Adding Z to Protect Amp (newbie question)?


2007-10-22 1:18 am
Is there a circuit which will add constant impedance over a wide frequency range?

I’m considering building MTM’s with 8-ohm mids in parallel, yielding nominal 4 ohms. My amplifier specifies 6 ohms minimum, so I want to add 2 ohms to the mids (assuming they dominate the impedance since they’ll consume most of the power). I want the extra impedance to have minimal effect on the frequency performance of the crossover.

As I (crudely) understand, a 2-ohm resistor in series will protect my amp, but its effects will vary with frequency.

Have I just accidentally described an L-pad circuit?
I'm surprised to see no answer yet, so I'll have a go. Though I'm no expert, I've found out quite a bit about extra resistance in the circuit.

First, what's the amp? Do you know why they say 6 ohms min - is it for safety or just their opinion of what sounds good? It's worth asking the maker.

Next, I'd say that putting a 2 ohm resistor in series with the bass drivers seems like a bad idea to me. OK, it would keep the impedance to about 6 ohms minimum, but it will have an effect on the sound. I'd only do it if I was *really* sure that a 4 ohm load would blow the amp up, and I rather doubt that this is the case.

My valve amp has about 2 ohms output impedance, and modelling my MTM speaker showed that this had a major effect on the bass response, and on the crossover performance. These effects can easily be modelled, and you could start by doing this using the data for your proposed drivers. This should give you an idea if it's worth going further.

Jeff Bagsby's crossover simulator allows a series resistor to be inserted.
Modelling the bass performance requires a bit of simple maths to work out the new Qes and Qts parameters, then use those in WinISD or whatever. Ask me if you want details.

Good luck.


2007-10-22 1:18 am
I should have known it wasn't that simple.

My amp is an Onkyo receiver/amp, about 8 years old, nothing high-end or audiophile-quality.

Would putting an L-pad across the mids have the desired effect?

I'd like to have a go at working out the new Qes and Qts. How do I do that?

To work out the new Q values with an additional series resistor.

First, Qes.
For your chosen driver, look up Qes and DCR (voice coil resistance). I'll call your added series resistor Rs.

New value of QES = Qes x (DCR +Rs)/DCR

Next, Qts.

Qts = (Qms x Qes) / (Qms + Qts)

and of course use your new value of QES.

An example: Peerless XT18WH09 with an added series resistor of 3 ohms.
The quoted data : Qms 1.38; Qes 0.46; Qts 0.37; DCR 5.5 ohms.

New QES = 0.46 x 8.5 / 5.5 = 0.71 ; quite a difference

New Qts = 1.38 x 0.71 / (1.38 + 0.71) = 0.469

Now, the effect of the increase in Qts.
I had a good looking model in WinISD for two of these drivers in a 40 litre cabinet, bass reflex, using the standard Qts. When I used the changed Qts, I got a large peak in the bass response ... this would not have sounded good.
Changing the box tuning to a lower value fixed the problem; 35 Hz tuning gave a peak of 1.3dB, and 30Hz gave 1.0dB.

In the end I used different drivers that suited my box volume better, giving even less of a peak (and they sound fantastic!); but I used exactly the same process. And also ended up with a low box tuning.

I still reckon you won't need to add a series resistor; and if you are still concerned you might be better off going for a single bass driver; but if you get WinISD, look up some driver data and have a go at some simulations, you'll learn a lot and I hope get some enjoyment from it.

Good luck!


Disabled Account
2007-12-31 1:16 am
i would either

1 - put midranges in series and simply pad down (attenuate) the tweeter

2 - get a better amp. good amps handle 2 ohms no problem.

this forum has an amplifiers section where people would guide you to a good amp. a good amp (if you don't already have one) is an investment.