Quicky Ruby Question

[armstrom]

I just have a quick question about the ruby amp and the gain pot specifically.

I'm using the ruby as a preamp section for a 15W power amp I'm building. I will be replacing the 10K volume pot with a standard Big Muff tone control... My concern is that the 1K gain pot specified may be too small. Would using a 10K gain pot be overkill?

Here's my reasoning: If you look at the LM386 datasheet one of the sample circuits is set up for a voltage gain (Av) of 50. They accomplish this with a 1.2K resistor from pin 1 to pin 8. The 1K gain pot will provide even less resistance than this at it's minimum value... that means a ruby will never have a gain of less than 50x if using a 1K pot.  On the other hand, a 10K pot between pins 1 and 8 would result in a gain of just a little over 20x  (10K paralled with the 1.35K internal feedback resistor gives a total resistance of ~1.2K) So, does this modification seem reasonable? Or would it make the gain too unresponsive? A 10K log pot might help to make the gain more sensitive to change while still providing the same range of control.

Any thoughts?
-Matt

[MikeH]

Not to dodge your question (because I don't know the answer) but the 386 is used in the ruby and many other small amplifiers because of it's capability to drive a speaker, not because of it's tonal superiority.  In fact, I'd say it ranks pretty low tone-wise.  As long as you've got the power handled and you're just looking for a preamp, (ie gain and tonestack, etc) there are a lot of other projects out there that will sound a trillion times better.  Notably many of the projects at runoffgroove.com which are based on the preamp sections of popular tube amplifiers.

[armstrom]

I've read reports from a number of people who have used the ruby as a preamp for a power amp section with good results.
Here are some of the reasons I chose the ruby:

1) I already have one built on a PCB...
2) It has a low output impedance making it suitable to drive pretty much any power amp stage regardless of its input impedance
3) wide range of gain control.
4) Seems to have a nice tone when it transititons into clipping. I've heard most op amp circuits (which could provide the same range of gain as the ruby) don't clip as gracefully.

I have tried using a professor tweed circuit to drive a 25W power amp I built a while ago and it just didn't have enough gain. I had to turn the 100K volume pot on the tweed all the way up to get any reasonble sound out of the amp, and even then it was only maybe at conversational speaking levels... I know the power amp is good because I tried it with a line level output from an MP3 player and it was quite loud.

Would you suggest some other simple preamp circuit I can toss together quickly? Here are my criteria:
1) Decent gain that's adjustable
2) Low output impedance so I can get good signal transfer to my TDA2005 based power amp
3) Nice distortion when the gain is increased.

The amp is being built as a gift for a blues harp player. He uses a Green Bullet mic so any guitar preamp section should work fine with it. Obviously crazy overdrive and diode-style clipping are not really needed here as the harmonica sounds "best" to him with just a bit of breakup... ("dirt")

[ambulancevoice]

I just have a quick question about the ruby amp and the gain pot specifically.

I'm using the ruby as a preamp section for a 15W power amp I'm building. I will be replacing the 10K volume pot with a standard Big Muff tone control... My concern is that the 1K gain pot specified may be too small. Would using a 10K gain pot be overkill?

-Matt

i think i read somewhere that 5k should be the max value pot you should put in there
ive never seen anyone use more than 5k, let alone 10k
also, according to run off groove, the 1k pot alone manages to increase the gain from 20x to 200x
also, that big muff tone stack will probably cause a volume drop (unless thats what you want)
id probably use this ruby circuit, just without the volume pot (but the volume pot i think would make a good pre-amp volume control, for more versatility)

[MikeH]

To answer your question about ROG: The prof tweed is relatively low gain compared to some others.  if you wanted to try one with more gain try the Thor, English Channel, Matchbox, and even the uno would work well from ROG.  Also the Dr. Boogey and BSIAB 2 are along the same lines.  Unfortunately none of them are what I would call "easily tossed together".  They generally need to be breadboarded and tweaked a little.