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Mosfet amps

I'm interested in a mosfet design amp on Ebay and wondered if you guys know much about them. Other than expecting a valve like sound but with reliability issues what else should I be aware off?

https://www.ebay.co.uk/itm/Kenwood-K5050R-MOS-FET-Amplifier/163550502860?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

Like pretty much any device they do not have any particular sound. All depends on topology. They are more reliable than standard (bipolar) transistors. Don't expect anything to be different to any other transistor amp.
 
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I own some MOSFET amps, and have used various ones for a long time. I agree with Arkless, it is all down to the details. There was period in the early 80s when the Hitachi lateral devices first became available, when lots of manufacturers used them, as they were robust and worked with a simple drive circuit. Cheap versions used a single pair of devices per channel, and had limited current output, and many sounded fairly average. Later on, other semiconductors with more current became available, and there were more design choices. Not all the later devices are as robust as the low current Hitachi ones.
 
There are huge differences between lateral and vertical mosfets. Laterals had better linearity and thermal stability, but also much higher price, on resistance and lower gain. They were often used in variations of the Hitachi circuit, which had pleasant slightly valve like characteristics and sound. Verticals are more transistor like and used sensibly, like in some Creek or MF amplifiers, make a good clean sound.
There are also plenty of poor designs with thermal stability trouble or a spray of crossover distortion.
 
There are huge differences between lateral and vertical mosfets. Laterals had better linearity and thermal stability, but also much higher price, on resistance and lower gain. They were often used in variations of the Hitachi circuit, which had pleasant slightly valve like characteristics and sound. Verticals are more transistor like and used sensibly, like in some Creek or MF amplifiers, make a good clean sound.
There are also plenty of poor designs with thermal stability trouble or a spray of crossover distortion.

I strongly disagree that lateral mosfets sound valve like.... they are also about the most thermally stable of all output devices.
All the Musical Fidelity mosfet amps that I know of used lateral not vertical mosfets.
 
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The Kenwood was a lateral mosfet design but was reviewed as a bit "toppy" and harsh. The example in the photo looks battered, so only of interest to me for parts and not at that price
 
Threshold Vgs on between 0.8 and 2.8V is lateral territory, though might be the much missed "logic level" vmos
Everything these days is around 3V
There are two threshold groups listed, so designed for paralleling
 
Threshold Vgs on between 0.8 and 2.8V is lateral territory, though might be the much missed "logic level" vmos
Everything these days is around 3V
There are two threshold groups listed, so designed for paralleling


Laterals usually start conducting at a couple of hundred millivolts, also the Toshiba parts increase in current with temperature for many Amps for a fixed Vgs which is more like VMOS.
 
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Yup. They are Toshiba ones specifically for audio... as I guess you will know! I used these in a class A design of my own, 3 pairs per channel.

Care to post the design?, always interesting to see what other people are up to. The Toshiba parts would be better suited to my non switching design using the cheap as chips IRF9540/240 but it sounds pretty good as it is.
 
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Care to post the design?, always interesting to see what other people are up to. The Toshiba parts would be better suited to my non switching design using the cheap as chips IRF9540/240 but it sounds pretty good as it is.

Sorry but I tend to keep my designs to myself just in case I have a commercial application for them one day....
 
Laterals usually start conducting at a couple of hundred millivolts, also the Toshiba parts increase in current with temperature for many Amps for a fixed Vgs which is more like VMOS.
The 0.8V to 2.8V is specified at a very high 100mA Ids value, so the threshold is questionable. A modern vertical of this power rating might be specified at 250uA.
Unfortunately the datasheet say nothing about temperature sensitivity, which is the easy lateral vs vertical detector
 
Sorry but I tend to keep my designs to myself just in case I have a commercial application for them one day....


Don't be shy, you know as well as I do when it comes to hifi marketing beats design hands down every single time and if you are you are using obsolete parts it will never be commercial anyway.
 


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