Transistors in high power and small power working state would be different.
This point and different MOSFET.
Transistor in the output current, large B very need more current.
Such as output 5 A, MJW21195 HFE about 50.
So the drive tube will work in 100 MA conditions.
If VCE = +-60 V. So the drive tube power consumption of about 6 watts
This means you prefer N mosfets in output stage for large current and high power ?
I have working NMOS Amp with +/-95 V DC, Sound is amazing and great from very deep bass to clear highs
Such a high voltage. The biggest advantage is can output more voltage..
That is the output more power, more big voice.
In fact, we usually use so big.
Now I only have a Chinese FORUMS.
Later, I may let my friend help me to get a English.
But my English level is poor
bbs.ljmaudio.net
This version after test results to get.
It is not compatible with other MOSFET.
Including IRFP250, FQA25N16
It is not compatible with other MOSFET.
Including IRFP250, FQA25N16
The Vbe multiplier should be mounted on an output transistor or at least on the heatsink.
I would also have used a pot in the Vbe multiplier to set the bias spot on i.e. no crossover distortion but not excessive bias current.
This provides a voltage VBE. Is constituted by two transistors.
Is a normal transistor, one MOSFET, MOSFET IRF640S to compensate for the temperature and IRFP240 VGS. .
IRFP240 work VGS bias voltage of about 3V.
In the same temperature, the IRF640 and IRFP240 have similar temperature VGS changes.
They are installed in the same radiator can control the IRFP240 VGS voltage.
They are installed in the same radiator can control the IRFP240 VGS voltage.
May be OT, but I need info on the L10 version
I am building an amp around the L10 version of LJM's circuit. This board was purchased on eBay, and came with no info what-so-ever. I need to know what the power requirements are. I believe it needs a split rail supply, and had intended to use a torroidal transformer rated at 225VA with a 24v + 24v output.
I apologize if this post is in the wrong forum, but was the only lead I could find apart from an all Chinese language forum.
Thanks,
- Tim -
I am building an amp around the L10 version of LJM's circuit. This board was purchased on eBay, and came with no info what-so-ever. I need to know what the power requirements are. I believe it needs a split rail supply, and had intended to use a torroidal transformer rated at 225VA with a 24v + 24v output.
I apologize if this post is in the wrong forum, but was the only lead I could find apart from an all Chinese language forum.
Thanks,
- Tim -
LJM 150 VERSIONS?
I have ordered a pair of these from Sep Store.China via Ebay. They arrived and appear to be a similair to the circuit BUT different set of resistor values. ie NO 100k at input .Their is ONLY one pair of 330R resistor on the board? and a 22R close to the input signal. The 68R and 100R are correct plus 1 x 150-R..seem the same however 10 x 100r and 8 x 2k2 ?
Did you revise this circuit or is this sep stores revised version.
I ask because the output devices x 6 one one amp have blown in 2 hours....
Dave
Dear LJM
I have ordered a pair of these from Sep Store.China via Ebay. They arrived and appear to be a similair to the circuit BUT different set of resistor values. ie NO 100k at input .Their is ONLY one pair of 330R resistor on the board? and a 22R close to the input signal. The 68R and 100R are correct plus 1 x 150-R..seem the same however 10 x 100r and 8 x 2k2 ?
Did you revise this circuit or is this sep stores revised version.
I ask because the output devices x 6 one one amp have blown in 2 hours....
Dave
Giving up on Chinese kit amplifiers
Well The second amplifier has now gone the way of the first. I belive All the output devices have failed.ie Both Boards.
Looking at numerous postings on this site I am beginning to think now that the significant posting bt Andrew about the L20 design using 4 x 2 plastic drivers is probably the most relevant.
These amps only worked for me at low levels into a pair of easy to drive speakers.
Once connected to the Volt speakers (less efficient) They ran for a short while and when I turned up the volume they blew the 3.15 amp fuse after about 10 seconds..
Initially only one amp seemed to have dead outputs and I ran the other amp for less than 30 minutes befiore that failed and now the amp is an instant fuse blow.Dead accross each of the pins on all the drivers with NO other apparant failings on the board.?
I had considered that an increase from L120 to L150 may have been enough for me but NO. My previous metal outputs did 105 watt for many years untill the removal men damagaed them.
It looks like the L20s could be the minimum size amp even with the huge amount of heatsinking that I use. Over 1 kilos per channel at 53v DC poss/neg.
