Threshold S300 output transistors help

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I need help, I have a Thresold S300 with some bad outputs transistos. Unfortunately the Motorola part number has been removed. If any ones knows the NPN and NPN part number I sure would appreciate it. Of course this amp was designed by Mr. Pass
a long time ago. It bit the dust trying to drive a pair of one ohm Apogee speakers.

Outputs on the S-500 were MJ15022 and MJ15023, drivers for the bank were one of the opposite polarity. As far as I know, the S-300 was just a scaled down S-500. I had a channel pop out on me once upon a time just because of cosmic rays (woofer panels on Magneplanar T-IVs, real tame 4 ohm resistive load--just playing background music levels at the time), and got the info from the tech critter at Threshold. No schematics on hand, unfortunately. I spent a bit reverse engineering the front end to figure out what all had blown and why, 'cause when the output went, it ripped all the way back up into the front end. After replacing all the transistors and resistors, it still didn't work. Turned out it was a tantulum cap with a voltage rating of something like 25V. When the front end blew, the voltage spiked (rails on the 500s are over 70 volts) and killed the silly cap. Given that the cap was something tiny like .15uF, I just put in a 100V Mylar cap.


You might consider calling Jon (sounds like Yon) Soderberg at Vintage Amp Repair. He's an X-Threshold engineer and his side business is repairing their gear...

I've spoken to him several times and he is very knowledgeable both from a topology standpoint as well as the actual products...

He has also always been willing to share info with me but keep in mind that this is also his business so tread lightly...

I have a Ta-300 which is basically an S-250 with a different faceplate. HPotter nailed the outputs - they are exactly what mine uses... The only reason that I would check with Jon just to be sure is because your amp is actually older and may have used different outputs than the later stuff...

I know he also has parts... As an FYI he also does mods to the older stuff that are supposed to be very good!!!

One last tidbit - I converted my amp to class A by switching the transformer secondaries to the 39v (if memory serves) pair that are there but capped off (green I think...) You simply re-bias the amp up into full class a after that. It made a HUGE difference in how my amp sounded... My amp essentially became an SA-3.9 by doing this. It dropped the output power down from 150 wpc to 60 and I have cranked the bias up as far as I dare... Sonically the change was amazing... You might seriously consider doing this to yours...

Hope this helps!

Good luck,

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Threshold Transistors

Thanks for the feedback, I've been going to fix this amp for years
now but started listen to another amp. I've repaired the amp several times before, but lost the transistors part numbers.

Anyway, the failure was caused by driving a pair of ONE ohm Apogee Sintilla's. I've given up on the Apogee speakers since the cat got the bass ribbon, their now room dividers.

I have the schematic for the Stasis series and wanted to share it. Mr. Pass sent it to me a long time ago. Back then, I was in the process of changing the collector output resistors from 1 to .68 ohm to to increase the output current land decrease the power loss, hoping this might increase the life of the outputs.

Thanks from your help


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One last little tidbit...

Should you decide that you want your room dividers to create beautiful sound once again, you might check out the sites below for information...

Graz in Australia is manufacturing new ribbons... I know he is in the process of developing the bass ribbons for several models. You might check with him regarding availablity. He also can guide you on possibly repairing yours...

I also know another gentleman who has some new old stock from when ADS dumped the Apogee line... They are expensive ~$300 each bass panel if he happens to have them... If you are interested I'll forward you contact information...

The trick is properly tensioning the bass panels if you replace them... There are several folks who have been actively working the solution to this problem and would be glad share info with you. Search the following links for info...

Lots of good info here -

The other source you might consider for a ribbon is Ebay. Every now and again damaged sets go through...

I wouldn't give up on the Apogee's... Newer technology can drive those speakers very well and with a few crossover mods there is very little on the market that can resolve (or sound) like they can...

Apogee Speakers


I e-mail Graz in Australia about the bass ribbon he said he was thinking about doing one for the Scintillas. He said he would get
back in touch but he never did. So, if you have another contact that might do a panel please let me know. I watched a web video about replacing the panel, it looks like you really need to have done this several times, plus you need some special jigs- fixtures to set and hold the panel. I guess I could build the jigs.

As for another amp, I have a late model Krell KSA 150 which
I like very much. It drives the Scintilla's very well.

At one time I though about ripping the bass panels out and replacing them with a somthing like a fast 8" Scan-Speak woofer, but reality set in and I built a pair of three way speaker. Having said that I would like to listen to them again they were great while they lasted.



The person that you need to contact is Bob Dragunas. He used to work for Apogee/ADS in their service department...

The ribbons that he has are new old stock from Apogee when they closed down...

Sometimes it takes a few messages before he replies back...

His email is:

I also have his phone number but would rather not give it out without asking first... If you don't hear back from him within a couple of weeks, let me know. Again, it usually takes a while to hear back but he does usually answer... My impression is that this is not high priority for him and I think he just does it on the side when he has time/feels like it...

I hope he has the panels that you're looking for. It would be much better to replace the pair than try to switch to dynamic drivers IMHO...

Good luck,


PS - Let me know if you don't hear back and I'll call him and ask if it would be ok that I forward his phone number to you... Just do me a favor and try his email first as that's how he's always communicated regarding Apogee stuff... Thx!
Transistor HELL

Oh dum me, I did not know that or friends at MOT had dropped
their transistor lines. However, I did find that there are several other companies making MOTOROLA parts. Anyway, if you need
the mpsu10 or 60 you will have to cross it to another part.

Motorola "Uniwatt package" transistors
NPN PNP Amps Volts min hFE @ mAmps fT (MHz)
MPSU01 MPSU51 2 30 50 @ 1000 50
MPSU01A MPSU51A 2 40 50 @ 1000 50
MPSU02 MPSU52 0.8 40 30 @ 500 150
MPSU03 --- 1 120 40@ 10 35
MPSU04 --- 1 180 40 @ 10 35
MPSU05 MPSU55 2 60 60 @ 250 50
MPSU06 MPSU56 2 80 60 @ 250 50
MPSU07 MPSU57 2 100 30 @ 250 50
MPSU10 MPSU60 0.5 300 30 @ 3 45
MPSU45 MPSU95 2 40 4000 @1000 100
Maximum power = 10 Watts @ 25 degrees Celcius


ECG240 MPSU60 use BF872

Claims to have stock
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