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Differential Tube Line Amp |
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The EB-894/211 is a high quality lineamp/power amp driver, using one tube of the type ECC83 as a differential amplifier and two ECC81 as low-impedance output stages. It operates without feedback. Because it has differential input and output, it can also be used as a balanced line amplifier.
Tube V1, an ECC83, is operated as a normal differential amplifier. Each half is operated at 1 mA, which is defined by the constant current diode D1 or D1/D2 in parallel. You can use a 2mA constant current diode for D1, or two 1mA diodes for D1/D2. For proper biasing D1 has to be connected to a negative voltage. This voltage can be derived from an independent 12.6V winding. A simple
bridge with a capacitor and a 79L12 TO-92 regulator will work fine. If you have problems providing a -12V regulated supply, you can use a 9 Volt battery; the current consumption is only 2 mA, which allows several months of operation. P1 and P2 are used to balance the DC conditions and the gain in the differential amplifier. Resistors R5 and R6 (plus the trimpot P1) are introducing a form of local feedback, which is reducing the gain. However, it is also improving linearity and large
signal handling. Used as a lineamp it has a single-ended-gain of approx. 22 dB.
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Tube V1 can also be operated at a lower current. Using a 1mA current diode for D1, each half is operating at 0.5mA. Resistors R3 and R4 need to be changed to establish the same DC conditions as before. Total gain goes up to approx. 25dB, but linearity improves as well, especially at higher signal levels.
If you use the driver with a single input the input signal should be connected to INP1 and INP2 needs to be grounded. If the overall gain of the power amp is too high you might want to use some of the excess gain for negative feedback. The feedback has to be connected to INP2. If you have a balanced input signal connect it to both inputs.
Not all manufacturer's ECC83 can be used for V1. The circuit was developed around the TUNGSRAM ECC83, however others can also be used. The Telefunken ECC83, the IE ECC83 and the ULTRON ECC83 worked also well. Matching the two triodes is recommended, but the circuit works ok with unmatched ones as well. You might want to experiment with other ECC83s to get the best sound from the lineamp/power amp.
The output impedance of the differential amplifier is very high (close to the value of the anode resistors), and can't be loaded very much without loss of gain and increased distortion. V2 and V3 are configured as White Cathode Followers. They are operating at approx. 2.9mA, and provide very low output impedance. They have very good linearity and add very little distortion of their own to the total distortion of the amplifier. V2 and V3 should be ECC81 or equivalent.
The filaments of the tubes are biased off ground with a voltage divider R14-R17 to protect the tubes from breakdown. If this is already done at the filament regulators, then leave out the components associated with it.
Typical specifications for the differential line amp:
Freq. response: 10Hz-40kHz, -0.5dB
Gain: 22 dB (Single-ended)
THD at 1 kHz: 3V: 0.04%
10V:0.14%
20V:0.45%
30V:1.25%
40V:saturation
Rise time: 0-50V peak: <2usec
Output impedance: <100 Ohm, including R12 and R23.
Recommended min. load is 20k.
PCB: 80x190mm.
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If possible test each amplifier module separately before installing it in the box. This simplifies measurements, adjustments, and if necessary, component changes. If you have access to a scope, connect it to the output of the module and check whether radio frequency (RF) oscillations are present.If you have a complete audio instrumentation in your workshop perform the usual gain, frequency response, noise, Total Harmonic Distortion (THD), Intermodulation Distortion (IM) measurements. Inputs should be shorted to ground under DC measurements/adjustments.
Short the input to ground and short the signal ground to power ground at the output. Connect +260V anode supply and 12.6V filament supply to the board. Check the anode voltages on tube V1:pin 1 and pin 6 should be approx. +125V. Anode 1 of tube V2A and V3A is approx. +225V, the cathode of V2B and V3B: pin 8 is at +1.65V. The DC voltage at the output (pin 3 and 6 of V2/V3) is approx. +138V.
The DC adjustment of the circuit is simple. Connect a high impedance DC Voltmeter alternatively to the two anodes of V1 and adjust P1 until the anode voltages are equal. Remove the short from the input of the module and connect an audio oscillator to the input. If you have a dual channel oscilloscope connect the two inputs to the two outputs OUT1 and OUT2. Connect the scope to “ADD” and observe the added signals, which indicate the difference between the two outputs. Adjust P2 until the ADDed signal is a flat line, or as close to a flat line you can manage to get. This indicates that the two outputs have the same amplitude, i.e. the gain is the same.
If you don’t have a dual channel scope you can do this adjustment with an audio mV meter. Connect the mV meter alternatively to OUT1 and OUT2 and adjust P2 until the two outputs have the same amplitude. This adjustment needs to be repeated when the board is installed into the power amplifier to compensate for the slight difference in the two halves of the output stage.
Connect a distortion analyzer to the outputs, one at a time and check the distortion at different output levels. If you have good balance between the two triodes of the ECC83 the distortion will be very low. You might want to experiment with different tubes to get the best distortion figures/best sound from the circuit.
For a complete description of the EB-894/211 line amp see the article: “A Differential Line Amp with Tubes” on the homepage under “Special Articles” or in Glass Audio issue 1/97.
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The EB-894/211 design is the property of Erno Borbely/BORBELY AUDIO. Commercial use and duplication in any form is not authorized without a license agreement with Erno Borbely/BORBELY AUDIO. |
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