agian once I have everything done and tested.
-Matt

I am brand new to the forum and have been trying to soak up as much info here as I can but I'm still in need of a bit of "spoon feeding" . So, please forgive my obvious "noobness".

I have built and tested an "AMP6 Basic" kit from 41Hz.com ( http://www.41hz.com/main.aspx?pageID=118 ).

So you don't have to read the website, here are the vitals:

• 25W x 2 output
• based on Tripath TA2020 chip
• powered by 12V DC "brick"

So, I want to turn this guy in to a little practice/small gig amp for a friend. I'm either going to have a single guitar input that just gets duplicated on both channels (one preamp) or a guitar preamp on one channel and a mic preamp on the other.

I am having serious problems deciding what to do about the preamp stage. Can I simply use one of the many pedals adapted from old tube amps? (like the kits found here: http://olcircuits.com/olckits.html).
Or, am I forced to build something custom? My goal is to cram the preamp and power amp into a smallish enclosure and run cables to satellite speakers.  My initial plan was to build a simple JFET preamp circuit found here: http://www.till.com/articles/GuitarPreamp/index.html but I'm not sure if that has sufficient gain, and it lacks a tone stack.

I would like something that can run off the same 12V power supply I already have for the power amp. 

Any suggestions? I can build a kit with the best of them, but get very frustrated with perfboard and trying to source obscure parts so I would be more than willing to pay a bit extra for a ready to assemble kit.

Thanks for your patience and any advice you can give.
-Matt Armstrong