SDS Labs Headphone Amplifier

by Sheldon D. Stokes

A couple of years ago, I bought a set of the now-famous Grado SR-60 headphones. I use them when I travel or when I don?t want to bother anybody with music at home. But my reference system doesn?t have a headphone jack except on my CD player, and it doesn?t have a volume control. And if that wasn?t bad enough, the headphone amp isn?t very good quality. So I needed a good quality headphone amp that I could use for all my sources.

A problem with the Grado head-phones is their low impedance. They present about a 32 ohm load on the ampli-fier driving them. My first thought was to use tubes, and drive the headphones with a cathode follower. but with the 32 ohm impedance, this isn?t very practical unless you?re building a large OTL amp. So I decided to build a solid state headphone amp. The circuit shown here is an adaptation of Walt Jung?s headphone amp described in the Audio IC Op-Amp Applications book. From what I?ve heard, this book is out of print.

The changes I?ve made in Walt Jung?s circuit are that I have replaced the bipolar output transistors with MOSFETs, and I changed the biasing method for the MOSFETs. I also designed the power supply. But the essence of the circuit remains unchanged. The heart of the circuit is a dual op-amp. Which drives a pair of push pull output MOSFETs. The output from the MOSFETs is included in the feedback loop of the op-amp, so the distortion is very low. The gain of the amp is set at a value of 2, and it includes an input pot for volume adjustment. [Editor: To increase the gain, increase the value of R11. For example, R11 = 10K will set the gain to 10.]

SDS amplifier schematic.

Figure 1

The headphone amp circuit is basically a simple non-inverting op-amp gain stage with external buffering (figure 1). I find that for op-amps to sound their best, they should not be operated at the edge of their drive capabilities. Many commercial headphone products use op-amps directly to drive a pair of headphones. While it can be done this way, I have found that adding a buffer to the output of the op-amp reduces the harshness and stridency dramatically (two common complaints about op-amp based designs). I rarely use op-amps for serious audio design, due to what I consider to be questionable sonic merits of op-amps. But in this circuit, the op-amp is running at a very low gain setting, and is not driving the headphones directly. These two factors make this headphone amp a very neutral and musical device. The headphone amp also uses a low impedance, regulated power supply sourced from a 25W toroidal transformer (figure 2). For such a small amplifier regulation is practical and the sonic benefit is quite noticeable. This amplifier is direct coupled as well, so there isn?t any capacitors in the signal path.

Power supply.

Figure 2

The amplifier uses a zener diodes to provide the bias current for the output FET?s. The output stage is biased fairly heavily for such a small amplifier. I find that FET?s sound their best when they have fairly high bias currents. This amp will run class A up to two watts. Each device is dissipating a watt of power at idle, and should be mounted on a heat sink (I used an AAVID TO-220 Power Cooler, Digikey #HS132-ND). The biasing portion of the circuit also has the provision for limiting the amplifier output by using a pair of LED?s (per channel). If voltage swing gets too large, the LED turns on and the output signal is reduced. I don?t use the LED?s in my prototype of this circuit, they shouldn?t degrade the sound quality of the amplifier, but I left them off just because I didn?t think I?d need them, and I try to keep as little in the signal path as possible.

This amplifier is powerful enough to also drive an efficient set of speakers. It produces about 4 watts of power before clipping. This amp clips asymmetrically (at 10V p-p), as the gates of the MOSFETs are not driven in their potential center. The reason I have done this is that I have found that op-amps sound the best when they sink a bit of output current. Many folks put a resistor on the output to one of the voltage rails to “bias” the op-amp into class “A” operation. By tying the output of the op-amp to one of the gates instead of the middle of the two gates, the op-amp output is sitting at about 4.5 volts above ground potential, and thus is “biased” into class A. This may seem to contradict what I said earlier about needing to buffer the output of the op-amp so it doesn?t drive difficult loads. In this case, the op-amp is still only driving the gates of the MOSFETs, and the load it?s output stage sees is essentially the same as if it were connected to the center of the MOSFET gates.

Schematic of output stage configured for center-bias.

Figure 3

The asymmetrical clipping is not a problem for a headphone amp because your ears will bleed long before the MOSFETs clip. If you are going to use this design for driving speakers, you should tie the op-amp output the center of the MOSFET gates using a pair of 450 ohm resistors (figure 3). Then the amp will clip at 15V p-p. I listened to this amp driving my Quad ESL?s and it?s quite promising. This amp sounds very good. If you own a set of Grado headphones and are driving them with an op-amp based headphone amp, I believe that the buffering the op-amp with a pair of MOSFET?s (per channel) will result in much improved sound quality. I haven?t heard a more transparent sounding headphone amp available at a reasonable cost.

Figure 4

Figure 5

View full scale version of PC board layout and placement diagram

The circuit board layout and population guide are shown in figures 4 and 5. The schematic only shows parts for one channel. On the circuit board, parts for the other channel are offset by 50 (e.g., R10 for channel 1 becomes R60 for channel 2). All of the parts, including the toroidal transformer, are available from Digi-Key Electronics and Radio Shack.

The International Rectifier MOSFETs that I used originally don’t always work. I’m now using Harris MOSFETs: RFP15N05 for the N channel and RFP30P05 for the P channel. But a wide array of the MOSFETs from the same Harris series will work as well. You can use others as long as you choose well-matched complimentary pairs with decent power handling and similar specs.

R5 and R55 are needed only if no potentiometer is used (e.g., it’s driven from a pre-amp or the variable outputs of a CD player). C16 and C66 provide feedback compensation for the op-amp and improve the amplifier’s stability. They are not nessesary with the parts I’ve specified, but don’t hurt anything. If left in place, they allow the dual op-amps to be switched out to try different ones without fear of stability problems. R12 and R62 are not necessary when using MOSFETs, but are necessary if the output devices will be bipolar transistors.

c. 1996, 1998, 2000 Sheldon D. Stokes.
From SDS Labs site. (Republished with permission.)

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