Choice of transformer, chip

[autir]

Hello all

I recently bought a Celestion Bulldog speaker, made in UK for Vox, 15W, 6 Ohm. I want to build an amp head just for this beauty.
I am unable to choose between TDA2030, 2040 and 2050, does anyone have any recommendations?
What transformer ratings should I go for? (THE AMP MUST NEVER REACH > 15W 6 Ohm)

Thank you in advance 

[Enzo]

TDA2030, 2040, 2050, as in 30, 40, and 50 watts.   But those are merely capabilities, the ICs don't MAKE the power, any more than using jumper cables from your car will make a 9 volt battery any stronger.

Power supply will be the limiting factor.

Keep in mind also that you probably will be playing music through it, not steady sine waves.  SO a 30 watt amplifier will not be putting a steady 30 watts on the speaker.  Music has momentary peaks of power - whuch the speaker will handle - but most of it is lower power.

Ohm's Law and stuff, applied, tells me your 6 ohm speaker doeosn;t want to see more than about 9v for 15 watts.  9v RMS of signal, and that means a peak voltage of about 13v.   SO if you ran any of those amp ICs on +/-15v, it would pretty much prevent you from exceeding power to the speaker.

Me personally, I'd design something with a bit more headroom, and use some personal discipline to not crank the thing to the sky.

Having said all that, why have you already settled on the TDA chips?  Nothing wrong with them, but you might consider a smaller amp.  a 5 watt amp is plenty loud, maybe not on stage when competing with a drummer, but your 15 watt would have trouble with that too.

[J M Fahey]

I'm not sure that your speaker has 6 ohms impedance.
It quite probably has 16 (sixteen) ohms impedance , which is a normal value, *or* you measured 6 ohms DC RESISTANCE with your multimeter, which indicates 8 ohms impedance.
Please confirm that before going any further.

[autir]

Thank you for your replies.

Enzo, using ST chips is not written on any stone    It's just that I am familiar with their chips, have used them in the past. Any recommendations are welcome. My issue is that I create all my circuits on stripboard, so my choices regarding packages are limited. I have searched for a 5W or similar chip, but the only solutions were DIPs (unable to be heatsinked properly, so I would have problem dissipating all the excess energy and would face thermal shutdowns) and the next step would be the TDA2003 (being extremely non-efficient at such low power levels).

J M Fahey, this is exactly what I did. The speaker's label did not mention Ohms so I measured it with a DC multimeter at 6 Ohms. I admit that I was not aware that it is not the proper method. All my other speakers, from 3 to 8 Ohm, when measured provide their stated values. Why not this one? Is it because of the increased inductance? (the other speakers were max. 3 inch). If so, wouldn't the resistance settle after the initial inductance effect ?

[J M Fahey]

Hi Autir.
1) First of all, you have confirmed this speaker is 8 ohms. Impedance, stated by a single number, is always nominal, and usually accepted to be around 20% above DCR (it's not a Physics Law but a convention) because to give a better portrait you'll need the impedance *curve*, which takes a lot of space.
2) That said, remember that most multimeters show a value above zero when the probes touch each other, which comes not only from the wire resistance but from the various connectors in the path (probe sockets and selector switch) so you usually get between 0.3 and up to 1.6 ohms, the latter value on *very* cheap multimeters with quite grimey contacts.
You must substract that value from what you read.
In your earlier tests, it somewhat compensated that "missing" 20%,
Most "8 ohms" should have around 6.5 ohms DCR but many Musical Instrument speakers have 5 or 5.5 ohms DCR so they appear to "sound louder".
Oh well.
Now you know that you should not go above a TDA2050, with +/-16V rails, which come from a 12+12V ; 25 to 45 V.A. transformer, plus a 2A (or more) bridge and 2 4700x25 electrolytics.
It's easy to build on stripboard, and you can bolt the chip to a heatsink or even the chassis if aluminum, or a flat sheet of same material.
If possible, post a couple pics of that speaker, specially showing the magnet and peeking inside (through the frame arms) to show the voice coil joining the cone and suspension.
Is it an actual Celestion or a "specially made for VOX " type?
Search and download the excellent "DIY layout creator" , killer software, it has a stripboard design option including a TDA2030 example.
Try to get the "old version" which is a straight program that runs in Windows machines; he later modernized it rewriting it in Java, so it runs also in Linux and Mac, but it's klunkier, and, worst of all, can't open the earlier made examples, contributed by hobbyists the world over.

[rowdy_riemer]

He should have used C++ and Qt. It might have worked out better than java.