I'm sure this has been brought up countless times on message boards such as this but a quick search of the history here didn't seem to yield much so I'll go ahead and ask.

with a solid state power amp, how do you decide what impedance speaker to use? I know all the simple electrical stuff that the same power amp will output a lower wattage through a higher impedance speaker (basic ohm's law, same voltage swing through higher impedance = lower wattage) and that you have to make sure not to exceed the maximum current capacity of the power amp... but how about the speakers themselves? Is there any tonal advantage to choosing the 8Ohm version of a particular speaker over the 4Ohm? (or vice versa).  How about loudness? Most manufacturers only give the SPL for one impedance, even if the same speaker is offered in other impedances...

So, here's where the rubber meets the road. I'm trying to choose an affordable(ish) speaker to use with my LM3886 power amp and I'm not sure if I should go with a 4ohm or 8ohm. I plan to keep the rail voltages at a reasonable level so the current draw should be fine regardless of which choice I make.

Obviously, the 4ohm will mean the amp is "putting out more watts" but will that make the 4ohm speaker significantly louder than the 8ohm assuming all else stays the same? How about the tone? I know this is highly subjective and varies from speaker to speaker, but is there a general guideline regarding the sound quality of 4ohm vs 8ohm drivers? One of the speakers I'm considering is the Jensen Mod 12-70 http://www.jensenvintage.com/mod12-70.htm which is available in 4/8/16 ohm configurations.

I know this is probably a vague question but I'm a bit lost. I know any of the choices will "work" with my amp (not going to blow anything up) but what configuration will extract the most from the amp (highest output while still maintaining good tone).

As always, thanks in advance for any and all advice you can give!
-Matt

Quick question... Did you use that Stage Center reverb unit in an amp or did you build it as an external pedal? If you used it in an amp, where did you put it in the signal chain? Before the preamp or between the preamp and the power amp? From studying old tube amp schematics it seems the reverb portion of the circuit is usually after the the preamp, but obviously the pedal configuration of the Stage Center would need to be placed before the preamp (unless you put it in an FX loop). Is there any advantage/disadvantage to choosing one location over the other? I am considering integrating that design into the cabinet of an amp I'm building (LM3886 power amp driven by a JFET fender blackface clone preamp).

-Matt

Choosing a higher wattage speaker will not damage anything. You should probably use the same impedance as the stock speaker. As for the tone, that is where you will see the most difference. Each guitar speaker has its own unique character so changing to a different brand or model can affect your tone (for the better or worse). Another thing to look at is the efficiency of the speaker. If you buy a speaker with a lower efficiency rating than the stock speaker the amp will not seem as loud as it was before, if you get a higher efficiency rating then the amp will seem louder.

-Matt

Well, dropping the voltage did the trick. I dropped the supply voltage to the ruby preamp circuit with a simple 50% voltage divider (two 150 Ohm resistors). The results are nice dirty, bluesy distortion (IMHO). Here's a clip:
http://www.mr2-power.com/Hurricane_test_2.mp3

Any feedback/comments/criticisms greatly appreciated!

-Matt

Here are some MP3 samples (about 2mb each) of the amp with the 12V power supply. The first clip is the amp itself. As my earlier posts state it's a ruby preamp, big muff tone control and TDA2005 power amp. The speaker is a 6" Weber signature ceramic.

Amp Test:
http://www.mr2-power.com/hurricane%20test%201.mp3

Amp through Bose PA system (Bypassing power amp and internal speaker):
http://www.mr2-power.com/hurricane%20through%20PA.mp3

Once I get the amp home and can take some pictures of it I'll make a new thread with the pictures and clips together. I'll also post new clips once I reduce the voltage to the preamp and see if I can get some dirt out of it

-Matt

Can I "fake" a low impedance load? Maybe something as simple as an 8Ohm resistor in parallel with the output and ground? I'm sure that will kill my volume leves though as it will act as a voltage divider along with the input impedance of the power amp. I'll try reducing the voltage first to see what that gets me. Thanks for the help guys! I'll post the results.

