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1/27/01: Gus Wanner has prepared an Microsoft Excel application for simulating the operation of the Ohman crossfeed filter, so that DIYers can test the effects of component value changes on the filter's frequency response, time characteristics and channel separation. Wanner provides simulations for 3 levels of crossfeed (3dB, 6dB and 10dB). He writes:

The attached Excel 5.0/95 spreadsheet is an electrical analysis of Ingvar Ohman's crossfeed network as published in Toni Kemhagen's article on construction of the Lindesberg Portable Headphone Amplifier With Crossfeed. The spreadsheet includes plots of left and right channel output levels, channel separation, interchannel delay time, network input impedance and network output impedance. I have also made included entries for source resistance and load resistance.



This crossfeed network is interesting in that it is the first one I have seen which deals with the problem of excessive high frequency channel separation when listening to normal stereo recordngs with headphones. Since the optimum value of interchannel crossfeed is different for different recordings, I have included additional network analysis sheets for 3dB, 6dB, and 10dB low frequency channel separation. The shape of the channel separation versus frequency characteristic for these alternative networks follows (as closely as available component values permit) that of the original Ohman network specified in the referenced article. Since the crossfeed resistor (R4) is the same for all networks, a simple 2 pole 5 position switch would provide the ability to switch between the different networks for different degrees of crossfeed.



I appreciate the need to implement source impedance matching for opamps which are sensitive to source impedance when used in the non-inverting mode. Obviously, if source impedance is not constant (as with a gain control potentiometer at the input of an amplifier or, as here, with multiple crossfeed networks), source impedance matching becomes a major problem. A better solution would be to use opamps which are not as sensitive to this phenomenon. The manufacturer's data sheets generally provide this information as part of their application notes.



One note about these networks - as currently parameterized they are very sensitive to load resistance. It might be desirable to reparameterize the networks by dividing all resistances by 4 and multiplying all capacitance values by 4, thus lowering the output impedance by the same factor. If FET-input opamps are used, the existing values will not be a problem.

The worksheets are protected (without a password) to prevent accidental deletion of calculations. Input signal magnitudes, source and load resistances are entered on the "original" network page and are automatically carried over to the other pages. Note that these spreadsheets make use of the Excel complex number capabilities which are included in the "Analysis Package". This comes with the Excel package but may need to be manually installed using the original Excel installation CD or floppies. Current editions of Quattro Pro also include complex number analysis capability and may be able to load this spreadsheet.

Download Gus Wanner's MS Excel Simulator for the Ohman Filter

5/29/01: Gus has prepared a new version of his Ohman Filter Simulator. Computationally, it is the same as the earlier simulator, but includes a schematic for adding a variable Ohman crossfeed filter to the Landesberg (or any other amp). Thanks to David Richard Meddings for the submission.

Download Gus Wanner's MS Excel Simulator for the Ohman Filter

10/21/2001: The author presents an update to his amplifier design incorporating a sound image width control that operates separately from the Ohman crossfeed filter. He does not recommend modifications to the Ohman filter itself as a means of changing the width of the sound image.

It is wrong to change the setup of the Ohman filter when you want to tune the stereo image. The Ohman filter is made to mimic a normal loudspeaker setup, and the differences in people's hearing normally deviate only few percentages from the Ohman filter response. Modifying the Ohman filter's response is not an option because then the filter is not an Ohman filter anymore; it's just an "effect-box" that does something strange that may sound good to the user. Only one setup, the original Ohman filter setup, is to be used as a "true headphone monitoring system."



My variable Ohman filter uses a stereo expander in the feedback path of the input buffer before the crossfeed processor, so that you will be able to tune the stereo image without changing the setup of the Ohman filter. Imagine that you are listening with loudspeakers and you listening to a recording with a narrow stereo image. When you put the loudspeakers farther apart, the time difference between the ears then has to increase. You see the same thing in the graphs below.

The filter takes the signal from one channel and puts it at the other channel's negative input and, therefore, amplifies the differences between the channels, like a spatial expander. When the switch is set to "original," you have the original Ohman crossfeed network working. When the switch is set to +1,+2, +3 and +4, both the stereo expander and the crossfeed are working, and the width of the audio image increases more and more. The widest sound image is at setting +4. When the switch is set to "stereo", both the stereo expander and the crossfeed are disconnected, and the amp is in normal stereo mode. The switch is a 6-position, 2-pole shorting type - for example the ELMA typ01 (Elfa order no. 35-493-18).

I am very pleased with the result of this Variable Ohman crossfeed filter. I find it most useful to fine-tune the stereo image when the image is too narrow. It works with all types of music. But the most recordings don´t need to expanding, only if the sound image is too narrow or "boring".


Frequency Response of Variable Ohman Filter (Freq. vs. Volts)


Time Response of Variable Ohman Filter


Channel Separation of Variable Ohman Filter


Frequency Response of Variable Ohman Filter (Freq. vs. dBV)

11/17/2002: Corrected polarity of electrolytic caps for negative power supply.

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