Member @BDWoody was kind enough to send two phono preamps my way for testing. One was the Pro-Ject Phono Box DS+, the other was an almost-antique (vintage 1983) Straight Wire Audio (SWA) Phono Preamp. The results and comparison were interesting...
Let's start with the Pro-Ject. Physically, it's quite nice, a heavy metal case, a front panel LED display showing the selected options, and some significant heft. It's powered by an external wall-wart.
The innards are mostly not a let down, either- a double sided PCB with plated-through holes, a ground plane, clean layout, and decent quality components (e.g., WIMA caps, LT1028 and OPA2134 opamps).
This is quite a contrast to the Lounge phono stage I posted on a couple months back, which was a sloppily constructed and poorly laid out mess. I do have a couple of concerns here: first, I don't like seeing the wires from the power supply connector (the red and black twisted pair) running across the length of the board near sensitive circuitry. Second, the RCA connectors are rather cheep'n'cheesy, with input and output grounds connected directly together. That's not optimum for hum and noise immunity- my concern here turned out to be a valid one, as we will see.
As you can see from the photo of the front, there's a lot of flexibility/adjustability. I first checked the gain settings, all of which were accurate to within 0.2dB. Channel to channel balance was within 0.1dB at all gain settings. Delightful!
Next, input capacitances. This was done as I described in the Lounge review by using a large series resistor to the input, driving it with a square wave, then calculating the capacitance from the rise time (taking into account the input shunt resistance). Again, the display values were fairly accurate, all within 5% of the measurements. This was a promising start.
Now here's where it started going downhill. When I connected the AP analyzer, I got a large 60Hz hum component. Its size scaled with selected gain. I tried grounding the AP and Pro-Ject cases together. No luck. I tried plugging them into separate branches of my house's mains circuits. Nope. I tried changing interconnect types. Nope. I tried measuring one channel at a time. Nope. I seemed to be limited to 40dB SINAD no matter what I did. So this seemed very much to be a ground loop, and likely caused by the input and output phono jack configuration. I tried emailing Pro-Ject for some suggestions but got no response.
I ended up putting in Jensen 1:1 isolation transformers at the inputs to break up the ground loop, which improved things quite a bit. Now in practice, a cartridge can have its grounds electrically isolated from one another and from the output ground of the preamp, but this kind of behavior is a red flag that in some setups, the hum will be difficult or impossible to vanquish. I don't see this behavior with my own phono stages or with the SWA preamp which will be the subject of my next post, so I can be pretty confident that it's the specific grounding layout in this unit. If I owned it, I'd be tempted to attack this problem with an X-acto knife, some wire, and a soldering iron...
With the transformers in place, let's see what the measurements look like. First, let's see the RIAA curve conformance at 45dB gain (it was the same at all gain settings):
There's about a 0.15dB difference in gain between channels at high frequencies, which is probably too minor to worry about. The subsonic filter (eristic note: actually infrasonic) has a small peak at about 32Hz before rolling off at 12dB/octave. The 0.05dB dipsy doodle at 200-250Hz appears to be an artifact of the AP's built-in inverse RIAA equalization. Again, this is negigible, so I haven't gone into their EQ table to fix it. So conformance with the subsonic filter off is within +/-0.1dB over the audio band.
The Lounge phono stage had a weird issue with RIAA conformance (and hence frequency response) being a strong function of level, likely due to their, ummm, creative choice of EQ method. That would not be expected to be an issue in a stage designed by a competent engineer, but just to make sure...
OK, things are as they should be. Let's look at noise in MC mode, where the source resistances can be closely approximated as resistive:
This is output referred noise- to convert it to input referred noise, divide the voltage noise density by 1000 (i.e., every micro becomes a nano). At 1kHz, the noise density is about 3nV/rtHz, which isn't outstanding but isn't terrible. For typical MC with 0.2-0.5mV/5cm/s outputs, this should be well below the mechanical noises of playback.
MM noise was measured with a Stanton 881S as a source, with a shorted-input measurement also shown (just to drive home the point that shorted input measurements of MM are NOT valid because of the significant inductance and DCR). These data were taken at 40dB gain, so to convert to input-referred, divide the voltage noise density by 100.
At midband, the noise density is about 10nV/rt Hz, which again isn't breathtaking but more than good enough to not be the limit in the reproduction chain. Note that there's 20dB difference in noise at 10kHz between shorted in and a cartridge source. Ahem.
Distortion is quite good:
At 1V out (10mV in), the distortion is dominated by the -90dB second harmonic. It drops as the input level is decreased. This is ridiculously better than any cartridge, so is of no concern. Sweeping the frequency (the 10mV is at 1kHz, so the input voltage increases with increasing frequency) gives us a good feeling that distortion is no big deal here:
Input headroom is also good, 130mV at 1kHz, and over 200mV at 10kHz on the 40dB gain setting. At 60dB gain, the 10kHz overload is better than 50mV. Headroom is not a problem here.
All in all, IF your setup is one where the ground loops aren't an issue, the build and versatility of this unit make the relatively low price worthwhile IMO.
