# Amp mod / repair questions



## TechMasterJoe (Sep 5, 2013)

hey AmpGeeks got a simple question

i have a ton of IRFB260NPBF left over from a inverter project, i did a ways back you think they better then IRF640P that my Crap Tactic Belva BB5000D is using 

just to help i will post some Gut Shots i was shocked by the guts of this 300$ amp



uses real name brand parts
Right side is Power Supply Left is output Drivers
Output Drivers Are 
IRF640 N type Mosfets total of 16

Power supply is STP75NF75FP total of 20 +
MURF1620CT RECTIFIERS 8amps per phase 16amps per chip 16 total

this amp has a input problem i get raw DC when remote boost knob 
is hooked up and nothing when disconnected settings on amp do nothing 
get about 1v raw DC on scope even with 3V clean 30~100 Sweep going in
and about 122~126V (on 8ohm test load) of raw power when gain knob is added this is Over the Specs for the amp every mosfet and so on test good bass knob works fine on other amp ran out of ideas 
had a pair of them beating the tards out of a pair of HDC4-15's
as you can tell this amp is clean nothing burnt. all looks very good.
any help met with open hands can't afford a new amp just found out i have neck cancer Blahh


----------



## ATOMICTECH62 (Jan 24, 2009)

When it comes to swapping mosfets out in the audio stage it almost never works.Its not as easy as a transistor where you can sub one with similar current,voltage,wattage and gain.
Mosfets are very finicky.In the power supply it is easy to upgrade from something like a IRFZ44 to a IRF3205 by simply changing the gate resistors to overcome the static gate charge or Miller effect because they are usually only driven by a square at lower frequencies of about 30 to 80khz.
In the audio section of a class d amp the mosfets are being modulated at a very high frequency and they biased so close to each other that there is very little room for error.
All techs at sometime have tried substituting mosfets in the output section and it hardly ever works.In some amps the IRF640 can be upgraded with the FB31N20D but they usually run hot.Replacing the IRF640 with the 640N will not even work.
Most amps that use the N-channel IRF640 will be paired up with the P-channel IRF9640 in a push pull configuration that makes it even harder to sub out the 640.

The bass knob circuit for most amplifiers use a JRC13600 transconductance opamp that sends a DC voltage to the knob and back to the amp.The audio signal is controlled by this opamp and goes in and back out through a built in buffer stage.So this is where DC could be bleeding through.I dont have a similar model amp here at the moment so I cant be sure if yours uses the same IC but if not follow the traces from the jack and there should be a controller IC.
However.
It is not uncommon for this style of amp to have a few volts DC on the output terminals.


----------



## TechMasterJoe (Sep 5, 2013)

ATOMICTECH62 said:


> When it comes to swapping mosfets out in the audio stage it almost never works.Its not as easy as a transistor where you can sub one with similar current,voltage,wattage and gain.
> Mosfets are very finicky.In the power supply it is easy to upgrade from something like a IRFZ44 to a IRF3205 by simply changing the gate resistors to overcome the static gate charge or Miller effect because they are usually only driven by a square at lower frequencies of about 30 to 80khz.
> In the audio section of a class d amp the mosfets are being modulated at a very high frequency and they biased so close to each other that there is very little room for error.
> All techs at sometime have tried substituting mosfets in the output section and it hardly ever works.In some amps the IRF640 can be upgraded with the FB31N20D but they usually run hot.Replacing the IRF640 with the 640N will not even work.
> ...




TL074C has 3 of them 2 by filter 1 by bass control. I have reflow eq you think replacing them might fix it ? they are cheap I might even have some have to see when I'm home from work.

it has no P-ch mosfet pure N type setup so I hope the control never folds back LOL or that's a big pop. I can see why output stage is picky with speed and miller impact. thanks for the reply.


Note: i'm not used to this type of amp and both amps have Sub GND tied to Power Ground one works just fine so i'm guessing this is normal on this setup. working amp clamped at 2867.3watts 1%THD on 1 ohm at 4v input on bench at 1ohm test load rise to 1.097ohm with heat 14.087v at the amp held at 37c case temp after 1hour honestly the power for the money is impressive if you get a good one. Might mod it and replace thin bus bars with good cable and beef up traces board is a tad dry on solder might add more rectifier's I typo before it has 8 and they are the hottest thing when bench testing looked at amp with my Thermal cam avg 18c higher them mosfets
never really worked on amps just tested them want to learn more and start fixing them and this one has me stumped..

I custom built my amp test bench with touch screen controls works like a champ power supply is 2X 150 amp 14.4V (240v in) units + 8 14V 100Ah AGM battery's using 1/2X4" copper bus bars
the large 16ohm resistors are bolt to a water cooled plate running a Honda alu rad+ 350gph pump far more then needed to cool this thing peek testing load is 0.5 ohm at 15Kw for small burst anything under 5kw I can test for hours if I start my geny to help charge the batts (gen is a 24hp Honda twin with 3 275amp mechman alts with volt control + 5kw power inverter so I can use it as a real geny)

if anyone wants to use or see it and lives around orange California. pm me a reason to trust you to see my house and I will invite you over other wise we can meet hand me your amp and I will do a video test at all rated ohm loads if you want to test below ratings I need a signed waver incase it eats crap on test bench unless it's a good amp like we know they can take it you know witch ones..


----------



## TechMasterJoe (Sep 5, 2013)

some photos of the preamp section 





power supply driver


----------

