Help with dissecting first amp

[mrbubbs]

I'm trying to figure out some things about my B&K M200 mono amps. Looking inside, I see 2 rather large capacitors rated at 15600 uF and 75 VDC. Since this is a mono amp, I'm wondering why it looks as if there are 2 symmetrical components of everything in the amp, except the power transformer, switch, and inputs/output connections. I see 2 caps, 2 PCBs with 2 identical layout of MOSFETs, etc. Since this amp can take a balanced input, I'm wondering if the design is 2 separate amps, that are joined into one, but I'm not sure.

I also see 2 PCBs with a bunch of black MOSFETs in a line, that say K405 on one side of the heatsink mount and green MOSFETs labelled J115 on the other side of the heatsink mount. I figured out by searching datasheets that 1 type is P-channel and the other is N-channel. However, since the datasheet is in Japanese, I'm having trouble figuring out what kind of output (in watts) these MOSFETs produce. I thought maybe 100W but I see a total of 10 K405 and 10 J115 MOSFETs, which is way more than the 200W specification (well, at 8 ohms). I think I read that one should never use more than 20-33% or so of MOSFET capacity, but I need help confirming that.

 

[jaycee]

It's probably a bridge mode amp.. so it has two channels, and the speaker is connected between them.

 

[AndrewT]

Hi,
Vgs is quoted between the limits of 0.8V to 2.8V.
I think this means they are lateral FETs.

Using the 8A and 100W spec, a 5pair output stage can deliver 100W into 8ohms, 165W into 4ohms or 180W into 3ohms.
It is not so hot with only 200W into 2ohms but just 45degree phase angle to stay within the DC SOAR for 55degC. All the previous could manage 60degree phase angle load.

If your amplifier is bridged, then it can deliver twice the power into twice the load.
Those earlier figures become
200W into 16ohms
330W into 8ohms
360W into 6ohms
400W into 4ohms
all these are fed from +-45Vdc supply rails with 15600uF smoothing.
If the supply rails are >+-45Vdc then I cannot recommend a load under 6ohms when in bridge mode.
Can you tell us the supply rail voltage when off load?

 

[ilimzn]

Hi,
Vgs is quoted between the limits of 0.8V to 2.8V.
I think this means they are lateral FETs.

K405/J115 are not lateral FETs. They are a variation of a trench MOSFET, which is in itself a variation of a vertical MOSFET. It is especially made to have a low treshold voltage, so in this way it is similar to the lateral MOSFETs, but this is where the similarity ends. The gm is (much) higher, and the tempco is that of a vertical MOSFET, positive - it needs a bias servo.

K405/J115 have been out of production for a number of years. K1530/J200 are commonly used as replacements, although they have a higher Id rating and proportionally higher Ciss.

 

[mrbubbs]

It's probably a bridge mode amp.. so it has two channels, and the speaker is connected between them.

Yes, I definitely think that's the configuration, based on AndrewT's analysis.

Hi,
Vgs is quoted between the limits of 0.8V to 2.8V.
I think this means they are lateral FETs.

Using the 8A and 100W spec, a 5pair output stage can deliver 100W into 8ohms, 165W into 4ohms or 180W into 3ohms.
It is not so hot with only 200W into 2ohms but just 45degree phase angle to stay within the DC SOAR for 55degC. All the previous could manage 60degree phase angle load.

If your amplifier is bridged, then it can deliver twice the power into twice the load.
Those earlier figures become
200W into 16ohms
330W into 8ohms
360W into 6ohms
400W into 4ohms
all these are fed from +-45Vdc supply rails with 15600uF smoothing.
If the supply rails are >+-45Vdc then I cannot recommend a load under 6ohms when in bridge mode.
Can you tell us the supply rail voltage when off load?

Thanks! This is quite helpful to me. I have used this amp with no problems at 4ohms. I gotta find my DMM to measure the rail voltage. Need to also find a meter probe with an alligator clip or at least an alligator clip lead since measuring supply level voltages is safer with one hand than two.

K405/J115 are not lateral FETs. They are a variation of a trench MOSFET, which is in itself a variation of a vertical MOSFET. It is especially made to have a low treshold voltage, so in this way it is similar to the lateral MOSFETs, but this is where the similarity ends. The gm is (much) higher, and the tempco is that of a vertical MOSFET, positive - it needs a bias servo.

K405/J115 have been out of production for a number of years. K1530/J200 are commonly used as replacements, although they have a higher Id rating and proportionally higher Ciss.

Yeah, I noticed the out of production issue. Good thing these parts are still working okay. I've noted the replacement models and will look into what affect the differences will make.

I don't think this amp has any sort of "turn-on protection". I think the usage of a toroidal transformer is the cause of my speakers thumping when the amp is turned on. My UPS connected to the same circuit briefly beeps when the amps are first turned on as well. I think this amp could benefit from some sort of resistor with a relay to limit the inrush of current, possibly lengthening the lifespan of the components connected to the secondaries.