Custom UPC1237 speaker protection board for Phase Linear 400

spymac

New Member
I made my own speaker protection board for my PL400 Series II. This was made completely out of parts rescued from scrap, except the UPC1237s. I ordered those for a couple dollars shipped from Hong Kong.

I made my schematic based on the UPC1237 datasheet, and values from this project at buildaudioamps.com:

http://buildaudioamps.com/other-audio-related-projects/

I am powering this board off of the display bias in my amp. Before putting it into my amp, I tested the board on my bench with DC supplies. The UPC1237 has a 1-2 second delay when 12V bias is applied. It trips pretty quickly if I put a few volts DC, positive or negative, into either speaker terminal. It latches off once tripped (no auto-restart.)

I was worried about the voltage, and reverse voltage that would appear on the 100uF cap. I thought I probably needed a 100V bipolar cap there. I tested by injecting + and - 30V into the circuit, as if the amp is putting out +/- 30V DC, and the voltage on the 100uF cap never exceeded +/- 0.6V. This verifies that the UPC1237 is clamping, and that small of a negative voltage should be nothing to worry about on a normal polarized electrolytic cap.

I am attaching the schematic and pictures. It seems to work well, but please let me know any comments or potential issues. I went a little crazy with the Zobel networks because all PL400s I have worked on had bad resistors.

upc1237_schematic.png

board_top_bottom.JPG

board_side.JPG
 
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You may want to do something about shut off - the relay will stay on until power supply voltages get low enough for it to disengage. Simplest cure is to run relay supply from separate diodes off transformer, with smaller capacitor, with appropriate sized resistor. Typical circuit will use a 50uF filtering cap, and resistor to relay ( Value based on voltage and coil resistance).

Edit - looked up the link for the project you based this on - they used D1 and C5 and AC input which gives them fast shut down. (doesn't have to be super fast- just faster than the supply voltages.
 
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Thanks Steve. In this particular amp, I am powering this board off of the display bias, which is a separate winding of the transformer, and rectified into a 470uF cap. This cap bleeds down right away under the load of the displays, versus the main +/- 75V rails which are 10,000uF and take many seconds to bleed down. This is a great point for anyone wanting to adapt this for different amps that don't have an auxiliary bias like this.
 
FWIW there is also information in the datasheet about using pin 4 for AC off detection.
 
My Rotel RB976 Mk2 uses 3 UPC1237 chips for output and speaker protection. In this case they have used 4 transistors monitoring output overload on each channel pair, heatsink overtempt and high DC offset. If any of these go high a trigger voltage is fed into pin 1 of the protection chip. The circuit is fed from the power amp main rails and latches the relay off until the power is removed. The 3 circuits are also initially triggered by the system start up delay circuit.
Might be worth a look as it's quite an elegant little circuit.
 
Thanks for the info guys. It would be nice to have an overload protection, I will look into how the RB-976 MK2 does it. I found the service manual with the schematic, will have to take some time to figure out how it jumpers to different boards so I can understand what is going on.

I tried the protection board in the amp today and found that the extra load on the display bias gave it lots of ripple, which caused the readout to not get down to 0 anymore. So, I replaced the 470uF with 2x 820uF I had available. The displays are back to normal now. I measure 106 msec after AC removed until the relay opens with this bigger display bias cap.
 
I made my own speaker protection board for my PL400 Series II. This was made completely out of parts rescued from scrap, except the UPC1237s. I ordered those for a couple dollars shipped from Hong Kong.

I made my schematic based on the UPC1237 datasheet, and values from this project at buildaudioamps.com:

http://buildaudioamps.com/other-audio-related-projects/

I am powering this board off of the display bias in my amp. Before putting it into my amp, I tested the board on my bench with DC supplies. The UPC1237 has a 1-2 second delay when 12V bias is applied. It trips pretty quickly if I put a few volts DC, positive or negative, into either speaker terminal. It latches off once tripped (no auto-restart.)

I was worried about the voltage, and reverse voltage that would appear on the 100uF cap. I thought I probably needed a 100V bipolar cap there. I tested by injecting + and - 30V into the circuit, as if the amp is putting out +/- 30V DC, and the voltage on the 100uF cap never exceeded +/- 0.6V. This verifies that the UPC1237 is clamping, and that small of a negative voltage should be nothing to worry about on a normal polarized electrolytic cap.

I am attaching the schematic and pictures. It seems to work well, but please let me know any comments or potential issues. I went a little crazy with the Zobel networks because all PL400s I have worked on had bad resistors.

View attachment 1268840

View attachment 1268841

View attachment 1268842
I tried to post my own threat but for some reason I couldn't get it to post so I found the most closely related thread from someone who sounds like they know what they are doing lol. I've got a pl400 that I'm rebuilding (eventually to include the white oak board etc) but right now it's stock other then new caps in the power supply (upped slightly from stock). Before I actually hook this up to my speakers I want to install dc protection and I just received my dc protection kit that I ordered on ebay. My question is, does the zobil network (2resistors+cap) get hooked up before or after the DC protection board?? I assume the only thing after the DC protection circuit is the speaker output terminals but I just want to me sure. Good 'ol ebay obviously didn't include much info on installing this board. Any info you could give me would be greatly appreciated, thank you.
 
