P3a No quiescent current!

[PetarLD]

Hello, just finished my first P3a. 😀

I don't have a suitable power supply yet, so I decided to run it off of 4x6v rechargable batteries, which gives me ~12V of rail voltage. I connected everything and I had the multimeter set to Ampers, and I had it connected to both collectors of the output transistors which are MJL4302A and MJL4281A.

Yeah so there is no quiescent current, the multimeter shows 0.000. But when I measure the DC offset of the output for the speaker it shows 200.4 mV. Is this something normal, because Rod says it should be set around 50mV. So then I decidet to connect a speaker to it and a source and it amplifies just fine, with a little distortion in the base, but I think that will go away when I get the real power supply.

As far as parts go I used everything that was suggested in the schematic, except the VR1 for which I used a 3k3 single turn out of an old cell phone station, so I tried to turn it and no difference.

Any help would be greatly appreciated. Thanks!!!


~Petar~

 

[Fritz]

Hi Peter,

I have a P3A and with a pot set to 2200 ohms at VR1 I measure no bias current. If I go down to probably around 1500-1800 I have bias. Could be your pot doesnt have the range to give the current you want? Id crank the pot all the way over and measure the minimum resistance, if your still around 2k you may need to change the pot?
My recolection is that you can mesure VR1 with it installed in the circuit.
I really like the amp and it sounds nice even on low voltage.


Fritz

 

[Mattyo5]

is the bias transistor collector base emitter all installed in the correct configuration? my anthony holton amps wouldn't bias because i had the bias transistor attached backwards....

-Matthew K. Olson

 

[selim]

Petar,

The collectors of the output devices are not supposed to be connected to the power rails in this amplifier.

Best regards

Selim

 

[PetarLD]

Hi guys, thanks for the replies.

I double checked the transistors and everything is the way it is supposed to be. The pot can be adjusted from 2.94 Kohms to 0 Ohms.

I am going to test it with the supply that I am building right now and I will give you the results.

Selim, I don't have the supply rails connected to the collectors of the output transistors, they are connected to the emitters.

Best regards,

~Petar~

 

[djk]

If you want to run a P3A on ±12V try changing R8 to 6K8 and R9, R10 to 1K0.

Change back to the original values for R8, 8, 10, when you build your permanent supply.

 

[millwood]

Hello, just finished my first P3a. 😀

I don't have a suitable power supply yet, so I decided to run it off of 4x6v rechargable batteries, which gives me ~12V of rail voltage. I connected everything and I had the multimeter set to Ampers, and I had it connected to both collectors of the output transistors which are MJL4302A and MJL4281A.~Petar~

by doing that, you kind of altered the schematic of the output stage, as the internal resistance of an ampemeter is very very low. Why don't you try to measure the voltage drop off those "collector" resistors (0.33ohm)?

Yeah so there is no quiescent current, the multimeter shows 0.000. But when I measure the DC offset of the output for the speaker it shows 200.4 mV. Is this something normal, because Rod says it should be set around 50mV. ~Petar~

at +-12v, the operating current on the VAS is considerly less (about 2ma now, vs. 6ma in Rod's design). so even if the VR1 is set at its maximum setting (of 2k), you will NOT generate enough voltage drop to open up the BE junction of Q9 (the Vbe multiplier). so there is not sufficient voltage drop on the whole Vbe multiplier, thus the output transistors never turned on: the Vbe multiplier is generating about 0.6v of voltage drop, hardly enough to open up two bE junctions (Q5 and Q6).

you have a few choices. You can replace the bootstrap resistors with smaller ones so tha tthe VAS runs at about 6ma (use two 1k resistors for R9 / R10 will do that. But remember to replace them when you go back to 35v, or whatever your desired voltage is). or use higher resistors / trimmer for R16 / VR1 (3K / 6K respectively).

looking at Rod's design, I would have used slightly different topology to accomodate "my" needs of replacing VAS transistors quickly. Rather than using a transistor as the current source on the tail, I would have replaced it with a simple resistor and use a current mirror. that way, I wouldn't have to worry about R6 when switching Q4. But that's for another discussion.

 

[millwood]

If you want to run a P3A on ±12V try changing R8 to 6K8

I have found those LEDs are pretty good even at low current. 0.5ma or slightly less isn't that bad for a regular LED. It should work.

and R9, R10 to 1K0.

Change back to the original values for R8, 8, 10, when you build your permanent supply.

you are right on.

 

[PetarLD]

Hi, I finally got the power supply finished, it has +-24V supply rails, because I don't need that much power anyway, I had it turned all the way up and I checked the voice coil of the 12" speaker in my setup and the coil was getting warm.

But I measured the voltage accross the bias resistor and it was as Rod said 0.7V, and the DC offset of the output was now down to 187mV, but still no bias voltage. How am I suposed to measure it anyway? My dad says that I should measure it for each output resistor (mine are .47Ohm 5W) separatelly.

Also when I play songs some of the mid-high frequencies are cut off, not played at all. My input capacitor is 4.7uF.

Best regards,

~Petar~

 

[PetarLD]

Hi gang, thanks for all of your help, but I found what the problem was, I went through and inspected all of the soldering joints and I found out that I haven't connected one of the ends of R6 to the point where the base of Q4 and the collector of Q1 meet.

Now I still don't have bias but at least there is 0 volts on both of the output resistors, before I had 10mV on the one that is connectd to Q7 and 0 volts on the one that is connected to Q8.

So tomorrow when I get back from school I will change R9 and R10 to 1k, and R8 to 6k8.

Thanks again.

~Petar~