AUDIO MAGAZINE March 1959

SEVERAL YEARS AGO, long before announcement of the stereo disc heralded a new era in audio reproduction, Bell was one of the very first - perhaps the first - to market an integrated stereo amplifier. The Carillon is the latest succession to Bell's original model and contains a number of refinements of features and design, provides about three times as much power, is good to listen to, just as good to look at (as handsome a unit as this reviewer has seen), and, considering its power, is relatively light and compact.

The Carillon provides almost all the special stereo features and functions that can prove useful. There is a balance control, a master gain control, and a function switch with three positions: stereo, reverse stereo, and monophonic. The first two positions are self-explanatory. In the mono mode, the two channels are combined, which is desirable when playing a mono disc with a stereo pickup, for this causes the audio signals (lateral information) to add and rumble (vertical) to cancel. If it is desired to feed one source to both channels, the function switch is set to mono and the balance control is turned to one extreme or the other, depending upon whether the left or right source is to be cut off; since the balance control permits infinite attenuation, this results in shutting off one source.

To balance the left channel against the right all the way from the program source (disc, tape, or tuner) to the sound emanating from each speaker, it is highly desirable to be able to alternate rapidly between two conditions: left channel on and right channel off; left channel off and right channel on. The Carillon allows one to do exactly this, although the instructions fail to mention the fact. There is a speaker switch that connects speaker A or speaker B or both; this applies to each channel. The original purpose is to enable one to connect an additional speaker, to each channel for the den, playroom, or wherever. On the output terminal strip for each channel, there are terminals marked A and B, intended for the speaker in each room. If instead one connects the left stereo speaker to terminal A on one terminal strip, and the right stereo speaker to terminal B on the other terminal strip, switching between positions A and B will alternate the sounds of the two speakers. In the A-B position of the speaker switch, both speakers will be on. (If one employs the switch for balancing at high levels, there is the possibility of damaging the output transformers if their loads are suddenly removed. Therefore it is advisable to connect a 50-ohm 10-watt resistor across each output transformer. Power consumption by this resistor will be relatively slight.)

The only significant omission in terms of stereo functions is that of phase reversal. Fortunately, this omission is easy to correct by installing a double-pole double- throw switch at the speaker or elsewhere to reverse leads to one of the speakers, although for stereo records this function no longer seems to be necessary.

For each channel, there are seven inputs. Three are high level, and two of these—TUNER and TAPE AMPLIFIER—have input level sets. The low-level inputs are for high-impedance microphonic (magnetic), tape head, magnetic phono cartridge, and ceramic phono cartridge. Although ceramic pickups are actually high- level, amplitude devices, in the Carillon the signal from such a cartridge is fed through a small capacitance, which in effect converts the cartridge into a low-level, velocity device, so that the same preamplification and equalization may be employed as for a magnetic cartridge. The amplifier can accommodate either a ceramic or magnetic pickup, but not both, inasmuch as the input jack for each leads to the same point on the selector switch.

There are no level sets for the low-level inputs. To this reviewer's way of thinking, a level set following the preamplifier stage would be useful for equating the volume obtained from a low-level source with that from the high-level sources and for achieving balance between the two sections of a stereo phono pickup (differences between sections can be as much as 4 to 6dB) or the two sections of a stereo tape head. Inasmuch as most tape amplifiers and most tuners contain gain controls, which in a pinch can serve the same purpose as input level sets on the amplifier, it would seem preferable if one of the two pairs of level sets in the Carillon had been used for the preamplifier section instead.

The Carillon follows the trend toward separate rather than ganged tone controls, permitting one to compensate for differing tonal characteristics of unlike speaker systems, for different effects of room acoustics upon each speaker, for different tonal balance in each channel of the program material, and so on. On the other hand, the low filter for reducing rumble and the high filter for reducing noise are ganged devices, each one controlling both channels.

The Carillon is rated at 30 watts per channel. This reviewer measured about 25 watts output at mid-frequencies before clipping became apparent on an oscilloscope. The difference between 30 and 25 watts is quite minor, less than 1dB. At the frequency extremes of 20 and 20,000 cycles, each channel was able to deliver 20 watts of a well-formed sine wave before clipping occurred, and this is very good. Clipping was symmetrical, and the amplifier gave no sign of distress, such as oscillation or radical change in waveform, when driven into the clipping region - also very good.

Circuitry of the Carillon is straightforward and follows design principles of proven worth. The unit may be termed an "all-feedback" amplifier inasmuch as there is feedback in the preamplifier (between two sections of an ECC83/12AX7) for equalization, feedback in the following stages (ECC83) in connection with tone controls and the high filter, and feedback from the output transformer over the remaining stages.

The left channel provides equalization for the LP and "European" phono recording characteristics as well as for the EIAA curve. The right channel supplies only RIAA compensation. The thought is that all stereo discs are recorded EIAA, so that LP and European equalization are needed only for mono discs, i.e., only for one channel. But, as previously pointed out, the best way to play mono discs with a stereo cartridge is to parallel the outputs of both cartridge sections in order to cancel vertical rumble. However, if one sets the Carillon's selector switch to the LP or European position, equalization will instead be EIAA on the right channel, with a consequent tonal unbalance between channels. This would be alleviated if the user switched cartridges to a mono pickup (properly wired to the head) when playing mono records.

