Hi all,
I recently finished assembling a pair of L20 amps I got off ebay. Running them at 68V into 4 ohms.
I used my 100W dim bulb tester as usual to check for shorts and as hoped for the bulb lit up and died down almost to no glow at all as the caps charged up.
I then started playing music from an ipod, I'm guessing around 1V signal into a pair of 4 ohm test speakers and let it play for a while, still hooked up to the bulb tester. The bulb tester started lighting up a bit during music peaks, then after about half an hour I blew both 2A fuses. After replacing the fuses my meter showed I had a short in the amps rails to ground and the bulb stays lit when switching on.
I imagine I have blown some output transistors or maybe driver transistors. My question is: Did I precipitate this by leaving the current draw of the bulb tester hooked up? By cheating the transistors of voltage could I have damaged them? I thought this amp would handle low voltages.
Thanks for any suggestions
I recently finished assembling a pair of L20 amps I got off ebay. Running them at 68V into 4 ohms.
I used my 100W dim bulb tester as usual to check for shorts and as hoped for the bulb lit up and died down almost to no glow at all as the caps charged up.
I then started playing music from an ipod, I'm guessing around 1V signal into a pair of 4 ohm test speakers and let it play for a while, still hooked up to the bulb tester. The bulb tester started lighting up a bit during music peaks, then after about half an hour I blew both 2A fuses. After replacing the fuses my meter showed I had a short in the amps rails to ground and the bulb stays lit when switching on.
I imagine I have blown some output transistors or maybe driver transistors. My question is: Did I precipitate this by leaving the current draw of the bulb tester hooked up? By cheating the transistors of voltage could I have damaged them? I thought this amp would handle low voltages.
Thanks for any suggestions
The amp fault is almost certainly NOT due to leaving the bulb tester in circuit.
However, once the equipment is "proved" to be wired OK, then you power up direct to mains and check the pre-use settings.
These would include checking and if necessary adjusting the output bias current/s and checking and setting the output offset voltage/s.
Did you make any checks for correct output set up?
However, once the equipment is "proved" to be wired OK, then you power up direct to mains and check the pre-use settings.
These would include checking and if necessary adjusting the output bias current/s and checking and setting the output offset voltage/s.
Did you make any checks for correct output set up?
A bulb tester can cause problems......
As the amp puts out current into the load (as you turn the volume up), the bulb starts to glow and light. When that is happening, the voltage rails of the amplifier are falling (and being modulated by the music).
That condition can cause an amplifier to "latch" swinging the output to one or other of the rails with consequent continuous current draw through the load. The amp is unable to recover because the bulb does not allow enough curent to be drawn for the rails to recover and the feedback loop to take effect stabilising things.
Whether that has happened here I wouldn't like to say but it is a possibility.
As the amp puts out current into the load (as you turn the volume up), the bulb starts to glow and light. When that is happening, the voltage rails of the amplifier are falling (and being modulated by the music).
That condition can cause an amplifier to "latch" swinging the output to one or other of the rails with consequent continuous current draw through the load. The amp is unable to recover because the bulb does not allow enough curent to be drawn for the rails to recover and the feedback loop to take effect stabilising things.
Whether that has happened here I wouldn't like to say but it is a possibility.
Hi, thanks for the replies
Did you make any checks for correct output set up?
I checked the rails to ground, (+-68 volts). I did not check for DC offset at the speaker tetrminals. I was playing music for half an hour with the bulb tester hooked up. Initially sounded fine but I was surprised at how little gain there was. The bulb started becoming brighter and brighter, and was fully lit with music peaks so I turned the input volume down. The bulb tester went back to being dimly lit. After about 20-30 minutes the sound started to crackle and then pop-pop, the fuses went to heaven. To me it was suspicious that the fuses went within a minute of each other. I had just stepped out to answer the front door when the first one went, by the time I started grilling my wife as to what it sounded like, (be careful what you wish for), I found out for myself, the other one went.
I won't be able to find out if this is indeed a case of latching as both boards are now shorted. I am going to remove the power transistors and test them for failure first. The person who sold me the boards was not interested (after much fruitless negotiation) in giving me an accurate schematic of the board, so I have half a mind to spend some quality time figuring out the circuit from the board tracks and posting it to the forum. It is an L20 amp I found offered on eBay. (Please let's not use this thread to bash the amp, etc. but if anyone has a schematic of the preamp, protection, and power sections, I'd appreciated it.)
thanks for the replies,
gary
Did you make any checks for correct output set up?
I checked the rails to ground, (+-68 volts). I did not check for DC offset at the speaker tetrminals. I was playing music for half an hour with the bulb tester hooked up. Initially sounded fine but I was surprised at how little gain there was. The bulb started becoming brighter and brighter, and was fully lit with music peaks so I turned the input volume down. The bulb tester went back to being dimly lit. After about 20-30 minutes the sound started to crackle and then pop-pop, the fuses went to heaven. To me it was suspicious that the fuses went within a minute of each other. I had just stepped out to answer the front door when the first one went, by the time I started grilling my wife as to what it sounded like, (be careful what you wish for), I found out for myself, the other one went.
I won't be able to find out if this is indeed a case of latching as both boards are now shorted. I am going to remove the power transistors and test them for failure first. The person who sold me the boards was not interested (after much fruitless negotiation) in giving me an accurate schematic of the board, so I have half a mind to spend some quality time figuring out the circuit from the board tracks and posting it to the forum. It is an L20 amp I found offered on eBay. (Please let's not use this thread to bash the amp, etc. but if anyone has a schematic of the preamp, protection, and power sections, I'd appreciated it.)
thanks for the replies,
gary
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