Infinity BU-1 sub low volume after repair - diyAudio
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Old 12th April 2010, 09:16 PM   #1
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Default Infinity BU-1 sub low volume after repair

Hello
I helped a friend om mine repairing his sub, it had som strange
buzzing transformer like sound.

I replaced all electrolytic caps, the bias adjusting pot.
And unfortunantly i messed up and destroyed the output
transistors. (also change 3 100uf 35v with 200uf 50v caps but why would this matter??)

1 irf530 and 1 irf9530 (replaced with same)

Now the buzzing sound is gone but the amp has very low volume.
I have tried adjusting the bias but it wont fix the low volume fault.

Its like that the old ones had higher gain, i dont know the ending letter
of the old ones, just says IRF530 i think. And the ones i bought had an
IRF540(N) to it. Dont know the difference, seemes like its just different
packages with the different letters.

Can i have destroyed something else when i accidently shorted the outputs?
Know its hard to tell without schematic but i have just found one that
dont match, thinks its the UK schematic i have and this is the european model i have. Not relly sure why they dont match its the same modell name.

If i connect the sub directly to the cd player i have more volume than i need. (dont need to have the knob more than halfways)
When i connect it trought my stereo amp (have tried 2 different) the gain is to low at normal sound volumes even with the knob
at max on the sub. This was not the case before...


What should i do? Need some guidance here Thanks!

Last edited by silversweden; 12th April 2010 at 09:28 PM.
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Old 12th April 2010, 09:45 PM   #2
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Join Date: Feb 2006
Location: UK
So, you replaced the IRF530 with a 540, and replaced the 9530 with a 9530?

You should have also used a 9540, as the devices should be used as a complementary pair.

With regards to the caps, it depends on what they are for, its hard to tell you without seeing the schematic. Normally increasing the voltage ratings is fine with replacements, but try to stick to the correct capacitance.

Are you using the speaker level inputs while connecting it to your stereo amp, and the RCA input when connecting to your CD player?
If so, it sounds like the RCA input is working OK, but the speaker level input is at fault. Check the values of the resistors in that part of the circuit.

I'm by no means an expert in these matters, but this might give you something to work on.

Last edited by ben12345; 12th April 2010 at 09:49 PM.
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Old 13th April 2010, 04:11 AM   #3
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Im sorry! I was tierd yesterday, i replaced IRF530 with IRF530N
and IRF9530 with IRF9530N. The brand was IR

I dont remember where the caps where placed so it would be hard
figuring out where they where, but why would a increment in capacitanse
lower the volume?

And i always connect the RCA, speaker level input is not something
i use, ever.
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Old 13th April 2010, 03:17 PM   #4
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need help here !
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Old 13th April 2010, 09:06 PM   #5
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I now replaced the caps back to its original values. 100uf instead of 200uf. (3of them).

The problem is still there, low gain if i connect the sub to a pre-out on the
stereo, same result with both my nad and my onkyo and volume at max on both sub and amp.

If i connect the sub to an cd player directly i get very good gain (volume) with only the knob on the sub turned 1/3.

This problem ocured after i accidently shorted the speakerwires when the amp was still switched on (no input). Dont really know why both outputs shorted when there was no output.

What could have been damaged when i shorted the speakerwires?

I replaced the output transistors with same modell, IRF530N and IRF9530N.
They should match very good or is it often transistors dont match? :S

Someone who have the schematic for the Infinity BU-1 230volt version??
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Old 14th April 2010, 03:28 PM   #6
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can someone help me understand
I think the output (gain) characteristics are different comparing this two mosfets.

Brand: IR Modell: IRF530N
https://www1.elfa.se/data1/wwwroot/a...s/07114929.pdf

Brand: Vishay Modell: IRF530PbF
http://www.vishay.com/docs/91019/91019.pdf

The problem right now is to small gain att low volumes, dont know if its the transistors or something else causing this. Thinking of swaping IR to Vishay to get more gain, but its hard to understand the diagrams on the datasheets.

Someone?
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