I am pretty sure that the Zobel network should be hooked up so that it is always connected to the amplifier. That is the way I did mine. My thinking was that it could prevent the amplifier from oscillating with the relay open.
 
I am pretty sure that the Zobel network should be hooked up so that it is always connected to the amplifier. That is the way I did mine. My thinking was that it could prevent the amplifier from oscillating with the relay open.
This makes total sense, thank you. Also maybe you could assist me with another issue in having. The DC protection board I got was from ebay and it requires 12-19vdc and I'm wondering if I can find a circuit already in the amp to supply this to the DC board or would I require a separate power source. I probably could find a xformer small enough to fit inside the case or build a small voltage regulator circuit. It would be nice if I could just piggyback onto something existing circuit with the 12-19vdc.
 
You can get 14V DC from the large capacitor on the back of the right display. The display is fed from a separate winding of the main transformer. It is rectified directly into that large capacitor (should be 470uF, 25V). I would recommend replacing that capacitor with 1000uF 25V if you want to do it this way.

I believe that there is a 1/2 Amp fuse for the display power, INTERNAL to the transformer. If you get everything hooked up and want to make sure it won't blow that fuse, you can use a DC power supply to feed in 14V into that large capacitor on the back of the right display, making SURE you have + and - correct. The displays should turn on and your protection board should do its thing and click after a few seconds. You can use this setup to measure the current from your power supply that the displays and protection board relay(s) are using. If this current is well under 0.5 Amps, you're good.
 
You can get 14V DC from the large capacitor on the back of the right display. The display is fed from a separate winding of the main transformer. It is rectified directly into that large capacitor (should be 470uF, 25V). I would recommend replacing that capacitor with 1000uF 25V if you want to do it this way.

I believe that there is a 1/2 Amp fuse for the display power, INTERNAL to the transformer. If you get everything hooked up and want to make sure it won't blow that fuse, you can use a DC power supply to feed in 14V into that large capacitor on the back of the right display, making SURE you have + and - correct. The displays should turn on and your protection board should do its thing and click after a few seconds. You can use this setup to measure the current from your power supply that the displays and protection board relay(s) are using. If this current is well under 0.5 Amps, you're good.
Thank you so much for your help, you are the best. I didn't realize this was exactly what you were talking about in an earlier post. I think I read up too the second or third last page before posting my question. I spent the last week reading thru different threads on this forum and I ended up with more questions then when I first decided to upgrade my pl400 s1. I first thought after pulling the faceplate off I would "just" install spkr/protection, larger ps/caps, and LEDs. Plus some minor gnd mods and replace the driver board caps. Now my list of possible upgrades is 2 pages long.
 
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Thank you so much for your help, you are the best. I didn't realize this was exactly what you were talking about in an earlier post. I think I read up too the second or third last page before posting my question. I spent the last week reading thru different threads on this forum and I ended up with more questions then when I first decided to upgrade my pl400 s1. I first thought after pulling the faceplate off I would "just" install spkr/protection, larger ps/caps, and LEDs. Plus some minor gnd mods and replace the driver board caps. Now my list of possible upgrades is 2 pages long.
Is yours series one or two. The display voltage is only available on series two. Series one has a 6 volt winding which runs the lamps, and no 14 volts. :(
 
Is yours series one or two. The display voltage is only available on series two. Series one has a 6 volt winding which runs the lamps, and no 14 volts. :(
Mine is a series 1, so I'm guessing I'm out of luck. Finding a small enough xformr to supply the 12+volts and stay on the safe side of having enough ma shouldn't be too hard. Thanks for the update. I was about to probe my display board but I couldn't locate the cap you mentioned so I came back to our post to find your added comment about series 1 or 2 lol.
 
Since I gotta make a trip to the surplus store to find a 12-19v xformr I was gonna spend the morning to replace all the caps on the driver board but after looking closer I see they are all nichicon (blue) caps that are in great shape. (looks like all the resistors were replaced also. I have no idea when these were changed, should I replace them anyways??? (ps. I did pull one and check it and it looks to be right on spec). I can not believe how clean this thing is inside. My 8yr old yamaha receiver has more dirt and nasties inside.
 
I'm sorry i don't know why I thought it was 12-29vdc, it's actually AC I need for the spkr protection board (12-10vAC)
 
Thoo
Mine is a series 1, so I'm guessing I'm out of luck. Finding a small enough xformr to supply the 12+volts and stay on the safe side of having enough ma shouldn't be too hard. Thanks for the update. I was about to probe my display board but I couldn't locate the cap you mentioned so I came back to our post to find your added comment about series 1 or 2 lol.
Thought outside the box - First, you can usually use DC instead of AC, but may need higher voltage, so you could try a voltage doubler circuit on the 6 volts AC.
May need to make some changes so it shuts off faster than the amp....YMMV
 
There is a well engineered solution for the Phase Linear 400's using the UPC1237. It works with both the Series I and Series II models and requires minimal effort to install (no extra transformers and no major surgery).

installedboard.jpg


Take a look at

http://home.earthlink.net/~dimlay/index.html

for more info.
 
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