woofers gone wild

Hi folks,

Just took the amp over to my mastering engineer friend's house, and we spend a long time taking measurements. The DC offset is -3 volts into a 4 ohm load. That can't be good. I'll call the Unison people tomorrow.

Thanks for all your help.

OK...Forget about the pot. It appears that the Unison amp has an active circuit to balance its output, because other aspects of its circuit design make it particularly prone to DC offset. Something has gone wrong with this circuit, or the DC offset exceeds its correction range and you need someone familiar with the Unison amp to work on it.

Lousyreeds1

Check out The Unico Series documentation.

Down at the very bottom of the paper it states

The amplifier is fitted with precision op-amp servo-regulation control of the output offset voltage.

If you bought the Unico new it is still under warranty (3yrs parts/labor). If you bought it used the warranty might be transferrable. You can check with the Unison Distributor for further information or check with your dealer.

FWIW, if the unit is under warranty let Vmax Services take care of it. If not it's your call! (Are you feeling lucky?)

Lousyreeds1...OK, maybe you better have someone who is familiar with electronics look at it. Hopefully it is just a simple adjustment which they might even do for free (if they want your further business).

A "Pot" is a Potentiometer, which is a variable resistor. It consists of a length of conductive material, often in a circle, with high resistance, and a sliding contact so that the length of material which the electricity has to pass through can be changed by moving the slider. Obviously this changes the resistance to the electricity passing through the material. Pots are used in many parts of circuitry, not just volume controls, although that is best known. The typical audio power amp output stage has two transistors, one connected to the (+) power supply, and the other to the (-) power supply. When there is a signal it causes the transistors to conduct, one more and the other less, so that the signal appears at the output. When there is no signal these two transistors should conduct equally, so that the output is neither (+) or (-). Since transistors and other circuitry are not all exactly the same an adjustment pot is provided so that a good balance can be achieved when there is no signal.

I hope this will help you avoid a snow job when you deal with the repairman.

PS: This problem is most often caused by playing too much dissonant music :-)

Hi,

How do I do that? And what's the pot?

Believe me when I say I'm a -complete- idiot when it comes to the basic technical workings of my audio system :)

Thanks again.

From the pictures I see it looks like a really nice amp for the price. Dare I say DK-like in concept :-)
If the problem is just misadjuctment of the pot to null DC offset it is a real easy fix. In fact, the circuit board may have markings to identify the pot (probably in Italian) and if so you might adjust it yourself. The amp, and your speakers, will sound even better after you correct this problem.

Just curious... would that have somewhat of a degaussing effect on the woofers magnets or not?

Thanks Shadorne, I'll do that.

For starters you could simply check the DC voltage at your speaker terminals with a digital voltmeter and see what this reads with power on and power off. I would suggest contacting the manufacturer with the results to find out what is an acceptable DC offset.

Thanks for the continued help. There are no dealers or technicians in the area, but I do have a friend who's an excellent mastering engineer and quite good with electronics. Is this something anyone decently versed in electronic repair can fix, or do I really need a specialist? Would someone be willing to explain to me in laymen's terms what the adjustment would involve?

Take it to dealer or qualified technician to check. Don't tinker with it yourself (unless you're reasonably comfortable with PS design).

If the woofers just make a click then you could be Ok ....but if they click and remain off from their center position then you probably have a DC offset as Eldartford correctly points out.

On closer examination it's more like a few millimeters. Still more than I'm used to. What adjustments might need to be done, and how can I do them?

Thanks.

Your amp has DC voltage on its output. If the cones move a centimeter this voltage is way more than normal. The amp needs adjustment.