Weird issue TDA7377 amplifier

[Sidecar Bob]

I bought one of these TDA7377 stereo amps on eBay a couple of years ago, assembled and (theoretically) ready to use but during testing I noticed some strange behaviour:
- If I connect its input to the computer's headphone jack before I power it up (there is no power switch) it works perfectly.
- If I power it first and then connect it to the computer there is no sound.
- If I power it up and touch a wet finger to the input there is no hum
- If I connect it to the computer, power it up and then disconnect it from the computer and touch an input I hear the loud hum one would expect.
- I wondered if it needed an impedance/resistance across the input in order to turn on so I tacked a couple of 1KΩresistors across the inputs but it made no difference.

I am completely at a loss to understand why it is acting like this.


The board says "HI-FI STEREO 40W+40W AMP Dian Xin audio"

The only information I've been able to find is the description from the eBay listing
Amplifier chip: TDA7377
Output power: 40W+40W
Channel Type: 2.0 Channel
Operating voltage: AC-DC 9-15V/20-40W
Frequency response: 20Hz-20KHz
Size: 100*110*35mm
weight:100g

 

[Sidecar Bob]

BTW: The title should be "Weird issue TDA7377 amplifier". I would appreciate if a moderator could correct that for me.

 

[Harald Kapp]

Done.

 

[Harald Kapp]

Do you have a circuit diagram? Check pin 7, this should be a standby pin which may put the amplifier in standby if not connected correctly.

 

[ramussons]

What is your power source? Check its ground return, Earthing...... Maybe some (un)wanted loop.

 

[Sidecar Bob]

Harald: Thanks for fixing the title. I can't find a circuit diagram for it but when I can spend enough time in the shop to look up the data sheet I'll try to figure out how pin 7 is connected. You mean the TDA7377 and not the 8 pin chip, right?

ramussons: I hadn't thought about ground. I am using a 12V 3 "brick" power supply that has a grounded power plug. I just ran down to the shop to check and the sleeve (negative) of the 12V plug is connected to ground.
The connection between the computer and the amp is a cable with a 3.5mm stereo plug and 2 RCAs. If I plug it into the computer the sleeves of the RCAs are grounded so its ground connections and the computer's are good and I read zero ohms between the power socket and the audio in jack of the amp.
When I can spend a bit of time trying things I'll have to try grounding the 3.5mm stereo plug's sleeve before plugging the power into the amp and see if it works.

The odd thing about this is that if it is a ground loop it is only turning on when the loop is present.

 

[Audioguru]

They lie about the output power. They use a 14.4V power supply and say 30 fake Watts or 20 severely distorted Watts.
40 real Watts in to 4 ohms is an output of 12.65V RMS into 4 ohms. 12.65V RMS is 17.89V peak and is 35.77V peak-to-peak.
The amplifier channels are bridged so each speaker wire must have 17.89V peak-to-peak. Much more voltage than the battery?

But not when powered from only 12V. The maximum output on each speaker wire will probably be only 10.5V peak-to-peak or bridged 21V peak-to-peak which is an RMS voltage of only 7.43V then the output power is 7.43V squared/4 ohms= 13.8 real Watts per channel.

The ebay people probably have the 0V connections of the inputs connected to the standby pin of the amplifier and a resistor to the positive supply. Then the amplifier is muted without inputs "grounding" the 0V input connectors.

 

[Sidecar Bob]

Yeah, I wasn't expecting anywhere near 40W RMS (= real watts) per channel.
Are you old enough to remember the "Lloyds watt"? Back in the day there was a popular consumer electronics company called Lloyds; Their stuff was well built but their specs were always suspect. Back then there was some debate about whether amplifiers, speakers &c should be rated in RMS watts or peak but Lloyds went beyond that and used peak to peak watts (1W RMS=2.83W P-P). Of course, we electronics students took it farther and disparagingly referred to any suspiciously high power rating as being in as Lloyds watts.
I believe the Chinese audio industry uses something similar and (assuming your calculations and mine are both right) the numbers would bear that out because 2.83 x 13.8 = 39.054 which the sales department would round up to 40W

I'd like to use it with a pair of PolyPlanar P20 speakers in the garage so 13.8W would be more than enough.

 

[Audioguru]

If your speakers are 8 ohms then the power is halfed.

