by Jan Meier
[Editor: This article is a follow-up to A DIY Headphone Amplifier with Natural Crossfeed by Jan Meier and resulted from an e-mail discussion with Chu Moy – the author of An Acoustic Simulator for Headphone Amplifiers.]
If you have an amplifier with a mono-switch, then here is a little experiment: listen to a stereo recording (by headphone) in stereo mode, and then press the mono-button and watch the bass. If you hear the same way I do, then you will notice that the bass suddenly seems to have weakened – it has become less pronounced. The effect is similar to that what is heard with a crossfeed filter, only much stronger. Listening in mono does introduce cancellation of low frequencies, but there is also cancellation higher frequencies (which is generally is even stronger since, with normal stereo recordings, low frequencies are more in phase). With the crossfeed activated, a weak cancellation will only be present at low frequencies, but at all frequencies, the sum of the sound pressures at both eardrums always equals the sum of the pressures in stereo mode!
At first I also wondered about the apparent loss of bass, but actually, it is this unnatural, larger then life-size, uni-directional bass, that counts for most of the annoying effects of headphone listening. I know, the crossfeed sound is nothing for a bass-freak. One should not expect a punchy bass, only a relaxation of the sound. It’s like listening to loudspeakers – a balanced speaker does not jump at you at first hearing but is rather colourless/neutral/unobtrusive. The rewards come while listening for longer periods of time. A good speaker does not fatigue, and this exactly is the strength of the natural crossfeed filter.
To add bass or not to add bass….that is the question. I believe that most bass-losses are due to psychoacoustic effects, but after thinking it over more carefully, enhancing the bass response of the natural crossfeed filter could be legitimate, because headphone sound is optimized without using crossfeed. If there really is a psychoacoustic effect (a uni-directional bass is unnatural and I believe that, with headphones, this emphasizes its existence), then the effect has been (unconsciously) corrected for in the sonic design of the transducers.
The crossfeed design by Siegfried Linkwitz (see An Acoustic Simulator for Headphone Amplifiers by Chu Moy) includes a bass boost to compensate for low frequency cancellation. Figure 1 is a graph of the frequency response of both the direct-signal and of a mono-signal that is given on both signals simultaneously. Responses were calculated for a 60 Ohm load (such as headphones) and for a very high output load (e.g., a headphone amplifier).
As with the natural crossfeed filter, the direct signal with the Linkwitz filter shows a signal loss at lower frequencies, (-1.0 dB at 60 Ohms, -0.35 dB at 50k Ohms). However, more important is that a mono-signal at frequencies below 700 Hz is increased by up to 1.3 dB at a 60-Ohm load and up to 1.9 dB (!) at 50k Ohms. The delay times for the Linkwitz design (figure 2) are fairly natural, as the crossfeed signal has similar filter frequencies and thereby should have similar delay times as the natural crossover filter.
I designed a modified version of the natural crossfeed filter that has a frequency response very similar to the Linkwitz filter. It can be found in figure 3. The crossfeed level is medium. It easily can be tested between a CD-player and headphone amplifier (there is no insertion loss). I guess it sounds very similar to the Linkwitz filter, but is a little bit easier to realize. I have never implemented this Linkwitz equivalent, being fully satisfied with the original natural crossfeed filter.
The Linkwitz equivalent can be substituted for the filter in my headphone amplifier design. The bass EQ switch (S1) was not intended to compensate for any apparent loss of bass due to the crossfeed. It simply should compensate for the natural roll-off of the transducers. Such filtering is nothing new. With my headphones, I use position “3.” In this position, only the very lowest frequencies are amplified. Even with the Linkwitz equivalent, it could be very nice to keep the bass extension switch. Since it can be switched off, it will not hurt and the extra work/costs are little. It is, however, a matter of personal taste. Hi-fi purists do not like unnecessary equalization and might want to leave it out.
With the Linkwitz equivalent filter, it is not possible to set the crossfeed level with a 2 x 6 switch. Not only do the resistors and capacitors in the two outer networks (Z1) have to be switched, but also the central capacitor to ground. A possible option is to use a 3 x 4 switch (see figure 4) and leave the first two crossfeed positions out (they have a very weak effect, and I never use them).
To customize the time delay and amplitude profiles of the Linkwitz equivalent, download the circuit simulation spreadsheets below (in Quattro Pro 1 and 3 formats). Change the values B4..C9 at the first page according to figure 5 and the corresponding lines in the pictures will change. Similarly you can change the values in D4..E9 for a direct comparison of the different options.
(Please remove the .xls extension on the files above.)
Yesterday, I did some listening with popular music with heavy bass using the original natural crossfeed filter. The bass in these recordings was more “centered”. As expected, I could not/hardly notice any specific loss of bass. I present the Linkwitz equivalent design as an example of how the various filters can be tested to personal taste, by putting them between a CD player and a power/headphone amplifier, before eventually building the headphone amp project.
6/22/99: Added figure 2.
7/26/99: Added figure 5 and instructions for using spreadsheet circuit simulator.
