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Erläuterungen zu diesen 1959er US-AUDIO Seiten

Die hier stehenden amerikanischen Artikel aus 1959 (aus der US-AUDIO) sind teilweise sehr gewöhnungsbedürftig, weil sie erstens aus einer längst vergangenen Zeit stammen und zweitens, weil dort in den USA ganz "anders" gedacht wurde als bei uns in Old Germany oder in Europa.
Vergleichbar mit unseren deutschen Hifi-Magazinen etwa ab 1962 ist jedoch, daß auch diese Zeitschrift ihre Anzeigen- Kunden und -Leser (be- oder ab- ?) werben mußte. - Weiterhin sind die Dimensionen des amerikanischen Kontinents mit den unseren hier in Europa nicht vergleichbar. - Ein Redaktions-"Trip" von New York nach Los Angeles oder gar in die Wüste nach Las-Vegas zu einer der Audio- "Shows" war - auch mit dem Flugzeug - immer noch eine Weltreise. Und jede Ausstellung oder "Messe" wurde als "Show" deklariert. Und natürlich, in USA musste alles "Show" sein, um beim Publikum einige Aufmerksamkeit zu erzeugen.

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EQUIPMENT PROFILE - - AUDIO • NOVEMBER, 1959

Wie bei uns auch, wurden neue Geräte recht ausführlich - selbstverständlich mit sehr kräftiger "Unterstützung" (diesen Text mußt Du bringen, sonst gibts keine Anzeige) der Hersteller einschließlich der davor oder danach "begleitenden" Anzeigen - hier vorgestellt bzw. ausführlich beschrieben.

H. H. SCOTT 299 STEREO AMPLIFIER

The Scott 299, Fig. 1, is an "integrated amplifier" - bei uns "Vollverstärker" genannt - (control amplifier and power amplifier combined on one chassis) for stereo, rated at 17 watts per channel, and offering the performance and refinement of design that may be expected these days of a piece of equipment intended for high fidelity use. Considering everything that a stereo amplifier must do, it is relatively simple in appearance and straightforward in operation. The use of a substantial number of slide switches in place of the rotary type helps cut down the "busy" look and makes for an attractive front panel.

Controls (die Knöpfe)

The 299 has a stereo-selector control, separate bass and treble controls for each channel but concentrically mounted, a stereo-balance control, a phase-reversing switch, and a ganged gain control.

The stereo-selector

The stereo-selector control has seven positions. To enable one to balance speaker levels, the first two settings, marked Balance A and Balance B, combine both input signals and feed them to either Channel A or Channel B. By alternating between these positions and adjusting the stereo balance control, one can equate the speaker levels. In the third position the stereo selector combines both input signals and feeds them to both channels, which is desirable when playing mono records with a stereo pickup. The fourth position provides true stereo, and the fifth position reverse stereo. In the sixth and seventh positions, either signal A or signal B is fed to both channels. If the speakers have previously been balanced  - by using identical speakers or by means of L-pads or similar devices - one can balance the input signals by alternating between positions 6 and 7 and adjusting the stereo-balance control; thus one might balance the two sections of a stereo cartridge when playing a mono record.

The stereo-balance control

The stereo-balance control provides equal level on both channels at mid-position. When turned fully to the left or right it eliminates one channel without affecting the level of the other.

The phase reversing control is a double-pole double-throw switch that reverses the connections on Channel B between the output transformer taps and the speaker terminals. This reversal can be achieved on any of the impedance taps (4, 8, and 16 ohms) because the "high" speaker terminal is connected to the desired tap by means of a jumper, as shown in Fig. 2.

To enable the user to know at a glance whether the unit is operating in the true stereo mode, reverse stereo, simulated stereo, or any other of the seven possible modes of operation, the 299 features a set of three pilot lamps that light up in various combinations, depending upon the position of the Stereo Selector control.

The "conventional" controls

The 299 provides the following "conventional" controls, all of which are ganged: loudness-volume switch, which converts the volume control into either a straight gain control or a loudness control with automatic bass and treble boost at low levels; rumble filter; scratch filter; input selector.

