Chipamp troubleshooting

[will_raymo2000]

Just put the finishing touches to a 3875 kit from audiosector (Thanks Peter for the super fast delivery!)

I am now experiencing slight sonic issues - The sound is really quite dull at the moment. What I mean by this is there is not much bass and the treble is really weak. Yet the high mids are almost overpowering.

I am currently using some old Sony 3way 6ohm speakers which sounded ok(ish) with my Yamaha AV Reciever. I was not expecting miracles using this new amp but I was expecting better sound than I have currently got!

Any ideas?!


PSU - 225VA 2x18V -> roughly 26V DC with amp connected

[AndrewT]

Hi,
what filter values have you fitted to the input?
What supply smoothing have you fitted between the rectifiers and the chip?
What Zobel or Thiele have you fitted to the output?
Are all the components you have fitted identical to Audiosector's?

[will_raymo2000]

Hi Andrew,

I got the standard kit from audiosector and have got it running stereo from one transformer and one rectifier. All the values of components are as the kit. I also have the feedback resistor soldered onto the chip legs as per Peters instructions.

I have found the sound has opened up a bit (I knew it took a while to burn in and settle down but I didnt realise it would make such a dramatic difference!)

But now the issue is with the dreaded hum/hiss. I am currently using the amp with an iPod as a source and when the ipod is plugged in and turned on the hiss is non existant. However when the ipod is off and turns on an almighty thump is heard from the speakers - i measured about 5/6 volts peak on the speaker outputs.
the problem is also evident when nothing is plugged in to the RCAs.

I have currently Star Grounded the power grounds from PG- to PG- on both boards and the signal/output grounds are in their respective pads on the board.

Many thanks for your help

[kscharf]

Peter's design follows the classic gain clone that does NOT use a capacitor in the Ci position or in the input of the amp. This means that if the ipod has any dc offset there will be a rather good size dc shift in the output of the amp when the ipod is turned on.

A capacitor in the input between the ipod and the amp will help somewhat. The usual value here is 1 - 4uf non polarized.

You should have a resistor from the non inverting input of the LM3875 to ground, I thought Peter's circuit had one.

Also, strange as it may seem, if your power supply is fed by a three conductor power cord with the green (ground pin) wire connected to ground, DISCONNECT IT!. Let your AC input FLOAT and use a pair of .1uf capacitors across the power line to ground provide a ground.
You will find that many high quality audio gear do NOT use a three conductor power cord, they use double insulated two conductor cords.

[AndrewT]

Originally posted by kscharf
if your power supply is fed by a three conductor power cord with the green (ground pin) wire connected to ground, DISCONNECT IT!. Let your AC input FLOAT and use a pair of .1uf capacitors across the power line to ground provide a ground.
You will find that many high quality audio gear do NOT use a three conductor power cord, they use double insulated two conductor cords.

DO NOT try to build a double insulated amplifier.

You MUST permanently connect all exposed conductive parts to Safety Earth.

[peranders]

kscharf, excuse me but all power cords are double insulated.

[will_raymo2000]

Yep I will definitely stick with the earth to chassis arrangement! What I may experiment with is an earth disconnect network though.

I also dont have any suitable capacitors for input at the moment. Can anyone suggest a decent type/model?

Finally - does wiring length affect the hum/hiss? At the moment the amp is built on a sheet of metal with no metal case, could this also be contributing to the degradation of the sound quality?

Many thanks

[kscharf]

NP capacitors intended for speaker cross over use will work well.
You don't need a very large voltage rating however, so a low 'wattage' cross over type will do. Most of these are polyprop's.

As for the unearthed chassis, take a look at the IEC power connectors used on many high end audio gear. They are TWO conductor, NOT three. And true, power cords ARE double insulated, the term also applies to power tools where the inerds of the tool are double insulated from the power line, I guess that doesn't apply to audio chassis.

The reason for the ungrounded power cords are to eliminate ground loops that can cause hum problems.

[pacificblue]

Originally posted by will_raymo2000
What I may experiment with is an earth disconnect network though.

The disconnecting network solves hum issues. Up to now you only have described hiss and thump. That network will not help with either. The datasheet shows more components than the audiosector kit, some of them marked as optional. Chances are that they will help with your issues.

Originally posted by kscharf
NP capacitors intended for speaker cross over use will work well.
You don't need a very large voltage rating however, so a low 'wattage' cross over type will do. Most of these are polyprop's.

You can use normal MKP capacitors. The ones for speaker cross-overs are usually for higher voltage, physically bigger and more expensive.

Originally posted by kscharf
As for the unearthed chassis, take a look at the IEC power connectors used on many high end audio gear.

You will find double insulated equipment more often in low and mid end audio than in high end. Anyhow the difference is that the manufacturers have the knowledge, how to make double insulated equipment and the means to test its safety. The safety test includes letting the equipment drop to the floor from a defined height and see, if the isolation is still intact, even if the equipment is physically broken. Are you sure that is a reasonable approach for DIY?

[will_raymo2000]

I have just found two old speakers in my cupboard which helpfully had 2 2.2uf 50v NP capacitors in them.

Soldered them between the rca and the input of the amp and the thump is still there but less so than before. I think I can live with that now. The hum when the ipod is unplugged is also greatly reduced/disappeared.

The only issues remaining now is a slight hiss.. It sounds like it is pulsing now

slowly getting there!

cheers for the help guys