Next up, the SWA (as soon as I can find a little time to write up the results).
Let's start with the Pro-Ject. Physically, it's quite nice, a heavy metal case, a front panel LED display showing the selected options, and some significant heft. It's powered by an external wall-wart.
The innards are mostly not a let down, either- a double sided PCB with plated-through holes, a ground plane, clean layout, and decent quality components (e.g., WIMA caps, LT1028 and OPA2134 opamps).
This is quite a contrast to the Lounge phono stage I posted on a couple months back, which was a sloppily constructed and poorly laid out mess. I do have a couple of concerns here: first, I don't like seeing the wires from the power supply connector (the red and black twisted pair) running across the length of the board near sensitive circuitry. Second, the RCA connectors are rather cheep'n'cheesy, with input and output grounds connected directly together. That's not optimum for hum and noise immunity- my concern here turned out to be a valid one, as we will see.
As you can see from the photo of the front, there's a lot of flexibility/adjustability. I first checked the gain settings, all of which were accurate to within 0.2dB. Channel to channel balance was within 0.1dB at all gain settings. Delightful!
Next, input capacitances. This was done as I described in the Lounge review by using a large series resistor to the input, driving it with a square wave, then calculating the capacitance from the rise time (taking into account the input shunt resistance). Again, the display values were fairly accurate, all within 5% of the measurements. This was a promising start.
Now here's where it started going downhill. When I connected the AP analyzer, I got a large 60Hz hum component. Its size scaled with selected gain. I tried grounding the AP and Pro-Ject cases together. No luck. I tried plugging them into separate branches of my house's mains circuits. Nope. I tried changing interconnect types. Nope. I tried measuring one channel at a time. Nope. I seemed to be limited to 40dB SINAD no matter what I did. So this seemed very much to be a ground loop, and likely caused by the input and output phono jack configuration. I tried emailing Pro-Ject for some suggestions but got no response.
I ended up putting in Jensen 1:1 isolation transformers at the inputs to break up the ground loop, which improved things quite a bit. Now in practice, a cartridge can have its grounds electrically isolated from one another and from the output ground of the preamp, but this kind of behavior is a red flag that in some setups, the hum will be difficult or impossible to vanquish. I don't see this behavior with my own phono stages or with the SWA preamp which will be the subject of my next post, so I can be pretty confident that it's the specific grounding layout in this unit. If I owned it, I'd be tempted to attack this problem with an X-acto knife, some wire, and a soldering iron...
With the transformers in place, let's see what the measurements look like. First, let's see the RIAA curve conformance at 45dB gain (it was the same at all gain settings):
There's about a 0.15dB difference in gain between channels at high frequencies, which is probably too minor to worry about. The subsonic filter (eristic note: actually infrasonic) has a small peak at about 32Hz before rolling off at 12dB/octave. The 0.05dB dipsy doodle at 200-250Hz appears to be an artifact of the AP's built-in inverse RIAA equalization. Again, this is negigible, so I haven't gone into their EQ table to fix it. So conformance with the subsonic filter off is within +/-0.1dB over the audio band.
The Lounge phono stage had a weird issue with RIAA conformance (and hence frequency response) being a strong function of level, likely due to their, ummm, creative choice of EQ method. That would not be expected to be an issue in a stage designed by a competent engineer, but just to make sure...
OK, things are as they should be. Let's look at noise in MC mode, where the source resistances can be closely approximated as resistive:
This is output referred noise- to convert it to input referred noise, divide the voltage noise density by 1000 (i.e., every micro becomes a nano). At 1kHz, the noise density is about 3nV/rtHz, which isn't outstanding but isn't terrible. For typical MC with 0.2-0.5mV/5cm/s outputs, this should be well below the mechanical noises of playback.
MM noise was measured with a Stanton 881S as a source, with a shorted-input measurement also shown (just to drive home the point that shorted input measurements of MM are NOT valid because of the significant inductance and DCR). These data were taken at 40dB gain, so to convert to input-referred, divide the voltage noise density by 100.
At midband, the noise density is about 10nV/rt Hz, which again isn't breathtaking but more than good enough to not be the limit in the reproduction chain. Note that there's 20dB difference in noise at 10kHz between shorted in and a cartridge source. Ahem.
Distortion is quite good:
At 1V out (10mV in), the distortion is dominated by the -90dB second harmonic. It drops as the input level is decreased. This is ridiculously better than any cartridge, so is of no concern. Sweeping the frequency (the 10mV is at 1kHz, so the input voltage increases with increasing frequency) gives us a good feeling that distortion is no big deal here:
Input headroom is also good, 130mV at 1kHz, and over 200mV at 10kHz on the 40dB gain setting. At 60dB gain, the 10kHz overload is better than 50mV. Headroom is not a problem here.
All in all, IF your setup is one where the ground loops aren't an issue, the build and versatility of this unit make the relatively low price worthwhile IMO.
Next up, the SWA (as soon as I can find a little time to write up the results).
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