In its first three positions, the input selector accepts the signals from the magnetic inputs and supplies European 78 phono, RIAA phono, or NAB (formerly NAETB) tape equalization. In the fourth and fifth positions it selects the high-level inputs, respectively tuner and tape amplifier. Intended for operation in conjunction with the input selector nre two slide switches marked Channel A Pickup and Channel B Pickup; they permit one to choose on each channel between either of two low-level.

Inputs and Outputs

There are four inputs per channel: two magnetic inputs and two high-level ones, the latter marked tuner and tape.

The magnetic inputs are intended for either magnetic phono cartridges or tape heads, with appropriate equalization determined by the position of the input selector. One chooses between Magnetic Input 1 or Magnetic Input 2 by the Channel A Pickup and Channel B Pickup switches. This meets the problem of the individual desiring to operate both a changer and a transcription turntable, or the one desiring to accommodate a tape head as well as a phono cartridge.

Magnetic Input 2 has two jacks, marked Low and High. The High jack is intended for cartridges with relatively high signal output and feeds the signal through a voltage divider to prevent the phono pre-amp and succeeding stages from being overloaded. The Low jack presents an impedance of 47,000 ohms, while the High jack presents 147,000 ohms. These impedances should be suitable for most pickups and heads. One can use either the High or Low jack on Magnetic Input 2, but not both at once.

No specific provision is made for piezoelectric cartridges, which typically require an input impedance of about 2 megohms to maintain bass response. On high-level inputs the input impedance is only 500,000 ohms, which is usually too low, although one might correct the situation adequately by turning up the bass control. Another possibility is to feed the cartridge into the magnetic input jack presenting a 47,000 ohm load. For some piezoelectric cartridges this will result in fairly correct equalization by converting the pickup into the equivalent of a velocity device throughout the audio range; in other words, by causing the signal output to rise with frequency in the same manner as for a magnetic cartridge.

The speaker output terminals

The speaker output terminals will accommodate 4, 8, and 16-ohm speakers. As previously discussed in connection with Fig. 2, connection to the desired tap of the output transformer is made by a jumper. This arrangement makes it possible to connect additional sets of speakers, or possibly a speaker for center-channel operation, to whatever terminals may be desired.

The "center channel output"

The 299 features a center channel output for feeding a third amplifier and speaker system. The signal is taken from the output transformer of each channel, via isolating resistors.

An output jack for feeding a tape recorder is provided on each channel. The manufacturer states that the tape recorder input should have an impedance of at least 200,000 ohms, which does not appear to raise a problem with most tape machines. Maximum recommended capacitance of the cable to the recorder is 200 uuf, which limits one to about 8 feet of low-capacitance cable, and to proportionately less if other cable is used.

Unused Outputs shorted (grounded)

It may not be possible to record on some tape machines if the output cable of the machine remains connected to the 299's tape input jack. The 299 grounds all incoming signals other than that fed through by the input selector switch. However, some tape machines fail to disconnect the output cable from their amplifier circuitry when the machine is in the record mode. Hence the signal is grounded before it can reach the record head, as illustrated in Fig. 3.

Some amplifiers meet this problem through a tape-monitor switch arrangement, which uses a special switch to accept the signal from the tape machine and at no time grounds this signal. In the operating manual for the Scott 222 (in the succeeding review), there is a diagram showing how to construct a simple external switching facility to meet this problem. Doubtless the manufacturer will furnish this diagram on request to owners of the 299.

Circuitry

Magnetic equalization (phono or tape) is achieved through frequency-selective feedback in the preamplifier stage. The signal from the preamplifier or a high-level signal goes through rumble and scratch filters of the losser type, followed by a stage of amplification, Baxendall-type tone controls, another stage of amplification, and the gain control (accompanied by the loudness-volume switch).

Die Gefahr der Übersteuerung der Inputs

Placement of the gain control at a relatively late stage raises the possibility of overloading the earlier stages due to excessive signal. To illustrate, some tuners may present as much as 3 volts on peaks. The writer introduced 3 volts equivalent sine wave signal into the tuner input and measured 1.2 per cent IM distortion at 5 watts output (equivalent sine-wave power).

