ParaBass Ruby type Amp

[strungout]

So, I've ben using my Ruby amp as a bass amp, which I leave at my friends' to play when I go there. It sounds good, very mid-range-y with a bit of lows. I run it on 12V.
I decided I was gonna build a new box for my it (folded-horn type (I blame Mark)), and while I was at it, change the circuit a bit to include a parametric bass control to offset the mid bump and hit the enclosure's resonant peak. I'm happy with the circuit, so far.

There's a 'bass boost' example circuit on the LM386 datasheet, but it's actually a treble cut, it seems.
I remembered reading Alfonso Hermida's talking points on how to build a hi-gain multi-stage distortion, when feu RDV was designing his Heavy Metal Pedal, and he made one point about the parametric 'bass' EQ: to use it to tune-in the 'cabinet' resonnant frequency. I thought it might work as well for this amp. Give it that much more Hmph, since it's a small enclosure, it needs it... With a para-EQ, I can dial in the frequency that makes my current enclosure boogie. I made it adjustable from 33Hz to 473Hz. But my bass's low B is at 31Hz. I should lower the range a smidge... I removed the Gain pot and just left pin 1 and 8 unconnected for the lowest gain (20), to keep it as clean as possible when the lows are boosted. It does go to the edge of breaking up, but I have to hit the strings real hard to get it there at max boost. If I need distortion, I'll plug in a pedal, heh.

For now, I'm still using the same enclosure (a metal, cylindrical, ex-cellphone-packaging box), till I can find/buy some wood. I was thinking 3/8" pine plywood for the folded-horn box. Will be light and solid enough, if I build it right. It would be W 6" x H 10" x D 8" (see picture). Small. Needs to be stored in a cupboard at my Bro's. The speaker is a 4" 4 Ohm subwoofer. Right now, the speaker is at the bottom of the cylinder, facing outward and down, like a subwoofer box, proped up with rubber feet. I expect the tone to change a lot in the new construction. For the better I hope.
I haven't got rid of all the noise, but it's not bad at all. What helped most was just adding a 100nF cap after the first 2k2, where Vref comes in through a 1M, for a lowpass filter at 723Hz. Tried other values, but 100n was good.

Anyway, I was looking for advice, suggestions, comments before I commit this to perf. See anything that can be improved? I mostly hack things together with my limited knowledge. It works, but if it could be better at what it does... The goal was just to add bass control/more bass while being able to tune back in to the mid-range-y sound.
I'm using a 10 Ohm 5W ceramic resistor in the zobel network. Does the resistance need to be x10 the output wattage, as I read, or will 5W do? This circuit outputs 1W with a 4 Ohm speaker, right?

I'm-a go play with it some more...


Schematic:


3D mock up:


Current Enclosure:

[Steben]

Cool.

Just a tiny thing: may I ask why a mosfet as input? A JFET works just as good (less parts!, perhaps less noise).

[niektb]

The 2SK30 is a JFET 

[Steben]

The 2SK30 is a JFET 

Crap, 2sk30 gave me mosfet results

[duck_arse]

I've only ever seen Zobel's with 1/4 or 1/2 W resistors.

[strungout]

I was thinking about what I wrote and it doesn't make sense... x10. Imagine the size of that resistor on a 100W amp...   

I've only ever seen Zobel's with 1/4 or 1/2 W resistors.

Ok, well, smaller board! But, seriously, I keep finding these threads about woofers and crossovers needing 10-30W resistor in the zobel network... Any idea what's up with that!?

[strungout]

Edit: This amp doesn't like my 32 Ohms headphones... or their sensitivity. I don't know. It hums when I plug them in (speaker out of the circuit)... What am I missing?

So, I haven't bought the wood yet and it's sunday. Will have to go tomorrow.

I ran into a problem I hadn't considered yet: shielding! I need to shield the circuit... The cylindrical enclosure is aluminum, so I didn't have to add anything to shield it. The other ideas I had either cost more money or were a PITA (conductive paint, copper tape,etc). The simplest solution was to make two cut-aways at the top/back and use a 1590BB sized box. Looks good:



I'm thinking I'll screw the box from the inside, to the top underside of the cabinet. I'll also make the left panel removeable, for easy access to everything.

Revised schematic:


Changed some values in the parametric stage. Added a headphones jack, which will disconnect the speaker when used. Added a reverse polarity protection diode.

I like having a project. Keeps me focus on other stuff. Stuff that's worthwhile thinking about. Coming along well.

[PRR]

Raise the headphone sleeve resistor to 100-300 Ohms. Does that drop hum?

Of course there shouldn't be hum in the system, if perfectly powered, filtered, shielded.

I note you bias the JFET direct from power, even though you have a well-filtered Vref in the system.

IIRC, pin 7 bypass can be a lot more than 10uFd and benefit full-bass speaker/phone (10uFd is for a gutless pocket radio).

I do not think that shoebox qualifies as a "bass horn". It may do best as a sealed box. Be prepared to try it.

[strungout]

When I first powered circuit, a few minute ago, there was no hum. At all. I unplugged it to change the 10 ohm resistor off the sleeve, and when I plugged it back in, it was humming again, though less with a 100 ohm resistor. I went up to 330 ohm and the hum is barely audible.
After that, I changed the biasing on the buffer. You're right, it was redundant. No change in hum, though.

One thing I forgot to mention is that the hum is a lot worse when I touch the metal on the Frequency pot and goes away when I touch the ground. If I touch both, I get hum, though not as loud. Do I just have a grounding problem? The enclosure should make it more silent. And with the knobs I won't be touching the bare metal. I just want to make it a silent a possible before that.

And I'm not expecting my friend's neighbors to complain! But I take your point.

[PRR]

> Do I just have a grounding problem?

Seems likely.

[strungout]

Everything worked out ok, all in glorious imperfection. It does sound bigger, I think, than the old one. It sure does have more bass. Because of the parametric EQ. It oscillates if I max everything, though. But, the speaker distorts if I do anyway. I think theresonance is too high... Otherwise, It's silent enough (have to put my ear right next to the speaker to hear noise), I can get some bassy fuzz and it gets clean enough. Could be worse. Will do the job.

Some Pics:







Some sounds:

https://soundcloud.com/user-165425177/parabassrubyamp01/s-JVK4PAUl8gi

https://soundcloud.com/user-165425177/parabassrubyamp02/s-Rm7qTCQW759

[PRR]

I like the look.

[strungout]

Thanks!