Some say that a Passive Preamp might just be the best value for the dollar, but for others it just doesnt work. Why? Ill show you a way of checking it out without spending more than a dollar.
The thread below and others like it contains many valid opinions and experience that serve as a warning that a passive might not work for all systems. Other sources warn you need to be careful when designing and applying it. A general summary that hits some of the low and high spots, starting with the lows follows:
1) A thinness of sound even though there is a greater detail.
2) A loss of base and punch (which might account for item 1.)
3) A loss of high frequencies.
4) A loss of gain that might be unacceptable.
The highs:
5) Magnificent detailing and sound stage equal to the best preamp.
6) It of course is not a preamp, which is an active circuit. The Passive is, a combination of wires, switches and resistors with capacitance from the interconnects, thus there is no added noise from transistors, tubes or power supplies. Distortion is zero, the noise floor is minus infinity. Because of the circuitry it acts as a high and low pass filter. Thus, if by good design there is no frequency loss, it is the standard by which the best preamp can be judged.
My personal concerns were:
7) Although my class A amp is reputed to have tube like sound I never heard it without the tube preamp. Would the sound be too harsh?
8) The black box mystery: how my equipment would react as the internal circuitry of the amp or source might cause low or high frequency loss?
9) Would there be enough gain?
Ref. 1 (see end) cautions:
10) Loss of base if the Passive Pot resistance is too low (Source to Passive side). This is due to capacitors inside the source reacting with the Passive resistance (the maximum resistance across the terminals.)
11) Loss of highs if the Passive Pot resistance is too high. This is due to the capacitance of the interconnects (Passive to amp side) and whatever capacitance in the amp reacting with the Passive resistance (terminal to wipe).
12) Ref. 1 recommended 10K Ohms for the pot while others recommended 25 K ohm for large stereo and 10K for portable devices to address Items 10 and 11.
13) In Ref. 1 the diagram for high frequency loss is for a 10K Ohms Pot. It says interconnects of 6-feet is acceptable. For 25 K Ohms shorter interconnects would be required.
14) What is the power rating of the Passive components? This is factional wattage and I used 22 g wire.
15) Item 14 indicates that base punch or speed is not lost because of a power drain on the source.
16) There is no gain loss thru the Passive, however the gain from the preamp is being eliminated. It is thus possible not to have enough gain with the Passive alone. I think both the 10K and 25K ohm pots will have the same gain. The current is higher for the 10K, the voltage (or gain) max out at 2 V.
17) There might be a power increase from the amp. The source to passive is a sub-base filter. Sub-base eats power that speakers cant reproduce and is not part of the musical program.
18) My favorite tapes and LPs have been transferred to digital. But new preamps have almost nothing on them so I would have to buy a phono-amp anyway.
So for $1 - I plugged the CD player directly into the amp - but there is no volume adjustment. Thus I burned movements 4 and 5 of Berlioz Symphonie Fantastique at five levels using sound editing software. (Some are available as free downloads). -24dB achieved low and -12 dB loud. Of course all systems have different gains. My class A is only 25 watts driving an 88db/watt/m speaker. The results of this first test were impressive (Items 5 & 6 were realized and Items 7 & 9 & 16 were not an issue.). Thus the next test: would adding a Passive eat the improvements?
So for $0 I built a Passive from parts I had in the basement. I converted the circuit to a step resistance design. Using a bread board and 1% resistors was able to test 10K and 25K design. The results of this more exhaustive and lengthy test was likewise positive for all types of music. I didnt notice any loss of highs or lows on either the 10K or 25K circuit. During this test I used 3 and 6-foot interconnects.
The next test required purchasing it cost about eighty dollars.
20) I used a 25K dual stacked log pot that had concentric shafts to adjust the balance found at guitar supply stores. The balance did need adjusting, nothing at high volume and more and more at low volume. This was due to a 300 ohm offset between pots which I later adjusted by adding a resistor. (Advice: take the average of the resistance difference in four or five positions under 3K ohms.) Now the balance is perfect regardless of volume. So you might buy a cheaper ganged log pot without concentric shafts and correct the offset resistance at low volume. At high volume (resistance) small offsets becomes insignificant. You could also use individual pots but it is a real pain getting the balance perfect every time the volume is changed.
21) I am using 3 interconnects from CDs and tuner to equalizer, 1.5 to Passive and 1.5 to amp. Finding a spec on a total (line and connectors) interconnect is almost impossible. Some comment that the connectors are a big part of the overall capacitance.
22) For two or more sources you will need a switch box. I am using an equalizer with by-pass switch. It was also a life line incase I lost lows or highs.
23) I did not lose base or punch in fact the base is more solid and better defined. (I wont talk of speed because I believe the speakers are so much slower than the electronics that it is a non issue.)
24) Some reviewers commented on the Rotel RCD-1072 (my test CD player) that because of the low noise floor it sounds like there is a black hole between instruments. I hope you dont think that is thinness. The Passive will increase this separation giving each instrument more stand-aloneness.
After a year of enjoying this home built unit I decided to write this article. To be honest I was so delighted by the improvement (musicality I have never heard in a high end store or my system) I stopped testing. Thus to complete the evaluation I needed a test to determine if any high or low frequencies were lost? This is the sticky part of this article. Unfortunately I no longer have access to a calibrated industrial sound meter. The speakers are rated 25 to 20K +/- 3 dB. That is a 6 dB change (= +/- 3) and is equal to a 50% loss of gain at high and low ends. I cant hear 18K and 15K has to be boosted lots, so if I lost the very highs I couldnt hear it anyway. The room is overdamped with a rug and a blanket hanging on the wall to control reflection. Most listening rooms cancel very low base. But here is how you can test your results.
25) Using sound editing software again I rerecord an audio test disc step frequency at different dB levels for a comparison of: a) CD direct to Amp b) CD to Passive to amp, and c) CD to preamp to amp. Also the various dB levels allowed me to compare the Passive Pot in maximum and listening volume position. Nothing disappeared within my hearing range; 40 12K were unchanged in all tests.
26) I achieved my personal goal greater purity of sound reproduction. Some want an amp or preamp that drives the base. My philosophy is that purity can never be restored, seek purity first then add the frills when required, like a sub woofer or equalizer that can be taken out of the system when not required.
I was fortunate in that my system responded positively to this experiment. It passed test Items 1-9 using cautions in 10-13. The Passive box (plastic outlet box) doesnt look great but it is small and hidden. It works great so I probably wont buy commercial Passive till something malfunctions. I hope this article encourages you to try something new for little or no investment. Good listening.
An after though for new product: We all like to tinker, it makes us sit up and listen. People role tubes, change phono pickups, switch interconnects and cables, all looking for a new angle on the music. What if a new preamp design came with multiple universally modularized slots so you could switch-out units, like cards in a PC? It might include four bays for: the glowing sound of tubes, the youthful fast sound of transistors, the pure priestly sound of passive, and a flaming designer sound, all available at the turn of a selector switch.
REF 1: http://richard-mudhar.suite101.com/use-a-passive-preamp-for-excellent-sound-quality-a128047 Shows circuits, and graph of high frequency loss for interconnect lengths.
Other: http://sound.westhost.com/project01.htm Better Volume (and Balance) Controls Rod Elliott - ESP / Bernd Ludwig
