Over the time I have built quite good audiophile class music system, which includes Oppo 205 used as physical media player and D/A converter for streaming, Accuphase C2400 control amplifier and ATC SCM100 speakers. This system plays very well, but as you know, audiophiles are always in search of the further improvement big or (in my case) small. I have reported while back that adding shielded power cables brought a positive improvement to sound. The logical next step was to add power line filter. The reasoning for that was based on the fact that I found quite a significant amount of power line noise from 60 Hz all way up to 40 kHz. Here the word “significant” should means that I was able to reliably measure it using tools I have in my disposal. I also followed numerous examples from people reporting a substantial improvement of sound after adding power line filter. Finally, I decided to add filter into my system hopefully to get improvement be it big or small. I chose a well-known power filter made by Furman, the company which specializes in power solutions mostly for professional audio. In building my music room I mostly follow examples used in professional audio and trust them more than consumer level audiophile products. Furman makes power filters, automatic voltage regulators and isolation transformers. Since I do not experience any major voltage variations or impact from ground loop (I use mostly balanced interconnect here), I decided to get their top filter only product P-1800PF-R. They advertise significant reduction of power line noise starting at 2kHz an up, with ultrasonic range frequencies suppressed as much as 50 dB. You can find specifications of that model here: P-1800 PF R: Power Conditioner for Instrument Amplifiers | Furman
I found a good deal for that unit and it arrived within few days. This model has three banks of outlets: two for low current devices (which they advertised as “isolated”) and one more for high current devices like power amplifiers. In my case I planned to connect Oppo and Accuphase to low current band and active ATC speakers to high current bank.
Furman P-1800PF-R feels solidly built, front panel has voltmeter display, which supposedly show power line voltage, button to increase/decrease brightness of said display (I found that it does not keep setting between on/off cycles) and power switch. The unit looks nice, if you like professional grade equipment.
First time I powered it up using 200W incandescent bulb as load and everything worked as expected using all three output banks. At that time I was convinced that this device is nt broken and I do not go through RMA.
I did a search, but could not find any internal picture of this particular model, thus I decided to open the cover and see what is inside. There is no protective seal on it, thus I was free to undo a dozen screws holding top cover and easily removed it. Here I am offering you a picture I made.
Overall internals of Furman filter were within my expectations. Filter board includes standard CLC filer with addition of DC elimination. It also has multiple elements to absorb high voltage transient to protect against random surges. I do not know exactly what soft start circuit is doing here and it was not advertised by manufacturer. RMS voltmeter board is connected to the display on the front panel. Furman has some models of power filters where they have current meter too, and I found some unpopulated positions on that board which probably reserved for components in current metering circuit. There is also large black component, which I could not identify. It looks similar to large value film capacitor but could be something else too. It likely serves the advertised function to supply up to 45 amperes of peak current to high current load, thus improving power factor. Though I do not have any knowledge or way to verify that claim. I found that the only thing that separates two low current outlet bands is a tiny common mode choke, made by ferrite cylinder through with both line and neutral wires are passing. This means that there is no real separation of two low current banks at frequencies below at least several megahertz. Front panel switch seems serving current protection duties – this component is either thermal or magnetic circuit breaker rated for 15 amperes.
Now it was time to make objective tests of this Furman filter. For that test I used my audio measurement system which can work up to 90kHz. Here I added a high voltage differential active probe (Micsig DP10013), which allowed me to safely connect measurement system directly to power line. Micsig device is designed to be used with oscilloscopes, with at beast 12 bits ADC, and I was using it here with more than 20 effective bits measurement system. As a result I found that probe adds random noise between 3 and 10 kHz and narrow peak at 24kHz.
But as you can see on the next picture, power line noise still exceeds measurement errors. This line measurement was done without load, and you will see that amount of current significantly changes the picture.
Power line noise is found mostly under 10kHz with some additional narrow peaks at 20kHz and above. Before doing these measurements in audio range, I used high frequency spectrum analyzer to see if there is any power line noise above 100kHz but found none.
I did tests with two types of load: 200W incandescent bulb and 1700W iron. Results were significantly different.