5/4/00: Jasmin Levallois built this version of the Pocket Headphone Amplifier (see article by Chu Moy). It features an input gain stage, the Meier enhanced-bass natural crossfeed filter and an output buffer. He writes:
Finally I got some free time to complete my project…. I got a lot of work to do for school during the last few weeks and I didn’t have time to work on my amp. This weekend I decided to take one day to transfer the amp from the breadboard to the pc board. I used about the same circuit as Jeff Medin. The input stage has a gain of 10, the output stage is a voltage follower, and in the middle I put the Meier bass-enhanced crossfeed circuit.
I used 2 OPA2132 opamps, but if I had to do it again I would use 2 OPA2134. An OPA2132 costs $6.99 while an OPA2134 costs $2.67. Since there is almost no audible difference between both opamps, I would go with the OPA2134 to save money. Since the second stage has no voltage gain, I decided to omit the capacitor in front of the output stage. I also removed the resistor in front of the output stage, and I don’t hear any noise from the output stage. The only noise I can hear, sometimes, is coming from my CD player.
As you’ll see on the photos, the inside of my amp is very messy, but, hey, its my first electronic project. Fortunately, even if it’s messy inside, the outside looks pretty good. I really like this Serpac Enclosure (Digikey part no. SRH65-9VB-ND); it looks ways better than the PacTec case.
The photo of the battery compartment is to show that the Serpac enclosure has a 9v Battery compartment with battery contacts. It’s easier to remove the battery with that kind of battery compartment than the PacTec Enclosure. Also the Serpac enclosure is just about the same size as the Pactec enclosure except that it’s a bit longer, and the height is a little bit less. This might be a problem for the electrolytic capacitors. I would recommend the Philips ones with this enclosure rather than the Panasonic Z series because the Philips electrolytic caps are much smaller.
5/6/00: Gus Wanner developed a low impedance version of the enhanced-bass filter to drive his Sennheiser HD600 headphones directly from a power amplifier, and has prepared a MS-Excel application to model the enhanced-bass filter (both low- and high-impedance versions). DIYers can change component values and instantly see the effects of their changes on the filter’s frequency response, time delay profiles, etc. Wanner writes:
Compared to my Sennheiser HD25s, the HD600s have a lower sensitivity (the HD25s produce about 105 db SPL at 1mW into 70 ohms, while the HD600s produce about 97 db SPL with 1mW into 300 ohms), a higher impedance, and a maximum input level of 200mW. With the HD600s, 200mW requires about 7.75 volts across each phone. I measured the impedance magnitude versus frequency for both the HD25 and HD600 headphones. The measurements were made using an audio oscillator in series with a 300 ohm 1% metal film resistor. By measuring the voltage across each headphone and the voltage across the 300 ohm resistor, it is possible to compute the impedance.
I have attached a spreadsheet which provides my data and the impedance plots. It is interesting to note that both headphones have an impedance peak around 100 Hertz; in the case of the HD600s this peak has a magnitude of almost 600 ohms! I also looked at both the current and voltage waveforms on a scope; for both phones the waveforms are in phase for the entire audio range. Slight phase shift can be seen at 20 Hz and at 20,000 Hz, but I would say that the impedance of both of these phones is predominantly resistive.
To handle the HD600 inefficiency, I decided to look at Jan Meier’s crossfeed networks. Jan’s networks have the advantage of low insertion loss, but tend to be sensitive to both source and load impedances. I created a new spreadsheet with an analysis of Jan’s enhanced bass crossfeed filter, using his original component values (designed for use within his headphone amplifier) and a low impedance version designed for use with a nominal 300 ohm load impedance.
This spreadsheet is attached hereto as well; you can see a performance comparison between my final low impedance compenent values and Jan’s original. Because of the large impedance variation of the HD600s at low frequencies, I incorporated their impedance versus frequency into the design as wel. I did not assume a constant headphone load impedance in my analysis of the network, but actually put in my measured data of impedance versus frequency to see how the response of the network would be affected by the Senn HD600 peak around 80 Hertz. For other headphones, you can substitute the impedance magnitude versus frequency (if you have it) into the appropriate column on the spreadsheet. Alternatively, you could substitute the manufacturer’s nominal impedance value (e.g., 120 ohms or whatever) at each of the test frequencies.
Components for my crossfeed networks were purchased from Digikey – the capacitors are Panasonic ECQ-E(F) series metalized polyester film types which are available in values up to 10 microfarads at 100 WVDC at reasonable prices. Non-polarized electrolytics are NOT recommended for this application. The resistors are Ohmite TA Series “Power Chip” ™ thick film on an alumina substrate 5 Watt rating. These resistors have basically no inductance (50 nanohenries at 1 MHz!). Non-inductive wirewound resistors could also be used, however standard wirewound resistors are not recommended due to their inductance.
With this network, the HD600s have plenty of volume with my 20 watt/channel monitor amp and sound wonderful – the closest thing to electrostatic speaker sound I have heard! Again, note that the low impedance version of Jan Meier’s network is more sensitive to load impedance than the low impedance version of the modified Linkwitz network. With the HD600s, to get 200mW requires about 24.5 volts into each channel of the modified Linkwitz crossfeed network, which corresponds to an amplifier output of about 75 watts into 8 ohms! I did, in fact, hook up the modified Linkwitz crossfeed network to my 150 watt/channel power amplifier, and the Hd600s sounded great. But I wanted to use the HD600s with my McIntosh C40, so I had to look for another solution.