A little background before I get into some questions.
I have a 2 channel lm3875 kit with 2 amps run in parallel for each channel (2 kits) and each one has a 18V 250 VA transformer.
When I turned it on I got some 60 hz buzz coming out of the speaker. I plan on putting a shield around the transformers and rectifiers so that should help cut down on the interference.
That is not the main problem though. The chips get real hot fairly quick, even with no load or input hooked up. I have them hooked up to some pretty heavy duty heat sinks (6 fins, 2"fit height, 3" wide and 12" long, 2 chips per sink).
The diodes in the rectifiers also get hot - could these be hooked up wrong? (I thought if they were backwards they would pop)
I let it cool down for a few minutes and was going to probe around to check some voltages from the rectifier and when I turned it on again the fuse blew. (it didn't blow turning it on 5 or so times and then it just went) I have both transformers wired into a 6 amp fuse.
I am using avel lindberg transformers and have the wires hooked up as follows
on the input (120 V) - blue and violet combines to L terminal
brown and grey combines to N terminal
and output (18V) - red - AC1 H, black AC1 N
yellow AC2 H, orange AC2 N
I have checked over the connections between the rectifiers and the amp boards and they are correct. Everything is grounded.
Does the Chassis Ground get tied back to anything other than the 3rd prong of the IEC plug?
I have a 2 channel lm3875 kit with 2 amps run in parallel for each channel (2 kits) and each one has a 18V 250 VA transformer.
When I turned it on I got some 60 hz buzz coming out of the speaker. I plan on putting a shield around the transformers and rectifiers so that should help cut down on the interference.
That is not the main problem though. The chips get real hot fairly quick, even with no load or input hooked up. I have them hooked up to some pretty heavy duty heat sinks (6 fins, 2"fit height, 3" wide and 12" long, 2 chips per sink).
The diodes in the rectifiers also get hot - could these be hooked up wrong? (I thought if they were backwards they would pop)
I let it cool down for a few minutes and was going to probe around to check some voltages from the rectifier and when I turned it on again the fuse blew. (it didn't blow turning it on 5 or so times and then it just went) I have both transformers wired into a 6 amp fuse.
I am using avel lindberg transformers and have the wires hooked up as follows
on the input (120 V) - blue and violet combines to L terminal
brown and grey combines to N terminal
and output (18V) - red - AC1 H, black AC1 N
yellow AC2 H, orange AC2 N
I have checked over the connections between the rectifiers and the amp boards and they are correct. Everything is grounded.
Does the Chassis Ground get tied back to anything other than the 3rd prong of the IEC plug?
ejn327 said:
I would recommend disconnecting the power supply from the amplifier boards, and seeing if your power supply voltages are correct. If they are correct, I would test 1 channel at a time. If the channels are not working when running 1 at a time, then your LM3875 chips are probably bad, and I can mail you some new ones.
You certainly shouldn't be blowing a 6A fuse for the amp, so something is definately wrong. Try testing the power supply and each channel individually, and take it from there.
--
Brian
I pulled out the power supplies and unhooked everything.
I am getting +27 and - 27 volts, so the power supplies are working.
When I hook up 1 amp it works and does not get hot. Its getting late but tomorrow I want to take the 1st windings from each transformer for one channel and the 2nd windings to the other channel. (red/black from each trafo to one channel and orange/yellow from each trafo to the other channel)
Could this be a cause to the heat, some sort of phase issue?
I am getting +27 and - 27 volts, so the power supplies are working.
When I hook up 1 amp it works and does not get hot. Its getting late but tomorrow I want to take the 1st windings from each transformer for one channel and the 2nd windings to the other channel. (red/black from each trafo to one channel and orange/yellow from each trafo to the other channel)
Could this be a cause to the heat, some sort of phase issue?
Banned
Joined 2002
I had a 0.1 ohm 5W resistor at the output of each amp.
The feedback resistors are all fine.
After the rectifier the power is DC, but doesn't it still have a slight phase? Could taking different phases to 2 chips and then combining them cause problems?
The way I had it hooked up when it got real hot was one transformer for 2 amps in parallel. Tonight I want to try using the same windings from each transformer for 2 amps in parallel
The feedback resistors are all fine.
After the rectifier the power is DC, but doesn't it still have a slight phase? Could taking different phases to 2 chips and then combining them cause problems?
The way I had it hooked up when it got real hot was one transformer for 2 amps in parallel. Tonight I want to try using the same windings from each transformer for 2 amps in parallel
almost blew a capacitor doing that toothe caps are all mounted correctly.
The problem with the heat and static noise seems to take place when I combine the amps in parallel (static noise only comes on one channel when in parallel, the other channel has no static).
With one amp running by itself it doesn't get very warm, but when it is combined it will run cool for a bit then heat up very fast. It also seems to heat up quicker when I have the source disconnected or paused.
Maybe I am not paralleling them correctly, but at the speaker level output I have a 0.1 ohm resistor on each amp and these are then combined togather (resistor in series with outout but outputs paralleled togather)
The problem with the heat and static noise seems to take place when I combine the amps in parallel (static noise only comes on one channel when in parallel, the other channel has no static).
With one amp running by itself it doesn't get very warm, but when it is combined it will run cool for a bit then heat up very fast. It also seems to heat up quicker when I have the source disconnected or paused.
Maybe I am not paralleling them correctly, but at the speaker level output I have a 0.1 ohm resistor on each amp and these are then combined togather (resistor in series with outout but outputs paralleled togather)
I had the amps hooked up to a portable CD player. The signal to the amp is running through regular wire for now and soldered to some female RCA connectors.
I'm sure that shielded coax would be best for the signal, and I will change it before I hook it up for good.
When I played a signal to the amps, I made sure to keep the output level the same when I checked the offset. I checked them one channel at a time at first and the hooked both up. I got pretty much the same measurements both times.
I also unhooked the output resistors while checking everything
I'm sure that shielded coax would be best for the signal, and I will change it before I hook it up for good.
When I played a signal to the amps, I made sure to keep the output level the same when I checked the offset. I checked them one channel at a time at first and the hooked both up. I got pretty much the same measurements both times.
I also unhooked the output resistors while checking everything
The reason I asked, is because when measuring DC offset, one has to make sure that no DC is also present at the input, otherwise the DC readings at the output might not be right. If your CD player has a DC blocking cap, everything is fine, but just to make sure, can you also measure what value of DC you are getting at amp's input (after RCA jack).
Since you did measure all channels from the same source, there is still substantial difference between the readings, and in case there is no DC passed from your source, 170mV is definitely out of ordinary and you have to replace the chip.
I measured at least 200 chips, and only once I encountered 170mV at the output (with 22k input impedance). The DC values of 120mV you can encounter occasionally (and more often with recent batch of chips), but on average it rarely gets past 70mV (with 22k input impedance; with lower impedances DC also goes down).
Since you did measure all channels from the same source, there is still substantial difference between the readings, and in case there is no DC passed from your source, 170mV is definitely out of ordinary and you have to replace the chip.
I measured at least 200 chips, and only once I encountered 170mV at the output (with 22k input impedance). The DC values of 120mV you can encounter occasionally (and more often with recent batch of chips), but on average it rarely gets past 70mV (with 22k input impedance; with lower impedances DC also goes down).
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