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A Tale of Two Phono Preamps, Part 1: Pro-Ject Phono Box DS+

SIY

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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.

front panel.png


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).

pro-ject innards.png


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):

Pro-Ject_ RIAA Conformance.png


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...

Pro-Ject_ RIAA Conformance vs Level.png


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:

Pro-Ject Noise Spectrum MC mode.PNG


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.

Pro-Ject Noise Spectrum.png


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:

Distortion Spectrum_ Pro-Ject 40dB Gain.png


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:

Pro-Ject THD+N 10mV In.png


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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BDWoody

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Thanks for the review! Looking forward to part 2.
I've got a Puffin now too if you want me to send that... It's been fun to play around with.
 

restorer-john

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What's with the 85Hz spike? micro/display?
 

pozz

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After you finish part 2, would you be willing to show off measurements of one of your own phono stages?
 
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SIY

SIY

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I may very well. My mm stage measurements appeared in Volume 8 of Linear Audio, but that was before the Audio Precision appeared here, so it might be time to revisit that.
 

daftcombo

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Nice to see my beloved 1more Triple Driver Over-ear on the picture (used with PEQ here).

And thanks for the double review!
 

Willem

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This is reassuring for me. I have a Pro-ject Optical Box E phono (MM in, optical out) that I bought even though I could not find any independent measurements. If Pro-Ject can do one pre amp properly I am hoping and assuming they can also do a different one properly.
 
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[ … ] decent quality components (e.g., WIMA caps, LT1028 and OPA2134 opamps). [ … ]

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.

Given the ~1nV/sqrtHz input noise voltage density of the LT1028, this is quite high. Are they using high resistor values in the feedback network of the LT1028? Or was the gain 66dB (which would result in ~1.5nV/sqrtHz)?

At 60dB gain, the 1kHz overload is better than 50mV.
This would result in > 50V output voltage, which seems very unlikely. 5mV perhaps … ?
 
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SIY

SIY

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Nice to see my beloved 1more Triple Driver Over-ear on the picture (used with PEQ here).

And thanks for the double review!

I reviewed them for AudioXpress a few months back, and still use them as my main headphones for traveling and hiking. Super comfortable for me, and very easy to listen to.

Not a double review yet, but I'll have the second part up this weekend.:D
 

anmpr1

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MM noise was measured with a Stanton 881S as a source...
It's too bad Stanton abandoned their phono cartridge operation. I know they make a few 'DJ' items, but even those appear to be on the back burner. Reports from the field suggested poor QC with their latter-day (but now long gone) 681 MI series; their MM 881 had been discontinued much earlier. The 681, once a respectable cartridge--not to mention the superb 881S, had become a bad joke. In product specs Stanton resorted to calling their nothing special .03x.07 diamond a 'Stereohedron', which was the name the old company gave to their line contact stylus.

QC issues and overall languishing can easily be attributed to the horrible Gibson management team. Think about it. With their core business, how can you bankrupt the Les Paul, one of the two most iconic guitars in the world? In that context, what they did to Stanton was small potatoes. Perhaps with bankruptcy and new management, along with the so-called 'vinyl revival', someone in corporate could see this as an opportunity to make something of Stanton's history.

In any case, thanks @SIY for the review. Much appreciated.
 
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SIY

SIY

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What I most regret is Technics abandoning their cartridge line- they were (IMO) the finest phono cartridges ever made, but couldn't find hold in the US for a variety of reasons.

When Gibson was looking for new management, I thought about contacting them and throwing my hat into the ring, but got talked out of it. Too late now. :cool: They not only had management issues, they also had someone in the US government who was out to get them, and that didn't help. Never piss off the Mafia or government bureaucrats.
 

anmpr1

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What I most regret is Technics abandoning their cartridge line- they were (IMO) the finest phono cartridges ever made, but couldn't find hold in the US for a variety of reasons.

When Gibson was looking for new management, I thought about contacting them and throwing my hat into the ring, but got talked out of it. Too late now. :cool: They not only had management issues, they also had someone in the US government who was out to get them, and that didn't help. Never piss off the Mafia or government bureaucrats.
You know things have gone too far, probably, when the Fish and Wildlife Service starts doing a Jimi Hendrix/Pete Townsend on guitars. As much as I was no fan of Henry Juszkiewicz, I think he had a point when he said that all of it could have been avoided had "a caring human being representing the government" (his words) simply contacted him instead of raiding the factory over 'illegal' wood. Not that Henry was an especially 'caring human being' (at least if you read what some of his colleagues and employees said about him), so maybe it was just karma. LOL

I know Technics had a fairly extensive range of cartridges. They were pretty rare in the general American marketplace. I never heard any, but always heard good things about them. I think the "P Mount" concept kind of labeled them as not being serious, at least within the hip and sophisticated cartridge community.
 
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Frank Dernie

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I know Technics had a fairly extensive range of cartridges. They were pretty rare in the general American marketplace. I never heard any, but always heard good things about them. I think the "P Mount" concept kind of labeled them as not being serious, at least within the hip and sophisticated cartridge community.
I had one on a Technics SL15 turntable. I wish I had kept it, since P-mount to ½" adapters are more available now but it went to the tip because the TT itself never worked properly :(
 

AudioSceptic

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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.
I hope that someone sends you a Pro-Ject Phono Box S2 or Phono Box S2 Ultra to test. These look like really good value and I love the tiny size. This one, the DS+, looks bigger than it needs to be. Re the construction, why didn't they just make the power supply wires a bit longer so they could run along the edges of the PCB? It also looks like some of the bolts came out of a random parts bin, being twice the length required.
 

BDWoody

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