Tiny giant amp cab

[Mgt280y]

Just finished my 2 x 10 bass cab running on a tiny giant works first time
Couple of things
If it's turned on with nothing on the input there is quite a hum, but plug the bass in it stops.

When everything is plugged in there is a tiny amount of noise (expected ? ) but if I touch the volume pot shaft the noise changes, is it also possible that I can feel a very slight tingle in my fingers when I touch the pot or am I imagining it?

[highwater]

Sounds like you have DC voltage somewhere that you shouldn't.

First step would be to make sure that it isn't something "obvious", like the circuitboard and/or component legs shorting against the jacks, pots, or enclosure.

If that doesn't turn anything up, use your meter to check for DC volts on the pot body and shaft, the input and output jacks (both signal lug and ground), and the enclosure itself. All of those should be at 0v DC... do the tests with the negative lead at the battery itself (or the ground pin on the power jack, whichever is applicable) - if you use the chassis instead, that could give 0.000v even if there *is* a problem.

What to do next is beyond my pay grade, but that ought to give you a good start. It will be helpful to give some additional details - did you use vero, stripboard, a PCB, etc., what kind of enclosure is it in, what type jacks did you use, that sort of thing.

[PRR]

> nothing on the input there is quite a hum, but plug the bass in it stops.

We usually wire the input jack so when nothing is plugged-in, the input is shorted.

You don't have that? Look at almost any guitar-amp plan.

[Mgt280y]

Didn't know that, the input is not shorted when not plugged in I'll try that tho.

[Mgt280y]

shorting jack works a treat 
also added a -ve lead to the pot casing and that has helped

with all plugged in there is still a little noise with the volume turned up but i think this is likley to be an induced noise from maybe the ac/transformer as i have mounted inside the cab ?

Some images for anyone who wants to browse
https://www.flickr.com/gp/141176322@N02/418w6E

[Kipper4]

Neat Dan. Very tidy build. Congrats.

Is your bass active?

[Mgt280y]

yes but it has the 3 way switch so i "belive" it can be passive?

oh i have also learnt today that the cab should be ported ?

if thats the case how do i work out the size of hole i now have to make in the box and how long the post needs to be 

[Quackzed]

WOW, that came out SWEEEET!!! looks solid as hell too. google port calculator, i've use pvc in the past for ported enclosures ,easy to cut and you can cut different lengths to test out, just some 1" pipe and a flange to fit it to and pvc glue (pvc dissolver really) . dry fit test easy enough... do the calc but test before committing: let your ears have the final say. 

[Mgt280y]

i have looked at a couple and they only seem to give you the length of the port when you enter the height and width or diameter(round) ports and what is the frequency for tuning ?

[Mgt280y]

Also can anyone suggest a tone stack to use in this still undecided as to if im putting any other effects in / pre amp keep thinking a twin castor as pre but the tone shaping on that i dont find to be that good either and would only be present while pre is on.

[Quackzed]

heres a good port calc , scroll down a bit. you'll need box volume but thats some basic hxwxl minus any bracing volume... theres a box volume calc as well, plus some other usefull speaker box building calcs and resourses. good page!
http://www.the12volt.com/caraudio/boxcalcs.asp

[PRR]

Guys, he *needs* a calculator that starts with DRIVER parameters. Qts, Fs, Vas.

Without that, box frequency is a total guess, surely wrong, leading to lumpy response.

You can NOT "improve" a small box by putting a hole (or pipe) in it. If you made your own drivers, a combination of odd driver parameters and a small ported box *may* be some improvement.

If the box is large you can trade-off the excess internal volume for a small increase of output over a narrow band (and much less output below that).

Dan's beautiful box is, IMO, just big enuff that a port MAY be better.

Having wasted many hours in both guesswork and laborious computations, I'd leave it be. A sealed box has the special advantage that sub-sonics will NOT over-excursion the suspension and rarely sounds "flappy".

[Mgt280y]

Thanks PRR, TBH I like the way it sounds and unless a port is going to make lots of good difference I'd rather not cut holes as leather covered wood not something I want to slip with the drill 

[anotherjim]

Beautiful cabinet work.
I'd avoid porting too.

I do wonder if you'll need some damping "stuff" on the interior walls. See it fitted in instrument cabs nowadays when once I only saw it in Hi-fi/Studio cabs.
What's best? High density fibre wool? Mineral loaded foam? Nothing at all?

[Mgt280y]

I was always under the impression correct if wrong but filling was to to stop the enclosure noise ie the box rattling if poorly constructed, i did consited if it had any effect on the sound bouncing of the rear of the enclousure but again happy to be corrected but most of the rear sound is sub low end which is non directional and "shouldnt" be affected by phasing (correct term?) 

[PRR]

> box rattling

That's fixed by proper construction and gluing.

Imagine a well-poured concrete box. Can't rattle. Now get your head inside and talk. Just like a concrete cellar, there are echoes and resonances. These can pass back *through* the cone and contaminate the perfect sound you intended. In hi-fi it is good to put a couple inch lining, or a wad of old wool socks, in the box. It is much less done in instrument amps. I think the extra resonances enhance the sound.

A sealed box's bass resonance goes by air-volume. But we always want a smaller box. It turns out that a carefully selected amount of "fuzz" makes the enclosed volume act bigger than the ruler says. It takes energy out of the trapped wave. The fuzz must be fine stuff, probably half the box to have any effect, but over-stuffing just reduces enclosed volume so is not better. Fiberglas attic insulation is very suitable. Dacron pillow-fluff is nicer to the hands. The improvement in effective volume is not large. And while an exact volume can give an exact frequency response, variation of volume just shifts response around without a large change of total output over the bottom octave.

I'd try half a wool (not poly) blanket, or a half square foot attic fuzz, tacked to the back. But I would not expect any "wow" difference. And it is small added weight and must be secured or it will fall down on a road-trip. So I probably would not bother.

[anotherjim]

Hmmm... A lot of acoustics stuff is often counter-intuitive.

So, the fibre "fuzz" either diffuses (pathways longer) and/or absorbs the wave so it acts like it's travelled further  -  as if the box were bigger? Conversely, if the fuzz is too dense/thick, then you have reduced the box volume?

But if the cab doesn't have an annoying honk then don't bother.