Mini Headphone Amplifier


Posted on Feb 22, 2013    6634

SW1 bypasses the crossfeed network. I have reconfigured the original crossfeed schematic so that now the 100k resistor always bridges the bypass switch and thereby reduces any 'crackle' or 'click' or whatever you may call them. Don't omit these 100K resistors as they form a part of the crossfeed network and omitting them would bear undesirable results. Note that R6 and R9 are indicated as 4.53k, however the use of 4.7k resistors will be perfectly adequate in practice.


Mini Headphone Amplifier
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I chose the NE5532 for this project. Since the source is a PDA's internal DAC, I didn't see the need to use premium opamps. Of course, if it makes you feel better, you can always use higher quality (expensive) opamps. Just make sure the opamp is capable of driving low impedances. LM6171, OPA2134, OPA2132, OPA134 and OPA4134 (dual) are some possible substitutes. It's likely that there are others. IC sockets are therefore a good idea if you have plans to upgrade the opamps. The volume pot should be a linear type and would give, with the 15k resistor in parallel, the benefits outlined in ESP's A Better Volume Control. The crossfeed is on a separate board in the prototype. I mounted it vertically on the main board using hot-melt glue. All the switches, jacks and volume control were also mounted on the enclosure using a hot-melt glue gun. I used generous amounts of hot-melt glue around the bases of all the capacitors as they are more susceptible to lead and track breaking due to vibrations. My prototype uses two 9V alkaline batteries to give 9-0-9V supply. I get around 20 hours of operation at normal portable listening levels. The effect of the demise of a few pairs of alkaline batteries on my wallet has decided me to switch over to rechargeable batteries. A battery charger is now under construction. This amp can also be powered by Project 05 using a 15-0-15V transformer. A 5VA transformer should have oomph enough for the job. The first thing that you notice about the sound is the authority of the low frequency. The sensation of the kick drum's 'kick' (pun intended) on the earlobes greatly enhances the listening experience. Only now do I realise the full potential of the Sennheiser PMX 60. With the crossfeed on, the vocal that used to seem to be right on top of one's nose is pulled forward. The perceived depth in the sound stage and the bass is very much dependent on the source material. For some materials, loss in bass is experienced with the crossfeed on. This is due to the cancellations of the unrealistic, out-of-phase signals. A star ground was not necessary for my battery powered version but is recommended for a mains powered one. Use the common point of the power filter caps as the ground return and employ a ground loop breaker if you use a metal enclosure. For the enclosure, I chose what used to be a part of a plastic school lunch box. I measured and marked the spots for the cuts I had to make. A sharp hobby-knife, a drill bit, a tabletop vise and a steady hand were all that I needed for the job. When working with plastic, it's a very good practice to measure twice and cut once (he spake from bitter experience).

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