Ruby circuit too clean.

[armstrom]

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

[teemuk]

12V will give some bit of extra headroom, so will running to higher impedance. In the datasheet, page 4, top right there's a diagram that should explain pretty much all of this. Basically, e.g. 9V supply and 8-ohm load would mean clipping at 6V output. Increasing the supply voltage to 12V and the load to kilo-ohms (anything above 16 ohms is listed "infinity" in datasheet) would mean clipping at about 10V.

How much will the approximately 4 volts count...? Don't know, pull out the supply and hook it up to battery to test.

If people run their Rubys to 4-ohm load instead of 8 ohms the difference will be even higher since since the output signal is clipped at approximately 3.5V.

If the increased headroom due to mods is not the issue then maybe the gain setting circuit between pins 1 and 8 has some issues.

[Jack1962]

Teemuk is dead on bro, I would say the high impedance is your problem.

                                                 Rock On

[armstrom]

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

[armstrom]

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

[armstrom]

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