finding solid state amp output resistance, ohms

[j.michael droke]

Hi there: Hornresp TH program perameter requires the amplifier (solid state) output resistance in ohms. When this parameter is unknown, how can the resistance be determined, without damaging the amplifier? ( no manual is available for a Harmon Kardon HK870 ) ...regards, Michael Droke

[djoffe]

For a typical solid state amplifier, you could assume about 0.1 Ohms. There is also a spec sometimes given, called damping factor. Damping factor is the ratio of some reference load resistance, typically 8 Ohms, to the output impedance of the amp. So, a damping factor of 80 would mean 0.1 Ohms of output impedance.

In general, the output impedance is a function of frequency, but you might not have to get that sophisticated.

If that's not good enough for now, I can respond with a procedure to make the measurement.

[cbdb]

Most SS amps have output resistors that limit the output impedance and are a good aproximation of it. They are usualy low value (.1 or.2 ohms) larger power resistors near the amp outs and there should be 1 per output transistor. If you have multiple output transistor pairs per channel than these resistors values are in paralell. So if the value of one resistor is .1 ohms and the amp only has one pair of output transistors the output impedance is about .1 ohms, two pairs would be .05 ohms, 4 pairs .025ohms etc.

[woody]

Yea probably be safe to just use .1ohm for your calculations except if you are going
to use a passive crossover you will also need to include the dc resistance of your choke too.

[Mooly]

Originally Posted by cbdb

Most SS amps have output resistors that limit the output impedance and are a good aproximation of it. They are usualy low value (.1 or.2 ohms) larger power resistors near the amp outs and there should be 1 per output transistor. If you have multiple output transistor pairs per channel than these resistors values are in paralell. So if the value of one resistor is .1 ohms and the amp only has one pair of output transistors the output impedance is about .1 ohms, two pairs would be .05 ohms, 4 pairs .025ohms etc.

These are "emmiter resistors" used for bias/temperature stability and have no bearing on the output impedance of the amp because they are (in 99.9% of amps) within the global feedback network.

[forr]

Originally Posted by j.michael droke

Hi there: Hornresp TH program perameter requires the amplifier (solid state) output resistance in ohms. When this parameter is unknown, how can the resistance be determined, without damaging the amplifier? ( no manual is available for a Harmon Kardon HK870 ) ...regards, Michael Droke

Fit a 10 Ohm, 10 W or more, power resistor at the amp output.
Set an input sine signal such as you get 10 V across it.
There is 1 A in the circuit.
Remove the power resistor, the amp output voltage should be a few mV higher, let's say 10.1 V.
This means than there is 0.1 V voltage drop across the internal impedance of the amplifier when 1 A is delivered.
The amp ouput impedance is then :
0.1 V / 1 A = 0.1 Ohm
This is Ohm's law

You can change the value of the resistor and the signal and do the calculations accordingly.
The ouptut impedance slightly changes with frequency, but, as already stated, if it is 0.1 Ohm or lower at 1 kHz, you can consider it as null.

[jacco vermeulen]

Originally Posted by j.michael droke

manual is available for a HK870

El manuál de servicio for Otala's Speedy Gonzales amp is here :
http://www.audio-circuit.dk/Schemati...870-pwr-sm.pdf

[j.michael droke]

Hi There: Thanks to all responders for the information about amplifier output resistance and the HK870 manual. ... Michael Droke