(IR) IRF530N vs (VISHAY) IRF530PbF - help me

[silversweden]

can someone help me understand

I am repairing an active Infinity Bu-1 subwoofer and after replacing the output transistors
IRF530 and IRF9530 i have to small gain at low input levels (low level output from the stereo, if i connect directly to cd player the volume is more than needed, but the signal is max all the time from cd player, from the stereo i can nearly hear the sub because the front speakers are playing much louder, have tried 2 different amps, 1 nad and 1 onkyo, i have to put the volume up to much on the stereo to hear the sub and then the front speakers are playing to loud)

I think the output (gain) characteristics are different comparing this two mosfets.

Brand: IR Modell: IRF530N <---- this is what i put in as a replacement.
https://www1.elfa.se/data1/wwwroot/a...s/07114929.pdf

Brand: Vishay Modell: IRF530PbF <---- maybe i should try this one instead?
http://www.vishay.com/docs/91019/91019.pdf

Can someone help me compare the specs! I want more gain at lower levels!
Hope you understand what i want, need som fast advice!

The sub sounds ok except from the low gain, and looking at a 100Hz
sine wave with the scope on the output the signal looks good.
(nearly) No distortion.

[Perry Babin]

The overall gain isn't determined by the output transistors. It's determined by the feedback components of the power amplifier and from the gain set in the preamp. Did you trace the signal through the amp to see if it was being muted/clamped at some point?

[silversweden]

Hello
Its hard to trace the signal without the right schematic, havent found the 230v schematic on this amp just the 120v and the layout is not even like.

But if i measure the input on the preamp board i have a nice sine wave,
and at the output from the preamp board the amplitude is changing if
i turn the volume knob on the sub.

And i also measured with scope on both driver trasistors and output transistors. The sinewave changes in amplitude if i turn the volume knob
on the sub.

One thing i notice is if a measure the output with a sinewave i have to have
the bias pot at max to get rid of crossover distortion, if i turn it 10% then 90% of the crossover dist. goes away. If i turn it to max the crossover dist is completly gone. Strange thing is that the amp is just drawing about 108mA at the primary side with the bias pot at max.

The thing is that i swaped the biaspot from a 502(5000ohm) to and 4700ohm because i could not find the exakt value at the moment. This is not the problem, but i also changed the outputs at the same time so i dont know when the strange bias fenomen occured because before when the amp was working and all the original parts where in there i could make the amp drav about 180mA with the bias pot at max.

One thing i can think of is that something else died with the outputs but its a very simple design and everything seemes to be working right now.

Before this problem with low gain i had no problem with the volume, but i accidently shorted + and - speakerwire wich made the outputs go up in smoke Dont know why this happend because i had no input on the amp and when i measured the ac and dc on the output just before i shorted it and i had nearly 0volts. Dont know what i should do right now, the only thing i can think of is that the new outputs dont seem to match.

Any advice? And as i said before, the sub works great with the signal (rca) directly from the cdplayer, but from stereo i get to low gain at normal listening volume.

[Perry Babin]

In the other thread (found when googling BU-1), you stated that you replaced the electrolytic caps. One of those caps was likely part of the voltage divider for the feedback signal. If there is a broken connection in that circuit (pad broken from trace...), the gain will be low. If you know where the preamp signal feeds the power amp section and the signal there is the same as the output level at the speaker terminals, this is almost certainly the problem.

[silversweden]

There is a big difference in signal amplitude if i compare the preamp signal and output signal, and the sub really shakes the house if connected directly to a cd player, the gain problem only comes with and pre out on a stereo connected to the sub, need to max the volume knob and its still to low volume on the sub at normal listening volumes.

But i will check the solder pads again to be sure. I already checked this
twice before so i dont think i will find anything here but just to be sure...

This is turning into a nightmare :P From just a small background hum to
no hum (wich is what i wanted) and to low gain (not what i wanted).

It should not be easy =)

[Perry Babin]

Are you sure that there isn't a problem with the preamp output of the receiver?

[silversweden]

yes i am sure - have tried 2 different wich i know is working propertly.
if i turn the volume knob to halvways on the stereo with volume know set on max at sub i get party... i have tested this with the speaker output disabled on the stereo...

[Perry Babin]

How different is the schematic diagram you have from the sub amp you have?

[silversweden]

its quite different: www.itxpress.se/bu1.pdf 120v version.

My 230v version is more simple (cheap?), it has no speaker relay.
The outputs is IRF530/9530, TIP34/35 on the 120v.

The 120v bias circuit is different also i think, can not find
any pot. in this schematic.

I have found the 230v schematic on manualsparadise, but it
cost nearly as much as the sub itself so im not to happy spending
that money as im repairing this for a friend, for free

The last exit is to buy the schematic, if i have to.

The problem right now is that i messed up again!
I accidently had my fluke in Ohms mode and tryed to measure
volt on the IRFs..... and both of the outputs toasted...

You should not work on electronics when you are tierd i guess!!!!

I will try replacing those two IR wich i bought previous with
the vishay ones just to try, the IR ones is speced higher
ampere and lower RDSon, but i think the vishay match more even
with the old ones that was originaly there.

Strange thing because original it had one Motorola and one IR.
Maybe repaired before, or just cheapest parts they could get at
the production.

[silversweden]

I emailed infinity asking for the schematic and here it is!
http://www.itxpress.se/bu1_230v.pdf

The thing is that i shorted red and black speaker wires, then the
irf530/9530 died. After replacing them i have TO low gain at lower
input volumes. If i play the direct low level from cd i get very good
gain.

What else could have been destroyed when looking at schematic.