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M-Audio AV 40

The right speaker of M-Audio Studiophile AV 40 (Active Audio 4 inch) speaker set that I have had started to go quite a while ago.  In other words, only the left speaker (which houses most of the electronics in the set) was making sound.  My online research showed that a lot of people have been having problems with these speakers.  Even the Amazon customer reviews for AV40 were clear of how these speakers can be problematic.  Considering that most people paid $150+ for these speakers, I thought it might be a good idea to fix my own speakers and try to help others as well.

Typical Problems with M-Audio AV40 speakers:

*** My conclusion is that the power supply circuitry's design is the cause of failure of these speakers in most cases.  More specifically, the power supply filter circuitry on the main board in the right speaker heats up too much!  This causes the capacitors to start to fail that expose the issue as units that don't turn on, have noisy sound since the capacitors have lost their useful life to filter out power noise, or no sound in one speaker since one of the internal connectors for audio signal is close to the overheating power supply ***

Type 1 Problem: Both speakers just die after a while and do not turn on:

The most likely cause besides the obvious (bad power cord, etc): the power supply capacitors may have gone bad and would need to be replaced.  You can physically diagnose this in most cases by opening the left speaker and looking at the internal power supply circuitry and looking for bodges in them.  There are four capacitor for the power supply and they are larger than the others toward the corner of the board near the 3-wire plastic connector header that come from the transformer inside the box.  You may also see some signs of over heating around the capacitors in forms of darkened circuit board, etc.  The cause of this problem is that by design the two large blue power resistors that are standing off of the board next to the capacitors along with the zener diodes that are next to them tend to pass quite a bit of current and are supposed to heat up.  However, besides the ambient air-cooling in the enclosure, there isn't much to cool them down.  The electrolytic capacitors loose significant of their useful life when operated in overheated conditions, which results in them breaking down in these speakers sooner than expected for a consumer electronic. You can read the value and voltage reading of the capacitor and replace them to fix the most power supply issues. Capacitors are about $1-$2 at RadioShack or even online at Amazon (see 220uf 25V capacitors on Amazon)

If the capacitors look to be fine, you may want to use a multi-meter, a DMM, or a voltmeter to check the output of the power supplies.  The following are where you would find these outputs and the voltages that I measured (green markings).

Type 2 Problem: There is a humming noise coming from the speakers:

This can be caused most likely because of two reasons.

  1. One some of the hot glue inside might have got cracked which is causing the parts near the transformers vibrate due to the electromagnetic signals. 
  2. The issue could be because of the power supply capacitor failure or near failure, which is resulting in the input power not getting filtered and the ripples from the 50/60Hz AC is leaking in the amplifier circuits. Follow the instructions under the "Both speakers just die after a while and do not turn on" section to see how you can diagnose and fix this.

Type 3 Problem: There is not any or there is only bass in one speaker:

This is very unlikely; but if it occurs, it is because there is an issue with the crossover circuit in that speaker. Most likely you can identify the problem by physical inspection of the circuit board in that speaker. The crossover circuit consists of two completely passive filters (one low pass and one high pass) as following.

Type 4 Problem: There is only sound coming from one speaker (typically the left one):

This is more tricky to diagnose.  The first time I had this issue it got resolved by opening the board and doing a deep inspection; and magically everything started to work again.  I suspect that the cause is a lose internal connector/header contact.  The second time that it occurred, I realized the faulty connector is one of the ones near the power supply.  See the above photo.

More on the troubleshooting that I did: I did not remove any of the internal header contacts or take any of the internal wiring apart since I was trying to maintain the original integrity and hot glues that were holding things together.  The way the circuitry in the left speaker works is that the signal from the audio inputs get pre-amplified on the main board.  Then they go to a header and with a set of the internal wires off of that board (may be to the volume nub in that speaker) and then back to the main board.  A similar thing happens at the inputs for the final Power Audio Amplifier.  If the wires/connectors are not the issue, I drafted the circuitry from what I saw as following.  Since the right and left channel circuitry are symmetrical, you can use an oscilloscope or a DMM (power/AC mode) to see where in one channel the signal is getting lost.  Note that for this diagnostic, you will need to have the circuit board powered up (WARNING: YOU NEED TO KNOW WHAT YOU ARE DOING SINCE IF YOU TOUCH THE A 110v/220v BARE WIRE, YOU CAN DIE).

The input amp circuit is the following based on what I observed on the circuit board:

The circuitry around the Output AMP is the following based on what I observed on the board.  The numbers in the squares are the pin numbers on TDA 7265 amplifier:

As a reference the following crossover circuit is what I observed in these speaker.  Each speaker has its own, and this circuitry is the only circuitry in the right speaker.  Note that it is pretty passive and it has a very low chances of failure due to its passive nature.  The crossover circuit is basically two passive filters, the low pass filter is for WF and the high pass is for TR, which results in the nice dynamic range of these speakers.  The circuitry for the left speaker is integrated in the main board, while the right speaker has its own circuitry in it.

Parts and Data Sheets:

Power Amp - TDA7265 , Data sheet: http://datasheet.octopart.com/TDA7265-STMicroelectronics-datasheet-14136201.pdf

Input Amp - 4558, Data sheet: http://www.ti.com/lit/ds/symlink/rc4558.pdf

OpAmp - TL074, Data sheet: http://www.ti.com/lit/ds/symlink/tl074.pdf

Block diagram for AV40, see appendix B of the user guide: https://cvp.com/pdf/av40_user_guide.pdf

Other Reference Links:

AV30 schematics - note this is somewhat different than AV40, but there are also some common parts: http://music-electronics-forum.com/t33027/#post302749


Was not applicable since my AV40 did not have the fuses that the user mentioned, but interesting: http://forum.allaboutcircuits.com/threads/advice-regarding-transformer-for-powered-speakers-studophile-av40.56757/

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