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.
Picture 1
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.
Picture 2
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!
Picture 3
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.
Picture 4
Picture 5
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.
Picture 6
Picture 7
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:
https://jestineyong.com/chauvet-4-bar-flex-repair/
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17 Comments
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James
May 11, 2018 at 3:14 pm
Hmm... this dude's a genius" 🙂
Parasuraman
May 11, 2018 at 4:22 pm
I have no words to say; such an excellent article, with indepth knowledge and professional expertise! Millions of thanks for sharing very valuable information!
Gerald Musy
May 11, 2018 at 5:49 pm
Congratulations Riaan. You know your area of expertise and this "repair-design improvement" is simply amazing.
Thank you for sharing such a great information.
Andrew F. ali
May 11, 2018 at 8:14 pm
A beautiful piece of re-engineering. Good information shared. Thanks.
Bill
May 11, 2018 at 8:54 pm
Loved the article.
Robert Calk Jr.
May 11, 2018 at 9:20 pm
Excellent work, Dr. Riaan! There's nothing in the datasheets to suggest using the MOSFETs in audio circuits - I wonder how the manufacturer's engineers missed that?
I had to add current sharing resistors to the transistors in my DC Bench Power Supply also. I use my Peak Atlas DCA 75 Pro to find matched sets of MOSFETs, transistors, etc.
Here is a link to Rod Elliot's "HEXFET In High Fidelity" article: http://sound.whsites.net/articles/hexfet.htm
Bernie Scott
May 11, 2018 at 10:28 pm
Very nice article...I don't understand why the original engineer(s) did not design a better circuit....I am very impressed.......
OYO MARK
May 11, 2018 at 11:17 pm
Thank alot . But i request for circuit of the amp.
Humberto
May 12, 2018 at 12:47 am
A very professional job, Mr. Riaan. Congratulations.
Albert
May 12, 2018 at 11:50 am
Very professional job that only can be done with the right knowledge of the matter! It is clear that you know your way in these special audio repairs.
caison
May 13, 2018 at 1:57 am
thanx that is a professional work
Yogesh Panchal
May 14, 2018 at 12:00 am
Riaan,
Thanks for sharing excellent modification tip.
Carlo Alberto
May 14, 2018 at 4:57 pm
A high-level fix: respect, man!
This board seems a 440-Series by Montarbo. Probably, the Factory knows some issue related with the heat ....
In the official "Care and Maintenance" notes, it put on evidence "Do not expose the enclosure to heat sources (lamps, lights, high power light sources, radiators or other products that produce heat)".
Maybe, somebody has noted an undersize of some heatsink ... 😉
Well done. Cheers!
Carlo
Justice
May 14, 2018 at 9:42 pm
Excellent article Riaan.
MB
October 3, 2019 at 11:28 pm
I wanted to really thank you.
I recovered a Montarbo W440A after its internal soundproofing took fire because of some heat problem on the tweeter amplifier. I made it work as a single-amplified system by purchasing the W440P crossover, and sending to the input of the bass amplifier the whole signal, taken from the circuit before it is divided into high and bass sections -obviuosly it would have had less power than before. It worked but it had a lot of background noise and the sound became orrible after ten minutes of operating -the distortion you described. I applied your guide. After it just needed a very very gentle low pass filter on the input because the highs and mids were really too much (about 6/7 dB too much at 10 000 Hz). Thank to you now the system is working like it always should have worked.
saber
May 8, 2020 at 9:22 am
any one plz have clear photo of w440 board
the 440F board
i need it to fix my montarbo
thnx
Wasim
July 8, 2020 at 5:30 am
Nella foto allegata c'è una temperatura molto alta
MONTARBO w440a
Vi prego di contattarmi per aiuto
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In the attached photo there is a very high temperature
MONTARBO w440a
Please contact me for help