Half Gain Headphone Amplifier Project

This is an experimental headphone amp using dual op-amp IC. The goal is for better design of headphone amp compares to the Cmoy amp. I am dig out any possibility to improve the typical headphone amp like RA1 clone, Cmoy LME, or tiny Cmoy amp like this and this. And here is the final layout, I named it the half gain amp. The IC that I use in this project is AD823 from Analog Device.

Basic idea of this project:
Half gain amp is a reduction of total volume from the original output or line out from the source. The main purpose of this design is to reduce the components that passes in the input signal to the IC input, especially the volume control. It is act like using a stepper attenuation or even better (because it not using any resistor in series). Think about typical Cmoy amp that have gain of 10 or even greater. The signal input are actually attenuate at certain level then the IC will back it up at the proper listening level.

The most issue that I've been facing when building any amp is the matching input impedance. Otherwise the sound will not perform like it should be. I should change the resistor on the input, matching with the source or even experiment with the resistance value of the potentiometer to get the best impedance level. It is effect to the sound after all.

Herewith the schematic:
Simple and using few components. It is worth to try and cost less.

The schematic & building concept:
The op-amp will act just like a buffer. I put a 47k resistor across output and inverting input without ground resistor. This method will make the gain around half to 0.7 times. The following resistor after the output act like a zobel network and give the benefit for the fine tuning total frequency output.

The circuit is build in the universal board, all connection are done by point-to-point wiring. I using hi quality components such as Takman resistor, Nichicon FG caps, and Wima film caps. The voltage divider is using TLE2426. The Alps pot is actually not connected to the circuit except I only using the built-in switch for on/off. Input caps are electrolytic caps, value are acceptable from 1uF to 2.2uF. The zobel caps are K40Y PIO caps.

Application:
This half gain amp do not have the volume control which could be problematic with the level output. My normal listening volume in 10x gain cmoy amp with 10k input impedance is around 9 to 11 o'clock. I never going lower or higher than that. This design is accommodate the level just like 10 to 11 o'clock which could be louder with high sensitivity headphone.

The main application with this amp is actually by pairing with any source or DAP with the output are controlled by the volume control such as the new iPhone 5S which is sound great over the headphone jack. This amp could make the sound better and powerful enough to drive bigger cans.

Sound quality:
For serious listening session, I just try this amp for less than an hour or two. The components are not breaking in yet. I write this blog while also listening with it. My source is iPod gen 5 with Apple lossless files on it.

I compare the sound by using and not using the amp and the difference is huge. Diana Krall voice when singing 'So Wonderful' might be overwhelming without the amp. It make the sound become harsh and mixed between frequency even I only using moderate sensitivity headphone like Sony MH-1 or Audio Technica ATH-M50X. The detail are blur and every notes overlap each other. Then I connected to the amp, I not experience that issues as before. The bass is significantly deep and loud. The vocal is clear pronounce and the high presentation clear without ear fatigue. This difference are noticeable better.

The second test is by using line out dock adapter via 30 pin iPod and guess what? the sound are incredible loud and clear. Very loud for more sensitive earphones. But all presentation are more clear, the vocal sound which I love most are laid back and far from ear fatigue. The bass level are huge and deeper. By the way, I am listening to Daft Punk - RAM album.

This amp drawback:
Okay, it is just only a short time listening session. I could not saying too much except I could say that it make a improvement over typical headphone amp such as the Cmoy and Grado. This sound improvement is come from clean signal path like I describe above.

One thing should be concern is how long is the battery lifetime. I using economic 9 volt battery and it is in used condition. I taken from my multi-meter battery for testing purpose. This amp drain so much power. For less than an hours or two, with the heavy bass music, the pilot led is dimmed then the sound start to distorted. later, I will buy the new battery and then test is over again until I post in this blog.

Another possible mods that the input drain resistors controlling the level input are using 47k from IC pin to the ground. This value can be reduce or increase to adjust the level output.

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.