All,
I’m having trouble diagnosing a fault here and would welcome some advice or a schematic, even of a similar amp. I have checked quite a few things.
Type: Velodyne SPL-R 1000 Active Subwoofer, circa 2005.
Symptom: Extreme distortion and low output level
I cannot find a service manual or schematic (asked Velodyne, no answer). I doubt a service manual even exists because the amp clearly was not designed to be taken apart! It is glued all over the place and needed components to be removed in order to open the metal shield. I have managed to set it up such that I can power-up the amp with a sig-gen as audio input and probe it with my scopemeter & insulated probes (the power amp is live and high voltage – no isolation).
It has a pre-amp which includes a DSP chip. This is powered from a small linear PSU of +/-15V or so. The preamp works OK. The remote control functions work and the signal output from the pre-amp is undistorted. The amp PSU is rectified 240V mains into a bank of 6 x 470uF electrolytics, setup as 2 series groups (I refer to the centre of this as a “0V” reference). The voltage is stable at 340VDC as expected.
The amp itself is a Class-D Half-Bridge with a pair of IFRP350A MOSFETs which feed a large toroid inductor, with the speaker connected from this to “0V”. These are driven by an IR2113S chip. There is a further pair of IFRP350A which are connected between the Bridge output and 0V and appear to be driven by two IR2101s chips. I cannot quite workout the schematic of this section; it seems rather unusual - it does not make a standard full bridge for instance.
The PWM is created by a circuit containing an HC11 and HC27 triple AND/NOR gates and HC14 schmitt trigger. It doesn’t seem to the “standard” triangle wave generator and comparator. Aside from these there is just one opamp (TL061). This appears to be setup as an integrating amplifier (capacitor in -ve feedback loop) which I believe is used to integrate the PWM signal to give -ve feedback overall. There is a negative feedback connection which goes back to the bridge output through a decoupling cap and also a 1M resistor which links this into the depths of the logic circuit.
The TL061 is supplied by its own makeshift PSU consisting of 2 resistors and 18V zeners giving a +/-18V supply centered on 0V. The HC logic chips have a 5V supply which is referenced from the -ve PSU rail (not the 0V centre). There are 2 x 78L15 regulators which I have not worked out quite how they fit in..
Here are my observations:- The main power, logic supply and op amp supply are all good and stable
- The whole amplifier is unstable. Under quiescent condition with no input it is NOT oscillating at a stable PWM carrier frequency. I am expecting to see PWM oscillation at a few hundred kHz.
- It sporadically bursts into oscillation, each time for only a fraction of a second. The frequency of this is not stable, some bursts are 500kHz. The bursts will happen about every second or so, but randomly.
- The MOSFETs gate drive and the inputs HIN, LIN to the IR2113 are all consistent so none of these parts appear to be malfunctioning.
- When I look at the TL061 the problem is apparent. The opamp is going into saturation. If I apply a low frequency audio signal, the output of the TL061 is clipped to max voltage. Without an input it typically drifts to hit the rails but the sporadic oscillation will bring it back. This is consistent with not seeing a stable PWM signal on the input or output (gate drive) of the IR2113.
- The TL061 is clearly going into saturation due to the feedback signal from the power amp.
I can attach a few sketches of the circuit that I have traced out. It is the same amp as this chap shows:
I have not replaced the caps he mentions, but they all read at least 33uF in circuit. Perhaps I should just try that. One did seem to have a bit of residue on the top. Unfortunately he doesn’t even say what problem he thinks this fixes.
So, I’m wondering if I now need to try to trace the circuit for the logic section as well as the input opamp. It is very hard going as it is tricky to access the components and many tracks are obscured.
I’m also wondering if it will tell me anything if I break the -ve feedback loop from the power amp back to the input integrating amp? On the other hand I’m also wondering if the fault it because this -ve feedback isn’t working properly.
Overall, I don’t understand how the PWM oscillation is meant to work in this circuit, and I don’t understand what the 2nd pair of MOSFETs which are not in a bridge are doing.
Thanks for reading!