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TonGiesberts

SO8 to DIP8 adapter - for OPAMPS and more

Status: Finished
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July 4, 2016
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A rather expensive opamp is used In the Supra 2.0 - High End Preamp for Record Player (150616), the LT1028. It has been around for a long time and today there are many alternatives available. The problem however is modern parts are often produced in SMT only. A possible alternative is the LME49990 from Texas Instruments. It’s available in SO8. The Supra phono amplifier is fully designed in THT. To use a SO8 package we designed a little adapter PCB of  10 by 10 mm. This corresponds with the size of a standard  8 pins DIP IC socket. Pinout is the same. The SO8  is mounted on the bottom  side of the adapter PCB and an additional power supply decoupling (only one capacitor between the positive and negative supply pins) is added on the top side. There are three pads for the capacitor so it can be placed between pin 4 and pin 7 (marked with S) or between pin 4 and pin 8 (marked with D). This way the decoupling capacitor can be used  for single and dual opamps. If the capacitor is not mounted the adapter can be used for any other SO8 IC. Placing the  package of the SMD on the bottom side means it has the shortest connections to the main PCB. Best is to solder the little adapter with 8 wires directly to the main PCB, but it makes servicing or experimenting more difficult We used expensive pin headers with rounded pins and put them in the already present IC sockets (see photo’s). When testing a very high parasitic oscillation occurred at about 45 MHz. To suppress this, a 100 pF ceramic capacitor is placed between pin 6 and pin 2 of the opamp by soldering it to the respective pins of the adapter. A small downside of the LME49990 is a higher offset in the Supra. This causes the first opamp in the second stage of the filtering (IC5AIC10A) to have a higher offset. In our prototype close to 0.4 V. This however doesn’t affect the specifications, which by the way are also excellent:
 
Measurements on Supra Phono Preamplifier with LME49990MA:
 
Input signal 2.5 mV at 1 kHz (source is 256 mV and divider 56 kΩ/560 Ω)
Power supply is DC/DC converter 150464-1
 
Signal to noise              > 73 dB (B = 22 Hz…22 kHz)
                                   > 78 dB(A)
                                   > 83 dB (input shorted, B = 22 Hz…22 kHz)
                                   > 94 dB(A) (input shorted)
Output level                  484 mV
THD (without noise)     < 0.001 % (B = 20 kHz)
Deviation from RIAA   < 0.1 dB (100 Hz…10 kHz)
                                   < 1 dB (20 Hz)
                                   < 0.2 dB (20 kHz, with additional 100 pF capacitor)
 
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