|27th March 2012, 06:48 AM||#1|
Join Date: Mar 2012
A2451 DYI subwoofer blows.
I've just installed my third A2451 amp module into my DYI subwoofer. The two previous modules both exhibited popping and clicking for a few months before they eventually died.
This makes 3 modules in just 2 years. Before you say turn the volume down... The unit has an auto on/off feature that I never use and I only turn on for limited applications and never/ever loud. So given my use I don;t understand why they are burning out.
The chaps at "The Loud Speaker Kit Company" (Perth WA) said they have had a few returned and have identified some resisters (that burn out) that have been upgraded in the replacement models.
Circled in picture attachment. In the original units these resisters were quite small and when I opened up the dead unit they were burnt black. In the second unit they are much larger presumably to handle the heat.
This doesn't seem to have alleviated the problem however. They are not black but the second unit exhibited the exact same symptoms the first module did.
Lets see how long the third one lasts!
Has anyone else had any experience with these amps?
Are there any suggestions!
|15th May 2012, 06:53 AM||#7|
Join Date: May 2012
I've had exactly the same problem. Replaced those resistors, but no luck. Also going to replace the other resistors and those diodes as well..
Anyway, the specs of the amp (available from Altronics) are:
Auto power on/off with signal presence
High and low level inputs
High level outputs
Short circuit output protection
Active crossover variable from 20Hz - 200Hz
Phase reversal switch
Power Output: 180 Max @ 4Ω (0.5% THD)
Frequency Response(Variable): 20Hz - 200Hz
Roll Off Rate: 6dB/Octave
S/N Ratio: >64dB
Auto Turn On Sensitivity: 21mV
Auto Power Off Time: 15-20 Minutes
Input Impedance: (Line) 47k Ω, (Speaker) 200 Ω
|7th June 2012, 10:02 AM||#8|
Join Date: Aug 2011
Location: Maryland USA
Looks familiar, I had a $99 plate amp do the same thing: the board turned black near 2 "large" resistors and stopped working.
I threw it out, which I don't like to do. Seems like they would use larger resistors; i.e. I'm not buying anymore plate amp, not the $99 ones anyway.
|16th February 2013, 07:09 AM||#9|
Join Date: Jan 2003
Not sure if you still have this plate amp, but I might be able to shed some light on what's happening, as I've had the same problem.
The 2 resistors are part of a voltage dropping circuit in conjunction with the two transistors (Q120, Q121) just below the resistors in your picture and the two 12v zener diodes between the resistors and transistors.
This drops a rectified voltage from + - 55v down to + - 12v and feeds the preamp board and what looks like some kind of thermal sensor on the main heatsink.
If your amp is the same components as mine, the transistors are S8550 and S8050. For reasons unknown to me (laziness? incompetence?) they are running them out of spec, with Vce around 43v, datasheet I found said 40v.
My amp was popping and burping about every 2 mins.
When I opened it up, I found that the -12v rail had collapsed to -0.75v, and the S8550 was dead.
I replaced these 2 transistors with BC640 and BC639 (be careful, the first datasheet I found said S8050 was PNP, it is definitely NPN, caused me some grief), and I replaced the 2 resistors with 5 watt types, stood off from the pcb. While I was at it, I replaced the nearest small electrolytic caps as well.
Seems to be working fine now, although the 2 transistors get pretty hot. If they fail again, the next try might be to add in a pre-regulator eg: Pre-Regulator and just use LM7812 and LM7912.
Of course, all these issues could have been avoided if they had just run a more appropriate second winding on the transformer. Starting with + - 55v seems ridiculous when you want to end up with + - 12v. The rails don't appear to power anything else
I guess the answer might be to not buy it in the first place, as it's pretty rubbish. It's cheap, but seems false economy. I won't be getting another, that's for sure. Maybe try a class d from parts express?...
Anyway, hope this might help somewhat. Good luck.
|5th April 2013, 09:14 PM||#10|
Join Date: Jan 2006
Using your info (thanks) I've installed the pre-regulator on the heat sink using the existing tapped holes on the heat sink for the MJE 3055 and MJE 2955 transistors. So far I've been reducing the output of the pre-regulator to plus and minus 20 volts and the transistors on the board run a lot cooler. I've bypassed the on board resistors. I'm thinking of reducing the volts even further by using 15 volt Zeners.
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