I also came to the conclusion that LJM has revised the L150 board as its similair in R values to the Later L15. If not then I have been sold two duds.however They are both Blue boards...Perhaps some one could revise or update this information somehowe?
As a standby I am using a 90 watt Toshiba which can go into shutdown when driven loud for and length of time so that establishes the speakers are a bit power hungry.The new room is at least 50 per cent larger than previously.
Not sure if I want to proceed to another LJM design as I think the Chines output devices MUST be pretty poor. (sonic change when warmed up and driven).
Plenty of spare parts going cheap one day...
Dave
Well The second amplifier has now gone the way of the first. I belive All the output devices have failed.ie Both Boards.
Looking at numerous postings on this site I am beginning to think now that the significant posting bt Andrew about the L20 design using 4 x 2 plastic drivers is probably the most relevant.
These amps only worked for me at low levels into a pair of easy to drive speakers.
Once connected to the Volt speakers (less efficient) They ran for a short while and when I turned up the volume they blew the 3.15 amp fuse after about 10 seconds..
Initially only one amp seemed to have dead outputs and I ran the other amp for less than 30 minutes befiore that failed and now the amp is an instant fuse blow.Dead accross each of the pins on all the drivers with NO other apparant failings on the board.?
I had considered that an increase from L120 to L150 may have been enough for me but NO. My previous metal outputs did 105 watt for many years untill the removal men damagaed them.
It looks like the L20s could be the minimum size amp even with the huge amount of heatsinking that I use. Over 1 kilos per channel at 53v DC poss/neg.
I also came to the conclusion that LJM has revised the L150 board as its similair in R values to the Later L15. If not then I have been sold two duds.however They are both Blue boards...Perhaps some one could revise or update this information somehowe?
As a standby I am using a 90 watt Toshiba which can go into shutdown when driven loud for and length of time so that establishes the speakers are a bit power hungry.The new room is at least 50 per cent larger than previously.
Not sure if I want to proceed to another LJM design as I think the Chines output devices MUST be pretty poor. (sonic change when warmed up and driven).
Plenty of spare parts going cheap one day...
Dave
Cheap amps are never cheap...
I have no idea what caused failure. It could be poor quality or fake parts. The design has some risk areas too. It looks like a prototype that has not been stress tested. It needs harsh testing. I suspect you may find the resistor in the Zobel network is running hot, indicating parasitic oscillation.
About fake parts: I have bought fake parts from China, some A1302s as I recall. Out of curiosity, I crushed a few with vise grips to see the die, and it was definitely a small signal transistor die. Should have been about 1cm square, was much smaller. I scrapped the lot of 100 with a cheap lesson learned: Buy only from known trusted suppliers and manufacturers. Cheap Chinese transistors are too often counterfeits. For this kit I would suspect the MOSFETS may be fake, so crush some to check die size. I'd be tempted to look at all the small transistors on a curve tracer to see if they are close to spec, which I rather doubt.
About the design: Where do I start? A possible schematic was posted earlier. Q11 was obviously connected wrong, base collector swapped. I edited that and added some comments below. The Baxandall diode seems pointless with MOSFETS, it's for bipolar EF. Resistors R7 and R11 should be about 680 ohm, not 68 ohm. There are more errors, but I think they are intentional to mislead copy cats in China. LJM has a sense of humor.
People are reporting that these amps self destruct, for unknown reason. It could be caused by fake parts, but design and physical layout are critical too. I have some thoughts about possible reasons, but without boards in hand I'm guessing a bit. Here we go.
Issue #1: Parallel MOSFETs have a tendency to ring oscillate with each other due to parasitic coupling to the gate. This can be destructive. ALL manufacturers recommend a ferrite bead on the gate lead to stomp this. I have seen oscillations in the 50-100 MHz range. I edited the schematic to add beads on the gate leads as a suggestion. (The beads look strange, not IEEE symbols, because I did this in Photoshop, not a schematic capture program. You get the idea anyway.)
However, that's a guess and may not do anything, but I suspect you would see self oscillation with a scope. Other thoughts: most working designs of this type have a huge capacitor (say 330 pF) from VAS to ground too, which is a desperate measure to quench oscillations. It usually works but seems totally wrong. The root cause needs to be discovered. Unless these are perfectly matched parts, they need non-inductive source resistors to encourage equal current sharing.