The high-level sources and the output of the preamplifier stage feed into the selector switch. Connected to the arm of the switch are: an output jack for feeding a tape recorder; the low filter circuit; and the high end of the volume control. The signal goes from the arm of the volume control to the balance control, to the function switch, and to the first grid of the ECC83 associated with the tone controls and high filter. Following this tube is the loudness control and then the power amplifier section, using an ECC83 as a voltage amplifier and split-load phase inverter. The output stage employs 6CA7's connected in Ultra Linear fashion and using cathode bias. This results in less power output than with fixed bias (some amplifiers obtain 50 watts and more with 6CA7's), but on the other hand one does not run the same risk of tube destruction if the bias supply should fail.

All heaters but those of the output tubes are d.c. operated. This is done by connecting the cathodes of all four output tubes (for the two channels) in parallel and running part of the cathode current through the heaters, which are arranged in a series- parallel configuration.

In a stereo amplifier, it is important not only that each channel provide good performance in terms of frequency response, distortion, equalization, etc., but also that the two channels be very similar to each other in performance characteristics. Following are measurements taken by this reviewer with respect to a number of functions, showing that on the whole the Carillon maintains excellent correspondence between channels, along with very creditable performance in absolute.

1. Treble controls. At 15,000 cps, the left channel provided a maximum of 9dB boost and a maximum of 21dB cut. For the right channel, the respective figures were 10dB and 20dB.

2. Bass controls. At 30 cps the left channel provided a maximum of 16dB boost and a maximum of 18dB cut; the right channel, 15dB and 19dB.

3. Master loudness control. Following were the amounts of bass boost at 50 cps relative to 1000 cps for various degrees of attenuation:

Unlike most other loudness controls, which provide unlimited attenuation, the Carillon's loudness control confines attenuation to a rated 40dB (actually 37.5dB according to the reviewer's measurements). This should be sufficient for virtually all circumstances. Limited attenuation makes it possible for the control to maintain the excellent balance indicated by the above figures. The importance of such balance cannot be overemphasized.

4. Master gain control. Its ability to maintain balance between channels was very good on the whole, but, because it permits infinite attenuation, not quite so good as that of the master loudness control. During the first 5dB of attenuation, balance in the reviewer's unit changed by 3.5dB, with the right channel dropping in level below the left one. But between 5dB and 55dB attenuation - which is apt to be the working range 99 per cent of the time - balance remained virtually within 2dB. Thus if one were to adjust the balance control for equality between channels with the master gain control at mid-setting, one would have inter-channel balance within 2dB at any normal setting of the gain control. It may be pointed out that even at 60dB attenuation one would have balance within 3.5dB.

5. Low filter. This provided the following attenuation at selected frequencies:

6. High filter. This provided the following attenuation at selected frequencies:

7. RIAA equalisation. The difference between channels was 2.5dB at 50 cps and only 0.5dB at 10,000 cps. The maximum absolute error was 3dB excess bass boost at 50 cps and 2dB insufficient treble cut at 10,000 cps.

8. NARTB {tape) equalization. The difference between channels was 2dB at 50 cps and only 0.5dB at 15,000 cps. The maximum absolute error was 3.5dB insufficient bass boost at 50 cps and 2dB insufficient treble cut at 15,000 cps. Since most playback heads have at least 1 or 2dB loss at 15,000 cps, the variation at the treble end is of no consequence.

9. IM distortion. The following readings were obtained at various amounts of equivalent sine wave power, using 60 and 5000 cps in 4:1 ratio:

10. Sensitivity. As measured on the basis of a signal fed into the tuner input and with the master gain control full on, sensitivity on the right channel was about 1.5dB lower than on the left channel.

11. Frequency Response. With the tone controls set to mid-position and with the master gain control at maximum, frequency response of each channel was extremely flat, better than within 1dB, from 20 to 15,000 cps. With the gain control set for 6dB reduction in volume, the worst position for high-frequency response, there was a drop of 1.5dB at 10,000 cps.

Based on 25 watts output at 1000 cps, the reviewer measured a signal to noise ratio of 74dB on high-level input, which is superior to the manufacturer's claim of 71dB. The ratio measured 55dB on magnetic phono input in EIAA position, 48dB on tape head input, and 61dB on microphone input. Crosstalk between channels measured 66dB isolation at 1000 cps, 61dB at 50 cps, and 45dB at 10,000 cps.

As a final measurement, the reviewer fed square waves into the amplifier and viewed the output on an oscilloscope for signs of ringing. None at all were evident at 100 and 1000 cps, and only one slight ripple was observed on a 10,000 cps square wave.

Considering the complexity of a stereo amplifier, a well-written instruction book is of considerable importance to the purchaser's successful use of the unit. The Carillon comes with such a book, which contains four drawings showing how to connect one set of stereo speakers, how to connect two sets of stereo speakers, how to wire a complete stereo system to the Carillon, and how to use the Carillon as a monophonic 60-watt amplifier. Even though the booklet is written on a completely non-technical level, it contains a schematic, a laudable recognition of a fact of life - that any piece of audio equipment may some day require servicing and that the serviceman to whom the equipment is brought may not have the schematic on hand.