| Stereo Hybrid Power Amplifier |
For many, the analog record has served its purpose and been replaced by the more practical and reliable CD. Sonically, though, a CD's reproduction is debatable. Some music lovers believe that CD's don't sound bad, but are missing musicality. Almost nobody can be more specific about this tenuous feeling.
This isn't the place for a discussion about digital recording procedures, as it could become philosophic and polarised. On the other hand it can be demonstrated physically that some technical details in CD players have a poor effect on the reproduction of music, but can certainly be alleviated. For that you don't need to suffer delusions of grandeur and buy a CD player for DM 20 000. Those with experience in CD technology can achieve impressive sonic improvements at much less expense. That doesn't mean that a modified bottom-of -the-line CD player can replace a more expensive model. A stock, good-sounding player is the more reasonable basis for tuning.
It doesn't matter if a CD player has a 16, 18, or 1 bit converter. The megahertz sampling rate on the digital end controls sound quality. In spite of more or less complicated filtering technology, some high-frequency garbage always appears at the player's analog output. As the human ear is almost insensitive above 20 KHz, this noise is not apparent as such. It appears as modulation of the audible signal and is responsible for intermodulation and TIM distortion. Two closely adjacent high-frequency noises within the same frequency band can generate interference in the audible range.
An improvement is an active low-pass filter in a separate housing to prevent induction and capacitive cross-talk so that high frequency noise does not sneak through the filter.
A filter circuit of this kind, based on a PCC 88 double triode is reviewed here. It is a four-pole Bessel filter with a 50 KHz cut-off frequency (-3dB) so phase shift under 20 KHz is negligible and does not negatively influence sound quality. The circuit's input impedance is 400 K; its output impedance is 330 ohms. The amplification factor is one. So this circuit can be viewed as an impedance converter. CD players with high-impedance outputs such as those with volume controls mate well with this unit. The CD filter is capable of driving a power amplifier, so a preamplifier is unnecessary where the power amplifier has enough gain at its input and the CD player has a volume control.
Constant Current Sources
The circuit reviewed here features constant current sources in place of conventional cathode resistors. In its basic form a constant current source is a field effect transistor (FET) whose gate and source are connected. Constant current is generated when a voltage is applied between the drain and source and is independent from the applied voltage over a wide range. It is only dependent on the impurities in the semiconductor (See Picture 1). The advantage this circuit has over other current stabilizing measures it that it does not require a separate power supply for the transistor. A current source based on a voltage reference, the most common configuration, has to be coupled to a separate operating voltage. This causes noise. Another advantage of the FET-based current source is its low heat sensitivity. However the user has no influence on the current capacity. He either buys a fet with a fixed current value such as those in a TO-92 housing with two pins from Siliconix, or takes the trouble to measure individual FETS until he finds the right one.
Listening Test
The sonic differences between a cathode resistor and a constant current source, as well as those between various fixed-value FET's and measured-value FET's were identified during extensive listening tests. For this purpose, R9 was replaced with current sources between 2.0 and 2.5 mA. In place of R10, a 2.5 to 3.0 mA transistor was installed.
The CD filter with cathode resistors lowered the noise level of our Onkyo DX6870 CD player impressively. Its slightly "foggy" reproduction cleared up entirely, and previously diffused or undiscovered details suddenly became recognizable. Instruments stood fast in their places with more space between them.
The difference between cathode resistors and constant current sources was again as much as with the addition of the CD filter. Reproduction was clearer; music sounded more natural and distant from the speakers. There was more detail, especially in the highs. The differences between various transistors was much less and to some extent unidentifiable. But fixed-value FET's generally sounded better than standard FET's. The exception was the Model BFW12 FET. This transistor was the best of all constant current sources. When thermally aged, it is superior to all other FET's.
Conclusion
There are various uses for the CD filter. As a separate unit it can be installed into an existing system. With its existing power supply on its PC board, it delivers excellent results, although a larger power supply wouldn't hurt (see the June 92 issue of K&T). If you use that power supply, the CD filter's board layout makes it possible to cut off the section for the power supply.
By adding a line stage and - if required - a phone stage, this filter can be turned into a preamplifier. The filter can be plugged into the CD input of an existing preamp, similar to an RIAA equalizer in a phono input. Integrated into the preamp reviewed in the January 92 issue of K&T, the existing power supply can run the CD filter.
