I’m needing some help, my first time using the TDA7294. I’ve installed them into a Yamaha R-300 which originally used a STK2030 output and STK3042II voltage amplifier. The amplifier has +/- 34.5v rails. The boards were built using all of the components I sourced, so every piece is an authentic device, not a kit from China.
On the first power up the protection relay clicked and I measured less than a mV of offset on each channel, woohoo! Then I sadly saw I had a very slight amount of hum, I’m measuring about 430mV AC on the outputs, so about 0.02 watts worth into an 8 ohm load.
Also, do the TDA7294 run hot when sitting idle? Or are they getting hot because of the hum? Most mosfet amps I’ve come across run hot, so I was wondering if it was the same since it uses mosfet outputs. Not sure how warm I should let the heatsink get, but after about 3-4 minutes I put a fan on it which kept it cool. Maybe not enough heatsink for two chips. The original amp was rated 30w x 2 and I’m getting a clean 50 watts, we’ll clean minus the hum.
All of the grounds on the amp connect on the same ground trace; those being ground for the power supply, negative speaker output, and ground for the input. Even though they all connect together with the traces, I also ran wires from their holes to each other on the board and all the grounds for both boards met at one wire that went directly to the chassis, right here. You can see the screw right under the heatsink. I went for as short of a run as possible.
The audio input for each board came from where the input pins of the old STK3042 would have been. The wire going from that position on the board to the TDA7294 board is a shielded coax cable, shielding being connected to the ground of the TDA amp board.
Here you can see the output of the amp, the trace is fuzzy since it has that 430 mV of hum riding on it.
Any suggestions on why I have that hum? Any suggestions as to how to get rid of it?
Secondly, should the amp chips be getting hot just sitting idle? If so then I won’t worry about it. If the hum is the cause of the excessive heat that would be cool.
Dan
On the first power up the protection relay clicked and I measured less than a mV of offset on each channel, woohoo! Then I sadly saw I had a very slight amount of hum, I’m measuring about 430mV AC on the outputs, so about 0.02 watts worth into an 8 ohm load.
Also, do the TDA7294 run hot when sitting idle? Or are they getting hot because of the hum? Most mosfet amps I’ve come across run hot, so I was wondering if it was the same since it uses mosfet outputs. Not sure how warm I should let the heatsink get, but after about 3-4 minutes I put a fan on it which kept it cool. Maybe not enough heatsink for two chips. The original amp was rated 30w x 2 and I’m getting a clean 50 watts, we’ll clean minus the hum.
All of the grounds on the amp connect on the same ground trace; those being ground for the power supply, negative speaker output, and ground for the input. Even though they all connect together with the traces, I also ran wires from their holes to each other on the board and all the grounds for both boards met at one wire that went directly to the chassis, right here. You can see the screw right under the heatsink. I went for as short of a run as possible.
The audio input for each board came from where the input pins of the old STK3042 would have been. The wire going from that position on the board to the TDA7294 board is a shielded coax cable, shielding being connected to the ground of the TDA amp board.
Here you can see the output of the amp, the trace is fuzzy since it has that 430 mV of hum riding on it.
Any suggestions on why I have that hum? Any suggestions as to how to get rid of it?
Secondly, should the amp chips be getting hot just sitting idle? If so then I won’t worry about it. If the hum is the cause of the excessive heat that would be cool.
Dan
Your amps are oscillating for some reason. I guess you were showing a 1 kHz signal, weren't you? The fuzzyness on the scope never do result from mains hum, but from a very high frequency (much higher than 1 kHz). This may also explain why your chips are getting that hot with no drive.
Best regards!
Best regards!
Thank you for the suggestion, I mentioned in my post above that there is less than 1 mV of offset on each channel.
Thank you, I was wondering if it was an oscillation, here is an image of it close up. And yes, the output signal I was showing was 1khz. I need to figure out how to get this scope to display the frequency it is seeing.
Would I be correct in thinking this is a little over 1 MHz, like 1.1 MHz? I’ll probe the power supply and the input, but I’m guessing it’s originating at the chip. Any thoughts as to what would cause this? I’ll post pics of the boards I used later.
The problem is genuine STKs are hard to find now and probably much more expensive than a 7294.
@saabracer23 temporarily connect a 47pf capacitor across the feedback resistor and see if that helps.
@saabracer23 temporarily connect a 47pf capacitor across the feedback resistor and see if that helps.
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Here is the schematic of the R-300. I’m taking the signal from pin 1 and pin 15 of the STK-3042 and the output is going to directly to the junction of the two 0.47 ohm emitter resistors. Emitter resistors have been removed.
Not sure they’d make them sound like dog poo as the 7294 is very highly regarded in its own right. The STK packages are ancient by now and good luck getting an authentic package. You’re rolling the dice.
Thank you, I will try that! Sorry, which one is the feedback resistor?
I did some testing and I found that if I disconnected the input from the left channel that I got perfectly clean output from the right channel.
No oscillating at all. As soon as I hook up the input wire for the left channel both chips go into oscillation, even if I wire the two inputs to the same signal source (essentially making it a two channel mono amp).
Dan
The resistor from output to inverting input.
Add a small ceramic across that resistor.
& yes, implementation is the key. When I successfully made my first tda7294 my impression was like OMG !
Very powerful performance, sound was also very nice. Although you need correct speakers & little tweaking to make it sound right.
Add a small ceramic across that resistor.
& yes, implementation is the key. When I successfully made my first tda7294 my impression was like OMG !
Very powerful performance, sound was also very nice. Although you need correct speakers & little tweaking to make it sound right.
As far as I can see the only components missing from the yamaha pcb are IC302/303 and resistors R320/330, everything else is still on the pcb. If you already have input reference and feedback resistors on the TDA7294 board, you have a problem...they will be in parallel with the input reference resistors R321/R322 and feedback network (R333//C331+C333, R323-C319) and the same for the other channel (R334//C332-C334, R324-C320).
Also, you have a problem with grounding and PS lines, but you will deal with that later.
Also, you have a problem with grounding and PS lines, but you will deal with that later.
Got side tracked and wanted to come back to this. So I placed a 47 pF cap across the 22k resistor you suggested.
The hum and oscillating is gone. The heatsink no longer gets warm. It plays audio, but there seems to be a fluttering to the sound. Best way I can describe it.
Here is the waveform, doesn’t look great.
Dan
Thank you for your input. Are you suggesting I remove those resistors and caps from the Yamaha board?
Dan