2 years ago

Tagged:
hi fi
solid state
mods

Here’s a hi-fi project which might be of some interest: mods to a Cambridge Audio 540P moving magnet phono preamp. Having recently swapped the 80s NAD preamp out of my main setup for a newer integrated amp, I no longer had a built-in phono stage. I looked around for an inexpensive, workable outboard phono stage and settled on the Cambridge 540P. The 640P is supposed to be nicer, but I don’t need moving coil inputs and I don’t think it really offers much more.  Both models are a 2SA970 discrete differential amp for the input stage, followed by an NE5532 opamp gain stage, and another NE5532 opamp for RIAA equalization.  Nothing super special, but the RIAA section is very accurate due to the use of 1% tolerance components around it. I use an AT440mla cartridge on a very modded Technics SL-1200MK2. The 540P uses a pretty typical MM loading of 47k-ohms, with 220pF of additional input capacitance.  With the AT400mla, this sounded way too bright in the upper treble. You can see this in the first frequency response plot below, which is the stock 540P with an AT440mla and an assumed 100pF interconnect capacitance (total 320pF). There’s an almost 3dB peak from 8kHz to 14kHz, with a steep rolloff that puts the response at -6dB at 20kHz.  Very audible. I first clipped the 220pF input caps, and then the sound was dull and recessed. You can see this in the second FR plot, which shows that while the peaking is gone, it’s still 3dB down at 20kHz.  In this case, the 47k loading is to blame, because it’s over-damping the cartridge for the capacitance. The solution was to change the input loading to get a more even response.  The third FR plot shows the response with the loading resistors (R14 and R22) changed out for 68k resistors.  Only about a 0.5 dB peak at 15kHz, then right at 0dB at 20kHz.  Very nice indeed! I’ve actually ended up at 62k to achieve this curve, since my interconnect capacitance is slightly higher at 125pf.  If I end up replacing it with a Blue Jeans Cable LC-1 at 2 feet (only ~50pF total capacitance!), I can change the loading resistors to 77k and get smooth treble extension to 25kHz.

Here’s a hi-fi project which might be of some interest: mods to a Cambridge Audio 540P moving magnet phono preamp.

Having recently swapped the 80s NAD preamp out of my main setup for a newer integrated amp, I no longer had a built-in phono stage. I looked around for an inexpensive, workable outboard phono stage and settled on the Cambridge 540P. The 640P is supposed to be nicer, but I don’t need moving coil inputs and I don’t think it really offers much more.  Both models are a 2SA970 discrete differential amp for the input stage, followed by an NE5532 opamp gain stage, and another NE5532 opamp for RIAA equalization.  Nothing super special, but the RIAA section is very accurate due to the use of 1% tolerance components around it.

I use an AT440mla cartridge on a very modded Technics SL-1200MK2. The 540P uses a pretty typical MM loading of 47k-ohms, with 220pF of additional input capacitance.  With the AT400mla, this sounded way too bright in the upper treble. You can see this in the first frequency response plot below, which is the stock 540P with an AT440mla and an assumed 100pF interconnect capacitance (total 320pF). There’s an almost 3dB peak from 8kHz to 14kHz, with a steep rolloff that puts the response at -6dB at 20kHz.  Very audible.

I first clipped the 220pF input caps, and then the sound was dull and recessed. You can see this in the second FR plot, which shows that while the peaking is gone, it’s still 3dB down at 20kHz.  In this case, the 47k loading is to blame, because it’s over-damping the cartridge for the capacitance.

The solution was to change the input loading to get a more even response.  The third FR plot shows the response with the loading resistors (R14 and R22) changed out for 68k resistors.  Only about a 0.5 dB peak at 15kHz, then right at 0dB at 20kHz.  Very nice indeed!

I’ve actually ended up at 62k to achieve this curve, since my interconnect capacitance is slightly higher at 125pf.  If I end up replacing it with a Blue Jeans Cable LC-1 at 2 feet (only ~50pF total capacitance!), I can change the loading resistors to 77k and get smooth treble extension to 25kHz.