The FE constant current sources used in this filter are excellently suited for other applications with cathode followers. When replacing resistors with FE's, remember that a FET can only drop a maximum of 30 volts and works best at 10 to 20 volts. First measure the voltage drop across the cathode resistor, then calculate current with I=E/R and find a FET that can handle the current. If necessary, use a dropping resistor to reduce the voltage drop across the fet to the optimum range.
Stereo Hybrid Power Amplifier
Even a high-end amplifier can be inexpensive
A good-sounding power amplifier doesn't have to be huge. Well considered sizing of a compact but efficient circuit can be responsible for a good-sounding amplifier of small dimensions.
The Hybrid power amp reviewed in the January and February 93 issues of K&T has been reworked by Holger Stein to reduce the number of components at sonically uncritical points. The power supply has been reduced in size. The input stage now consists of a grounded cathode circuit and a cathode follower on the basis of a PCC 88 double triode operating at 72 volts B+. This voltage is supplied through a series of resistors and capacitors that drop some voltage from the rectifier, but supply a stable operating voltage. The heater voltage is supplied by a separate, 13 volt, winding and regulated by a 7812, using two diodes (D19 and D20) as a voltage reference to increase the voltage by 1.4 volts. If either an E88CC or a 6DJ8 is used in th circuit (both tubes are plug-in replacements for the PCC88), a built-in switch shorts one of the diodes to lower the heater voltage to 12.7 volts.
Output Stage
The gate voltage for the MOSFET's is not supplied by an active source as in the Hybend mono blocks, but through series resistors R19 and R20. The output transistors, IRFD 530 and IRFP 953 from International Rectifier are mounted on a common heat sink with transistor T1 that regulates the gate voltage. All transistors have mica insulators. R24 and C12 at the output load this stage at high frequencies and suppress vibration before it can occur.
L1 and R23 dampen voltage spikes for capacitive loads such as electrostatic speakers. L1 consists of 25 turns of 1.4 mm diameter lacquered copper wire. This choke is wound by hand around a 10mm diameter dowell rod or pencil. R23 is installed within the choke.
Alignment
100mA idle current runs through resistors R21 and R22 and is set via pot P1. To set it, measure the voltage drop across the resistors. The idle current is correct at 22 mV. The output is balanced via pot P2, whereby DC is measured at the output with a millivolt meter.
Listening Test
Our test speakers have Eton Hexacone basses and Focal kevlar tweeters. This combination exhibits very tight imaging and excellent detail throughout its entire range. We plugged an Onkyo DX 6870 CD player directly into the power amp. The Hybrid sounded dimensional, pleasant, and neutral. Bass was tight and authoritative. This amplifier's strength lies in the mid high range. Music sounded transparent; the wealth of detail surprised the listeners. Even at high volume levels the amp seemed to play with ease, although the speakers are not very efficient at 86 dB.
Only at extremely high levels did the amplifier run out of steam. But it did demonstrate that in spite of its nominally-low rating of 20 watts at 4 ohms, it is sufficient to fill a living room with music.
Conclusion
For 400 marks Holger Stein's stereo Hybrid amplifier is extremely dimensional and detailed. It's sonic quality to price ratio is astonishingly good. Purchase of a housing would lower this ratio somewhat. Those who can build their own won't find a better power amplifier for their money.
Price/Performance Ratio:
sound - exceptional
equipment - satisfactory
Production quality - good to very good
Measurements by the K&T Lab:
Power output:
measuring frequencies: 20 Hz 1 kHz 20 kHz
load angle: 0 Grad 0 Grad 60 Grad 0 Grad
at 2 ohms: 21 W 30 W 32 W 24 W
at 4 ohms: 20 W 25 W 26 W 20 W
at 8 ohms: 17 W 20 W 21 W 14 W
sensitivity at full volume:
at 2 ohms: 0,92 V
at 4 ohms: 1,2 V
at 8 ohms: 1,5 V
for 1 volt output: 0,12 V
Voltage amplification factor: 8,3
Input impedance: 12,3 kOhm
Range (-3 dB): <1Hz...53 kHz
Operating voltages:
Heater 13.4v
B+ 72 v (input and driver stage)
B+/B- +/- 21v (output stage)
Features
+ Excellent sound quality to price ratio
+ ideal unit for multi-channel systems, i.e. home theater
+ can be bridged for mono