A guide to building the Pass F4 amplifier

[MASantos]

I keep repeating myself, because almost no one is listening.
Stop using the ground word. It just adds to confusion.

The screen wire is a screen wire. It is not a ground.
Connect the screen wire to Chassis using the lowest impedance connection you can manage.

Well, I'm not an EE so it might be normal for me to confuse some technical terminology. I'll change that configuration then.

 

[AndrewT]

Neither are most of us.
EE = electronics engineer and/or electrical engineer.

I doubt if they make up 0.1% of our Membership.

 

[2 picoDumbs]

Well, I'm not an EE so it might be normal for me to confuse some technical terminology. I'll change that configuration then.

Anything that gives Andrew a brain melt down is ok with me.
Keep up the great work.
Hahahaha

 

[MASantos]

Assembly is finally finished!!
Initial setup and testing on saturday! Regarding the light bulb contraption to power up for the first time, I only have acess to 60W light bulbs, any problem with using one of those?

Air temperatures are around the 30ºC right now so it's a good time to bias it and make sure it handles high room temperatures!

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

 

[ColinA]

Tidy.

 

[Zen Mod]

60W bulb is ok , if amp is not biased , simply because amp will idle with just small power consumption

 

[MASantos]

So now the troubleshooting begins...

I fired up the amp with the 40W light bulb in the cable and the trimpots are turned all the way counterclockwise. The light bulb doesn't fade out.

I am getting no light on the psu and amp pcb diodes and only 1.2V rail to ground.

I may have made a silly mistake and mounted the diodes reversed polarity, would this prevent the psu rails from raising to full voltage?

 

[2 picoDumbs]

Did you measure voltage drop across source resistors?
You could also test just the powersupply in isolation (disconnect amp from supply) and retest with light bulb.

 

[MASantos]

Did you measure voltage drop across source resistors?
You could also test just the powersupply in isolation (disconnect amp from supply) and retest with light bulb.

I tested it a couple of days ago and go a nice +-33.4VDC on the rails. After that I finished wiring the amplifier pcbs.

 

[2 picoDumbs]

Ok
Measure to see if there is a voltage drop on source resistors.
You could also measure the voltage across tl431.

 

[ColinA]

So now the troubleshooting begins...

I fired up the amp with the 40W light bulb in the cable and the trimpots are turned all the way counterclockwise. The light bulb doesn't fade out.

Do not rely on the trim pots working this way!
Check and measure.
Then try turning them and watch the result on meter.

Also try 100w bulb.

 

[MASantos]

Ok, I just removed the psu wiring to the amp pcbs and the psu measured just fine again, with the diodes lighting up. I'll reconnect everything again and retest with light bulb, maybe it was a short somewhere on the wiring. Will do that later today, wedding time now!

 

[2 picoDumbs]

As Colin says don't always assume counterclockwise is off
Measure voltage across TL431

 

[AndrewT]

Don't change up to a higher wattage bulb yet.
Find out why the small bulb is glowing first.
Some part of the circuit is drawing too much current. You have to find out why.

 

[MASantos]

I'll connect one channel only and power up to see if it is only on one channel or if both are having problems. Any measurements I should make before applying power again?

Here is the datasheet for the trimpots, I believe they increase resistance with clockwise turns. http://www.bitechnologies.com/pdfs/67.pdf

 

[ColinA]

I'll connect one channel only and power up to see if it is only on one channel or if both are having problems. Any measurements I should make before applying power again?

Here is the datasheet for the trimpots, I believe they increase resistance with clockwise turns. http://www.bitechnologies.com/pdfs/67.pdf

Don't take ANYTHING for granted!
Be ready for the trim pots working the wrong way, mine did on an F6.

 

[MASantos]

I tested between the legs of the TL431 and they do indeed have the minimum resistance when fully counter clockwise. Left channel is connected to the psu, LED's are emiting light and the light bulb went dimm after a few seconds. It never stoped emitting light. Now on to the right channel.

 

[MASantos]

I believe I may have found the problem... On the right channel I mounted the mosfets the wrong way... P's are on the N's location and vice versa... I hope I did not fry them...

 

[MASantos]

Sucess!! Changed the mosfet's position and everything started up correctly!!

I have 250mV running through the mosftets for about 0.53A of bias current per each one!
So I have 27W of nice class A power feeding my ~6ohm speakers!!

Now off to finish the BA3 preamp!!

 

[ColinA]

Well done, satisfying when they get sorted isn't it.