But when the input signal was cut down to 0.5 volt, IM distortion at 5 watts output fell to the much lower figure of 0.23 per cent. The 299 has no input level-sets, so the incoming signal must be reduced at the source, for example by means of a gain control on a tuner.

After the gain control the signal goes into the power amplifier section, consisting of a phase splitter and the push-pull output tubes. In these days when the majority of high fidelity amplifiers appear to use either the split-load or long-tailed phase splitter, it is interesting to find in the 299 a reversion to one of the forms of floating paraphase inverter, which was common years back. A pentode-triode is used as the splitter. The output tubes, 7189's, are a relative newcomer on the scene and are tetrode operated with fixed bias.

Trimmpotis im Verstärker

A full quota of adjustments is provided in the power amplifier section, namely output tube bias, d.c. balance of the output tubes, and a.c. balance of the signals fed by the phase splitter to the output tubes.

Heaters of all the tubes in the control amplifier section are d.c. operated. The same d.c. source is employed to provide fixed negative bias to the output tubes.

Performance (Ausgangsleistung)

At 1000 cps the reviewer measured 16 watts of power per channel on the 16-ohm tap before the waveform became noticeably distorted (merklich verzerrt) on an oscilloscope. This is but a fraction of a decibel below the manufacturer's 17-watt rating. At 30 and 15,000 cps, which are practical bounds for high fidelity reproduction, maximum "undis-torted" power was about 7 watts, slightly more than 3 db down from power at 1000 cps.

Considering that audio energy at the frequency extremes is ordinarily well below the energy at mid-frequencies, and considering that 7 watts will drive speakers of medium and high efficiency to very loud levels, the 299 should provide adequate power in the majority of situations.

  • Anmerkung : Das ist natürlich ganz großer Unsinn oder die an den Werbe-Entscheider (Auftraggeber der Anzeigen) der Firma H.H.SCOTT gerichtete "Schleimscheisserei". Die Kenndaten werden so gewaltig unterschriten, mit 17 Watt wird geworben und 7 Watt kommen nur raus, daß es schon hahnebüchen einen totalen Verriss wert wäre. Aber das machten zu der Zeit alle Amerikaner so, viel Watt war (nur im Prospekt) gefragt. Karl Breh hätte dieses Teil vollkommen zerissen - wie später so manchen exotischen Japaner.


It may be noted 10 watts of "undistorted" power (wieviel Klirrfaktor ist denn "unverzerrte" Leistung ?) was available at 10,000 cps, while at the low end 16 watts were available at 100 cps, 14 watts at 50 cps, and 13 watts at 40 cps.
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Der Autor mißt zwischen 1,5%, 3,4% und 4,2% Klirr

It was surprising to find Channel A of the 299 originally producing 1.5 per cent IM distortion at 1 watt equivalent sine wave power, 3.4 per cent at 5 watts, and 4.2 per cent at 10 watts; these readings were obtained by using just enough signal input to drive the 299 to full output with the gain control at maximum.

Almost identical results were obtained on Channel B. Adjustment of the d.c. balance controls produced virtually no change, but a radical improvement was obtained by adjusting the a.c. balance controls.

The results then were 0.19 per cent IM at 1 watt, 0.23 per cent at 10 watts, 0.7 per cent at 10 watts, and 1.4 per cent at 15 watts; even (slightly) better results were obtained on Channel B.

From this experience it appears that the purchaser is well advised to have his 299 thoroughly checked and aligned by a competent technician before installing it in his audio system.

  • Anmerkung : Das heißt doch "durch die Blume gesagt", daß der Verstärker ab Werk gar nicht einsatzbereit ist.


Of course, this is a rule that applies to all audio equipment; the reviewer's experience with the 299 is far from exceptional. (It may be noted here that in measuring IM in Scott's 222 amplifier, reviewed below, adjustment of the a.c. balance control produced no improvement on one channel and only slight improvement on the other.)

