AMP1 HF Sound.

[magpietim]

Hi Jan,

I've finally finished my AMP1 and i'm very pleased with the results except for a HF whistle coming from the speakers...it is only just audible but annoying all the same....it doesn't increase when i turn the volume up and it disappears when i remove the inputs....it seems to be there whichever source i use but disappears the moment i remove the interconnects.
Do you have any idea what the trouble is and how i can remedy it. I was thinking of a small cap and resistor on the inputs to filter it out....any idea on values or another idea.

Cheers,

Tim Collins.

[Jan]

Hi,

I have heard this sometimes. It is very weak and comes and goes so it is hard to diagose. But I think it is a grounding problem. Try connecting a floating sound source like a portrable CD or MP3 player and check if the whistle goes away. If it does, then try placing a small HF inductor choke on the ground connection between your sound source and the amp. Increasing the filter cap on your +5V supply could also help.

Let me know if this helped.

[magpietim]

Hi Jan,

I've tried that by connecting a portable CD player and the sound is still there. If i remove the CD player but leave the interconnects in it disappears. I'm thinking it might be picking up some interferance from the digital circuits.
I will try paying attention to the +5v supply.
Tim.

[Jan]

Tripath recommend using 1000pF from cable connections to ground (casing) just where the cables leave the casing, to remove conducted HF from transmitting out with the cables, but it does not seem to be the problem here. You should should preferably use shielded cable for the +5V like the signal cables. And all of these should be grounded at the J2 jumper only.

[magpietim]

I'll give that a go and let you know how i go on.
Cheers,
Tim.

[magpietim]

I've used shielded cables for my +5v supply but the High frequency whistle is still there.
I've checked my PSU and all seems well, my +5v is grounded at J2 only, i'm at a lose as to how to proceed.
You say in an earlier reply 'Tripath recommend using 1000pF from cable connections to ground (casing) just where the cables leave the casing, to remove conducted HF from transmitting out with the cables'
Do you mean at the RCA inputs or the loudspeaker outputs?
Hope you can help me Jan.
Tim.

[Jan]

Hi, I mean at the RCAs for the signal connection. Hope this helps!

[magpietim]

Well placing a screwdriver near L4 changes the tone of the whistle so it's something to do with that choke. I know Jan is investigating it but if anyone has any ideas please post here.
Tim.

[Jan]

The TA2022 chip uses a negative gate drive voltage, which is 10 volts higher than the negative rail. There is a small SMPS in the chip that generates this voltage and delivers about 100 mA. It works at 300 kHz. There is an 100 uH inductor that is part of this SMPS. The inductor supplied with the AMP1 and AMP1-B kits is identical to the one suggested by Tripath and used on their demo boards. However, I suspect this axial choke is involved. Maybe it generates a leakage signal that is picked up by other components. As this effect is usually very weak, and for most builders, seems not audible at all, it is hard to diagnose. I can provoke this sound by "bad" grounding and using unshielded cables in the wrong locations so it should be possible to tell if this solution has an effect. I will test replacing it with a toroid choke and let you know.

[magpietim]

Thanks for the help Jan, finally after finishing all my other jobs that her in doors as got me doing i have re-wired my amp with completely shielded cables and the high frequency hiss as gone....silence is golden at last...and it sounds amazing....all i'm looking for now is time to listen to it!!
Cheers,
Tim.