Design of attenuation phono equalization circuit based on 6N3 dual triode (Part 2)

In the previous article (Part 1), we talked about the design method of the common attenuation-type phono amplifier. We only talked about it briefly. In actual production, there are definitely more things to consider, such as the selection of tubes, the calculation of total gain, the difference in circuit structure (divided into attenuation type and negative feedback type), distortion control (for small signals, this is not the focus, but it still needs to be considered), frequency response requirements (this is very important. For the phono amplifier equalization circuit, its frequency response usually reaches 100KHz), the error of the equalization characteristics, whether the circuit also needs to take into account some functions of the MC cartridge amplification, etc. In fact, in this short article, we can't cover everything. The most important thing is that we need to master some basic circuit knowledge ourselves, and then we can involve the design and production of the phono amplifier circuit on this basis.

Let’s get back to the topic of this article, which is the 6N3 attenuated phono amplifier circuit designed for a friend.

In the design involved in this article, I will not give a detailed introduction to the amplification factors and output impedance of each level. If you are interested in the basic knowledge, you can download various books to review it.

The record player used by my friend uses an MM cartridge, but considering that he may upgrade to an MC cartridge in the future, although this circuit design is suitable for amplifying MM cartridges, a 10x cartridge input transformer can still be added before the first stage of this circuit to achieve compatibility with both cartridges.

Figure 1 is the basic characteristics of the domestically produced 6N3 dual triode

Figure 1

For this attenuated phono amplifier circuit, the first main idea is the form of a two-stage common cathode amplifier plus a cathode follower circuit. The attenuation network is arranged between the second-stage common cathode amplifier circuit and the last cathode follower circuit of the circuit. The advantage of this is that the impedance matching effect of the cathode follower circuit can be better utilized.

For the phono amplifier circuit, in order to minimize the adverse effects of the time constants of the RC coupling circuits at each stage on the equalization network, the best way is to use direct coupling between stages (except the output stage). Therefore, before designing this phono amplifier, it was decided that this circuit would be directly coupled throughout except for the output coupling capacitor. This may seem to involve too many working points at each stage, but in fact it is of great benefit to the control of the RIAA equalization characteristics of the entire machine.

The original circuit without specific network parameters is given, as shown in Figure 2

Figure 2

My friend hopes that the sensitivity of this phono amplifier will be higher, that is, the amplification factor of the circuit will be larger, so this circuit sets the plate load resistance of 6N3 to 82K.

To calculate the exact parameters of R8, R14, C2, and C7 in the balancing network, as mentioned in the first article, we must first find the output impedance of the second-stage circuit, and first find the internal resistance of the tube:

Ri=µ/S=35/5.9≈6KΩ

Circuit output impedance: Rsc=6k//82K≈5.6K

We set: R8 = 120K

Then: C7=2187/(120+5.6)= 17.412n

C2=750/(120+5.6)= 5.971n

R14=318/17.412=18.263K

Then the complete circuit including the parameters of the equalization network components is shown in Figure 3 below:

Figure 3

After obtaining the complete circuit of Figure 3, we must definitely check the RIAA equalization characteristics obtained by the circuit to verify the accuracy error between the equalization characteristics of this circuit and the RIAA standard equalization characteristics.

The green line in Figure 4 represents the actual equalization characteristic curve obtained by the circuit in Figure 3, and the red line is the RIAA standard equalization characteristic curve.

Figure 4

The characteristics of Figure 4 seem good at first glance, but if you compare them carefully, you will find that the low-frequency characteristic curve obtained by the circuit in Figure 3 has a slightly higher gain than the standard curve. The amplitude of about 10Hz is slightly higher than 0.000 dB, and the high-frequency gain at 100KHz is lower than the standard equalization characteristic curve. In order to see more accurately, I enlarged the error part in Figure 4 and checked it. The difference between low-frequency and high-frequency gain is shown in Figures 5 and 6 below.

Figure 5

Figure 6

As can be seen in Figure 5, the gain of this circuit at the low frequency of 20Hz is about 0.135dB higher than the standard RIAA equalization characteristics; as can be seen in Figure 6, the high frequency gain of this circuit at 20KHz is 97.6mdB lower than the RIAA standard gain, which is about 0.1dB lower (but the gain at the high frequency of 100KHz is about 0.8dB lower).