Linearität :

With gain full on and the controls at mid-position, frequency response was "relatively" flat, as measured on Channel A, remaining within ±2db in the 40 to 15,000 cps range.

Response was down 2db at 20 cps and 3db at 20,000 cps. With the gain control at a position 6db below maximum, response was down 3db at 15,000 cps and 5db at 20,000.

Störabstand :

On high-level inputs, a signal-to-noise ratio of about 72db was measured, based on an input of 0.5 volt at 1000 cps, sufficient to drive the amplifier to rated power. Noise was measured with the input shorted. The manufacturer specifies a ratio of 80db; 72db is still very good.

Die Magnet-Eingänge :

Excellent results were obtained on the magnetic inputs, based on signals sufficient to drive the unit to rated power. On RIAA phono position, about 7mv input was required for full power output at 1000 cps, and the signal-to-noise ratio measured 63db.

Inasmuch as most magnetic cartridges will deliver substantially more than 7mv on signal peaks, a signal-to-noise ratio better than 63db can be expected. This is, however, very good. On NAB tape position, the signal-to-noise ratio again measured 63db. An input signal of about 8mv was required to drive the amplifier to rated power at 1000 cps. Since the maximum signal delivered by a tape head at this frequency is much more likely to be around 4mv than 8mv, the indicated signal-to-noise ratio for tape heads is in the vicinity of 57db, which is still excellent.

Die Kanalgleichheit :

The master gain control provides quite good tracking between channels. During the first 40db of gain reduction, the tracking error of Channel B relative to Channel A was ±2.5db in the unit tested. Tracking error rose to 3.5db at 45db gain reduction and 5db at 50db reduction.

Die schaltbaren Filter :

Ideally, a rumble filter removes the extremely low frequencies without affecting the moderately low ones. The rumble filter in the 299 does fairly well along these lines. With the filter on, response at 100 cps was reduced only 2db, while response measured 5.5db down at 50 cps and 9db down at 20 cps.

  • Anmerkung : Wie bei den späteren Japanern ist die Filter-Steilheit geradezu lächerlich und im praktischen Betrieb völlig unbrauchbar.


The scratch filter is specified as having a cutoff frequency of 5.000 cps, and measurements bore this out. Response was down 3.5db at 5000 cps, 7.5db at 10,000 cps, and 10db at 15,000 cps with the filter on.

Die "loudness" Entzerrung :

The loudness switch introduces almost as much treble boost as bass boost when level is reduced. At 30db gain reduction, bass boost reached 9db at 50cps and treble boost reached 8.5db at 10,000 cps. Further reduction in gain brought no additional boost in bass or treble.

Die Phonoentzerrung :

Phono and tape equalization proved to be very accurate. At the high end, both were within 1db of the RIAA and NAB curves respectively. RIAA bass boost was only 0.5db below the RIAA curve at 5cps, while tape bass boost was only 1.5db below the NAB curve at 50cps.

There are still many amplifiers that depart substantially from the NAB curve despite their manufacturers' claims. It is a pleasure to be able to report on an amplifier, the 299, that conforms accurately to standard tape equalization and at the same time is able to maintain an excellent signal-to-noise ratio, not at all easy to do when full NAB bass boost is provided.
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  • Anmerkung : Auch dieses Lob ist dem Anzeigen-Auftraggeber geschuldet. Karl Breh hatte mal bei einem Japaner als wirklich allerletztes alleiniges Highlight die 1,8m Länge des Netzkabels herausheben können. (Im Umkehrschluß : der ganze Verstärker ist eigentölich eine miserable Krücke.

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Operating Manual

The complexity of stereo compared with mono equipment has heightened the importance of the operating manual in assuring proper installation and operation of equipment. The Scott manual supplies full, clear instructions and a number of helpful diagrams showing how to connect the various input sources to the 299, how to connect the speakers, how to use the 299 as an electronic crossover, how to place the speakers relative to the listener, and how to connect to the center channel output jack. Also included are a schematic and instructions for making the bias, d.c. balance, and a.c. balance adjustments.  - H. B.

  • Hier hat sich also der Tester H. B. nicht mit Ruhm bekleckert. Die US-AUDIO brauchte damals in 1959 auch dringend jede einzelne Anzeige.

