hiraga class a 20w. hum

[moochinazz]

hey. finally got to work my hiraga class a amp. www.tcaas.btinternet.co.uk/hiraga3fig1.gif. its powered by rod elliots power supply +-24v. sound.westhost.com/p89-f1.gif. the problem is that the amp hum, the hum changes depending on engine revs, electric windows etc. where should i look for a solutution to my problem. thanks

[KatieandDad]

If you are using the SMPS Rod Elliot Car PSU the "Hum" is almost certainly not supply related.

Try rerouting your cables, I would suggest that you are picking up hum on the cabling.

[moochinazz]

maybe you have some guidelines to rerouting cables? here is the photo to make things more clear ImageShack® - Online Photo and Video Hosting

[KatieandDad]

Try disconnecting the INPUT phono leads and shorting the INPUTs.

This will tell you wether the problem is coming from outside of the box.

I also note that the amp is not yet in a vehicle.

If you are powering it with a 115-24V adaptor, that could be the source of your hum. Try just using a car battery.

[moochinazz]

the problem is that the hum is actuallly beginning when the amplifier itself is in the car and when the headunit is connected. there is no hum when the haedunit is off. but the problem is not in the headunit. because the car amps that i have installed at the moment do not hum.

[KatieandDad]

Sounds like you have a cabling problem as I originally intimated.

Check the cables carefully and make sure that the audio cables are run down the opposite side of the vehicle to the power cables.

You might also like to think about interconnects that don't use the shield for signal.

ie. Use a twin core screened cable, using the two cores for signal and signal return, connect the shield ONLY at one end to signal return.

[moochinazz]

the audio cables are separated from power cables. seems that the problem is in the amp. more thoughts would be great.

[pergo]

ground loops


Do you have power supply input ground and signal ground (secondary ground) decoupled?!

[moochinazz]

Originally Posted by pergo

ground loops


Do you have power supply input ground and signal ground (secondary ground) decoupled?!

i'ts my first project, could you provide more info on ground decoupling?

[pergo]

12V input is referred to power ground (- pole of the battery) but the signal ground cannot be referred to that.

You have a center tapped transformer, do you?

Your signal ground is the center tap, so the Higara's GND is the center tap of the secondary.

So you've a secondary floating ground. Good. Now you've to connect the signal ground (center tap) and the power ground (12V side) with 1k resistor // 100nF capacitor (in parallel).

With this solution, grounds are decoupled by 1k//100nF and you won't have any ground loop.


This method is use in every modern car amplifier.

See this: