Manufacturer's Specifications
Power Output: 100 watts per channel, both channels driven, from 20 Hz to 20 kHz, 8-ohm loads: 175 watts per channel into 4-ohm loads:
350 watts bridged mono into 8-ohm loads
Rated THD: 0 05%
SMPTE-IM Distortion: 0.05%.
Input Sensitivity: 10 V rms for rated output
Damping Factor: 300.
Frequency Response: 2 Hz to 100 kHz. +0, -1.0 dB
S/N Ratio: 100 dB below rated output
Input Impedance: 10 kilohms.
Slew Rate: 50 V/uS
Power Requirements: 100, 120, 200, 220, or 240 V a c.: 50/60 Hz, 500 watts maximum.
Dimensions: Rack version. 19 in. (483 mm) W x 5 1/4 in. (133 mm) H x 14 in. (356 mm) D: with wood cabinet option. 20 in. (508 mm) W x 6 1/4 in. (159 mm) H x 14 1/2 in. (368 mm) D.
Weight: 35 lbs. (15.9 kg).
Price: $995.00
Company Address: 5923 North Nevada Ave., Colorado Springs. Colo. 80907.
The Spectrascan Avatar BPA-100B (as well as the meter-equipped BPA 100M) is one of the most unusual amplifiers I have ever had the opportunity to test. I still suffer from somewhat mixed emotions concerning this novel design. For more years than I care to tell, it has been my contention that amplifier evaluations must include thorough bench testing as well as listening tests. For virtually all the amplifiers I have tested, this philosophy has been borne out, and I have run into relatively few problems in pursuing that approach. Spectrascan's BPA-100B is one of the notable exceptions. Here's a blow-by-blow description of what I ran into the first time I tried to measure this amplifier.
Things went smoothly enough as I started to measure harmonic distortion at low output levels. I usually plot THD and IM, beginning at 1-watt output per channel and working my way up to and beyond rated output, to produce the familiar curve of Fig. 1, using a mid-frequency test signal of 1 kHz and two end-frequencies of 20 Hz and 20 kHz.
When I got to about 10 watts per channel, I noted that something strange was happening. Distortion, as read on my THD analyzer, was beginning to rise while I watched a steady-state signal at a fixed level! Examining the distortion components on the 'scope. I noticed, to my amazement, that "spikes" were showing up (and growing) at the points corresponding to the zero-axis crossings of the sinusoidal test signal. After a few seconds of this, I was convinced that there was some thing wrong with the amplifier, perhaps some sort of thermal runaway. Had I had a complete owner's manual and some of the literature which Spectrascan's president and chief designer, Jan C. Hoigaard, sent me later. I might not have jumped to that conclusion so quickly. I quote from that literature:
"A word about BENCH TESTING:
This unit was designed for ultimate performance with even the most demanding music material, rather than with steady-state laboratory test-bench signals. For this reason, the automatic bias-control circuit, which is one of the advanced features of this amplifier, is optimized for music signals, rather than steady-state sine waves. This remarkable control circuit is so fast that the amplifier will be fully stabilized and reach optimum operating conditions in approximately 60 seconds from cold turn-on. With the Spectrascan, Inc. amplifiers, there is no need to wait for one hour or more for optimum performance! "This amplifier, therefore, does not require, nor benefit from, the IHF [sic] prescribed one-hour conditioning at 1/3 power, prior to bench measurements.
Such conditioning may cause excessive heat-sink temperature and cyclic shutdown. (NO DAMAGE WILL RESULT!) "To perform bench measurements, simply turn the unit on, leave on for a minute, and proceed with the measurements."
What the above paragraphs fail to mention is that after each measurement, you had better turn off the signal, allow some time for the self-adjusting bias circuitry to readjust itself, and proceed to the next measurement! That is what I ended up having to do in order to come up with the results shown in Fig. 1, and the process, as you might guess, took several hours! I have no problem with all of this, especially since everything stated regarding the sonic excellence of the amplifier under music-signal conditions turned out to be true. What concerns me, just a little, s the fact that this amplifier would not, under any circumstances, be able to meet the power rule requirements of the FTC (not the IHF, which simply adopted the Federal Trade Commission's wording in its standards). I'm not talking about the preconditioning part of the rule, which Spectrascan dismisses so lightly. I'm referring to the requirement that rated output be sustained for at least five minutes, and during those five minutes rated THD must not be exceeded. There's no way this amplifier could meet that requirement. Still, if that doesn't bother Spectrascan, I guess I'll say no more about it. It is. after all, the musical reproduction capabilities of an amplifier that count, Federal rules notwithstanding....
Panel Layout
AC On and "DC On" indicator lights are found at the upper left and upper right of the front panel of the BPA-100B. A "Power' switch is located below the "AC On" light, while a headphone jack can be found below the "DC On" light. A mode selector switch at the center of the panel selects nor mal stereo, mono, or bridged mono operation of the amplifier. Left- and right-channel clipping indicators are located on either side of the mode switch. The front panel is equipped with the usual pair of rugged handles associated with rack-mountable equipment.
The BPA-100B has a single pair of input jacks on its rear panel. In the mono or bridged mode, only the right input jack is active. Three pairs of five-way binding posts are used for speaker connections. Each pair is on 3/4-inch centers so that dual banana plugs may be used. The top pair is for the right-channel speaker, the center pair for the left channel, and the bottom pair for bridged operation. The red binding post of the left channel is actually connected to chassis ground. Because the two channels of this amplifier are out of phase with each other (to simplify bridged mono operation), the red terminal must, nevertheless, be connected to the "hot" or "+" terminal of the associated speaker to maintain proper acoustical phasing of the two channels. This arrangement also precludes using a common ground wire for speaker connections. In home use, this seldom poses a problem, but audio dealers having speaker switching panels that employ a common ground.
LOOK OUT! In addition to two massive heat-sinks, the rear panel also contains an unswitched a.c. convenience outlet arid a fuse-holder containing a replaceable 5-ampere slow-blow fuse.
Circuit Design Philosophy
The following circuit information was condensed from the owner's manual supplied with the BPA-100B. There are essentially two important and novel de sign techniques used in this amplifier: Nested, multiple-loop feedback net works and an electronically regulated power supply. Using nested, multiple feedback loops makes it possible to employ poles and zeroes positioned so that open-loop phase shift remains less than 90° beyond the loop gain crossover frequency, even though overall gain roll-off may exceed 12 dB per octave. This would seem, at first, to fail the Bode criteria for stability, which state that each 6 dB of roll-off adds 90° of phase shift to the feedback loop's desired 180°, that more than 90° (or 6 dB) leads to marginal stability, and that 180° (12 dB) turns the circuit into an oscillator. However, it can be shown that it is the relative roll-off closure rate between adjacent loops that deter mines stability and not the absolute roll-off rate. Expressed somewhat differently, each successive pole is can celled by a zero in the next nested loop, so the accumulated loop phase shift remains less than 90°.
All feedback loops originate from the amplifier output terminal. The outermost loop is fed back to the input-stage summing junction, while the remaining inner loop feeds back to inter mediate stages. Unlike conventional single-loop designs, the early stages within the outer loops of a multiple-loop amplifier receive the least amount of feedback, while the output section receives the largest amount of feedback.
According to Spectrascan, this is an ideal situation, since most open-loop distortion originates in the output stage.
The output section of the amplifier uses a symmetrical, mirror-image com pound stage that yields high linearity and high efficiency. With a regulated supply voltage of ±45 V. the output stage is capable of a swing of ±43 V into 8 ohms before the onset of clip ping. That is precisely the power level I measured for this amplifier before it reached rated distortion.
Although the advantages of a brute-force "soft- power supply versus a stiffly regulated one can be argued (and have been, endlessly). Spectrascan cites the following advantages for the "stiff" or regulated-supply approach they have elected to use in their BPA-100 series of amplifiers:
1. Specified rated output power is always available, even with music waveforms that are square-wave shaped:
2. A regulated power supply reduces TIM phenomena caused by the shift in transistor operating points which results from supply voltage variations:
3. Power-line voltage variations (brownouts, etc.) will not affect the amplifier's audio power-output capability, nor will high line voltages cause damage to the amplifier, and
4. An electronically regulated power supply offers new possibilities for circuit protection without introducing TIM associated with conventional current-limiting circuits that come on gradually.
A regulated power supply may simply be electronically shut down on command from overload-sensing circuitry.
The obvious disadvantage of a regulated supply (and there are those who don't consider it a disadvantage at all) is that amplifiers employing this design exhibit virtually no dynamic headroom.
Maximum output power is the same for short-burst signals as it is for steady state signals.
Measurements
Fig. 1 Power output vs. THD.
Using the rather unusual measurement methods described earlier, I measured exactly 115 watts per channel of output power, for 8-ohm loads, at all relevant frequencies. At rated out put (100 watts per channel). THD measured 0.018% at 1 kHz, 0.01% at 20 Hz and 0.03% at 20 kHz. SMPTE IM for rated output measured 0.035%, while CCIR IM measured a much lower 0.0016%. IHF IM was below the measurement limits of my test equipment (well under 0.03%), and since it appears that IHF IM is a good indicator of the presence or absence of TIM, this would tend to confirm Spectrascan's claims regarding dynamic forms of distortion. With 4-ohm loads, the amplifier was able to deliver just under 200 watts per channel for its rated THD of 0.05% at mid-frequencies. At rated power output (175 watts per channel), THD measured 0.01% at 1 kHz and 20 Hz and 0.05% at 20 kHz. SMPTE IM under those conditions measured 0.04%.
Frequency response extended from 2 Hz to 100 kHz for the-3 dB roll-off points. Input sensitivity, referred to 1-watt output, measured exactly 100 mV, Since 1 watt is exactly 20 dB below the rated output of this amplifier (into 8 ohms), that sensitivity corresponds exactly with the 1 V (for rated output) claimed by the manufacturer. Unweighted signal-to-noise ratio referred to 1 watt measured 72 dB, increasing to 79 dB when an A-weighting filter was introduced. Adding 20 dB to that figure (to reference the S/N to rated output, as Spectrascan chose to do) brings the figure to 99 dB-close enough to the claimed 100 dB.
Dynamic headroom, as I expected, was almost nonexistent, measuring less than 0.5 dB. Damping factor, referred to an 8-ohm load and measured with a 50-Hz test signal, was in excess of 200. The amplifier was unconditionally stable under a variety of load conditions, including application of high capacitance values across the open circuit output terminals.
Use and Listening Tests
Despite the BPA-100B's inability to pass FTC power rule tests, it ran remarkably cool when fed with actual musical program signals, reproduced with peaks approaching maximum available power levels. Those arguing that the FTC rule is obsolete and doesn't represent real-world conditions can have a field day with this great-sounding amplifier. Not only did it coast along at relatively cool temperatures, but it never shut down during any of my rather demanding listening tests, during which there were several moments when the clipping indicators were flashing merrily away (deliberate y, of course). I felt that reproduction of some of my most demanding CDs was uncompromised, with good, solid bass and extremely good definition and transparency. In fact (and I am almost reluctant to admit this), some of the CDs which I had previously felt were a bit on the harsh side actually sounded a bit smoother when played through this amplifier.
Certainly $1,000 is a fairly high price to pay for 100 watts per channel of amplifier power. But to the very discerning and critical listener who is likely to favor this amplifier, no price is too high if the resulting sound is as good as it was during my several evenings of very pleasurable listening.
-Leonard Feldman
( Audio magazine,Jul. 1984)
Also see:
Sumo Model Nine Amplifier (Equip. Profile, Dec. 1983)
Streets Electronics Model 950 Power Amp (Jan. 1985)
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