ESP P68 Subwoofer Amplifier - Need Help


2012-04-01 9:53 pm
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




(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

Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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.


2012-04-01 9:53 pm
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

Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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?
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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?
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2012-04-01 9:53 pm
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


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


here the pictures:




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?
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Extra Heatsink out, now it gets warm

voltage drop: D2=656mv D3=648

are you sure about this 550mv? ou the build article says it should be between 30mv and 80mv, and yes, i messed up Q1,2,3 are BC546, Q4 MJE340 Q5,Q6 MJE350

Q7 and Q8 are in the main heatsink, and it says that Q1 to Q3 could be BC46 ou MPSA42, just to watch out the pins, since the are inverted, the silkscreen on the board are for the BC546

Quote from the build article:

Note that if you use MPSA42 devices (and some other common variants), the pinouts are reversed! This means that the PCB overlay is not correct for these devices, so take great care when installing to make certain that you have the correct orientation.

Base Emiter: Q1 = 53V Q2 = 53V

Between terminals: R5 = 20mv R6 = 720mv R7 = 1350mv R16/17 = 210mv

and i'm using a standard green led
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hey i have 8 pair 2sc3858/2sa1494 sanken..sir,is their any problem if i use this instead of mjls in esp 68 project???help me....nd i have also 2 car subwoofer each 4 ohms..if i use 40 0 40v 800va transfomer for esp 68 nd connect it into 2ohms load (2 sub connect in parallel).is there any problem??

That i don't know, but 2ohms is too low, will overheat/burn output transistors