From the actual simulation results of Figure 5 and Figure 6, the accuracy of our circuit in calculation is about 0.135dB higher at the low frequency of 20Hz and about 0.1dB lower at the gain of 20KHz. Compared with most phono amplifiers, this value is quite good, but what if we want to pursue higher accuracy of equalization characteristics? For example, why is the low frequency gain of 20Hz 0.135dB higher? The high frequency gain of 20KHz is 0.1dB lower? Can we make this circuit achieve a smaller error with the standard equalization characteristics through more accurate calculation or measures? I believe that this higher goal is the ultimate purpose of our design of this phono amplifier.

We will solve the problems of low and high frequency gain errors one by one, and first analyze the causes of low frequency gain errors.

In the tube phono amplifier circuit, if we follow the normal design idea and calculate the output impedance value of the voltage amplifier stage before the equalization network and substitute it into the circuit, the low-frequency gain still increases or decreases. There is no doubt that it is due to the deviation in the calculation of the output impedance of the amplifier circuit (as explained in the previous part of this article). In the circuit of the equalization network in Figure 3, the output impedance of the second voltage amplifier stage has been calculated to be about 5.6K according to the requirements of the circuit. From the simulation results, it is obvious that this value still has a large error. Where does the error appear?

When calculating the internal resistance of the tube, the internal resistance of the tube is 6KΩ. We get it according to the basic formula of the physical characteristics of the tube, that is, the internal resistance of the tube is equal to the ratio of the amplification factor of the electron tube to the transconductance Ri=µ/S. Is there anything wrong with this? This formula is obviously correct, but the root of the problem is that the transconductance value is not a constant for a triode. When the transconductance changes, the internal resistance of the tube naturally changes.

Friends who know the basics of vacuum tubes should know that for a triode, its µ amplification factor is usually a constant, but its transconductance and internal resistance are not constant. The internal resistance of the tube in the manual usually refers to its typical value at the smooth rising section of the anode-grid characteristic curve, but when the tube is in the curved section with a smaller screen current, its transconductance and internal resistance will be significantly different from the values ​​in the straight rising section. At this time, its internal resistance is usually higher than the typical value, and the transconductance becomes lower (the product of the transconductance and the internal resistance usually does not change at this time, that is, the µ value will not change significantly). Carefully analyzing the reason why the low-frequency gain of the phono circuit in Figure 3 is higher than the standard, there is no doubt that I substituted the tube internal resistance under the typical working state of the electron tube into the calculation formula, that is, the 6K ohm tube internal resistance is the typical value given by the manufacturer for this tube under the recommended working state (smooth rising section of the curve), but it may not be the exact value of this tube under the working state of a smaller screen current. In order to obtain a more accurate internal resistance value of the 6N3 tube at the operating point of the circuit in Figure 3, the characteristic curve of the 6N3 tube was found and accurately solved. In this circuit, the second stage works at a smaller value of about 2.5mA at the screen current, as shown in Figure 7:

Figure 7

The negative gate voltage at the working point of the second-stage circuit is about -3V, so in Figure 7 I chose the -3V gate voltage positive current curve, and selected the two points corresponding to 2mA and 3mA. According to the change of positive voltage divided by positive current, it is calculated that in this range, the internal resistance of 6N3 is equal to:

(141V-130V)/(3mA-2mA)=11KΩ

From this, it can be seen that the internal resistance of the tube under a smaller screen current is much larger than the 6K ohm tube internal resistance value we previously substituted into the circuit.

We substitute the internal resistance of the 11K ohm tube into the circuit and recalculate the output impedance of the second stage circuit:

Rsc=11K//82K≈9.7KΩ

When calculating the circuit in Figure 3, we substituted Rsc=5.6K into the circuit for calculation. Now the actual 9.7K output impedance is 4.1KΩ more than the previous 5.6K value, so we have to deduct this extra 4.1K value from the 120K (R8) resistor. In this way, we can ensure that the values ​​of C2, C7, and R14 in the circuit in Figure 3 remain unchanged to obtain more accurate low-frequency characteristics, as shown in Figure 8 below: