SA-12/100 Repair Options 
 
 
    

Repairing an SA-12/100 with a blown output stage

The MOSFETs for the SA-12 or SA-100 are no longer available in sufficient quantity to create matched sets. Because these devices are operated in their positive thermal coefficient range, thousands are needed for matching in order to come up with pairs that are matched closely enough to avoid "current-hogging." (More information here.) Equivalents with the required low (2A or less) zero temperature coefficient point do not exist.

I can offer three possible suggestions:

1. Order a repair kit for the SA-100 (same as the SA-12) and have a local technician or dealer install it. One kit has all the parts required to repair one channel of an SA-12/100: four matched MOSFETs, four gate resistors, and the protection diodes. However, the MOSFETs are not new: they have been taken from amplifiers that have been upgraded to NP100s. There is no warranty on the MOSFETs -- I cannot warrant these used parts as MOSFETs can be internally weaked without any external indication, and I am not installing the parts. But they have been tested and matched. The cost is $220 / kit. I have no idea what the dealer will charge to install the kit. To order, see here. For installation and biasing instuctions for repairing a failed SA-type amplifier, click here.
   
   
2. Your second option is to send your amplifier to me to have the kit installed. The cost is $300 ($80 labor) plus return shipping. Please contact me first for an Repair Authorization number prior to shipping. See also http://www.altavistaaudio.com/Shipping.html for information on shipping this unit. Note that the replacement MOSFETs used are not new: they have been taken from amplifiers that have been upgraded to NP100s. They are warrantied for 90 days only.
   
   
3. Your third option is to have your SA-12 or SA-100 upgraded to an NP100. NP100 amplifiers come with a five-year warranty, are very hard to damage, and sound better than the SA-12 or 100 ever did.
   
   
4. A former fourth option which is probably pretty much impossible is to use Exicon MOSFETs. Their ECF20N20 and ECF20P20 parts (no longer listed on the website?) are designed for audio and appear to have sufficently high voltage and current ratings. The manufacturer describes them as "High Quality/High Preformance Lateral Mosfets for Audio." They come in T0-3 (metal) package like the original SA-20/220 parts. (This information courtesy of Steve Ardillas, thank you Steve.)
       Some caveats are neccesary:
       Unlike the original parts, these are "lateral" types, not vertical types (how the part is constructed) so they are not an identical match to the original part. But as manufacturer points out, matching is not required for these Lateral MOSFETs. This is normal for lateral types, they do not typically require matching.
       The manufacturer does not provide biasing information (such as Vgs(on)) so it is possible that the bias network in the amplifier may need to be changed to properly bias the output stage. Lateral-type MOSFETs that I've worked with require more voltage to start to conduct current.
        The manufacturer also does not provide information about the internal losses (Rds(on)) of the parts; your amplifier may not be able to provide as much peak current as it originally did. Lateral types almost always have higher internal resistance.
        UPDATE 4.10.07: A client sent an SA-220 to us along with some Exicon MOSFETs (20P20 and 20N20, parts not listed on the website) that he requested we install. We determined that these parts are not wired the same as the original parts, having the drain and the source reversed. The original parts have the drain connected to the device case, while the Exicon parts have the source connected to the case. This appears to be true for all the metal can TO-3 parts listed on the website. The transistor socket wiring is incorrect for the Exicon parts and in the case of the SA-220, where the transistor sockets are pc-mounted, substantial modification would need to be done to the circuit boards. We did not attempt the modification because it is quite involved. The SA-20 would be easier to reconfigure because the sockets are connected with wires. We have not tried that. So at this time, the idea is still theory.
   
   
   
5. A fifth option is to install Toshiba MOSFETs 2SK1530 (n-channel) and 2SJ201 (p-channel). These are 200V, 12A parts whose biasing requirements seem electrically compatible with the bias network in the amplifier. However, they are TO-3 plastic case, rather than TO-3 metal case parts like the originals, so a little extra work will need to be done to mount and wire them: their source and gate pins are in the right locations to pass through the holes already drilled in the heatsinks, but a bit of work will need to be done to get the drains wired up. I have no personal experience with these parts, but I've been told that some are using them successfully. However, it should be noted that like the original parts, these have a positive temperature coefficient and will most certainly need matching to assure equal current-sharing. The SA amplifiers are not wired up to allow easy monitoring of individual device drain currents, so the DIY installer should keep this in mind and consider wiring in source resistors to monitor currents to avoid current-hogging and possible thermal runaway.