Here we start with measurements at 200W. Picture below shows what filter is doing when 200W bulb is connected to low current output bank. In this and all other pictures orange graph shows noise found with load directly connected to wall outlet. This allows an easy comparison. As you can see, Furman filter practically eliminates any noise above 30kHz but apparently adds noise starting at 5 kHz. I am not sure what the reason for that is. It could be some resonance in filter circuit or additional non-linearity introduced by DC elimination. At the same time there is almost no difference for harmonics of power line frequency below 2kHz. Aggregated THD of power line also was not any different when filter was used.
Next measurement was for 1700W load, which is close to maximum rating of Furman filter. Also I found that voltage drops significantly in my house when that amount of load is introduced: from 121V without load to 109V at 14 amperes consumed from wall outlet. I do not use that much power regularly in that room and my music room has a dedicated power circuit which only drops to 117 volts under 1700W load. I observed that Furman filter activated a low voltage warning light when I connected 1700W load.
As you can see now raw power line has a significant random noise above 5kHz. This noise is now suppressed by 10-15dB by filter. There is also no additional noise observed with lower load. Though some ultrasonic peaks are higher than what they were under low load, but still noise suppression is obvious. It seems that Furman filter works better under high load when using low current output.
Next test was done for high current output bank, again with 200W and 1700W load. With 200W bulb connected to high current output there is now no noise additional noise bump above 5kHz and Furman filter effectively suppresses ultrasonic noise above 20kHz.
With 1700W load, result is similar to what was observed with that load connected to low current output bank. We again observe 10-15dB reduction in ultrasonic noise, but a bit of additional noise in 2-5kHz range.
From what we can see here I cannot say that low current output bank is better filtered than high current bank. At lower load high current bank seems to do better job.
While objective results showing that Furman P-1800PF-R unit is actually work as power line filter in audio range, the ultimate assessment would be to hear if that makes any difference in my music system. Thus, I conducted a listening test by playing the same track when all my components were connected directly to wall power and repeating that play right after I connected components to Furman unit. For obvious reason it was not a blind test, but I also invited a second person – my wife, who has no stakes in that audiophile game but demonstrated her ability to hear subtle difference in sound quality before.
I played mostly high-res tracks recorded at different periods from 1960s to 2020s. Types of music were also very different: from classical to jazz to electronic.
Both me and my wife agreed that this was a very hard exercise with sound differences much smaller than all other comparisons we did before. In the end we agreed that there was a difference, but we could not come to conclusion that Furman filter made sound consistently better or consistently worse. It was on case by case basis. Overall, I found that bass frequencies got a bit more diffused when filter was used, high frequency percussion sounds were also somewhat smoothed out. One particular track that was from live recording had applause sound which felt further back from speaker plane with direct connection, while it felt closer while using filter. Other tracks which had close miked vocal sounded rawer (in a good way) without filter.
Subjective evaluation resulted in my decision to ship Furman filter back for refund. It may help some people when they have major power line noise issues but, in my case, it does not make any appreciable difference.
I found a good deal for that unit and it arrived within few days. This model has three banks of outlets: two for low current devices (which they advertised as “isolated”) and one more for high current devices like power amplifiers. In my case I planned to connect Oppo and Accuphase to low current band and active ATC speakers to high current bank.
Furman P-1800PF-R feels solidly built, front panel has voltmeter display, which supposedly show power line voltage, button to increase/decrease brightness of said display (I found that it does not keep setting between on/off cycles) and power switch. The unit looks nice, if you like professional grade equipment.
First time I powered it up using 200W incandescent bulb as load and everything worked as expected using all three output banks. At that time I was convinced that this device is nt broken and I do not go through RMA.
I did a search, but could not find any internal picture of this particular model, thus I decided to open the cover and see what is inside. There is no protective seal on it, thus I was free to undo a dozen screws holding top cover and easily removed it. Here I am offering you a picture I made.
Overall internals of Furman filter were within my expectations. Filter board includes standard CLC filer with addition of DC elimination. It also has multiple elements to absorb high voltage transient to protect against random surges. I do not know exactly what soft start circuit is doing here and it was not advertised by manufacturer. RMS voltmeter board is connected to the display on the front panel. Furman has some models of power filters where they have current meter too, and I found some unpopulated positions on that board which probably reserved for components in current metering circuit. There is also large black component, which I could not identify. It looks similar to large value film capacitor but could be something else too. It likely serves the advertised function to supply up to 45 amperes of peak current to high current load, thus improving power factor. Though I do not have any knowledge or way to verify that claim. I found that the only thing that separates two low current outlet bands is a tiny common mode choke, made by ferrite cylinder through with both line and neutral wires are passing. This means that there is no real separation of two low current banks at frequencies below at least several megahertz. Front panel switch seems serving current protection duties – this component is either thermal or magnetic circuit breaker rated for 15 amperes.
