dc Offset

An ideal dc coupled power amplifier has zero measurable dc voltage at the output terminals.

All real dc coupled power amplifiers will have a measurable dc voltage at the output terminals.

Most tube amplifiers remove the dc component by using an output transformer.

Many older designs incorporate a blocking capacitor to remove the dc component.

Most modern transistor amplifiers have neither transformers or caps to block a dc voltage at the output terminals. Instead, dc sensing protection circuits will work to shut down the amp or remove the speakers from the circuit if a certain dc threshold is reached.

What is the threshold at which our dc voltage at the speaker terminals is "bad?"

There is no clear answer to this question. The most common tolerance I have seen suggested is 100mV, but I have also seen suggestions that a volt or more of dc output can be "OK" for certain amps. Occasionally, service manuals will specify a maximum allowable offset. As a repair tech, I play it safe and consider anything beyond 100mV to be a potential problem and worthy of a closer look.

When could a volt or more be OK? The reasoning would be that 1V across a 4 ohm load is "only" 250mA. If the particular array of speakers can withstand their voice coils being subjected to 250mW of constant dc, then it may be "OK." A low watt single speaker may burn out under such conditions. It depends on the speaker, and speakers aren't generally spec'd for maximum constant dc. The manufacturer's suggested maximum constant dc would probably be "near 0mA."

At the much more conservative 100mV, only 25mA will be heating up a 4 ohm coil. This is only 2.5mW, and should be nothing to a typical voice coil.

So if you've got an amp, and the dc offset is exceeding whatever your threshold is, how do you go about lowering it?

Old "Totem Pole" Amps

"Totem pole" output amps use two identical output halves "stacked" on top of each other. The dc offset here is entirely dependent upon the two halves being matched. The output transistors, and everything connected to them must be equal. This means |V+| = |V-| as well. If replacing transistors, make sure to match their Vbe as tightly as you can. Often the resistors will drift in value over time. Replace the resistors with fresh metal film 1% types if you can, and of the same or higher wattage rating as the originals.

Differential Input Amps

Modern power amps are all designed like op amps. There is a differential input stage, and a portion of the output is fed into the input as negative feedback. The dc path should be arranged so that the "big op amp" of a power amp has unity gain at dc. With this arrangement, an ideal power amp will have zero dc offset, so as long as the input is at 0.000V, the output and negative feedback input should also be at 0.000V.

Some amp designers go ahead and give you a dedicated trim pot for dc offset. This is relatively rare though since the dc output tolerance is such a grey area and a good design will inherently have a low dc offset. If you are lucky enough to be able to trim your offset away, go for it.

Fig 1 - A section of the Hartke HA1200 amp. VR201 is the dedicated dc offset trim control.

In the schematic above (Fig 1), a dedicated trim, VR201, is found in the lower left quadrant. This injects a small amount of either positive or negative voltage into the differential input to adjust the final offset. D201 and D202, along with the resistor divider network, ensure that the trim range is only going to be about a 1/2 volt in either direction. I found this practical example within the Hartke HA1200 amp.

A large offset may be due to a mismatched differential input. This is usually a pair of transistors arranged in the "long tailed pair" configuration. Adding a trim pot to balance the emitter circuit can adjust the offset.