# I have a stupid question.

Discussion in 'Amps and Cabs [BG]' started by Thunderbird91, Nov 2, 2005.

1. ### Thunderbird91

Oct 24, 2005
Landisville, PA
what is convection cooling mean? Is it neccessarily a bad thing? If so, how do you fix it?

2. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
Convection cooling doesn't use a fan.

3. ### Eric Moesle

Sep 21, 2001
Columbus OH
There are no such things as stupid questions . . . only stupid people . . . I forget where that quote came from, probably a sitcom.

Seriously, no question is stupid. Questions are the way we learn.

4. ### Thunderbird91

Oct 24, 2005
Landisville, PA
ok heres another not stupid question... what are ohms? why does it matter what ohms you have?

5. ### westland

Oct 8, 2004
Hong Kong
Convection cooling = hot air rises, so make a space for it to rise, and it will suck in cooler air to cool your electronics (BTW, before air conditioning, public buildings were built with a huge atrium, to let the hot air rise, and suck in cooler air from the streets)

Ohms = it varies with whether you are talking about direct current or alternating current

Ohms are a measure of resistance in DC; a measure of reactance, which is a function of inductance and capacitance in AC.

6. ### Mcrelly

Jun 16, 2003
Minnesota, USA
Knowing your speaker "load" or combined resistence measured in ohms is important because there is only a limited number or cabs that an average amp can handle. for example my stewart can handle a 4ohm "load" in mono mode. if I have one 8ohm cab, the load is 8ohms which is o.k. as you add cabs the "load" will drop. for example two 8ohm cabs connected to the same output on the amp will equal a 4ohm load. 3 8ohm cabs would be 2.6 ohms I think. the lower the load number that more current wants to leave the amp until the amp is unstable a blows a fuse or overheats.

THAT is why you should know about ohms.

running higher load, say a 16ohm speaker is o.k. but volume output will be wasted as heat withing the speaker.

7. ### Thunderbird91

Oct 24, 2005
Landisville, PA
i'm not getting it... explain so thoroughly it hurts. please.

8. ### billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
The lower the impedance load your speaker has, which is measured in ohms, the more power your amp can put out. However, if you take the impedance too low your amp will at best overheat, at worst blow completely. So the idea is to have the inpedance load low enough so that the amp will give all that it's able to without destroying itself.

9. ### Thunderbird91

Oct 24, 2005
Landisville, PA
how do you know what ohms to get though? like is 8 better than 4?

10. ### MunjibungaTotal Hyper-Elite MemberGold Supporting Member

May 6, 2000
San Diego (when not at Groom Lake)
Independent Contractor to Bass San Diego
No, 4 is better than 8. But 2 isn't necessarily better than 4, unless you're trying to let the smoke out of your amp.

11. ### PilbaraBass

It's all about delivering the power to the speakers...the better you match the amplifier's output impedence with the speaker impedence, the better you'll get that power delivered to those speakers...

as Munji says, if the impedence is too low (=load too great) the amplifier will have to work very hard by delivering much more current ...it is the CURRENT that generates the heat and hence...too much CURRENT = too much heat = SMOKE!

12. ### MunjibungaTotal Hyper-Elite MemberGold Supporting Member

May 6, 2000
San Diego (when not at Groom Lake)
Independent Contractor to Bass San Diego
BTW, the above is a gross oversimplification of matters, and there are many other things to consider such as the minimum impedance rating of your amp, how many cabinets you're going to run, amp power and what-have-you.

13. ### haujobb

Dec 16, 2004
Wallaceburg
Munji, are you feeling okay? I'm not seeing the Munjibunga humor that I'm use too...

Anyways... It's like the price is right, you need to get as close to the minimum impedance of your amp as possible without going under.

Remember though, that a lower impedance gives you more power, even though it would seem other wise. Also, in %99.9 of cases, daisy chaining two cabs will lower the impedance.

14. ### PilbaraBass

impedance is inversely proportional to load...so if you add a cabinet, you increase the load (lower the impedance)...

increasing the load, increases the demand on the power amp...hence, more power is generated by the amp....however, that is NOT to say necessarily that more power is delivered to the speakers...if you are within the rating of the amplifier, then SURE...but if your load is TOO great (impedance too low) then the amp will saturate and you'll turn your amplifier into a portable stove top...

15. ### permagrin

May 1, 2003
San Pedro, CA
Not so.

There are three types of heat transfer: conduction, radiation, and convection. Generally heat transfer involves more than one of these types.

Conduction refers to transfer of heat from one medium to another by contact. Touch a hot surface and heat transfers to your skin.

Radiation refers to energy, uh, radiating from a surface, like the fins on the back of your amp. Radiation doesn't require a medium (e.g., the only way to dump heat from a satellite is via radiation, since there is no medium in space to allow conduction or convection).

Convection refers to heat transfer to a moving fluid (liquid or gas), like air from a fan (this is forced air convection, there's also the other kind that westland describes). Forced convection is generally the most efficient means of heat transfer.

These crude definitions are what come to mind from mechanical engineering classes I took some 15-20 years ago, better definitions should be easy to find (hint: Google).

16. ### jnightmare

May 12, 2004
Carson, CA

i remember my teacher saying "theres no such thing as stupid questions but i might give out stupid answers"

17. ### Thunderbird91

Oct 24, 2005
Landisville, PA
ok thank you all for answering my question, but is there specifie examples? like if the head is 8 ohms then a 4 ohms cab will... yadda yadda yadda..

18. ### shanmag

Jul 27, 2005
most heads will be able to run 8ohm aswell as 4ohm, some even go down to 2ohm but i dont think ive ever seen a strictly 8ohm head

19. ### steveksux

Mar 23, 2005
Detroit area, Troy, MI
Yup, usually solid state amps will support 8 or 4 ohms, and often 2 ohm loads.

Just make sure your cabs don't present a lower impedance (ohms) than your amp is rated for. And the closer your cabs get to the lowest ohm rating your amp will support, the more power it will put out.

Remember, 2 cabs connected together with the same ohm rating look like 1/2 that ohm value. So 2 8 ohm cabs = 4 ohms. 2 4 ohm cabs = 2 ohms. Keep that in mind so you can add a cab later. So if your amp goes down to 4 ohms, buy an 8 ohm cab. Later you can add another 8 ohm cab to get the max power out of the amp. If you got a 4 ohm cab for a 4 ohm amp, you can't add a 2nd cab.

Randy

20. ### gottawalk

Jul 4, 2005
Woodinville, WA
It's been TOO many years since "basic electricity" class. It still throws me that putting two cabs together LOWER the ohms instead of adding MORE resistance. Is that because of the "series" vs. "parallel" aspect of how they're linked?

I would have thought that "parallel", they would stay the same ohms and in "series" they would add together the resistance, hence doubling the amount of ohms...

Time to get a book on it!