 

[Sidecar Bob]

True, but this is bridged so the output should be in the 15W RMS range which will be fine.

OK, back to troubleshooting:

Testing ground related stuff was easiest so I tried that first. I'll post my findings here even though it proved to be a dead end.
- I tried connecting the shield of the 3.5 plug to the computer's chassis with a clip lead before plugging the power supply in and the amp worked (wet finger test).
- I tried connecting the shield to the ground of the power bar above the bench (same as the PSU is plugged into) before powering the amp and it didn't work.
- I connected the shield of the 3.5 to ground of the receptacle the computer is plugged into before powering the amp and it worked.
This is very strange because my tester indicates that the AC plugs are wired correctly and the resistance between the grounds is negligible.
- I tried a different power supply that is not grounded and the amp did not work whether the 3.5's shield was grounded or not.

The datasheet confirms pin 7 of the TDA3777 is the standby. It is connected to V+ via the recommended 10KΩ resistor but there is no 10μF capacitor as recommended in the schematics in the datasheet (see below).
Hmmm.....
So I tacked on a suitable cap and plugged in the PSU and it works.
Then I tried the PSU that isn't grounded and it still works.
Problem solved. Now to figure out where to connect the cap properly.

Thanks.

 

[Audioguru]

You show the datasheet schematic for stereo plus bridged subwoofer. Then the single-ended stereo outputs produce only 3.7W each into 8 ohms with low distortion when the supply is only 12V.
Also, the single-ended outputs are out-of-phase with each other.

 

[Sidecar Bob]

Oops.Here are the other 2 drawings from the datasheet. Mine is connected like Figure 2.
Anyway, the standby part is the same for all 3 and adding the cap fixed it.

 

[ramussons]

Are'nt we digressing from the problem stated?

 

[bertus]

Hello,

The mentioned 40 Watts will not be reached.
This is from the UTC datasheet:

Also check the voltage at pin 7 of the chip (standby pin), as @Harald Kapp stated in post #4.
See page 6 of the attached datasheet for more info.

Bertus

 

[Harald Kapp]

Are'nt we digressing from the problem stated?

I think not. Since adding the missing cap fixed the issue, it's good now, isn't it?

Or do you refer to the discussion about power output? In that case I agree.

 

[Sidecar Bob]

Now that the amp works I'm happy to talk about whatever anyone wants to talk about. You should see some of the digressions we have on the motorcycle forums sometimes; At least this is still related to the amp

And as for the output power, we should all be aware that sales departments everywhere want to publish the numbers that make the product look as good as possible. This was true when I was designing speakers in Canada in the '70s & '80s and it is true in China now. I only paid $4.79 CAD with free shipping in Jan 2020 so I didn't expect that the power rating would be in real RMS watts.
I only mentioned 40W because it is printed on the board and might be useful if anyone wanted to try to look for it online.

What amazes me is how much more they are selling for now. The cheapest one I could find on eBay or Aliexpress today is $14.12 CAD including shipping
https://www.ebay.ca/itm/224457748819?hash=item3442b97153:g:8zsAAOSwMhlgmi7l

And if the pics are accurate the cap is still missing

 

[Sidecar Bob]

Here's something related to the amp and its power output we can discuss:
2 years ago I wasn't thinking about that big heatsink and how many watts it might be dissipating so I planned to mount this inside a closed plastic box (I was going to use it in a room where the music would never be terribly loud).
Now I'm planning to use it in the garage (larger space and I do sometimes turn up the volume so I can hear it over power tools) so I'm thinking about that heatsink dissipating as much as 30W and wondering if it would be a wise idea to add some vent holes in one end of the box and a 40mm fan in the other.
What do you guys think?

 

[bertus]

Hello,

You might be interested in the following articles on heatsinks:
https://sound-au.com/articles/heatsink-amp.htm
https://sound-au.com/heatsinks.htm
https://sound-au.com/articles/diy-heatsink.htm

Bertus

 

[Audioguru]

The TDA7377 amplifier IC was designed a number of years ago by a company in Taiwan. I do not know if ST Micro bought the design.
Modern European and American audio amplifier ICs are class-D for high efficiency and fairly low heating.

 

[Sidecar Bob]

So do you think I should add a fan or maybe just some vents?