Build a moving coil phono pre-preamp (Feb. 1978)

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SPECIFICATIONS:

Gain: 26 dB (x 20) ±1 dB with a 9-volt power source.

Input Impedance: 100 ohms.

Output Impedance: 2000 ohms.

Lower-3-dB Frequency: Less than 1 Hz.

Upper-3-dB Frequency: 150 kHz.

A-Weighted S/N Ratio: 85 dB referenced to a 1000 Hz, 1 mV input signal.

THD: Measured at 1000 Hz with a source impedance of 10 ohms: At 10 mV the second harmonic and third harmonic are both -73 dB. At 30 mV the second harmonic is -73 dB and the third harmonic is -70 dB. At 100 mV output the second harmonic is -73 dB and the third harmonic is -50 dB. Note: The limit of the spectrum analyzer is -73 dB.

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Measured with an RIAA equalized preamplifier following the pre-preamp, and the pre-preamp input jacks shorted. The figure is corrected for noise contributed by the preamplifier.

Moving-coil phono cartridges have probably been regarded with mystique by most audiophiles who have never used them. Not only are they expensive when compared to a standard magnetic cartridge, but they require either an impedance-matching transformer or a pre-preamp which must be inserted into the reproducing chain between the cartridge and the RIAA preamplifier. Either of these necessary items can cost as much or more than the cartridge itself. Therefore, most audiophiles have probably never owned a moving-coil cartridge, and many have never even heard one.

The operation of moving-coil cartridges is based on the same laws of physics on which magnetic cartridges operate, that is, a mechanical force which is proportional to the groove modulation on a phonograph record is used to change the magnetic flux linkage in a coil of wire. The corresponding voltage induced in the coil is equal to the number of turns in the coil multiplied by the time rate of change (or time derivative) of the magnetic flux linkage through the coil. This voltage is then applied to the phono input of a preamplifier which performs three basic functions. The preamplifier must have adequate gain to boost the input voltage to a level that is sufficient to drive a power amplifier. It must correct for the time derivative response of the magnetic cartridge.

Lastly, it must equalize for the RIAA recording characteristic used by the record industry today. With a moving coil cartridge, a preamplifier can generally perform only the latter two functions.

The principal difference between a moving-coil cartridge and a conventional magnetic cartridge is the mechanical mechanism which is used to vary the flux linkage in the coil. In a Georgia Institute of Technology Atlanta, GA 30332 conventional cartridge, the coil is stationary and some element of the magnetic circuit is attached to the phono stylus. The motion of this element changes the flux linkage through the coil and generates the phono signal. Thus, we have moving magnet, variable reluctance, etc., cartridges. In contrast, as the name suggests, a moving-coil cartridge has the coil attached to the phono stylus. The motion of the coil itself changes the flux linkage and generates the phono signal. However, there is one major exception. The output signal from a moving coil cartridge is 20 to 30 dB less than the signal from a conventional magnetic cartridge.

The output impedance characteristic of moving-coil cartridges differs radically from that of conventional cartridges. In conventional cartridges, it is typically 2,000 to 3,000 ohms resistance in series with 500 to 1000 millihenries inductance. This very large inductance causes the output impedance at high frequencies to become very large. The input impedance to preamplifiers which have inadequate high-frequency loop gain can become capacitive at these frequencies. In combination with the cartridge output impedance, this can cause some weird frequency response aberrations. The cartridge inductance and preamplifier capacitance will generally cause a severe loss of high frequencies. Depending on the damping in the circuit, a resonant peak can exist in the high frequencies just below the rolloff frequency. This is one of the reasons that different cartridges can sound so different with a given preamplifier. The sound differences are caused by the interaction of the different cartridge output impedances with the preamplifier input impedance.

In order to eliminate all cartridge preamplifier impedance interactions, two approaches can be taken. The first is to use a phono preamplifier which exhibits a very low input capacitance.

The author has described such a unit which has no more than 20 picofarads input capacitance [1]. (R3 in this reference should have been 39 ohms and not 390 ohms.) Even with this low input capacitance, the phono cable capacitance in combination with the preamp capacitance can cause interactions with some cartridges. Thus, the second approach is to reduce the output impedance of the cartridge until it is low enough to drive a capacitive load with no impedance interactions. A moving-coil cartridge has this characteristic. (The particular cartridge used by the author will drive a 2 microfarad load with no measured loss below 12 kHz.) Unfortunately, the output signal is too low to drive a normal phono preamplifier stage directly.

There are two approaches to interfacing a moving-coil cartridge to a preamplifier. The first is to use a step-up transformer which steps up the voltage to a level sufficient to drive the preamplifier. Unfortunately, this is not always a good solution for two reasons. First, the transformer is very susceptible to hum pickup, a problem which has caused many to abandon moving-coil cartridges. Second, the output impedance characteristic of the transformer can be as bad or worse than that of a conventional cartridge.

