Need help with bias current adjustment

[Alan Meyne]

Dear Wise Friends,

Am a very rusty DIY guy getting back to the passion after a 15 year break!
As a come back project, I am building a power amp using a kit from Analog Metric called Passion 15 MKII monoblock. In the construction guide it says:
INTRODUCTION
Passion 15 MKII mono block uses a complementary high power MOSFET 2SK1529 and 2SJ200 which is capable to deliver 100W average output power. Two resistor trimmers can be used to adjust the biasing current so as to set either class A or class AB amplification with unity gain,(My Question: How to do this?) and also for output DC offset cancellation. Also, it requires either AC or DC input power with wide voltage ranges.

PROCEDURES

For AC power supply, apply two 20-27V AC to the AC 0 connector, short VCC to VCC2 and VEE to VEE2 by a wire, accordingly; for DC power supply, apply positive and negative 24-38V DC to VCC2 and VEE2, respectively. (My Question: What are the advantage disadvantage of using either AC or DC?)


After power on with limited current, adjust the biasing current by resistor trimmers VR1 and VR2. (Q: Where and how to measure?)The current can be calculated by Ib = V/R7 or Ib = V/R8. Also, adjust the VR1 and VR2 so as to cancel the output DC offset. (Q. Dose it mean that the previous adjustment may have to change?)

HELP!

Regards,
Alan

 

[ark]

Dear Wise Friends,

Am a very rusty DIY guy getting back to the passion after a 15 year break!
As a come back project, I am building a power amp using a kit from Analog Metric called Passion 15 MKII monoblock. In the construction guide it says:
INTRODUCTION
Passion 15 MKII mono block uses a complementary high power MOSFET 2SK1529 and 2SJ200 which is capable to deliver 100W average output power. Two resistor trimmers can be used to adjust the biasing current so as to set either class A or class AB amplification with unity gain,(My Question: How to do this?) and also for output DC offset cancellation. Also, it requires either AC or DC input power with wide voltage ranges.

PROCEDURES

For AC power supply, apply two 20-27V AC to the AC 0 connector, short VCC to VCC2 and VEE to VEE2 by a wire, accordingly; for DC power supply, apply positive and negative 24-38V DC to VCC2 and VEE2, respectively. (My Question: What are the advantage disadvantage of using either AC or DC?)


After power on with limited current, adjust the biasing current by resistor trimmers VR1 and VR2. (Q: Where and how to measure?)The current can be calculated by Ib = V/R7 or Ib = V/R8. Also, adjust the VR1 and VR2 so as to cancel the output DC offset. (Q. Dose it mean that the previous adjustment may have to change?)

HELP!

Regards,
Alan

Hi Alan,

To answer your first question about DC or AC, if you have an existing DC source which can provide clean DC with required voltage and current, then you can use that -for example a good battery. In this case you wont be using the rectifier circuit present in your board consisting of the bridge rectifier and the large capacitor. If dont have another DC source, you are better off powering your amp using a suitable transformer to supply the required AC voltage.

Your second question is a bit tricky . If you post a the circuit diagram of the amps output stage , I will be able to tell you how to measure and then tune the voltage and current to the required level.
In any case , before you connect your actual load, which is the speaker , you have to make sure that there is no DC offset present at the output. Here is how you could measure that.

Connect a multimeter across the speaker terminal ( without connecting the speaker ) and measure the DC voltage present.
Then you would adjust one of the pots till you get a near zero reading on the meter. DC offset should be less that a few milli volts.
Then you connect a dummy load to the output such a 25W 8Ohm resistor and again measure the DC voltage across the resistor ( same as your terminals) , reading should again be near zero volts.


(Q. Dose it mean that the previous adjustment may have to change?)

Yes. Once you completer the above exercise , you can start your tuning exercise by the adjusting the two pots such that the amount of trimming you did on both pots are identical and the DC offset is zero.

By adjusting the resistance value of the pot , you are actually changing the bias for the output transistor and hence the current flowing through them. In class AB config, you will adjust the bias just enough so that the transistors start conducting , but wont drain a lot of current. In class A you will actually bias the transistor such that it is at the mid point of its operating region. So you decide what mode of operation you need.

Coming back to the pot adjustment , imagine the system as a typical balance that you use for weighting stuff.
If you add some weight on one side , you have to put equal weight on the other side to get your balance back. Same logic applies here. When both your output transistors are conducting equally, your dc offset will be zero. By adjusting one pot you are increasing or decreasing the current and hence voltage appearing at the out of one transistor.( I am assuming that one trimmer controls the bias for one output transistor and you have two output transistors, hence two trimmers) To get it back to zero , you need to adjust the other pot by the same amount.

 

[quad]

Found a schematic on an auction site -

 

[Alan Meyne]

Hi Ark,

Thank you so much for your very comprahensive reply! I am indebted.

You are right, There are two trimpots. I haven't fully digested the trimming procedure. I hope to pick your brains as i complete the board and the powersupply. Thank you for being so helpful.

Warm Regards,

Alan

 

[Alan Meyne]

Hi Quad,

Thank you so much !!!

You are so helpful, I really appreciate this.

Warm Regards,

Alan