Mark Hammer´s miniamp short build report and speaker/inductor question

[Morocotopo]

Hi people, build report and more questions for you!!
I built the mini-amp at http://hammer.ampage.org/?cmd=lt&xid=&fid=&ex=&pg=2
I used a LM384 instead of a LM380, to get more power out of it (it´s a 5W chip, same pinout). I tested it with a 18V 1A power supply. Works good, still have to test it with a 22/24V supply to see if it puts out more power.
General impressions: The treble control has to be at max or near to get a nice balance, otherwise it gets a little too dark (tested with humbuckers and single coils). The gain control adds very subtle clipping, I expected more clipping (the original is to be used with a 12V power supply, maybe that´s the reason?). Now onto speaker matters...

I tested it with a 8" 8 ohm 30W speaker, made here in Argentina, don´t know the specs (sensitivity, etc.) beyond that, but it´s a guitar speaker, not an audio one.
With the speaker free standing it sounded thin and weak in the lows, so I put the speaker facing up in a 8" wide 10" tall cilyndrical thrash bin to test the response in a sort of "cabinet" (thrash bin speaker!!). The lows improved markedly, now it sounded fuller, much better in the lows, quite nice. The highs didn´t change much. Sparkly, nice for cleans.
Then I connected it to the speaker in my Fender hot rod Deluxe (stock 12" eminence). Much more volume, much more body, fuller, more "big amp"-like response. But.. The highs were less prominent, not as sparkly as the smaller speaker, almost bordering on dark. When connected to a 4X12 (Crate, I think), the same thing happened (more vol/body, less highs, bordering on dark).

Then I turned on my BSIAB II (od/dist box), drive on full...

The situation reversed, thru the 8" speaker it sounded fizzy (the lows were OK considering the speaker size, though), but thru the other speakers it sounded VERY good (yum!), the highs were nicely balanced. Since I´m planning to make a little combo a 12" or a 10" is out of the question, so I will try to attenuate the highs in the 8" speaker to make it work well in clean AND with a dist pedal. From what I´ve read, the way to do it is with a inductor in series with the positive speaker lead.

Soooo, the question is: what do you think? would that work? I don´t know much about inductors. What should I get? what value? What type? any other suggestions?

Any help appreciated.

Morocotopo

[Jaicen_solo]

Before you go messing with inductors or anything, i'd suggest tweaking the values of the caps in the tonestack to see how much they affect the sound through your chosen speaker. If you find things too bright, you could try a 330pF cap in parallel with the 47K reistor in the feedback loop of the 4558. Conversely if it's too dark, try upping the value of the resistor tied to the cap to ground on pin8 of the 380, or make the resistor a smaller value. 

[Morocotopo]

Jaicen, thanks for the tips. Will try.

Morocotopo

[Morocotopo]

OK, as usual, I didn´t follow other people´s advice... 
I went ahead and tried the inductor idea, with a (I think) 500 micro Henries one, little round thingy made of some sort of (copper?) heavy wire wrapped around a donut shaped core (so I guess it´s an "iron core" one). You know what? It works! I read some more on inductors on the net, and what I have is a first order lowpass filter (6 dB /oct). It´s less fizzy with distortion pedals, but not yet quite right, so I´m gonna try a second order filter (12 dB/oct), made with an inductor and a cap. Not really sure about where the cutoff frequency is, I´m going to experiment and let my ear be the judge.
This might be a useful idea for people who want to use non guitar speakers for miniamps like the ruby, little gem, etc. and attenuate the highs...

Question: Why guitar amps manufacturers never use inductors to shape freq response, AFAIK? Cost (one less part)? Not really necessary, not even in acoustic amps?

Morocotopo

[Jaicen_solo]

Because to make inductors in the right values rated for the wattages you'll find in a typical guitar amp is both difficult and expensive!

[MR COFFEE]

Two suggestions\ideas:

Put your speaker in a cabinet of some sort or the the bass will always be pitiful sounding because of the dipole effect (rear wave canceling front wave) to test it. Trash can is fine for testing <grin>.

On the high end fizz... Yes, a small toroid coil in series will roll off the fizzy highs of small full-range speakers.  So will high end roll-off filters in the amplifiers, probably at lower cost if you want something that will always be connected to that particular speaker.

Adding mass to the cone, as in painting the cone with some suitable material, will also tend to roll off highs. Amazing what can be done  to kill fizz with that stuff you coat tool handles with.  Some schemes to add mass in non-uniform ways do affect the breakup response in interesting ways if you are pushing them a bit hard.

mr coffee