Power amplifier output matching from 8 ohm to 16 ohm SOLVED


I have an old solid state audio power amp which needs a new driver. Problem is that it's originally designed for 8 Ohm driver and the replacement is 16 Ohm. I know that it can be safely connected but output current will then drop to half and volume will be reduced as well.

My question is, how I need to change the circuit to rise output current back to original with 16 Ohm load? Could it be simply done by changing resistor R88 and R87 values to let more current flow through transistors Q13 and Q14?

by valotus
November 29, 2018

1 Answer

Answer by mikerogerswsm

A 16 ohm load needs more voltage, say root two times, but less current in the same proportion. So it requires +/- 60V dc output stage supplies and probably better Q13/Q14 with higher voltage ratings, at least 120V. The previous stages also need redesign.

An alternative is to use two 16 ohm drivers in parallel.

Change of drivers may upset the compensation, which often is tweaked to suit a particular driver.

Surprising that a treble amplifier has such power, the main amplifier must be absolutely ginormous.

ACCEPTED +1 vote
by mikerogerswsm
November 29, 2018

Thank you very much for clarifying things to me! I'm able to test and change faulty components from electronic devices, but understanding a whole circuit design goes sometimes a bit over my skills... This particular circuit is from a Leslie organ amplifier cabinet, which has 8 Ohm 1" compression driver for treble and 4 Ohm 15" woofer for bass. Crossover board separates input signal from 800 Hz and despite of different impedances, both bass and treble has identical power amp circuits based on two 2N3055. However, my replacement driver is now 16 Ohm and treble amp volume needs to be turned full open to get them balance.

by valotus
November 30, 2018

That is very interesting, presumably it is a Hammond organ with Leslie speaker. Truly vintage equipment and well worth preserving.

The amp is very complex with three feedback loops, and both speaker voltage and current are fed back.

Best to try to keep this classic equipment in original condition.

I would try an initial test of intercepting the signal as it comes into P7/S8 and adding some ac gain, say 2x. This will effectively move the volume pot R74 further round without affecting anything else. It will increase the level but there may be a point where distortion becomes a problem. Worth a try.

by mikerogerswsm
November 30, 2018

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