Need to determine output impedance

Al Carey · From: Dublin, NH USA

I want to make a dummy load for this Sano amp I'm working on, but I don't know the output impedance, and the schematic doesn't say.

It uses 1x15" & 2x8" speakers, all wired in parallel, and none of the speakers are marked with an impedance. there is also a 4uf cap between the 15" and 2 8" speakers, I assume as a "crossover".

I put an ohmmeter on the wire and I get 5.5 ohms, so I'd guess 8 ohms is safe, but I'm not sure how the cap will affect the resistance reading?

Thanks in advance,
Al
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc...

Jim Pitman · From: Waterbury Ctr. VT 05677 USA

The simple answer - put an 8ohm dummy load across the output if you just want to turn it up for testing.
The not so simple answer -
From what you describe, and just my own sensibilities, the cap essentially removes the two 8" speakers when making a DC ohm measurement with a meter at the wire. The 5.5 ohms you measure sounds like the typical DC resistance for a speaker that calls itself 8 ohms in fact. So you are measuring the 15" speaker only.
In reality speakers have an impedance/freq curve. At their resonant frequency, which is typically less than 200hz, they are the most resistive, even as high as 50 ohms.
The capacitor's formula for it's resistance is frequency sensitive too, and is as follows 1/(2 x Pi x frequency)The capacitance value is in farads. Frequency is in hz and Pi is 3.1414...
So the 4uf cap resistance at 1000hz for example is 39.81ohms. At 100hz it would be 398.1 ohms. So think of the cap as a variable resistance that changes with frequency that's going through it - that's why it is useful as a crossover device. It's killing the current that could flow to the two smaller speakers at low frequencies because it is in series with them, therefore, they only get the highs.
Since the cap impedance is so high at low frequencies it is as if the small speakers are not there at low frequencies and their parallel effect can be ignored. A high freqs all speakers are in parallel, however, the 15's impedance is rising with frequency so the net impedance is not much less than 8ohms.
Hope this is helpful.

Jim Pitman · From: Waterbury Ctr. VT 05677 USA

Somewhere on the net is a diagram for making an RLC, ie resistance/inductance/capacitance network that simulates the changing impedance of a speaker and is the truest dummy load.
Typically, this is only use to verify an amp design can handle the changing impedance. It also is used in cabinet simulators to get the EQ effects of a speaker load.
For testing a regular old 8ohm resistor or 4ohm, if the amp is rated for it, is fine.

Al Carey · From: Dublin, NH USA

Thanks for all the great info, Jim.

Yeah, I'm just looking for a load for testing purposes on the bench. I use a 8 ohm resistor on other amps, I'll do the same with this.

Realizing that a speaker's impedance changes with frequency, does an ohmmeter test with DC, a frequency of zero?
_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc...

Jim Pitman · From: Waterbury Ctr. VT 05677 USA

Yes,it does.
Note that if you plug zero into that cap formula you will be dividing by zero - which is one of those calculus anomalies as the answer would be "infinity". (Hey that's the brand of my pedal steel guitar! lol)

Al Carey · From: Dublin, NH USA

hmmm... now that I think about it, that was a dumb question- a meter runs on a battery, so it must be DC.. Embarassed

_________________
Sierra U-12, Gretsch 6120, Fender Tele, old tube amps, etc...

Forum Index > Electronics		Next oldest topic :: Next newest topic

All times are GMT - 8 Hours	Jump to: