Preamplifiers

Lamm LL1 Signature Stereo Tube Preamplifier - An Audiophile's Dream

ARTICLE INDEX

On the Bench

For distortion measurements, I used an 80 kHz bandwidth, and the volume control set half way (12 'o clock, straight up).

With a 1 kHz sine wave test signal, distortion never went above 0.1% at 100 kOhms or 600 ohms load, 2 volts or 5 volts output. This pretty much covers the range of output voltages you are likely to encounter, and far below any input impedance combination you are likely to have with your downstream components. Notice that in all cases, the 2nd order harmonic predominated. In the first graph, the 2nd order harmonic is 70 dB below the fundamental, and the 3rd harmonic is 44 dB below the 2nd harmonic (so the 3rd harmonic is less than 1/100th of the level of the 2nd harmonic). This is what gives the LL1 such an incredibly sweet, delectable (I am writing this just before dinner, so I am using food adjectives) sound quality. It does not make itself known. It is just there, wafting on the background of the music, and adds a richness that is absent with most solid state products. Purists will say that is not high fidelity.

Well, if your desire is to absolutely reproduce the recording, it isn't going to happen. No system can do that without at least a bit of distortion. The issue is what you are willing to put up with. Good solid state preamplifiers and power amplifiers have very low distortion, so low, it is inaudible, and therefore, it does not matter whether it is mostly even-order or odd-order. With a preamp like the LL1, that little touch of 2nd order harmonic is like having your hot chocolate with whipped cream on top. It does not change the flavor of the chocolate, but it gives you an extra sensory experience, one which makes up for the fact that no hi-fi system can really make it sound like the orchestra is playing in your living room.

In the graphs below, you can also see some noise peaks. The coaxial interconnect is picking this up as electromagnetic interference, and is one of the drawbacks of a single-ended design. With a balanced design (also called differential), the interconnect is called XLR, which has three conductors, one positive, one negative, and one ground. The positive and negative conductor signals are complete waveforms, and are mirror images of one another (i.e., inverted phase - 1800 out of phase with respect to each other). The two signals are combined in the output stage by inverting one of them, and this eliminates the electrical interference noise picked up in the cable, as well as in the amplifier before inversion and combining the two signals, by common mode rejection ratio (CMRR). The process also increases the gain, because the two signals are added together.

The disadvantage of a balanced circuit is that the output stages are partially connected in a loop, resulting in the possibility of self-oscillation, which would show up as "ringing" in the output. Also, the input stage of a balanced amplifier tends to have more 3rd order harmonic distortion, while the input stage of a single-ended amplifier tends to have more 2nd order distortion. The pro industries, such as recording studios as well as musical groups performing on-stage, pretty much use balanced equipment throughout, because of its common mode rejection. This is very important when microphone cables and interconnects are long.

However, as you can see below, the noise in the LL1 is all below 100 dBv, which is inaudible. Hum from the AC supply (60 Hz) is also a problem in single-ended designs vs. balanced designs, and that is why the power supply in the LL1 is very sophisticated.

Another design that is different from the single-ended amplifier is the "Push-Pull" amplifier, where, again, there are two circuits side by side, passing the signal. At the input, the signal is passed through a splitter/inverter so that there are two paths, with each path having a complete signal, 1800 out of phase with respect to the other. At the output, the phase of the bottom signal (the red sine wave) is re-inverted by connecting the anodes of each signal path to the ends of an output transformer that is center tapped, with the center tap connected to a DC high voltage supply. (In the case of a solid state amplifier with no output transformer, the two signals are handled by PNP transistors on one side and NPN transistors on the other. The emitters are connected together and to a DC voltage supply.) During the process of combination with the top signal (the black sine wave), each signal is allowed to be "on" for just a bit more than half the waveform, so that one does not turn off just at the point the second one is turning on. Otherwise, crossover distortion would occur. The advantage of a push-pull circuit is that you get more power, and much of the harmonic distortion is cancelled out. However, it is the even-order harmonics that are diminished, while the odd-order harmonics remain. This makes the amplifier a bit harsher sounding, because even-order harmonics are euphonic, while odd-order harmonics are unpleasant. But, if the leftover odd order harmonic distortion is very low, it will be inaudible.

Before the first LL1 graph results are diagrams of a sine wave signal passing through a single-ended, balanced, and push-pull amplifier. The waveforms on the left are what passes through the amplifier, and the waveform to the right of the blue arrow is the output. For a balanced circuit, notice that the amplitude of the output represents the amplitude of the two input signals added together after one of them is inverted, and thus, twice the amplitude of each of the two input signals. For push-pull, you can see there is overlap between the positive and negative portions of the waveform (light blue line) when the signal is passing through the amplifier, in order to prevent crossover distortion. In general, preamps are single-ended and can be bridged to make them balanced. They are usually operated in Class A for part or all of the signal path. Some are push-pull. The LL1 is single-ended.

Power amplifiers can be single-ended, push-pull, balanced (either with single-ended or push-pull output stages), and can be set to operate in Class A or Class AB (and variations of AB). This is controlled by the amount of voltage, called the bias, on the tube's grid. Class A means that enough current to power the music at full level is flowing in the circuit even at idle (no music playing), and when a music signal passes through, the current is delivered to the output. The current that is not being used to drive the music signal is dissipated as heat. Class AB is where there is enough current flowing all the time to deliver a portion of the preamplifier's (or power amplifier's) output capability, and when that current demand is exceeded, the transistors or tubes then have to be turned on to a higher state so that the additional current can be delivered, rather than simply diverting current to the output that was already flowing. This turning on/turning off condition of Class B operation causes distortion.

Class A is very inefficient, and the power supplies have to be large (and expensive). Most power amplifiers are Class AB, and are biased into Class A for part of their power output. High end amplifiers will have a higher percentage of the total power capability biased into Class A. Mass market receivers tend to have only a very few watts biased into Class A. The LL1 preamplifier is biased as Pure Class A, meaning that its total output current capability is flowing all the time. It gets very warm because of this, and needs good ventilation. The power supplies also have to be large, and the combined weight of just the two power supplies for the LL1's two channels is more than 40 pounds.

single-ended-signal-diagram

balanced-signal-diagram

push-pull-signal-diagram-showing-splitter

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lamm-ll1-preamplifier-1-khz-2-volts-600-ohms

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At 10 kHz, the following graphs. Notice again, that even at the tortuous 600 ohm load, which you will never encounter in a real setup, it was still the 2nd order harmonic that predominated.

lamm-ll1-preamplifier-10-khz-2-volts-100-kohms

lamm-ll1-preamplifier-10-khz-2-volts-600-ohms

lamm-ll1-preamplifier-10-khz-5-volts-100-kohms

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Here are graphs using a combination of 19 kHz and 20 kHz sine waves as the input signal. The 1 kHz B-A peak was about 60 dB below the fundamentals in all cases.

lamm-ll1-preamplifier-19-khz-20-khz-2-volts-100-kohms

lamm-ll1-preamplifier-19-khz-20-khz-2-volts-600-ohms

lamm-ll1-preamplifier-19-khz-20-khz-5-volts-100-kohms

lamm-ll1-preamplifier-19-khz-20-khz-5-volts-600-ohms

IMD stayed below 1% for all combinations of output voltage and load impedance.

lamm-ll1-preamplifier-imd-2-volts-100-kohms

lamm-ll1-preamplifier-imd-2-volts-600-ohms

lamm-ll1-preamplifier-imd-5-volts-100-kohms

lamm-ll1-preamplifier-imd-5-volts-600-ohms

THD+N vs. Frequency indicated that distortion remained constant with a 100 kOhm load, and had more distortion at the low end with the 600 ohm load. In general, the LL1 sounded very neutral.

lamm-ll1-preamplifier-thd-plus-n-vs-fr

THD+N vs. Output Voltage showed that, even at 600 ohms, the LL1 was not straining. I set the volume control all the way up for this test. For both impedances, clipping (1% THD+N) occurred at 50 volts output. However, you will never need more than 10 volts, even for transient peaks.

lamm-ll1-preamplifier-thd-plus-n-vs-output-voltage

The measured frequency response was 20 Hz - 20 kHz, - 0.1 dB at 2 volts and 5 volts into 100 kOhms. At 600 ohms, there was rolloff below 100 Hz, but again, you will never encounter this level of downstream impedance. The LL1 wasn't designed to work with a 600 ohm load, and that's why the cut-off frequency in the bass region is much higher than it has to be (this has to do with the limited µF value of the output coupling capacitors, which were not designed to work with a load of 600 ohms; this low impedance, in turn, leads to the limitation in reproducing the full spectrum of low frequencies). In the LL1, when working with a 600 ohm load, the lower cut-off frequency is within the 66-70 Hz region. When working, for example, with a 10 kOhm load, the lower cut-off frequency is around 4 Hz. We show the 600 ohm spectrum here simply to illustrate the real potential of this incredible product.

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