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Die Bilder:
Fig. 1. H. H. Scott Model 299 Stereo Amplifier.
Fig. 2. Channel B output connections on the Scott 299.
Fig. 3. Example of a tape recorder where the input signal is grounded when used with an amplifier such as the Scott 299.
Fig. 4. H. H. Scott Model 222 Stereo Amplifier.

Der nächste Verstärker .....

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FISHER X-101A STEREO AMPLIFIER

Combining two 20-watt power amplifiers and a complete stereo preamplifier-control unit into one cabinet always results in an interesting unit, and in these days of miniaturization - to whatever extent is possible  - it shows a certain amount of ingenuity on the part of the manufacturer.

Not that it is unusual for a complete system to be so compressed, but in many instances it is accompanied with such cost- and space-cutting gimmicks that the performance may suffer as a result. This is not the case with the Fisher X-101A, however, for its performance is high up on the scale.

six inputs

Each channel has six inputs - two for phono, two for auxiliary devices, and one each for tuner and tape head - making twelve in all. Two recorder output jacks are provided, and a recorder-monitor jack is furnished on Channel A for use with monophonic tape recorders equipped with feed-through circuitry.

These connections are ahead of both volume-loudness and tone controls. Output impedances of 4, 8, and 16 ohms are provided on both channels for loudspeakers. There are two level-set controls on each channel - one in the Aux 1 circuit, and one following the preamplifier section to control phono and tape-head inputs.

The normal operating controls are: input selector, mode selector, balance, volume-loudness combined with the a.c. power switch, dual bass and treble tone controls, rumble switch, and loudness contour switch.

The input-selector switch has four positions for magnetic phono cartridges - two for monophonic and two for stereo use. The first two provide equalization for 78 and LP curves, and connect to the phono jack of Channel A; the remaining two connect to either phoxo 1 or phono 2 jacks of both channels for stereo use, and provide RIAA equalization.

The mode selector connects the output amplifiers to either Channel A or Channel B for mono inputs, and to both channels for stereo and reverse-stereo positions, making it possible to play mono records through both output channels when desired.

The tape-head input position sets up the correct equalization for 7 1/2-ips tapes, while tuner and auxiliary inputs are fed directly to the tone-control amplifier tube sections. The aux 2 position has a high input impedance for use with ceramic cartridges if desired.

The tube line-up in each channel is as follows: 12AX7 (or 7025 or ECC83) as preamp, with feedback equalization; 12AX7 as tone-control amplifier; 12AX7 as booster amplifier and phase splitter; and two EL84's (7189 or 6BQ5) as pentodes in the output stage.

Plate power is furnished by a GZ34 (5AR4), with the plate current of the four output tubes serving as heater current for the two preamp tubes  - with the double benefit of d.c, on the preamp heaters and reduction of heating by the elimination of cathode resistors for the output stages. Physically, the unit is 15y2 in. wide, 13 in. deep, and 4 13/16 in. high in chassis form. Ventilated wood cabinets are available.

Performance

Specifications for the X-101A call for an output of 20 watts per channel at a rated harmonic distortion of 0.7% and an IM distortion of 2%, with the usual 20-20,000 cps flat frequency response.

Measurements indicated an output of 21 watts on one channel and 23 on the other at 1% harmonic distortion. Intermodulation measurements were in accordance with specifications.

  • Anmerkung : Bei dem Scott Verstärker (gemessen mit nur 7 Watt pro Kanal) wurden die Leistungen bei tiefen und hohen Frequenzen angegeben, hier nicht! Warum nur ? Wollte oder durfte der Autor das nicht ?

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Nicht änderbare Unsymmetrien in der Ausgangsstufe

Since the phase-splitter circuit of each channel is provided with a means of adjusting the drive to the output tube grids, any minor differences in the amplification of the output tubes can be compensated readily. The type of phase splitter is that commonly known as the cathodyne or split-load circuit, and it is inherently balanced if the plate and cathode resistors are of exactly equal values.