Edit: Based on the chart it looks like a supply voltage of ~7V with an output impedance of infinity will clip at about the same point as a 9V supply driving an 8ohm load. I'll probably drop in an LM319 set to 7V and see how that affects sound. A 5V supply might get things to break up even sooner. I could then use a SPDT switch to change from the 12V supply for super clean sounds to the  regulated 5V (or 7V) supply for distortion... Hmm... have to give it a try.
-Matt

So I built a ruby circuit (slightly modified) to use as a preamp to my TDA2005 power amp and while it has good tone and is quite loud I can't seem to get the amp to "break up" or create any overdriven distortion even with the volume and gain cranked.

Here are the differences between what I built and the ruby schematic:

-removed the filter/zobel network on the output formed by R4 and C7 (not really needed since I'm not driving a speaker directly)
-reduced output cap to 1uF (again, not driving a low impedance load so such a huge cap isn't needed)
-Added a "standard" Big Muff tone control between the preamp (ruby) and power amp.
-Running at 12V
-Added the "grit switch" mod from the noisy cricket

Other than that, it's a standard ruby/cricket circuit. My initial tests were done without a grit switch. Due to the frustratingly clean output (loud as hell, but clean) I decided to add the grit switch and have my buddy try it again... I'm still awaiting results but I'm not too hopeful. I'm using a National LM386-4 and have tried a number of MPF102 fets in case I'm getting too much attenuation from the buffer.. no dice.

Does anyone have any idea what could be keeping the circuit clean? Could it be the 12V giving lots of clean headroom? Or the fact that the amp isn't driving a low impedance load?

I would like to be able to achieve results like this: http://www.youtube.com/watch?v=vk6nd3YcKDc

I want to run the TDA2005 circuit at 12V minimum to maintain good output to the speaker but may try a voltage divider or regulator to get 9v to run the ruby preamp and see if that helps give some overdrive/clipping sound.  Any other suggestions?

Thanks in advance,
-Matt

It should be said that a little hiss at full gain is to be expected. In my case I'm feeding the output into a 15W power amp and even then I barely hear the hiss at full volume and full gain, but it's still there.

You can do some testing to find out where the noise is coming from.
Here are my suggestions in easiest to hardest order.

-Try a different power supply. IF you're using a battery then this won't make any difference but if you're using a wall-wart you may be getting noise from there. Either try a different wall wart or switch to a battery.

-Swap out the buffer JFET. Assuming you socketed the JFET just try another one. I've had problems with MPF102 FETS being noisy in the past. Using a different batch or a different manufacturer cleared up the problem.

-Swap the LM386 chip. I've never encountered a bad one, but if you socketed the chip it will be super easy to try another (assuming you have a spare).

-Double-check all your solder joints. You may have a cold joint or a small solder bridge causing interference from another part of the circuit.

-Remove the input wire coming from the jack (desolder it at the board, not at the jack). If the noise goes away then your input wire is acting like an antenna picking up RF interference that then gets amplified by the ruby. Either make the wire shorter or use shielded wire.

-Remove the wires leading to and from the volume pot (again, at the board). Same story here as the input wires.. shorten or shield if they're picking up RF

-Change out the power supply filter CAP (100uF on a Ruby) Either try a different cap or increase it to a 220uF like the Noisy Cricket has. Again, not likely, but who knows.

-If all else fails, rebuild the circuit. This is what I ultimately did.  I was trying to eliminate RF interference in my ruby and somehow "broke" it. Probaby created a solder bridge or something but I have no idea what caused it. Since the ruby is such a simple circuit I decide to just build a new one. It took me all of 30 minutes and it works like a champ.

I hope you find your issue.. the Ruby is a great little circuit.

-Matt

Anyone?

I have a couple of basic questions regarding the use of shielded cable within an amp. Here goes.

1) Which end of the shield should be grounded? I know this is a generic question but I'm struggling with it. I know I should only ground one end of the shield to avoid ground loops, but which end? Lets say I'm running a shielded wire from my preamp board to a volume pot. Should the shield be grounded at the pot or at the board?

2) How many leads for a mono 1/4" audio jack? Since the barrel of the 1/4" plug is ground, can I just connect the shield and use a single conductor inside for the signal (tip)? Or, would it be better to run two-conductor shielded wire from the jack to the preamp board with the ground and signal on shielded conductors with yet another connection to ground for the shield itself?

