Noise Issues

[mrskinny]

When I got my Kit5 going,the first thing that I noticed was the loud hiss through the speakers- a LOT of hiss. Initially I had used the Rin/Rfeedback combination of 22K/47K. I changed this to the 22K/22K combination which reduced the hiss considerably,but,having efficient speakers,I still find the amount of hiss objectionable. I could reduce the gain again, but I'm planning to try different (less efficient) speakers with the amp, and I might end up with too little gain. Any ideas? I also noticed a high pitched noise through the speakers. This is at a low level- I have to put my ear to the speaker to hear it, and it stays at the same level regardless of the volume setting. I tried using a larger value capacitor for C34 (a 470uf), but this didn't do anything. I'd assume that this noise shouldn't be there,and don't know what would be causing it.Suggestions?

[retaeht]

Could the noise be power supply related ?

[mrskinny]

I'm not sure if this would have anything to do with the power supply, but, who knows? One of the suggestions made in the assembly instructions was to try connecting the heatsink directly to the chip,then connect it to ground. I tried this today, but found that this increased the high pitched noise quite a bit, but didn't do anything with the hiss. I'm wondering if this has something to do with the grounding scheme of the amp, or radiated EMI from the chip?

[Jan]

Check the soldering of the 0.1 uF caps, and especially C48 and pin 27 of the Tripath chip. This is the Biascaop which is part of the analogue section of the chip and poor connection is a shure way of getting hizzzzzz. Also C49 and pin 16 on the chip which are for the 5V.

[mrskinny]

I tried a different pair of speakers with the amp- a pair of two-way "bookshelf" speakers. These are quite a bit less efficient than my normal mains (90db vs 98db) and are likely closer to what would be "typically" used with these amps. As I'd expected, the hiss from the speakers is much less noticeable, and the high frequency noise is not heard- this is probably being filtered out by the inductors in the speaker crossovers. I think that retaeht has a valid point about the power supply though- with the bookshelf speakers I still noticed some hum from the power supply. I know that this has something to do with these particular transformers, as I used them previously in another project with the same result. However, due to the way that this power supply is configured, raw AC from the power transformer secondaries is, quite literally, being fed into the ground plane due to it's "balanced" configuration. I'm completely new to this type of amplifier, so I'm not really sure what to expect- perhaps this is normal- I don't know. I have been using "Gainclone" type amps up until now, and I've gotten used their almost dead silent operation.

[Jan]

This amp should also be almost dead silent. However, the switching nature make them a bit sensitive to cable arrangements etc. There are some poits, possibly apart from your transformer which I do ot know anything about;
- Use shielded cable for signals
- Make sure they lead away from the board, not making extra loops and windings in the box
- If you have a sensitive preamp, put it in a separate box and/or put the AMP5 in a shielded (metal) eclosure.
- If the noise is a faint high pitch beeeep, around 1.8 - 2 kHz, it can originate from the onboard SMPS for the gate voltage. The axial choke L4 seems to be the source. It is the part number recomended by Tripath but changing it to a shielded choke may help. I have some and can send one if you want to try.

[Jan]

This amp should also be almost dead silent. However, the switching nature make them a bit sensitive to cable arrangements etc. There are some poits, possibly apart from your transformer which I do ot know anything about;
- Use shielded cable for signals
- Make sure they lead away from the board, not making extra loops and windings in the box
- If you have a sensitive preamp, put it in a separate box and/or put the AMP5 in a shielded (metal) eclosure.
- If the noise is a faint high pitch beeeep, around 1.8 - 2 kHz, it can originate from the onboard SMPS for the gate voltage. The axial choke L4 seems to be the source. It is the part number recomended by Tripath but changing it to a shielded choke may help. I have some and can send one if you want to try.

[mrskinny]

The amount of hiss is related to the gain setting. I tried changing the two feedback resistors from the supplied 8.2K to 10K. This cut the amount of hiss quite a bit, but oddly kinda ruined the sound of the amp. Nothing that I've tried yet has changed the the high-pitched noise though. If you can send one of the shielded inductors, I would like to try it.

[mrskinny]

The amount of hiss is related to the gain setting. I tried changing the two feedback resistors from the supplied 8.2K to 10K. This cut the amount of hiss quite a bit, but oddly kinda ruined the sound of the amp. Nothing that I've tried yet has changed the the high-pitched noise though. If you can send one of the shielded inductors, I would like to try it.