Enter the pre-preamp. This is an active circuit which boosts the moving coil cartridge output signal by 20 to 30 dB, while exhibiting a purely resistive output impedance in the range of the output resistances of conventional cartridges. Thus, the impedance interactions are eliminated, and the hum problem can be virtually eliminated. Is there anything audible to be gained? The answer to this question depends a lot on the preferences of the listener and his system. However, a moving coil cartridge and pre-preamp can reveal details in recordings that can be a surprise to hear. This is particularly true with recordings of voices and acoustic instruments. The bow of a violin, the plucked strings of a guitar, the sheen of brushes on cymbals all exhibit the detail, depth, and definition of a live performance. The reproduction can also become so clean that previously unheard distortion on a record will become audible. Even the tiniest dust ball on the stylus becomes annoying long before it would have on the stylus of a conventional cartridge.

This article describes the construction of a push-pull pre-preamp which is both inexpensive and simple to build.

It has a gain of about 26 dB and a resistive output impedance of 2000 ohms. Since it uses no negative feedback, it cannot oscillate or exhibit slewing and TIM distortions. To eliminate hum, it is battery operated.

In addition, it is d.c. coupled to the cartridge for reproduction of the lowest recorded frequencies. An unusual feature of the circuit is its isolation of all d.c. bias current and voltage from the cartridge, even though the cartridge is d.c. coupled to the circuit.


Fig. 1--Complete circuit diagram for one channel of the head amplifier.

Note the ungrounded input and output jacks. The leads of capacitor C1 should be as short as possible.


Fig. 2--Circuit board foil pattern for a two-channel head amplifier.

This view is from the copper side of the board.

Fig 3--Component layout for the foil pattern of Fig. 2. Note that this view is from the opposite side of Fig. 2.

Circuit Description

The first stage in the design of any circuit is that of deciding on a circuit configuration or topology which will, we hope, meet the design objectives.

In the present case, the circuit must be designed to interface a moving-coil transducer which has a very low output impedance of 3 to 30 ohms to the high input impedance of a phono preamplifier. At the same time, the circuit must have a gain in the range of 20 to 30 dB. Some moving-coil cartridges have an output impedance higher than 30 ohms with a corresponding increase in output signal level. Thus, it is desirable that the circuit gain decrease with cartridge output impedance so that the phono preamplifier will not be overloaded by the higher level moving coil cartridges.

One transistor configuration which meets these requirements is the grounded-base amplifier. This circuit has a very low input impedance, operates with a current gain of unity and, thus, has an extremely wide bandwidth; its voltage gain can be easily set by varying its load impedance, and its input impedance characteristics can be adjusted so that its gain decreases for cartridges whose output impedance is greater than 30 ohms.

However, there is one problem ... the very low input impedance of a grounded-base amplifier requires very large input coupling capacitors for acceptable low-frequency response.

The requirement for an input coupling capacitor can be eliminated if there is no d.c. offset voltage at the circuit input. One way of achieving this is shown in the circuit of Fig. 1. This shows two complementary grounded base amplifiers which are connected in parallel between the signal input and output. The circuit is powered by a single 9-volt battery which floats with respect to ground. Thus, it is impossible for a d.c. current to flow through a cartridge connected to the signal input.

The operation of the circuit in Fig. 1 is very simple. Once switch S1 is closed, a current flows from battery B1 through resistors R3 and R4. This current slowly charges capacitors C2 and C5 until the base voltages of transistors Q1 and Q2 reach their cut-in voltage. At this time, Q1 and Q2 begin to conduct and the circuit will amplify.

Quiescently, Q1 and Q2 conduct approximately 125 microamperes. This low current insures long life for the battery B1. To understand how the circuit amplifies, consider a positive going input signal. Since Q1 is an n-p-n transistor, its current decreases. The current in Q2 increases since it is a p n-p transistor. This action unbalances the circuit and causes a signal current to flow through coupling capacitors C6 and C7, and to ground through resistor R5. This in turn generates an output voltage. A negative-going input signal causes an output voltage in a similar way.

Since an input signal always causes an increase in the conduction in one transistor and a decrease in the other, the circuit operation is true push-pull.

This makes the distortion very low and eliminates the need for negative feedback for good linearity. SMPTE IM distortion measurements on the pre-preamp in combination with a Crown IC-150 preamp show an IM distortion of 0.015 percent and 0.017 percent in the two channels at 1 V rms output from the tape output jack on the preamp. These figures increase to 0.024 percent in each channel at a 4 V rms output level. A 4 V rms IM test waveform has a peak-to-peak value of 13.72 volts. This is far greater than any cartridge would produce. The Crown gain in each case was 40 dB at 1000 Hz.

Since the pre-preamp circuit is used to amplify very low signals on the order of one millivolt and less, r.f. interference suppression is an important consideration. Capacitors C1, C3, C4, and C8 in Fig. 1 serve this purpose.

These capacitors are small enough so that they are open circuits for all audio frequency signals. However, they are low impedance elements to r.f. signals, which prevents radio, TV, ham, and CB transmissions from interfering with the circuit. Should r.f. interference be a problem, ferrite beads can be installed around the center conductor of each input and output cable. These are sometimes hard to find, and a simpler solution may be to install a 0.1 mF capacitor across the phono input jack for each channel. Since the output impedance of a moving coil is so low, this will in no way interfere with the circuit.

