Analogue Stage Upgrade with Operational Amplifiers.
With many CD players, including the more expensive models, the analogue stage is driven by operational amplifiers. Unfortunately, these op-amps tend to be of a poor quality and a sound degradation becomes inevitable. The effect on sound quality is an aggressive midrange in powerful passages. In complex music sections the instruments merge together causing the sound clarity to become unclear. Further more, poor quality op-amps can sometimes be responsible for a cold and edgy sound. These less expensive and inferior quality op-amps are typically one of the following;
NE5536, NJM5532, NJM2114, NJM4560, NJM4570, NJM4580, AD712, LM833.
If these components are changed to a better type, the following results can be achieved;
Treble and midrange becomes more liquid and easy flowing.
After our analysis of data sheets and listening tests of different op-amps,
we have found which parameters are important for sound quality. These include
low THD, settling time and the open loop gain (as low as possible). Other
parameters, such as input noise and unity gain stability, are also of importance,
if a more practical use of the device is required. We found two op-amps
with very promising sound qualities. These being LM6172 from National Semiconductor,
and the AD825, from Analog Devices.
We also developed a totally different approach, the ZAPfilter, which is better than even the best op-apms, but also comes at a higher pricetag. The AD825 still gives very good value for very little money.
Other op-amps we tested were found to have some potential, but did not come up to the high end level. These were;
OP627, OP2132, AD811, LT1122.
Further down on our list of test results we placed these components; AD712, OPA275, OP2604, NE5534, AD846. Any component that achieved a performance result below these others would not be suitable in good audio equipment.
As AD825 is only available from Analog Devices in SMD version, we have
built appropriate modules to carry it in a standard DIP footprint. So now
it is possible to fit AD825 into most CD players, where DIP, SIP or single
DIP packages are normally used.
Type 1..DIP Dual Opamp, replaces NE5532, NJM2114, JRC4580, AD712,
OPA2132, OP275, OPA2604 a.o.
Compare AD825 data with the most commonly used types:
(THD data listed, even thoung we do not consider it very important for sound quality.)