My first Build (pics & clip)

[armstrom]

This is my fist stab at building an amp from soup to nuts. It's all SS (sorry tube guys) but I'm happy with the results. It's quite small and powered by 12V DC.
Here are the details:

• LM386 based preamp (based on the Ruby and Noisy Cricket designs)
• Big Muff tone control between preamp and power amp stage
• TDA2005 Power amp
• 12V 2A power brick power supply
• Ted Weber Signature Series 6" speaker
• Custom wooden cabinet and aluminum chassis

Also, here's a sound clip a friend recorded for me. Fair warning, it's about 2.5MB.
http://www.mr2-power.com/Hurricane_test_2.mp3

[armstrom]

Hmm... no comments at all? Must be pretty bad

[loudthud]

Cabinet looks really nice. You need to find a source of industrial grade screws though. Sorry I don't have the bandwidth to listen to your sound clip. What does the Category control do?

[armstrom]

Thanks so much for the comment. Yeah, the screws are all from Home Depot/Lowes. In the future I'll probably order everything from McMaster so the heads sit more flush with the finish washers. But I did make sure there are no screws into wood. Everything is either bolted through with a nut and washer or a machine screw into a threaded insert secured in the wood. I hate wood screws... They always end up stripping out.

The category control adjusts the gain from about 20x to 200x. Since the theme of the amp was "hurricane" we played around with some of the terms. The "Storm surge" switch above the input jack provides an additional gain boost but in a different way. The LM386 chip that I used for the preamp actually sounds pretty good when clipping (as opposed to most normal opamps). I'll have to cut down the clip to some smaller segments so you can have a listen.
The guy I built it for has been using it quite a bit for direct recording (just routing the preamp output into a protools direct in box) so I'm going to throw together another box that just has the preamp section in it. Should be tiny and serve his purposes well.

[armstrom]

Ok, I've cut down a couple of snipits from the larger mp3. They should be managable for pretty much any connection. All of the clips are with the first volume control up about half way so much of the guiar signal is being attenuated. There is quite a bit more drive and grit available if you crank the volume knob up but my friend was recording this in his condo and didn't want to annoy his neighbors

-First clip is with the gain up about half way (category 3 or so).

-The second clip is with the category (gain) all the way up (Cat5)

-The final clip leaves the gain all the way up but turns on the "storm surge" switch for an additional gain boost.

[loudthud]

Sorry I didn't comment earlier, I lost track of this thread. I listened to the mp3's and these are my comments. Overall the amp sounds pretty good. The dynamic response of the power amp is good. There is no blocking distortion and transients are crisp. I didn't hear any bad things from the power supply. The speaker sounds good. The preamp is very clean. Sounds like an opamp with plenty of headroom. There is a certain distortion you get running a clean guitar thru a 12AX7. If you train your ear, you will start to hear it anytime someone plugs directly into a Fender tube amp with the volume on 2. Your amp doesn't make this sound and I'm not saying that's a bad thing. I think this is one of the things people mean when they say an amp sounds "warm". One way to warmup your amp would be a preamp made with JFETs or some other not so perfect devices or circuits.

[armstrom]

Thanks so much for the comments. Yeah, this amp was really designed for a blues harp player but the only person I could have test it plays guitar The preamp is indeed similar to an opamp. It's actually a very low wattage single chip power amp. It's designed to produce about 750mW through an 8ohm speaker and will do so quite happily from a 9V battery. There is actually a fet in the preamp but it's just a source follower directly after the guitar input. All of the preamp gain is provided by the LM386. The amp can get much much cleaner. Since my preamp chip was designed to drive an 8ohm load I had a very hard time getting it to clip when driving a 15-20K ohm load at full 12V supply. At 12V the amp would just get louder and louder as you crank the gain and never really get dirty. For the clips you heard I went back and added a simple 50% voltage divider to drop the supply voltage into the preamp down to 6V and that really helped get it to break up. I considered adding a switch to toggle betweel 12V supply and 6V supply to simulate a clean and dirty channel but there just wasn't room on the little control panel.

My friend has been doing some direct recording using the preamp out jack and I'm hoping to get my hands on some of those clips to share

[loudthud]

I was thinking of making a source follower that would add the right kind of distortion to warm up the signal a little. The problem is that FETs are not spec's very tightly, IDss can vary over a 5 to 1 range. Any circuit can go from good to bad even with the same company's parts. FETs distort more when Drain current varies. In oscilloscopes they like to use JFETs with constant current sources for low distortion.

What FET are you using? By changing the biasing and source resistor you should be able to dial in a little warmth.

[armstrom]

It's an MPF102 . I considered converting it into a common source gain stage but it's too late now, this amp is done and out the door to its new owner A good example of this is the "Big Daddy" overdrive pedal from runoffgroove.. http://www.runoffgroove.com/grace.html The grace pedal on that same page uses an input stage like my amp while the big daddy has gain on the input buffer to drive the LM386 harder. That wasn't the sound I was going for though