Distortion Tests - HifimeDIY T2 / LJM L20D / SURE TDA7498

[trevmar]

The L20D has some protection, under and over-voltage.
This is one of the reasons I have been using my modified TDA7498 boards in my 'stereo' here. The integrated chip amps do have more protection for the speakers built-into the chip. Over-current, supply rail levels, load impedance, etc. But the L20D has a cheap, easily replaceable output chip. At least, I think that is what LJM was thinking.

IMO, there is a limit to how accurate the protection on a component board can be, in any case. You almost certainly want to invest in a speaker protection board.

What I ended up with was documented here:
Trevor Marshall - Class D Audio Amplifier Design - TDA7498 Output filters
and here:
Trevor Marshall - Ground Effect Omnidirectional HiFi Loudspeakers

Sorry I took so long to reply to the posts above. I am totally happy with my setup now, and don't read the board very often.

Trevor..

[dgta]

Trevor, in reading the datasheet it seems to me the TDA7498 doesn't have any specific internal mechanism for sensing DC at the output. Is the current limiting what you depend on for speaker protection? Or do you use the 7498 board with separate speaker protection?

While DC at the output would cause massive output current, I'm not sure most speakers can take the 7A current limit of the 7498. Now with 32V supply and 8 ohm speakers, you're probably limited by the dc resistance of the speaker itself, which will probably allow less than 7A. So no benefit from the current limiting.

[trevmar]

Oh Heavens - I remember looking up this scenario once, and deciding it was dealt with in the chip, but I can't remember how. Let's look at the data sheet...

OK, I remember - I decided that the over-temperature protection was really quite important at minimizing problems, and because the amplifier was bridged, with both halves of the bridge in the one chip, the speaker would only be harmed if the TDA7498 "half-failed"

I did test over-temperature by running without a heatsink. It shut down cleanly. Took quite a bit of power to overheat the chip, though, the spkrs were quite loud...

So not a really rigorous protection, but I wanted the amp really small and lightweight, sitting here in my tiny office. I did buy a few spare Peerless speakers, just in case They are an amazing driver... (I can easily get spare SEAS woofers, don't need spares of them).

[dgta]

Originally Posted by trevmar

because the amplifier was bridged, with both halves of the bridge in the one chip, the speaker would only be harmed if the TDA7498 "half-failed"

If an output device in either amp fails and shorts to either rail, you have DC on the speakers. Yes the other amp is still switching, and with 32V B+ and 50% duty cycle it only puts 16VDC across the speaker. That's better than 32V, but still not desirable. Might be OK with some 8R speakers.

The only speaker I ever lost was connected to a well respected piece of Luxman gear. Playing at moderate volume, running cold. One of the final transistors failed short and put the rail voltage on the speaker. Amp had overtemp protection, etc. but no DC sensing.

Yes, disposable speakers make a difference, but not my favorite protection method

[trevmar]

Originally Posted by dgta

Yes, disposable speakers make a difference, but not my favorite protection method

For me, Performance, Size and Weight were most important, roughly in that order. I really didn't have any room for the speaker protector boards (those relays are massive) and so decided to go the expendable speakers route.

I did expect to be blowing up those tiny Peerless, but I haven't had that problem (yet). My ears would give out first, I think. Which is kind of interesting, and surprising...

[dgta]

Thanks Trevor. I am also specifically looking at this type of board mostly for the performance and size, although weight is not important to me. But I will be using it with nice speakers and the external protection boards basically screw up the "size" part of my requirements. Oh well...

[panson_hk]

I also got one SURE TDA7498 board recently. I found that output snubber RC on my board is not in series connected output + and -. Both R and C are connected to + and - (in parallel). When I put 0805 22 R there, the resistors smoked during test.

[panson_hk]

I believe TDA7498 is a self-oscillating class d. The switching frequency decreases to zero when output near clipping as shown in the figure below.

CH1 & 2: output + and - (after output filter); CH 3: inductor current; CH 4: output PWM waveform. Math channel = CH1 - CH2

As can be seen, PWM frequency drops to DC during output clipping.

[panson_hk]

Zero Hz is better term than DC.

[panson_hk]

I am going to test the SURE TDA7498 board in parallel bridge operation. Although data sheet does not mention this operation mode explicitly and availability of TDA7498E, the board layout shown in the data sheet was prepared for this mode. Are TDA7498, TDA7498E and TDA7498MV share the same silicon?