CFP?: I am concerned about the use of a complementary follower using PNP bipolar driver and N channel MOSFET. This topology has a high risk for possible instability and oscillation. I would use P channel MOSFETS here and keep to a known reliable topology. A small cost savings by using N channel parts means nothing if the amp blows up, or worse takes out an expensive speaker.
Physical Placement: Do you see how the output inductor and connector are right next to the input connector? That is a big error in physical placement. The high current output wiring can easily couple to the input wiring. Keep them far apart, and use shielded or twisted pairs for them. As a minor point, the Power Connectors have a similar issue. Power leads need to be twisted with Power Return leads all the way from the supply to the board, and connected at both ends. This is basic stuff, twisting power leads and always using a return, but if you want low noise it is a must.
Those are some thoughts. Firstly, I do suspect fake MOSFETS. But sometimes just bad physical layout can kill. The front end of this amp is pretty standard stuff, and well understood. I suspect the VAS and its current source may need different Miller compensation, but I'd wait. I'm pretty sure the problems people have are caused by the output stage, either bad parts, bad layout, poor soldering, or poor wiring practice.
I have no idea what caused failure. It could be poor quality or fake parts. The design has some risk areas too. It looks like a prototype that has not been stress tested. It needs harsh testing. I suspect you may find the resistor in the Zobel network is running hot, indicating parasitic oscillation.
About fake parts: I have bought fake parts from China, some A1302s as I recall. Out of curiosity, I crushed a few with vise grips to see the die, and it was definitely a small signal transistor die. Should have been about 1cm square, was much smaller. I scrapped the lot of 100 with a cheap lesson learned: Buy only from known trusted suppliers and manufacturers. Cheap Chinese transistors are too often counterfeits. For this kit I would suspect the MOSFETS may be fake, so crush some to check die size. I'd be tempted to look at all the small transistors on a curve tracer to see if they are close to spec, which I rather doubt.
About the design: Where do I start? A possible schematic was posted earlier. Q11 was obviously connected wrong, base collector swapped. I edited that and added some comments below. The Baxandall diode seems pointless with MOSFETS, it's for bipolar EF. Resistors R7 and R11 should be about 680 ohm, not 68 ohm. There are more errors, but I think they are intentional to mislead copy cats in China. LJM has a sense of humor.
People are reporting that these amps self destruct, for unknown reason. It could be caused by fake parts, but design and physical layout are critical too. I have some thoughts about possible reasons, but without boards in hand I'm guessing a bit. Here we go.
Issue #1: Parallel MOSFETs have a tendency to ring oscillate with each other due to parasitic coupling to the gate. This can be destructive. ALL manufacturers recommend a ferrite bead on the gate lead to stomp this. I have seen oscillations in the 50-100 MHz range. I edited the schematic to add beads on the gate leads as a suggestion. (The beads look strange, not IEEE symbols, because I did this in Photoshop, not a schematic capture program. You get the idea anyway.)
However, that's a guess and may not do anything, but I suspect you would see self oscillation with a scope. Other thoughts: most working designs of this type have a huge capacitor (say 330 pF) from VAS to ground too, which is a desperate measure to quench oscillations. It usually works but seems totally wrong. The root cause needs to be discovered. Unless these are perfectly matched parts, they need non-inductive source resistors to encourage equal current sharing.
CFP?: I am concerned about the use of a complementary follower using PNP bipolar driver and N channel MOSFET. This topology has a high risk for possible instability and oscillation. I would use P channel MOSFETS here and keep to a known reliable topology. A small cost savings by using N channel parts means nothing if the amp blows up, or worse takes out an expensive speaker.
Physical Placement: Do you see how the output inductor and connector are right next to the input connector? That is a big error in physical placement. The high current output wiring can easily couple to the input wiring. Keep them far apart, and use shielded or twisted pairs for them. As a minor point, the Power Connectors have a similar issue. Power leads need to be twisted with Power Return leads all the way from the supply to the board, and connected at both ends. This is basic stuff, twisting power leads and always using a return, but if you want low noise it is a must.
Those are some thoughts. Firstly, I do suspect fake MOSFETS. But sometimes just bad physical layout can kill. The front end of this amp is pretty standard stuff, and well understood. I suspect the VAS and its current source may need different Miller compensation, but I'd wait. I'm pretty sure the problems people have are caused by the output stage, either bad parts, bad layout, poor soldering, or poor wiring practice.
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