- Input Impedance=8
- Output Impedance=7
- Required Attenuation=5
It doesn’t appear that much is needed to balance the sound of the drivers, so the first design used on-hand components. What was possible with the readily-available parts I had in my parts drawer was a -6.69dB attenuator, pretty close to the estimated -5dB. With a bit of solder and some jumper wires, I fashioned this attenuator and attached between the driver and the crossover network. The measured results are shown below.
This measured nice despite the loudness mismatch and HF roll-off above 8KHz. Subjectively, the sound was pretty dull and the ambience in concert halls was fairly suppressed, although present if you strained your ears and listened through these mismatched levels. The tininess of the overall sound was completely gone and had a very attractive warmth especially in the woodwinds. The timbre of drums was excellent and vocals natural and unstrained. The quality of the sound was acceptable although not what one would consider to be high-end (yup, this means the design is junk; back to the
Next was a properly designed attenuator again using available components and -3.98dB of attenuation. Also added was a 0.01uF Teflon shunt capacitor across the 4.0uF polystyrene crossover network capacitor to help with the high-frequency roll-off issue above 8KHz.
The level measured a pretty decent match with a surprising smoothing in the 4-8KHz region (non expected but greatly appreciated). An interesting side effect was the additional smoothing of the midrange in the 500Hz-2KH range. These are the “trickle-down” effects of a well-designed attenuator on the sound of the entire speaker. Now the sound of this attenuator is still a little on the bright side with sibilance noticeable but not objectionable. Hall ambience is very rich and full creating a large sound stage, one of the largest the system has ever produced. But after prolonged listening, it was clear that the minor over-emphasis in the 2-5KHz region needed to be tamed ever so slightly. This attenuation level is pretty “close” to what I audibly perceive as a good quality level of sonic reproduction so I knew I was getting close to the final design worthy of the term high-end.
The next attenuator was about -1dB quieter. Although consisting of more than one resistor per leg (there are three legs in a T-pad), all of the resistors used in the attenuator were of the non-inductive type and one was matched to only 11.5% of the required value (not as precise as I wanted but the only values I had on hand). Now I suspected that this minute level of change would be inaudible so I anticipated that measurements were really where any differences would be noted and my ears would be able to subjectively detect nothing. Let’s first see first what the meter tells us.
The meter shows a pretty significant change in the overall character of the system. Starting with the bass, there is a new peak at about 250Hz and a more pronounced dip at 500Hz. The midrange between 500Hz and 2KHz is not as uniform as in the well-matched -3.98dB attenuator, and the tweeter is not really attenuated as anticipated. The sound is very different where the emphasis of the system is shifted to the regions around the crossover point at 2.8KHz. There is still clarity in stringed instruments and sibilance is a little worse and less controlled, but nothing to write home about. What is most depressing is that there is a definite non-uniformity in the overall sound again with an overbearing dullness not indicative of a true high-end system and something that is a definite mistake. Cat Steven’s “Father and Son” is very revealing of this attenuator’s shortcomings so much so that I need not ask my wife if “this is a good change” for indeed it is not.
What did the calculator predict with the mismatch? Let’s see and use what it tells us to correlate what I heard against what was predicted. For this attenuator, input impedance=8, output impedance=7, and attenuation=4.97. Optimal values are therefore 3.084, 1.149, and 12.391. I used 3.0, 1.0, and 12.0 resistors causing the VSWR to change from 1 to 1.026 and 1.039, apparently minor shifts by the numbers (less that 4%). But the significant change in the sound told more than the VSWR would be a good indicator of how it would sound (good or bad). The VSWR for the better-matched -3.98dB attenuator was 1.013 and 1.003 (less that 1.5%). Putting it another way, a 4% VSWR is not good enough and less than 1.5% is audibly much, much better. Back to the drawing boards.
I have returned the attenuator to the -3.96dB version and I am far more pleased with the overall sound. While still not pleased with the entire performance of this attenuator, the sound is far superior to the -4.97dB more than 10% mismatched version. One day, when I order matched non-inductive resistors for this attenuator version, I will re-visit it to confirm the suspicion that it was the mismatch that caused the huge change in tonal balance. Hopefully the shortcomings of the present -4.97dB attenuator will also be addressed and this particular link in the chain will be made as strong as it can possibly be.
This exercise hopefully demonstrated to you the need to invest serious time in precisely matching values in if nothing else the modest audio speaker attenuator. If you correlate these findings with other potential sources of similar sonic anomalies in amplifier and preamplifier design (transistor matching, tube matching, resistor matching, capacitor matching), you can begin to appreciate what it takes to produce a thoroughbred piece of audio gear. What reared its ugly head in the audio attenuator is a symptom of what care must be taken when designing and even more important in manufacturing a fine piece of audio gear regardless of type (speaker, amplifier, CD player, active equalizer, etc.). High tolerance capacitors and resistors are essential as are matching active devices. This will assure all of the sound each piece of gear is capable of producing will in fact make it unaltered to the output.
Like any chain, your audio system is only as strong as its weakest link. Extreme attention to detail AT ALL LEVELS must be made to assure the entire chain is as strong as it can possibly be. Hand matching components in the circuitry of your existing equipment to within 0.1% (or better) of optimum values should be something DIY hackers could easily accomplish and improve the sound coming from an average piece of gear (not from your high-end gear since hopefully this has already been done).
Yours for higher fidelity,
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