8ohm Amp, 4ohm Monitors

  • Thread starter Thread starter Slackmaster2K
  • Start date Start date
Slackmaster2K

Slackmaster2K

Gone
Yes, a cute little hafler power amp is certainly in my future. However I need both better monitors and a power amp, and right now the monitors take priority.

So here's the problem. I have an amp that wants an 8ohm load, and am going to get a pair of 20/20's which are 4ohm.

To avoid damaging the amplifier, I was thinking of wiring in a 4ohm non-inductive resistor. The question is, how? I'm no electronics expert. Talking with a friend, it was sort of determined that I would wire the resistor in series (one per channel of course), in which case I should be able to put it on either the ground or the lead. It would make sense to me sound-wise to put it on the ground, but maybe it doesn't matter...what the hell do I know.

I'm also curious as to the power rating of the resistor. If my amp does 40W per channel, do I need a 40W resistor?

Finally, this seems like an odd though somewhat common problem. Are there any products out there designed just for this sort of thing? The resistors that I've used in the past like this have all had somewhat delicate leads and would be somewhat difficult to wire inline. That's of course assuming that wiring it inline would be optimal....

Thoughts, comments?

Much appreciated!

Slackmaster 2000
 
It's a dicey issue...

Theoretically, you can just add a resistor as you mentioned, however, in practice, this would be impractical to say the least.
At the levels you would be working at (or in any audio situation), you would be dropping ONE HALF of your amp's output across that resistor (and all in the form of heat). For example, with a 40 W input, your monitors would be pushing 20W, and the resistor dissipating 20W. As well, since the resistor is not an inductive load, your amp would be working 4 times as hard to give you the required current to make that 20 watts (since in an inductive load, the required power is dependant on frequency - the lower the frequency, the more power required to drive the load). That resistor would be glowing. As well, the resistor would affect the frequency response of the monitors by restricting current flow.
As well, a 40 Watt resistor is about 4 inches long by 1/2 inch wide.
(It's been a long time since I've dealt with the theory behind all this - should look through some old texts when I get home).

Unfortunately I can't offer you an answer to solve your problem - unless you bought 4 20/20's and connected two in series on each side....


I'll get back to you if I find out more,
Mike
 
The resisor is called a dummy load. and It shouldnt be nessesary because the amp should be able to handle the 4 ohms anyway.(all amps made nowdays can).
 
The purpose of the dummy load is to be able to test and troubleshoot preceding stages of an amplifier (you use the same concept for RF amplifiers as well). It provides an accurate, known quantity for adjusting power levels, bias, etc.


I've been trying to dig all this theory up from the dredges of my memory, and without bringing out my box of Fourier transforms, I'll try and work this out...
To get maximum power transfer, you want the output impedance of the amplifier to match the input impedance of the speaker. Impedance is made up of resistance, capacitance and inductance and is dependant (or varies, anyway) with the frequency of the signal applied to it.
Dragon talks about impedance matching here:
https://homerecording.com/impedance.html
but this is a bit different - the objective in input circuits as opposed to output circuits, is to get maximum Voltage transfer (you're not as concerned about power - but that's another discussion).

I think darrin_h2000 is right when he states that most amps can handle variations in impedance - the only possible problem lies when you start to turn up the volume. According to Ohm's law, Current equals Voltage divided by Impedance (I=E/Z). Therefore, at a constant voltage of say 8 Volts, with a nominal impedance of 8 ohms, you have a current of 1 Amp. If you decrease your impedance to 4 ohms - there's now 2 Amps. Current is what is going to fry your output transistors. So, at lower levels, 4 ohms should be great.
I ran my 8 ohm amp like that for a while with 4 ohm Paradigms on the B channel - Boy, it sounded good, but I chickened out past about 4 on the volume (plenty loud tho').

As for your comment about feeling the load, that's right, the amp is going to be happier from an operational perspective - I guess I was thinking more of the loss of half the power to your monitors (where the hell did I get 4 times from in that other post ??). Anyway, adding a resistor is also going to change the whole frequency response of your monitors (which is why we buy them in the first place). You will have effectively changed the impedance of the speaker (by adding more resistance, it affects the capacitance and inductance of the speaker and how it responds to the amplifier). I'd be more inclined to just use them as they are, and watch that volume control until I can get a new amp.

Just $.02 worth...

Cheers,
Mike
 
Last edited:
Slackman - go for the 20/20 bas speakers and frig' the rest of it. You guys are making my head hurt. (Of course I come from the plug it in and jam school.)
 
mine hurts too - I'm gonna play my guitar...

just plug it in and jam...good one...I think I went there too...
:D
 
teknomike,

Thanks for the input! I think I'll take your advice and just watch the volume. It's an NAD amplifier so it should be of reasonable quality, and perhaps it will work just fine.

dachay2tnr,

If I had the money for the 20/20bas system, I'd have the money for a small hafler and the 20/20's. Call me crazy, but I don't like the idea of things built into other things :)

Slackmaster 2000
 
Something makes me think NAD produced (or produces) constant current amplifiers.
Anyway, NAD have also been known for overdesigning power supplies, so you may be all right.
Tell me, does the amp specify 8 ohms for speakers, or does it just give the spec (e.g. 40 W into 8 ohms)? If you do a search on the model, you may get lucky and find some detailed info about rated output with different impedances (as opposed to just the specified rating).
Cheers,
Mike

This is from the NAD site (see the ISC info) :
http://nadelectronics.com/Technical.html
 
Hey...I was just looking at some stuff on Deja...my amp is an old NAD 3020 and apparently, it's rated to do 20W into 8ohms or 40W into 4ohms. I'm not certain though, this is just from a few conversations that I 'overheard.' I'm not going to worry about it, thanks guys!

Slackmaster 2000
 
Ok, from the product brochure manual for my amp (which I didn't think I'd find):

"The power supply circuit for the output stage is only loosely regulated, so it is free to supply the high voltages needed for a high IHF headroom factor (at 8 ohms) and also to supply the large currents at lower voltages needed for driving low-imedance loads (down to 2 ohms)."

Thanks again technomike...at least we learned something! (like check the manual)

Slackmaster 2000
 
Careful you dont underpower those event 20/20s. Even if you can go 4 ohms not enough power will cause the amp to distort and damage the tweeters.

a good rule is to have at least 100% of the power rating in amp power but dont turn them up all the way. that way you get enough power and little distortion
 
You must log in or register to reply here.
Back
Top