Posted - 09/02/2005 : 18:44:54
| Before this latest technology from Analog Devices, OPAMPs do not have enough power to drive headphones directly. Some OPAMPs had enough powet output, but they either had too high noise, too high distortion, or were too slow or too fast for audio. Until AD8397, headphone amps (including SuperMacro-3) had been relying on external buffers (BUF634 e.g.) to add more power.
What make AD8397 a perfect headphone driver, especially for portables?:
* Very high output power - 300mA to 32 ohm load, 3-24V. Some other OPAMPs have similar power output, but they all have very high distortions when driving 32 ohm headphones. Based on my test, AD8397's benchmarks (including THD and IMD) are all the same good as OPAMP+BUF amps.
* Just fast enough for audio - if lower, IMD will be high and sound will be slow and less transparent; if higher, easy to go instable resulting into crappy sound.
* Low enough noise level - only 1 dB higher noise than the best OPAMP+BUF amp.
* Very battery friendly - work all way down below 3V and, the best part, its output is rail-to-rail(RR). OPAMP+BUF is almost perfect except that no buffer is RR. Without RR, what an amp can put onto headphones is always 2 to 4V less than a power supplies. Yes, you waste 2-4V of your precious batteris. In order to drive hi-impedance phones, an OPAMP+BUF amp has to use much higher voltage (more battery cells) for headroom. Now, thanking to RR, AD8397 can really put all the 3V into headphones; or all the 12V of SuperMacro-3 into the K1000 monster.
All well, it sounds good too. Of course, a good OPAMP can only guarantee on paper, the headphone amp design still defines how well it performs in real world. Especially for portable amps that do not use two 9V batteries, the virtual ground has to be done right, in addition to many other details that make great sound.
Why use AD8397 in SuperMacro-3?
The benifits of using AD8397 in SuperMacro-3:
* More headroom. As mentioned above, AD8397 can use all the 12V of SM3 onto headphones. This is a very good thing to cans like the K1000. More headroom is always a good thing to audio.
* Much longer battery life. Compared to a standard SM3 that has 2x buffers, battery drain is reduced from 40mA down to 20mA. Plus, AD8397 can work all way down to 3V. So battery life will be at least doubled.
* Buffers can be removed. Less components in signal channel is always a good thing to audio. AD8397's designer has done an outstanding job to isolate the hi-power output and small-signal input stages.
* Its sound might just be your type. To my ears, the ON Semi SA5534-OBCA is like a vast, clear sky; AD8397 adds some clouds on to it. Whether these clouds are like those before a storm or those brought your childhood imaginations, is subjected to your own ears. Sweet, sweet, and sweet, is the least I can say about its sound.
This is a revolutionary audio opamp in history. I have no idea why Stereophile, and all DIYers, backed up that stupid TI TPA6120, but ignored this one. Even the opamp guru Walter John, who also works for AD, has not promoted it yet.
How to use it in SuperMacro-3?
SuperMacro-3 is designed mainly for single OPAMPs. AD8397 is dual and it does not have a single version. Therefore, a dual-to-single adapter is required to use AD8397 in SM3. I'll offer AD8397 mounted on such adapters very soon. Although AD8397 is very cheap (less than 3 dollars when buy 100 pcs), these adapters and the very labor intensive and tricky wiring are not.
Although you don't have to remove the onboard buffers, but AD8397 really does not need them. Like any other OPAMPs, AD8397 will work with buffers just fine, but by doing so you lose the key point. If you are interested in this AD8397 and other latest tweaks, you may send your amp back for free upgrading (except parts). But please wait until I come out a full plan and detailed instructions to be posted here.
This should reminds us how flexible SM3 is, it's ready to embrace the new technologies to come. All to be shipped SM3s will have no soldered-on-board buffers; the OPAMP and buffer sockets will allow PlugNPlay add-ons. Future SM3 will have both single and dual sockets to take single and dual opamps directly.
Edited by - Xin on 09/03/2005 09:24:49