Matching transistor Aleph 1.7

[loovet]

I don’t need to match any of the transistors in my Aleph 1.7.

Am I right here?

/loovet

[kristijan-k]

Hello loovet,

You can use unmatched transistors for the Aleph P 1.7.
But, the matching is very easy, and if you use matched
transistors it would not hurt either.

Best regards,
Kristijan Kljucaric
http://web.vip.hr/pcb-design.vip

[loovet]

Originally posted by kristijan-k
Hello loovet,

You can use unmatched transistors for the Aleph P 1.7.
But, the matching is very easy, and if you use matched
transistors it would not hurt either.

Best regards,
Kristijan Kljucaric
http://web.vip.hr/pcb-design.vip

Thanks Kristijan,

I think that I will match them. As you said, not that much work.

Best regards,

/loovet

[swede]

Hi,

I just changed a set of output FET's on a SOZ for a friend in sweden. On one channel I used FET's matched by me, which resulted in about 100mV DC on the output. Not much and within margin of error. On the other channel I used two FET's matched by UrSv (frequent on this forum). The DC on the output was less than 10mV, which probably is on the "wrong" side of the accuracy of my DMM (digital multi meter) anyways.

So, UrSv should tell you how he did it. He usually uses constant current and measures the Vgs on the FET's. I used constant Voltage source and measured the current. UrSv won! ;=)

Best,
//magnus

[loovet]

Shouldn’t I match them the way Kristijan and others (even N.P. himself) usually recommends? Look at the picture below.

Is there a better way to do the matching? Maybe UrSv can give us an answer to that question?

/loovet

[swede]

Hi,

Well, matching is just a matter of knowing Ohm and Kirchoff.

If you want to match the devices at a "specific" current, you have to use a variable voltage source. You start by connecting the devices like in the picture you posted above. Measure over the resistor and make sure that the current though it is exactly the current you like to match your devices at. When you are at the right current (for instance 5mA or 100mA or whatever current you like to match at), you measure the voltage over the gate and the source. This is what UrSv did, and you get an exact measure of the Vgs for the device at a given current.

When I matched my devices I was lazy and I didn't have a variable supply handy. This is not a big deal, though your devices will differ slightly in current. You will be quite close though, and devices with the same Vgs will have the same current flowing through them anyways (kirchoff). Hence, you will be able to get matched devices, but the different sets will be matched with a slightly different current (which in any case will not be too different after all).

It is just a matter of how picky you are.

When I built my first board for my Aleph P I just picked random FET's from my big scrap box, just making sure that they were TO220 and that I put N-channel and P-channel FET's at the right places. (i.e IRF9620 and IRF644 instead of 9610 and 610) It made sound, which was the goal for my exercise. Your exercise might be just a bit different. Some day I might upgrade it. ;=)

My advice, as usual: Try and build it. Make it work. Then, after all the hassle, start tweaking. Othervise, you'll never finish the project.

Best,
//magnus

[loovet]

I think I’ve got your point of view.

Of course, my first aim is to have the thing make any noise at all. And after that I will try to make some adjustments to make it play lovely music.

Different current will flow through different transistors. Am I right here? But then, wouldn’t it be possible to do it the other way around. Keep a stable voltage and then change the value of the resistor, maybe with help of a pot? I migth be verry wrong here ;-)

Best regards

/loovet

[jh6you]

Hi loovet

The following is a quote from http://www.passdiy.com/pdf/balzenpre.pdf. I think the comment is also applicable to Aleph P rev1.7.

¡°It is not essential to match the MOSFETs used in this project, but it doesn't hurt either. I have tested the circuit with matched and random parts with insignificant performance differences. There are other circuits that take advantage of matching, but because of the separate bias sources for each device and the high value of resistance between the Source pins of the two devices, this circuit is indifferent to matching. The exception is the case where R15 has a small value for the purpose of very high gain. Under this circumstance, matching will improve performance, and I recommend Vgs matching to within .1 V or better.¡±

Enjoy your DIY.

JH