|Zarcon Dee Grissom's Idea Page|
|The waveguide below cut-off is not the only approach, there is another way involving a grounded screen of sorts. The trick involves using the smallest opening possible, and stuffing it full of ground wires, or shielded wires with the shield grounded. The abundance of grounded wire going threw the small opening acts as a screen of sorts, not allowing an opening large enough to let RF enter the opening easily. This dose not guarantee that the ambient RF can not induce some current on the wires going into the enclosure, it will only prevent most of the ambient RF from passing right through the opening.|
I specify EMI in the diagram rather then RFI, as RFI sometimes implies high frequency radio emissions, rather then interference magnetic and/or electric in nature of ANY frequency including DC.
|Pictured left is C-RAM LF, it is RF absorbing foam that will not shed carbon powder all over the PCB, and is available in weather resistant varieties. Ideal for use in this particular headphone amplifier.|
no larger size.
|The other features on the bottom of the board are not ancient magic glyphs, nor some mixture of Maxwell's equations and wizards magic. There "Distributed Element filter" folded line capacitors, and "folded line low pass filters". I placed these filters on the board to take over at frequencies above the 56mH inductors optimum operating frequency. I didn't create the folded line low pass filters, I adjusted the dimensions of a 1.5GHz low pass filter on CST's website to operate at the required frequency for this filter.|
The inductors are shielded to reduce there magnetic emissions, however they are still bar-inductors. If an external changing magnetic field is properly aliened with the inductors, they may act like tape-heads. To prevent this from happening I wanted to place a MuMetal box around each channels inductors. MuMetal sheets are not cheep (around Two-Hundred USD and up for "Engineering sample Kits"), and this may never happen. It all depends how sensitive they will be next to my monitors. The ferrite encasing around the inductors may be addiquite for the job.
I adjusted this filter to absorb the reflected signals to tame the SWR induced increase in the sound cards upper frequency range. The 10k-ohm resistor across the first inductor is to reduce it's reflectivity a bit, and the 22k-ohm resistor and 147 picofarad (100pF+47pF) capacitor before the first inductor is to absorb what is reflected by the first inductor. The 470k-ohm resistor across the fifty picofarad capacitor after the first inductor, is to tame some phase angle effects in the 7MHz range. I will eventually make a new board to accommodate the additional components. However, the current filter PCB seams to respond fine with the placement of the additional components.
|There is a small amount of crosstalk starting at 5kHz and up, that is probably from magnetic coupling of the inductors. The crosstalk disappears when this board is bypassed, and is insignificant compared to the crosstalk of the source sound card. About 0.0dB@5kHz to 4dB@43kHz over the baseline of the soundcard.|
|HPA17r16i Revision-1B is in the works, more parts need to be ordered at this point. The new 15mH inductors, and a replacement for the 50pF capacitors have already arrived. I need more resistors at this point|
|Old filter||Revision-1a Output RFI Filter|
The output filter was easier to get a lower frequency cut off, Due to the much lower impedance. However it needed to work with a wide range of impedances from 40ohms to 100 ohms. The other requirement is not excessively loading down or reflecting signals back at the amplifiers output buffers within there operating frequency range (DC to 210MHz).
|It took considerable time adjusting values of parts to get the filter close. I was not entirely happy with the VSWR spike at about 15kHz, however I thought this was irrelevant at audio frequencies. In lieu of the flat response on the other charts, I was not inclined to change the filter until I saw how the VSWR effected the output at 15kHz. No one I asked new if the VSWR spike would adversely effect an audio amplifier, now we know.|
The 1.35 to 1 VSWR at 15khz, caused a +7dB increase at 15kHz in the amplifiers feedback loop. The other +1.5dB spike just before this filters cut-off was caused by the old revision of the 133kHz input filter.
|PCB sim-cap||Old filter||New filter (revision-1a)||Placement shot|
858x453 eg 300dpi
|Glamor shot||Placement shot|
|PCB sim-cap||Glamor shot||Placement shot|
|Home > Ideas > HPA17r16, Intro.|
HPA17r16 Operating Environment.
HPA17r16 RFI filters.
The Main Amplifier Board.
HPA17r16 Vmon & Sources.
HPA17r16 Tests & Afterthoughts.
|My Email link. copy and past. email@example.com |
or is the status "coffee is good" on yahoo messenger