Must Read- Italian Made Powered Speaker Improvements
One of my clients imports a range of Italian made speakers. He brought me one of their popular models. There was a weird distortion happening on the speakers and ultimately this could lead to the amplifier failing. After some tests, I concluded that the distortion was crossover distortion and was most audible when injecting a 440Hz (A) signal into the inputs. The degree of distortion was also very temperature dependent.
I set out to find the cause and solution to this problem. It isn’t a known fault with these speakers, but almost all of them that I have seen (almost 40 units and counting) have this very same issue. Investigating and reading countless articles, I came up with a fix to remove the distortion and make these speakers more reliable.
First of all, these speakers use a Mosfet output stage, using modern output transistors. They are IRFP240 and IRFP9240. These Mosfets were originally designed for fast switching applications such as switch mode power supplies and are known as “Vertical” or “Hexfet” mosfets. They were never intended for use as audio amplifiers like their early ancestor the “Lateral” mosfet. These mosfets aren’t as linear and have a higher gate capacitance and are prone to thermal runaway. They don’t exhibit a good negative temperature coefficient as is believed (at least not in their linear range). With these mosfets it only “kicks –in” at a much higher current level. This is useless if you want to use them for audio.
But, there is a workaround!
Because of this believed negative temperature coefficient designers don’t add current sharing source resistors. These resistors should be added and should be high enough to force a degree of current sharing and help with temperature stability. This will also make life easier on the bias servo circuit.
In picture 1 you can see the absence of source resistors on the output board. The woofer (4 mosfets), and tweeter (2 mosfets) output stage is shown.
To install some 5W 0,47ohm source resistors, some traces need to be cut and cleared to expose the copper. See picture 2 below as reference.
In picture 3 below, you can see the resistors added. Note they are also added to the tweeter circuit even though it uses only one pair of mosfets!
I had one more issue with this circuit. In order to keep stable bias, the bias servo transistors need to be mounted on the heat sink to thermally track the output mosfets. Unfortunately these transistors are mounted on a separate driver board. I decided to use thin wires to mount two TO-126 packages flat on top of one of the output transistors, one for the woofer and one for the tweeter circuit. I used the same mounting holes and used thermal paste between the packages. I used BD139 transistors. See picture 4 & 5 below.
Note not to use too thick wires when doing something of this sort. They can carry too much thermal mass and possibly influence bias. Also keep them as short as possible.
In picture 6 & 7 you can see the wires for the bias transistors soldered to the board where the original TO-92 packages were.
Finally the unit needs to be re-biased. To do this I measured the voltages across each 0,47ohm resistor. Bias is set for average of 25mV across each resistor by turning the trimmer pots.
This yields good results with the IRFP240 & 9240’s.
No audible crossover distortion, no thermal runaway and a minimal loss in output power (<1W).
What’s better is you’ll have a reliable amplifier. I have not had any returns of a single unit!! The unit used in example above, is a Montarbo W440A powered speaker. I hope this helps someone designing or repairing such a circuit.
NOTE: Wait for the mosfets too cool properly after soldering on their terminals, before biasing. If you have to replace any of them, use matched sets. Batch numbers are not a reliable indication of matches!
For more info on this topic, check out Rod Elliott’s amazing article:
Using HEXFETs in High Fidelity Audio by Mitch Hodges.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs.
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You can also check his previous repair article below: