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Oliver Acoustic Research AR-94 specifications
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Introduction

While in a discussion on a webforum on headphones, someone invited me to make to make an headphone amplifier. Nothing too fancy, an affordable and decent sounding mains powered amplifier for use with a soon-to-buy Sennheiser HD600 or similar.

I accepted the invitation and built an amplifier with two dual opamps, configured in such a way that one half of each buffers the output signal, so the amp gets a little more current-headroom. I got the info out of a Burr-Brown application note, app note ab-051.

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Opamp headphone amplifier

The schematics for the power supply and the amplifier itself are as follows:

The amp runs from a dual 13V powersupply. The PSU is based on the PSU for the Szekeres headamp, with a few small alterations. 12V zeners are used, a smaller transformer (2x12V 8VA), an extra 1nf cap on the output, and smaller supply caps; 470uF on the inputs and 220uF on the outputs. I still used lm317/lm337 regulators, simply because I had them lying around. But lm117/lm137 are also usuable (the amp draws less than 100mA from the supply)

Some time ago, I received an email from Antti Penttala that he has built his own version of this amplifier. He has also designed a pcb for the circuit. You can read all about it on his webpage

The numbers around the ICs in the schematic correspond to the pin numbers of the opa2134 (if I didn't make any mistakes). The primary function of the 1K resistor at the input is to protect the amp from oscillating. When the amp starts oscilltating, a current will start to flow, but as soon as it does, the 1K will drop the voltage, so the amp will stop oscillating.

The amp has a gain of 11 ((100/10)+1), which is recomended for a high impededance headphone like the HD580/HD600. For lower impedance headphones, simply bring down the R5/R3 ratio.

Construction always starts with a big pile of parts:

Mounted onto the protoboard, it looks like this:

I used application note an-202 from analog devices as a reference for the grounding scheme in this amp. There are two ground paths, one is a power-ground, and one is a signal-ground. These two only meet at the transformer. The input caps are basic foil types.

I also wanted that this amp looked good. That's why I put some extra effort in preparing the front and backpanel of the enclosure. I measured all the parts that would be mounted on these panels and made a CAD drawing of the layout. I then carefully drew all the cut-outs on the aluminium faceplates and made centerpunches for the drillholes.

I then drilled all the holes and cut out the square holes with a Black and Decker multitool (dremel look-a-like). some edges might still look a little jagged, but luckily they disappear behind the mounting surfaces of the parts.

Now that most of the work was done, all that's left is putting the amp inside the enclosure and close it up! Here's the amp topless:

And it's behind

And a nice frontal view, with the buttons and led in place

Results

I almost forgot: The sound. I still find it difficult to judge the sound quality of different devices, and especially the ones I made myself. So I will just try to give some observations. The amplifier is dead quiet. Even with the volume turned all the way up and with or without a cdplayer connected there is no audible hum! That's a good thing. Also, it can drive a sennheiser HD-580 to really high volumes, which means that my original thought of using two dual opamps to buffer outputs was a good idea. The amplifier also plays distortion free, even at high volumes, so I guess it has enough reserves. That's all you're gonna get from me :-) No dropping veils, placements of instruments or getting intimate with the performers. Of course this little critter does all that, but I'm not the person to hear it, or to comment on it.....

Raymond van Weeghel