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Slobodan Super
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MC phono preamp

 

 

All tube preamp with unique step –up transformer

MC input sensitivity     0,3mV

MM input sensitivity     3mV

Line input sensitivity    300mW

Output level                   1V at 75 ohm load

 

This preamp is the best preamp which I ever made. Main development goal was to minimize the compromises which, in general, can’t be avoid in the phono preamps. Construction of good MC phono preamp is maybe the heaviest task in the whole field of the audio electronics, especially with tube circuits. Here we need to deal with the microvolt signals and in the same time preamp has to be able to accept very high input signal level without saturation. 

OK, high input saturation level is not so big problem with the tubes but obtain really low noise input stage is extremely difficult task for tubes. In fact, only very few types of tubes can be used in the MC input stage. 

Lot of tubes were on my test bench and I can said that even the best between them has limitations. EC8010, 417, C3g, D3a, 6C45, ECC88 and 6SL7 (4 parallel sections – 2 tubes), 6H30 (both parallel sections), all of them and some others are checked in searching for optimal input tube. Unfortunately, 2 nice candidates (EC8020 and 437) are not checked – they are extremely rare and very hard to find.

EC8010, 417, ECC88, 6SL7 and some other tubes can be used if you have high noise tolerance level. It works, sound is very nice when music plays bit noise is still audible. Not so strong, but….

Also, all of them have a bigger of smaller problems with the microphonic effect. Not so big trouble, this problem can be (almost) successfully suppressed with use of  the proper anti shock tube sockets.

6H30 with parallel connected sections is very good in terms of noise and microphonics effect but sonically is little bit down. Scope’s screen is reveal why, parallel connection of the 6H30 sections lead to the VHF oscillations. Generated frequency of around 150MHz has not so low level and create intermodulation with the music signal. Unfortunately, fight with the parallel connected 6H30 sections oscillations is very difficult due to a high internal inductance in the cathode.

Only one section is fully stable but noise became a little bit more audible

C3g, very good in terms of noise but exhibits strong microphonics effect  even in the MM input

6C45, practically only one which hasn’t troubles with the microphonics effect, even with the only 0,25mV input level. Excellent in terms of noise level but shows fully different noise spectrum than any of other tested tubes. Sonically, noise is “roughly” and despite the low level, can be little bit irritate for the listener.

D3a, clear winner in the terms of noise, even better than 6c45 but bellow the 1mV input level exhibits some trouble with the microphonics effect. Anti shock tube sockets is helpful but sensitive to the  microphonics is significantly higher in compare with, as example, EC8010.

 

So, what is solution?

We have a couple of different ways, depend of designer’s preference. The simplest way is to use the low noise FET in front of the tube. This solution is good but maybe is possible to find equally good (or even better) noise solution. Yes, answer is simple, transformer followed by D3a. As is mentioned earlier, D3a hasn’t mycrophonics trouble at input levels higher than 1mV. It means that transformer need to have transfer ratio of only 3 to 4 times for use with, let’s said, the most popular MC cartridge Dennon DL103.

Ok, with this small transfer ratio is not so big problem to construct very high quality step up transformer but very high quality doesn’t means desired quality (my MHz criteria). Even with the best possible transformer is difficult to expect that bandwidth can be higher than 200, 300 kHz if we want to avoid phase shift.

How to proceed? Idea is logical and simple but realization is fully different story. Basic idea is to use 2 transformers, one for lower frequencies and one for higher frequencies, and put them to operate together. It works virtually good, amplitude peaks on the crossover frequency and self resonance frequencies are relatively small but phase shifts inside the bandwidth are terrible.

So, is it such idea possible in the practice? Yes, after lot of development work, result taken from the scope's screen is shown on the images bellow. Combination of  2 transformers (made on ferrite core) and compensation network has bandwidth of around 1MHz and phase shift is negligible  bellow the 700KHz.

Total transfer ratio is 4,4 times which means that total gain of the MC stage is 11 times (D3a with very low anode resistor has gain of 2,5 times).

Only one “negative thing” with this topology is that transformers and compensation network need to be optimized for the particular MC cartridge. Who cares, I am not a mass producer and optimization is done for the Dennon DL103 cartridge.

Rest of the preamp is a MM / line stage. MM input is created as a split passive RIAA stage. Input tube is again D3a but here D3a has high gain (40 times). Choice of such gain of D3a in the input stage is excellent to ensure maximum possible overall S/N ratio and very high input saturation capability (around 350mV). Next stage is 1st RIAA filter (3180uS and 318uS). Second gain stage use one half of  the excellent Russian 6H30 tube with the gain of 12,5 times. Nothing less, nothing more, this is a optimum value.

Both D3a (MC and MM) are triode connected, of course.  

Logically, second RIAA filter (75uS and 3,18uS) should be located directly after the second gain stage but not in my preamp. Here is this filter located after the volume pot. Reason for this very unusual topology is to avoid additional cathode follower stage and ensure possibility to use small ohmic value pot (10K). OK, some penalty is paid, deviation of the RIAA curve is around 0,25dB instead less than 0,1dB in the traditional approach but it is very small and negligible victim.

As is shown on the schematic, preamp has 4-gang pot and 2 chains. Both of them can be called line stage. Upper one is used as line gain stage after the second RIAA filter and lower one as line stage for standard line inputs (CD, tuner…). Commutation is done after these stages, just before the output stage with one half of 6H30.

This output stage seems as ordinary cathode follower on the first look but is much more. First of all, this stage is intended to drive load of 75 ohms and enable precise termination to the 75 ohms coaxial cable as interconnect. It is lot of reasons for this arrangement, in practice is much more important that care about interconnects (length and quality) became past.

This is also fully unusual in the audio field but it is not a my problem. It is simply the best way of signal transfer between 2 points (well known from RF / video technique).

Precision output impedance of this stage is ensured by right choice of tube (6H30) and small additional resistor.

Another fully unusual thing (in audio) about the output stage is a implementation of the neutralization. This is also well known from RF technique, neutralization eliminate influence of the capacitance Cga and Miller effect became zero. It means that pot see only the couple of picofarads capacitance Cgk of the line tube (ECC88). Result is the linear frequency characteristic of the pot (independent of the pot’s setting) deep inside the MHz range.

Just for cosmetic reason, preamp is equipped with 2 small magic eyes (DM70) as visual level indicators.

And finally, all of them mentioned above doesn’t make sense without a very good power supply. Such supply doesn’t need to be complicated (voltage stabilization is not an issue) and according to them, power supply is created like simple (but with a lot of components) unit which consist only multiple RC cells in the filtering. But every preamp’s stage has its own power supply to ensure clean and hum free DC high voltage for tubes (total of 10 independent power supplies for both channels). This seems complicated and expensive but in reality it hasn’t practically nothing more components than good stabilized power supply with necessary and unavoidable decoupling RC cells between preamp’s stages. Cost of the 1 A bridge rectifiers and resistors are practically zero. Used electrolytic capacitors are also not so expensive due to relatively low supply voltage. 350V rating electrolytic capacitors are more then enough.

Heather supply is common one per channel (total of 2) and has voltage stabilization

 

So, that ‘s that. All design goals are reached

 

 

 

 

 

 

 

 



 
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