Actually there is some unbalance because of the capacitance between heater and cathode, but this affects only the frequency range from about 8.000 cps up, and has very little effect on listening quality.

However, even though the phase splitter is perfectly balanced, there may be a difference between the amplification provided by the two output tubes, and this can not be balanced out (in any practical circuit) in the output stage itself, so users must resort to tube matching to obtain exact balance between tubes in the output stage.

The use of a variable cathode resistor in the phase splitter results in feeding different values of signal to the two output tube grids - that is, signals that are not perfectly equal in amplitude though they are exactly opposite in phase.

But such an intentional change in the drive to the output stage can result in extremely low over-all distortion, and in an amplifier using comparatively small tubes, it is important that every step be taken to keep distortion to a minimum.

Checking the amplifier under test, we found that a variation of the control (chassis mounted) from the optimum position to one extreme changed the IM distortion from the rated 2% to as high as 8%.

The phase splitter circuit, which employs one half of a 12AX7, has a plate load of 100 k ohms and a cathode load consisting of an 82 k fixed resistor and a variable resistor of 50 k ohms. The grid resistors of the output stages are 470 k ohms. The circuit has the advantage of being inexpensive, yet capable of balancing the amplifier to a minimum of distortion.

Kanalgleichheit

Maximum variation of the volume-loudness control between channels was noted at 3db, and the tone controls were observed as tracking within 5db throughout the entire range. EIAA equalization for phono was within 1db throughout the range.

Considering the relative simplicity of the X-101A, together with the flexibility of control operation, the over-all design must be credited with following a system philosophy which is in keeping with the requirements of a satisfactory - in our opinion -  amplifier.

Lobhudelei extra

Among the things we believe to be necessary in any control amplifier are: a loudness control, separate tone controls for each channel, and means for paralleling the output sections to either input section. The Fisher X-101A has all those.

Quality of construction on the unit tested was up to usual Fisher standards. While the flexibility of the unit is not as great as with the Model 400, which is a preamp-control unit only, it could hardly be expected. We would still like to see a phase-reversing switch in the speaker circuit, and we believe some provision should be made to change the tape-head equalization as needed for the common tape speeds  - 3" and 7". The X-101A is still a fine unit, however, and it is not likely that the individual who buys Fisher equipment would be satisfied with 3 3/4-ips tape anyhow, so that is not too much of a problem.

As to the phasing switch, it could be mounted elsewhere in the system - even on one of the speakers. L-33

H. H. SCOTT 222 STEREO AMPLIFIER

The 222 is a slightly smaller brother to the 299, with the similarities greater than the dissimilarities. The 222 is rated at 12 watts per channel instead of 17 watts, otherwise has virtually the same performance specifications at the 299, and retains most of the latter's features.

  • Anmerkung : Also anstelle der 17 Watt (es waren ja nur echte 7 Watt) hat der 222 jetzt laut Prospekt 12 Watt -  das wären dann in etwa echte 4 Watt pro Kanal.


The 222 has one magnetic input and two high-level inputs, whereas the 299 has an additional magnetic input, with a choice between High- and Low-level jacks. The 222 lacks a rumble filter and a phase reversing switch, does not have the three panel lamps to indicate mode of operation, and does not provide European 78 phono equalization.

However, it is like the 299 in having a ganged gain control, volume-loudness switch, stereo balance control, separate bass and treble controls on each channel, a stereo selector switch providing the same seven modes of operation as the 299, a scratch filter, and a center-channel output.

Circuitry is largely the same, the principal difference being the use of 6BQ5's instead of 7189's in the output stage. Self-bias is employed. The same kind of phase inverter is used, along with an a.c. balancing control. No means are provided for adjusting bias and d.c. balance in the output stage.

The operating manual for the 222 provides not only instructions and diagrams similar to those for the 299 but also has the following useful diagrams:

1. Showing the audiofan how to construct external switching facilities to increase the number of low-level inputs.
2. Showing him how to construct external switching facilities for tape recorders that do not disconnect the output cable from the tape amplifier in the record mode.
3. Showing him how to construct a phase reversing switch. L-31
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