Construction Details

The circuit construction is very simple and requires no special instructions. The pre-preamp cannot oscillate since no negative feedback is used, therefore, component layout is not critical. Figure 2 shows the recommended stereo circuit-board foil patterns. The corresponding component locations are shown in Fig. 3. The view in Fig. 2 is from the copper side of the board while that in Fig. 3 is from the component side.

For proper hum and r.f. interference rejection, the circuit board and associated components should be mounted in a shielded enclosure as is shown for the author's unit. The input and output phono jacks must have their grounds isolated from chassis ground if proper r.f. and hum rejection are to be realized. Phono jacks with floating grounds are available on the market. However, the unisolated ones can be easily isolated from ground by installing them into an appropriate size grommet between the phono jack and chassis ground. This was the case with the pictured author's unit. In this case, the lip on one side of the grommet was cut off to form an extruded washer, after which it was inserted into a proper size hole in the chassis. A flat metal washer was installed on the phono jack, and the jack was inserted into the grommet. A flat insulating washer and the signal ground soldering lug were then installed on the phono jack inside the chassis, and the mounting nut was used to secure the assembly.

[E-B-C] Fig. 4--Pin configuration for transistors Q1 and Q2. This view is from the end of the transistor to which the leads attach.

Although it is not absolutely necessary, transistors Q1 and Q2 should have current gains that are matched within 10 percent. This insures that the circuit will be balanced quiescently with respect to ground potential.

To insure good continuity between circuit board ground and chassis ground, the circuit boards should be mounted on metal standoffs with inside star lockwashers installed on each end. The mounting screws should be tightened securely so that the lock washers will be firmly engaged. This will insure good continuity from circuit board ground through each stand-off to chassis ground. Capacitor C1 for each channel should be installed at the phono input jack. The leads to this capacitor should be as short as possible to prevent r.f. interference.

Final Circuit Checkout

Before using the pre-preamp the circuit should be inspected thoroughly to make sure that each component is correctly installed. In particular, Q1 and Q2 should be checked for proper lead connections. The polarized electrolytic capacitors C2, C5, C6, and C7 should be checked for proper polarity.

If all visual checks are satisfactory, turn the unit on by closing switch S1.

Wait two minutes, then measure the battery voltage. This should be between 9 and 9.5 volts. Next, measure the voltage at the collectors of Q1 and Q2. This should be within 0.5 volts of ±4.5 volts, respectively. Voltages outside these ranges indicate a wiring error, a faulty component, or a bad match in the current gains of Q1 and Q2.

If the preceding tests are satisfactory, the unit can be turned off and installed between the cartridge and preamp. To minimize hum pickup problems, the connecting cable lengths should not be too long. A standard two to three foot phono cable should give no problems. Initially, the turntable ground lead should be connected to the main preamp grounding screw. Turn on the system and the pre-preamp. After 60 seconds, the unit will be fully operational and a record can be played. If hum pickup is a problem, the unit should be moved away from any nearby transformers or a.c. motors to eliminate it. Connecting the turntable ground to the pre-preamp chassis ground may help, but in the author's experience, the turntable should be grounded to the main preamplifier.

There are no turn-on or turn-off thumps in the circuit, so no special precautions need to be observed in its use. When not in use, the power switch should be turned off to maximize battery life. A weak battery will cause a drop in gain. Therefore, batteries with an operating voltage less than 9 volts should not be used. Happy listening!

Reference

1. W.M. Leach, "Construct a Wide Bandwidth Preamplifier, " Audio, Vol. 61, No. 2, Feb. 1977, pp. 38-48.

Parts List Parts listed are for one channel only.

Metal film resistors are preferred.

However, carbon film resistors are much easier to obtain and will give comparable performance. Q1 and Q2 should have current gains that are matched within 10 percent. Do not substitute parts.

Q1-2N4401 Transistor.

Q2-2N4403 Transistor.

R1-100 Kilohm 5% Resistor.

R2-510 Kilohm 5 % Resistor.

R3, R4-1 Megohm 5% Resistor.

R5-2 Kilohm 5% Resistor.

C1-0.1 Microfarad Ceramic Capacitor.

C2, C5, C6, C7-100 Microfarad, 16 volt

Electrolytic Capacitors (Radial Leads).

C3, C4, C8-220 Picofarad Ceramic

Capacitors.

Miscellaneous DPST switch (serves two channels), 9 volt battery (alkaline cell preferable), battery mounting clips, battery terminal clips, phono jacks with floating grounds, 1 inch metal circuit board standoffs, No. 4-40 by 1/4 inch and 1/2 inch screws, No. 4-40 nuts, No. 4 inside star lockwashers, shielded cable, and No. 22 stranded hookup wire.

A complete kit of parts for the pre-preamp with anodized and silk screened box, metal film resistors, and matched transistors, excluding batteries and wooden side panels is available for $70.00 plus $2 shipping from Components, P. O. Box 33193, Decatur, Ga. 30033. Solder plated and drilled circuit boards are available for $5 each. A matched n-p-n/p-n-p transistor pair is available for $2. Shipping charge for circuit board and transistor orders is $1.

(Source: Audio magazine, Feb. 1978, W. Marshall Leach)

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