Now it was time to make objective tests of this Furman filter. For that test I used my audio measurement system which can work up to 90kHz. Here I added a high voltage differential active probe (Micsig DP10013), which allowed me to safely connect measurement system directly to power line. Micsig device is designed to be used with oscilloscopes, with at beast 12 bits ADC, and I was using it here with more than 20 effective bits measurement system. As a result I found that probe adds random noise between 3 and 10 kHz and narrow peak at 24kHz.
But as you can see on the next picture, power line noise still exceeds measurement errors. This line measurement was done without load, and you will see that amount of current significantly changes the picture.
Power line noise is found mostly under 10kHz with some additional narrow peaks at 20kHz and above. Before doing these measurements in audio range, I used high frequency spectrum analyzer to see if there is any power line noise above 100kHz but found none.
I did tests with two types of load: 200W incandescent bulb and 1700W iron. Results were significantly different.
Here we start with measurements at 200W. Picture below shows what filter is doing when 200W bulb is connected to low current output bank. In this and all other pictures orange graph shows noise found with load directly connected to wall outlet. This allows an easy comparison. As you can see, Furman filter practically eliminates any noise above 30kHz but apparently adds noise starting at 5 kHz. I am not sure what the reason for that is. It could be some resonance in filter circuit or additional non-linearity introduced by DC elimination. At the same time there is almost no difference for harmonics of power line frequency below 2kHz. Aggregated THD of power line also was not any different when filter was used.
Next measurement was for 1700W load, which is close to maximum rating of Furman filter. Also I found that voltage drops significantly in my house when that amount of load is introduced: from 121V without load to 109V at 14 amperes consumed from wall outlet. I do not use that much power regularly in that room and my music room has a dedicated power circuit which only drops to 117 volts under 1700W load. I observed that Furman filter activated a low voltage warning light when I connected 1700W load.
As you can see now raw power line has a significant random noise above 5kHz. This noise is now suppressed by 10-15dB by filter. There is also no additional noise observed with lower load. Though some ultrasonic peaks are higher than what they were under low load, but still noise suppression is obvious. It seems that Furman filter works better under high load when using low current output.
Next test was done for high current output bank, again with 200W and 1700W load. With 200W bulb connected to high current output there is now no noise additional noise bump above 5kHz and Furman filter effectively suppresses ultrasonic noise above 20kHz.
With 1700W load, result is similar to what was observed with that load connected to low current output bank. We again observe 10-15dB reduction in ultrasonic noise, but a bit of additional noise in 2-5kHz range.
From what we can see here I cannot say that low current output bank is better filtered than high current bank. At lower load high current bank seems to do better job.
While objective results showing that Furman P-1800PF-R unit is actually work as power line filter in audio range, the ultimate assessment would be to hear if that makes any difference in my music system. Thus, I conducted a listening test by playing the same track when all my components were connected directly to wall power and repeating that play right after I connected components to Furman unit. For obvious reason it was not a blind test, but I also invited a second person – my wife, who has no stakes in that audiophile game but demonstrated her ability to hear subtle difference in sound quality before.
I played mostly high-res tracks recorded at different periods from 1960s to 2020s. Types of music were also very different: from classical to jazz to electronic.
Both me and my wife agreed that this was a very hard exercise with sound differences much smaller than all other comparisons we did before. In the end we agreed that there was a difference, but we could not come to conclusion that Furman filter made sound consistently better or consistently worse. It was on case by case basis. Overall, I found that bass frequencies got a bit more diffused when filter was used, high frequency percussion sounds were also somewhat smoothed out. One particular track that was from live recording had applause sound which felt further back from speaker plane with direct connection, while it felt closer while using filter. Other tracks which had close miked vocal sounded rawer (in a good way) without filter.
Subjective evaluation resulted in my decision to ship Furman filter back for refund. It may help some people when they have major power line noise issues but, in my case, it does not make any appreciable difference.
