Building for Tweakability

Virtually every hole in the board is on 0.1 in (2.5 mm) spacing relative to all the related holes. Also, each resistor’s holes are 0.3 in (7.5 mm) apart. This means you can use DIP IC sockets, SIP socket strips and SIP header pin strips in the board. If you did this for everything where it makes sense, you would in effect have a PIMETA breadboard for maximum tweakability. :) Here’s a list of all the places you can do this and ideas for why you might want to do each one:

• R3-R8 block — Use a 300 mil DIP-20 socket or two rows of socket strips to allow easy swapping of resistors.

• Input and outputs — Solder SIP pins in the holes, and attach matching female connectors to the ends of the hookup wires to allow easy disconnection of the panel jacks. (The Molex KK series of connectors works well for this.) This might be useful for disassembling your amplifier for servicing without removing the jacks from the panels — just disconnect the jacks from the board, lift the board out, and the panel components can stay in place.

• Power pins — Similar to above.

Stacking Buffers

The BUF634 buffer is stackable. This means you can literally stack them on top of each other and solder their pins together to run multiple buffers in parallel. This makes the amp sound better, for a number of reasons:

1. It improves the slew rate of the buffers into loads like capacitive headphone cables, which lowers distortion. See the load vs. slew rate graph(s) in the buffer’s datasheet. With stacked buffers, the load is divided among the buffers. Treble response benefits the most from this.

2. It lowers the output impedance of the amplifer. With most headphones, bass improves as you lower output impedance.

You don’t need to stack buffers to get good sound. It’s a “last 10%” tweaking kind of thing.

As far as I can tell, you need to double the number of buffers each time to get a noticeable improvement, and with each additional level of buffers the improvement diminishes. Adding a second buffer to each channel gives a small but noticeable improvement. Doubling that to four per channel is a bit “out there” in terms of bang for the buck. I haven’t tried going beyond 4 per channel yet, but I suspect it would amount to a pretty tiny improvement.

You might think stacking power supply buffers would help. After all, the benefits mentioned above should help the output ground driver as well, since this is where the amp’s output currents return. Listening and bench tests don’t bear this out, however. I’m not certain why this is, but I suspect it’s because the output ground buffer doesn’t have to slew a voltage like the left and right channels’ buffers do.

Removing the Buffers

Just as you can add buffers to optimize the amp by spending more money, you can save money by removing the buffers. All three buffers are technically optional. Just jumper pin 3 to pin 6.

If you remove the output buffers, the op-amp chip will have to be strong enough to drive the load all by itself. Even if the chip is capable of a farily high output current, it won’t sound as good as when it’s insulated from the load by a buffer. Also, you’d have to rearrange the R3-R6 values since the multiloop feedback configuration won’t work without a buffer.

Biasing the Op-Amp into Class A

“Class A” refers to configuring an amplifier so that its output devices remain turned on all the time. This reduces thermal variation and eliminates crossover distortion, which makes the amp sound better.

Adding Crossfeed