tda7560 input problem

[jf3160]

hello, im new here but i have learned alot from this website. i just built my first amplifier a tda 7560, it works to a certain extent, but I'm having some problems with the inputs. If I plug anything in the speaker come just bottoms out, and gets hot, but if I just barely put the tip of the plug in it works fine. Also if I touch the heat sink the sound is all scratchy, so that might have something to do with it.
I'm using regular chrome plated rca jacks. And the input is from an android tablet with a split rca headphone cable. Thanks in advance for anyone who can give me any advice.

[wintermute]

It sounds like you have DC on the output of the android, and no coupling capacitor on the amplifier. Is there a capacitor in series with the input to the amp? A series cap on the input will block DC.

You should never hook up your speakers until you have checked for DC on the output with a multimeter or risk blowing them up!!

Tony.

[!]

Post a schematic of the amp circuit.

Measure DC offset coming from the audio source with it playing but not plugged into the amp. I am expecting that the source does not have significant DC offset, that it was trimmed or there is an output coupling cap in the source.

Is your case metal and grounded? If so, you've created a ground loop if the RCA jacks aren't insulated from it. Is tda7560 electrically insulated from the heatsink? If so ignore the warning but it is generally good to avoid touching chipamp heatsinks, some are at negative power rail voltage without insulation from the chip 'sink tab... though in this case the particular chip has lower rail voltages than some.

[jf3160]

thank you Tony, I am using .1 uf capacitors the only problem is that I'm stupid and hooked them up wrong. I'm not worried about the speakers, I was just using them to test, they're nothing special, just something I have laying around. Thank you for your quick response.

[jf3160]

Thank you the heatsink is not isolated, but I just won't touch it again. I don't have a case yet. I'm just in the testing stage right now. When I get to it three case will be made out of plastic

[jf3160]

Tony, thank you for your help. The input capacitors were wired in parallel. connecting them correctly fixed the problem. The amplifier sounds great now.

[wintermute]

excellent, glad it was simple

Make sure you take heed of !'s warning of the isolation from the heatsink. Mica washers and heatsink compound, or other thermal pads should be all you need. It may be fine now, but once you have it in a chassis, you should have the chassis earthed (mains safety earth) including the heatsink. If it is not isolated you may blow the chip, your PS, or both!

Tony.

[!]

What I opted to do once was let the heatsink be at -V and isolate it from the chassis instead with polycarbonate mounting brackets, but that build had it completely internal to the amp instead of outside.

Never thought about the importance of grounding a heatsink, is that a safety precaution in case something odd happens?

[wintermute]

Yes if the heatsink is exposed to the outside world, if a fault develops where mains comes in contact with the heatsink and it is not grounded to safety earth then there is an electrocution hazard. This may seem unlikely, but its within the realm of possibility. Any mains powered diy project should have all exposed metal components connected to safefty earth.

The other consideration is that if it is integral to the chassis (eg side or top) then provided your entire chassis is metal it will help with RF shielding (think faraday cage)

Tony.

[jf3160]

thank you all this is going into a car, metal case is out of the question, art least for right now. I will definitely try to find Mica washers though. The heatsink will be internal, vented at the top. Any idea where I can find Mica washers. Radio shack or hardware store maybe?