Thanks for any advice.

-Matt

Here are some pictures of my first and still on-going build:
The amp is a bridged TDA2005 with a modified ruby circuit as the preamp. Power will come from an external 12V power brick. The speaker is a 6" Weber signature ceramic. The cabinet started out life as an IKEA flower pot and was trimmed and shaped to its current form. The "chassis" is half of a Radio Shack aluminum enclosure. I'm having a leather handle made for it tomorrow since none of the ones I've found online are small enough (box is only 8" wide).

I hope to have it all buttoned up before the weekend... Hopefully everything works 
Opinions? I know it's kinda retro looking... but it's being built for a retro kinda guy.

Thanks for the feedback. I will probably remove the filter circuit on the output.

Just a comment about the LM386 outputting 1/2W... If my understanding is correct, amplifiers don't "generate" or output wattage. The LM386 is simply a voltage amplifier. The wattage rating is simply indicative of the maximum current that the amplifier can pass safely. If at a particular input voltage the 386 chip is outputting 1V through a 4Ohm load then the wattage is 0.25W..

One thing I want to understand though... For a fixed input voltage (say .1 V) will the output voltage be the same (2V assuming Av = 20) regardless of the impedance of the load being driven? I ask because on the LM386 datasheet there is a graph showing maximum peak-to-peak voltage vs. supply voltage and there are four curves, one for 4Ohm, 8Ohm, 16Ohm and infinity... Does this just indicate the maximum possible output voltage at those load levels? In other words, the point at which the output will clip.

So other confusions aside... The question still stands... does the schematic I posted seem reasonable? Will the 10K trimmer pot (after the volume pot) be sufficient to attenuate the signal enough so as not to overdrive the power amp?

Thanks again to everyone who has responed. I really appreciate it.
-Matt

Teemuk and Darwin,

Thanks for you help. Here's a partial schematic of what I plan to do (note I've only shown the signal path from input to the power amp (including the input of the power amp board) with my comments in place. This is not a complete schematic, I know I forgot the cap from pin 7 of the 386 to ground

So, what do you think? Will the 10K trimmer be sufficient to allow me to attenuate the output of the LM386 to keep from clipping the power amp with the 100K volume control at maximum? I could always add a series resistor before the 100K volume pot (maybe a 50K?) and that should increase the attenuation across the board (even when the pot is at maximum volume)

Any feedback is much appreciated as I need to get this built ASAP but don't want to waste time chasing my tail on a bad idea
-Matt

P.S. The schematic image is pretty wide, so you will probably have to scroll to see it all.

One thing I wanted to add is a preamp output jack so the amp can be plugged in to a PA for bigger sound. To do that I think I'll need to keep the 10K trimmer and add a new volume pot between the 220uF output cap of the ruby and the input of the power amp. I will probably reduce the size of the output cap though, maybe a .1uF as you suggest.

Do you see an issue with leaving the 10K trimmer after the 100K master volume control? I'm assuming enough signal will still make it to the power amp.

I can post a schematic of what I'm talking about later today.
-Matt

I'm doing something similar. I will be using a modified ruby circuit as a preamp to a TDA2005 15W power amp. I will make the following modifications to the ruby circuit:
-Replace the 10K volume pot with a big muff tone control
-Add a "master volume" control on the output as the other poster recommended.

My power amp board has a 10k trimmer that acts as a sort of master volume so I may just replace that with a 50K pot as the master control rather than have two pots, one right after the other.

Here's a schematic of the power amp (this was drawn by our own N9VOC and posted over on the diyAudio forums.. I hope he doesn't mind my re-hosting it) http://www.mr2-power.com/downloads/fk607.pdf

As you can see in the schematic the VR1 10K trimpot would act like a master volume control. Do you think just replacing the trimpot with a 50K log taper will be good enough to act as a master volume? (remember, there will be no volume control between the JFET buffer and the input of the LM386)

I do have one question that maybe someone can answer... can I reduce the size of the output cap on the ruby? The 220uF seems pretty big for just coupling to a fairly high impedance power amp stage.

-Matt