AMP15 design notes
Design notes for AMP15.
AMP15 was designed with a few different things in mind:
- There was a gap to fill between models the "30V" amps AMP1, AMP5 and AMP10 and the "60V" high power models Truepath and AMP2.
- To push the limits of sound quality
- To use hole-mount components
- Be an integrated solution, all on a single board if possible
One key to D-class amp performance is minimizing the lenght and inductance of power traces and switching traces. Combining short traces with hole mount components is a challenge and calls for a design that may be unconventional. However hole mount components allow solutions that are different from surface mounts, and if designed right, hole-mounted components are not a drawback.
It was early decided to use the Tripath TA3020 chip, which is hole mounted, and withstands quite high voltages. This is a well proven chip and used for our Truepath kits. However, the AMP15 was to be different, optimized for lower power levels, using hole mount components only if possible, and be an integrated solution on one PCB. The TA3020 chip has been around for quite a while. The TA3020 is has advantages for DIY:ers in that it is hole mounted and that it is quite a massive chip, with a large metal surface back, with considerable heat dissipation capacity. (Tripath later developed the TK2150 and TK2350 chipsets with similar data. My guess is that this was done for two reasons; production costs are lower when splitting into two chip, and secondly the addition of a small SMPS for the gate drive current).
For AMP15, a relatively new dual FET, IRFI4019H-117P from IRF, is used. These are very easily driven FETs, optimized for switching audio amplifiers. These FETs are packaged in a half-bridge fashion, in one 5-lead TO-220 package, i.e. two FETs in one package. The low gate charge and short connections of these FETs allow fast and clean power switching. Placing the FETs on the PCB bottom side, allows very short connections to the gate drives of the Tripath chip and at the same time the bottom mounting surface can act as a heat sink. This simplifies construction of the amp as a whole. As mentioned, the length of the switching traces and power traces on the power nodes where to be minimized. For the output filter, there are two inductors in parallel, which allowing placement very close to the FETs. The dual arrangement minimizes inductor resistance, decreases the load on each inductor. Right along the inductors, are the two output node polyprop filter capacitors. In all, a very compact design. The intrinsic diodes of the FETs are ultrafast so external protection clamping is not required, which simplifies the design. The FETs maximum current is limited to 2X9A continuous, compared to the 34A FETs used for Truepath and AMP2, so in comparison, AMP15 is better suited for moderate current loads. This amp is therefore not a heavy duty woofer amp, but rather a full range, very suitable for relatively high impedance speakers of 8 ohms or even 16 ohms. For a very high power system, you can use an AMP15 for the mid-high range, combined with a high current woofer amp. Of the same reasons, AMP15 is not optimized for bridging, even if it is possible. Consider bridging for 16 ohm speakers only.
The AMP15 power supply section is built with a rectifier based on two TO-220 dual series connected ultrafast diodes. Also these are in TO-220 packages on the PCB bottom side. These diodes are a slightly hard to find, but the solution is very handy and high performance. Another key component is the bulk cap for the power supply. Here, two massive 27.000uF low ESR/high ripple caps are used. For mechanical stability, they mount in a 4-hole pattern.
With the used FETs, the required gate drive current is moderate, so linear step-down regulators can be used for the gate drives. There are two voltage pre-regulators, one for the gate drive ("VN10"), on the negative power supply, and one for the +5V for the signal section on the positive supply. Each have a simple first stage transistor-zener based pre-regulator followed by a second stage 100mA regulator for the final voltage.
Initial listening and measurements have confirmed; this is a very clean playing amp!
As a summary, AMP15 is an amp with high performance, compact footprint, decent total cost
If you havn't built it you do not really own it