I acquired an HP 8903B Audio Analyzer a year or two ago, but hadn't really used it. Initially I used it to check a Dynaco PAT4 I bought cheaply off EBay. This PAT4 has a nasty scar and bend in its faceplate. I bought it with the thought of using it to do some experimentation, but that's a different story. Anyhow, this PAT4 tested really well for an original unmodified preamp. When I opened it up I was surprised to find it was really well built - whoever built it did a really professional job - so neat and tidy. THD over the 20-20kHz range was below 0.049% and SINAD measured 66 to 72 db.
Next I tested another Dynaco PAT4 that I had modified using kit bought from Update my Dynaco. New power supply, reduced line level distortion, gold jacks, etc. This one tested better than the one above. Distortion was down in the 0.01 to 0.02% range, SINAD was in the upper 70 db range. BTW, initially it tested poorly on one channel. This was traced to bass tone control was just on the edge. The PAT4 does not have a switch to switch out the tone controls. Instead the bass on treble pots have a "dead" position at the center, which if set to effectively takes the tone circuits out. This control was right on the edge. Once I move it, the preamp worked fine.
Ok, now to the PL4000. I restored this unit - a late model which doesn't have the 4 channel decoder - a few years ago. Front panel is soldered to motherboard. Performance looked good initially at low frequencies until I got up to about 10kHz. Then THD and SINAD takes off. Below 10kHz THD is 0.01 to 0.02%, SINAD starts at almost 80db, but falls to low 60's by 10kHz. At 20kHz THD is almost 2% and SINAD falls to 55dB - both channels. I started digging into it. Long story short my conclusion is the signal tracks that carry the signal to the Correlator boards cause the increase. For the record, these tests were conducted with the tone controls and the Correlator switched out (also the Peak Unlimiter and the Active Eq were off). I've attached a simplified diagram. In this configuration the Correlator boards are not being used - so I removed them - same measurements. Volume control removed - ditto. At this point the preamp is down to just the Input Selector switch and the initial Emitter follower. I ended up cutting the signal trace on the front panel that feeds the signals to the Correlators (just after the Tone Control switch). Distortion and SINAD measured good - 0.03%. As the photos below show, I tried moving the Correlators input resistor (1.2k) to where I had cut the trace. No joy. In the end I decided I don't care about THD at 20kHz as my hearing rolls off nicely below 13kHz and the first harmonic would be at 40kHz - maybe my dog cares. I repaired the cut tracks, put it all back together and retested that everything was ok. It makes impressive eye candy.
Finally I tested an Audio Research SP16 preamp I picked up at a Thrift shop last year. Tube preamp - no tone controls - no balance, but does have a remote control. Measures very well - left/right output is with in a couple of mV (hence no need for a balance control), SINAD is in the low 70 dB range, THD is 0.02% rising to 0.03% at 20kHz.
Next I tested another Dynaco PAT4 that I had modified using kit bought from Update my Dynaco. New power supply, reduced line level distortion, gold jacks, etc. This one tested better than the one above. Distortion was down in the 0.01 to 0.02% range, SINAD was in the upper 70 db range. BTW, initially it tested poorly on one channel. This was traced to bass tone control was just on the edge. The PAT4 does not have a switch to switch out the tone controls. Instead the bass on treble pots have a "dead" position at the center, which if set to effectively takes the tone circuits out. This control was right on the edge. Once I move it, the preamp worked fine.
Ok, now to the PL4000. I restored this unit - a late model which doesn't have the 4 channel decoder - a few years ago. Front panel is soldered to motherboard. Performance looked good initially at low frequencies until I got up to about 10kHz. Then THD and SINAD takes off. Below 10kHz THD is 0.01 to 0.02%, SINAD starts at almost 80db, but falls to low 60's by 10kHz. At 20kHz THD is almost 2% and SINAD falls to 55dB - both channels. I started digging into it. Long story short my conclusion is the signal tracks that carry the signal to the Correlator boards cause the increase. For the record, these tests were conducted with the tone controls and the Correlator switched out (also the Peak Unlimiter and the Active Eq were off). I've attached a simplified diagram. In this configuration the Correlator boards are not being used - so I removed them - same measurements. Volume control removed - ditto. At this point the preamp is down to just the Input Selector switch and the initial Emitter follower. I ended up cutting the signal trace on the front panel that feeds the signals to the Correlators (just after the Tone Control switch). Distortion and SINAD measured good - 0.03%. As the photos below show, I tried moving the Correlators input resistor (1.2k) to where I had cut the trace. No joy. In the end I decided I don't care about THD at 20kHz as my hearing rolls off nicely below 13kHz and the first harmonic would be at 40kHz - maybe my dog cares. I repaired the cut tracks, put it all back together and retested that everything was ok. It makes impressive eye candy.
Finally I tested an Audio Research SP16 preamp I picked up at a Thrift shop last year. Tube preamp - no tone controls - no balance, but does have a remote control. Measures very well - left/right output is with in a couple of mV (hence no need for a balance control), SINAD is in the low 70 dB range, THD is 0.02% rising to 0.03% at 20kHz.
