ESP P68 Subwoofer Amplifier - Need Help

[PedroDeMio]

Hello Guys

I've build a P68, the sub woofer amp form Rod Ellitot, i had purchased the pcb and it's verry easy to assemble

but i have three problems:

1- very low output power, nowhere near the 250w, barely make my subwoofer move

2-the driver transistors (Q4,Q5,Q6) doesn't heat, this is a class A driver, they should heat even with no signal

3- the Quiescent Current, mesurring R16/R17 i get 200mv = 50ma, this should be 30=mv to 80mv =16ma


i already checked all the board with a magnifier glass, replace Q1 to Q6 3 times, checked all the resistors, desoldered all output transistor and checked, re-done all the soldering

i have no ideia what more to do, here the schematic

I would apreciate any help

Thanks

Pedro



(if it could help e have pictures of the amp here, just let me know if it will help and i post them)

[Ian Finch]

Hi
This schematic is copyright material, Pedro. It is convenient to have it here but you could link to it.
BTW, do you have a multimeter and have you tested any voltages, such as that between the output
terminals (DC offset), and between driver transistor bases on Q5-Q6? (bias voltage). These will
narrow down the problem a little, if the parts are not showing any damage in your measurements so far.
Just be sure to remove input and speaker leads before testing.

All transistors should show a voltage of ~0.6 from B-E terminals when operating correctly. Is this so?

Are the rail voltages present at the appropriate output transistor connections and is input ground and
speaker ground returned to star ground at the power supply?

Pics are always appreciated, if you show your wiring to the power supply and speaker too, so we have
an indication of how you are checking things.

[PedroDeMio]

Hi, Thanks for the reply

here the link with all the details 300/500W Subwoofer Power Amplifier

The dc offset keeps below 50mv, oscilating by a few milivolts

i will test all driver transistors and post pictures as soon as possible, all the rails have power +-110vdc

just to advance things, the amp is not assembled in a enclosure yet:

Transformer ---- bridge rectifier ------caps (2x4700uf per side) ----- amp

the ground from speaker is conected to the capacitor ground, but for now its disconected, first i want to solve this 200mv voltage

[Ian Finch]

Q4 may only pass ~10 mA or dissipate around 1W. That's a TO220, it may not feel warm for a while, if on a heatsink.

The high bias current (R16,17) is set by V(D1+D2). Checking the voltage between the driver bases will show if much is wrong there.

[Ian Finch]

More tests: Bias is one issue apparently, so you can short either D2, D3 or even both to half or zero bias and see what happens to the R16,17 voltage . This should also drop. similarly.

The low power level suggests something different, like problems around the Voltage Amplifier Q4, since the input stage is showing fair balance and operation with only 50 mV offset. Is D1 glowing correctly?

[lineup]

You should measure R18-19-20-21 for bias.
Not R16-R17

[TANDBERGEREN]

For diagnostic purposes it would suit fine to check the current through R16-R17.

Also check the voltage drop over the two diodes D2 and D3.
Those are the ones responsible for the Bias.
Are they in contact, thermically, to the heatsink?
If not, the bias will just run away.

You are sure You have used the excact transistors wich is designed?

[PedroDeMio]

Here the tests:

Q5 base - Q6 base = 1.32v

Emiter - Base (Q9,Q10,Q11,Q12) = 195mv (i think we have something here)

Emiter - Base Q4 = 595mv Q5 = 476mv Q6 = 443mv Q7 = 565mv Q8 = 570mv

Q1,Q2 and Q3 get just warm

Shorting one of the two diodes and i get 5mv between R16/R17

No messurement on R18-R21, i think its to low for my multimeter

D1 is lighting up

TANDBERGEREN :

the drop between the diodes is ok, i'm thinking here, my heatsink for the drivers is to big? becouse of that the diodes doesn't heat up and no bias is ajusted?

For pre driver i'm using BC546, for driver MJE340/MJE350, and output MJL4281/4302

Thanks

here the pictures:

[TANDBERGEREN]

Yoiu should dismantle the extra heatsink on the drivers.
Thats for sure.
I was curious on what voltage drop there is over each diode.

The voltage over R16/R17 should be at least 550mV.
It seems as if the voltage drop over the diodes are too low.
Predriver is BC546?
Eh, I think You got it wrong.
Q4 is the predriver.
Q5,Q6, Q7 and Q8 makes the drivers. This is a bit special type of connection alowwing the temperature compensation be made at the first driver pair, Q5 and Q6. and by this removing the need for sensin directly to the output transistors: Q9.Q12
Q1, Q2 and Q3 should be the MPSA42, not BC546. There is very different characteristics.
I guess the Q7 and Q8 is mounted on the main heatsink?

Q3 is the constant current source for the differential input transistors Q1 and Q2

Could You check the voltage between Collector and emitter of the Q1 and Q2?

[TANDBERGEREN]

By the way.
You could check the voltage over R7
The voltage drop over R5 and R6 should be approximately the double of that.
The led shall illuminate, but remember: a standard green led must be used.
The voltage over it is probably around 1,8V