Hi All,
After reading about these little amps they just looked like too much fun. Got the first one a few days ago. Not bad in stock form but like everyone has said, lots of room for improvement.
The amp is now dissasembled for inspection and circuit analysis. Like pointed out in many places the bass response is lacking. The main culprit causing the weak bass is the input capacitors that are too small. In the Tripath datasheet they recommend 2.2uf. The parts in the Sonic Impact measure .33uf.
Here is a schematic of the input circuit the Sonic Impact.
As this input circuit is driving the inverting input of an opamp the input impedance is low. The impedance of an inverting opamp circuit is the value of the input resistor, in this case R1 and R2 at 20K. These are in parallel with R01 and R02 bringing the input impedance down to 6.67K.
Add the 50K pot and the impedance drops down to 5.8K. This is getting kind of low for some sources. Not a problem for the portable CD players that it is designed for as they are setup to drive headphones directly.
The path I think I'm going to take is to try and see how good the performance can be while keeping the constraints of the stock case.
1. Input caps must go. Just for grins they will be replaced with Black Gate BGN 4.7uf caps. The ceramic caps will be removed and the caps soldered to the SMT pads. I don't think the vias are large enough to get the cap leads into.
2. R02 and R02 will be removed to ease the load on the source components.
3. The power supply cap will be replaced with a larger low ESR cap (choice of what kind has not been made yet). It looks like if the 4 inductors are raised up the leads can be bent sideways enough to get a larger cap in the space available.
4. Board surgery- I don't like the way the 12 volt supply is routed. The measurements that panomaniac has made shows that the supply is very noisy with high frequency switching noise. This noisy supply is routed all the way around the board including up to the pot/switch and directly under the input circuitry. My plan is to add a small SPST switch to the rear panel to handle the power switching. The 12 volt supply will then be fed into the circuit at the power supply cap. The run that loops around the board will be cut and soldered to ground instead. This will keep the supply noise away from the input circuitry and provide a touch more ground plane.
5. more board surgery. A couple of different ways to go here. The input signal runs along the edge of the board and is not shielded. Option 1 would be to cut the runs on the board and run coax from the inputs to the pot then use more coax back to the board. The other choice would be to wrap some grounded copper tape around the edge of the board where the input signals run providing some shielding and use coax from the existing connectors to the pot.
6. More capacitance to help things sound good while running from the AA batteries. There is just enough room to add a pair of 16x35mm capacitors, one beside each battery compartment. I’ve found caps up to 6800uf @16 volts in this size. Don’t know if maximum size or lower ESR is more important here as the cap on the main board will be a low ESR type.
7. The value of R4 and R5 will be reduced to 20K. This will reduce the gain of the amplifier. In my system the amps are too sensitive. The transformer volume controll is almost at minimum to get the level I like to listen at.
All these ideas will have to wait until the next units get here. Just ordered a pair of amps from Partsexpress. Due date is Mar. 1st.
Gary
