Sound Design Philosophy: The RM-10
By Roger A. Modjeski
The RM-10 came into being in early 1990, when I noticed I rarely played music above a few watts on my Vandersteen 2C speakers. Later, I acquired a pair of the old QUAD electrostatics and achieved the same listening levels at even lower power. Although I loved my RM-9 with its 12 glowing tubes, I wondered if I could do my low-level listening with something smaller and simpler which consumed less power. I drew out the classic two-stage, split load driver, connected it to a push-pull output stage, and lashed up a power supply. Little did I know the time and effort that would be expended in tuning that simple circuit to make it sound like a little RM-9.

I wanted to incorporate the design philosophy that made the RM-9 the well respected amplifier that it is today. Most important of those criteria is the performance in overload. Since the RM-10 was going to be only one quarter the power of its big brother, it had better behave well in clipping. People tend to push small amplifiers harder, and I wanted it to be known as a gutsy little amp. As the split-load phase inverter is not known for its grace in overload, its character was corrected by choosing output tubes with low drive requirements and stabilizing the bias when grid current is drawn. The coupling capacitors do not charge and block off the output grids, as in the Dyanco ST-70. I also chose fixed (yet adjustable) bias over the commonly used cathode bias for better overload stability.

Note that this is the first amplifier to achieve 30 watts from one pair of 6BQ5/EL-84s. The classic circuit (Dynaco ST-35) typically achieves only 17.5 watts from the same pair. Current EL-84 amplifiers achieve 30 watts but use two pair per channel--just more tubes to heat up and replace later. They also consume far more power (220 watts at idle vs. 70 watts for the RM-10), which means more heat and shorter life for all the components. Long term cost of operation and freedom from repairs were handled simultaneously. At the recommended bias current of 30 mA. per pair, the idling dissipation is nine watts, or 75% of rating. One popular British amplifier employed EL-84s at 15 watts with rather short tube life. I estimate the tube life to be 5,000 to 10,000 hours if the amp is played below clipping at the recommended bias setting. Although higher idling current will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, and allows for more peak current when needed. The only loss is about 20% of the power rating or 1 dB. (This is fully covered in the section "Connecting Your Speakers".)

The RM-10 has some miraculous transformers, though not MAGIC. The output transformers have insertion loss, as all transformers do. They are a carefully chosen balance of copper and iron that achieve wide bandwidth and low magnetic distortion. Compared to the traditional output transformers, they have less iron to magnetize and therefore less magnetic distortion. They are the culmination of my last ten years' musings on output transformer design. The power transformer is rather large by comparison. In it, I am not concerned about magnetizing a large amount of iron. It uses the same grain-oriented, audio output quality M-6, in a winding configuration that provides a tighter supply than any other EL-84 amplifier I have measured. This high degree of regulation allows the RM-10 to achieve stable imaging and solid impact with transients. By standard transformer ratings, it is capable of 250 watts, which translates to 400% over its demand at typical music listening levels.

Two output taps are provided. Amplifiers with only one tap are too limited for my use. After a complete examination of the currently available binding posts that would fit the cost criterion of the RM-10, I decided to have my own made of solid brass. These allow the use of a 3/8" nut driver to tighten them. I have seen too many plastic posts strip out or shear off! I set the posts at 3/4" spacing and offer "banana caps" (optional) for those of you who use banana plugs. The Tiffany input jack was chosen because of its good design, high quality and cost effectiveness. Those three qualities are the criteria for all the parts in the RM-10.

Working our way back to the input, you will find it carefully considered. Although it is DC coupled, you can easily insert a capacitor right at the input jack to eliminate subsonic or any other low frequencies that might bother your speakers. The input section of the manual gives values for these capacitors. If you are planning to use the RM-10 as a midrange or tweeter amplifier, you can achieve a simple 6 dB/octave crossover by putting a single capacitor at the input jack. Your dealer or our technician will be happy to calculate the value. The only thing in the path to the first grid is a simple radio frequency filter to prevent interference. It is set at 100 KHz so that it will cause no audible effects. You may be curious about the reason for the two series plate load resistors on the first stage. While one would be within dissipation rating, I have found that high value metal film resistors must be operated well below their full rated voltage to achieve long life. All the resistors in the RM-10 are thoughtfully derated for long life. The critical ones are the same premium Resista brand we use in the RM-9. I believe that the superior sound of this brand is due to its low voltage coefficient, which puts harmonic distortion down at -120 dB. Coupling capacitors are high quality polyester which were found to sound better than any other type. (Top)

Since the total value of the first plate load is over 600 Kohms, it is virtually a current source. This allows the 12AX7 to operate in its most linear and highest gain mode. A neon lamp is used to conduct the large positive grid voltage and protect the second stage during warm-up. This often overlooked detail makes the 12AX7 much happier and extends its life markedly. Unlike the classic split load converters, the RM-10's runs at lower current to extend life. Drive is more than adequate, yet not so much to overload the output tubes. Note that V1 is a RAM grade AB (section 1 selected for low noise) and V2 is grade BA (section 2 low noise). It is essential to have a grade A section as the first amplifier to achieve the very low noise of which the RM-10 is capable.

Taking a lead from its big brother, the RM-10 power supply uses a voltage doubler to achieve best regulation from the transformer. This also conveniently provides a stiff and exact half voltage for the screen grids of the EL-84s without using a complex regulator. Since it is tied to a fraction (1/2) of the main supply voltage (B+) it is always just right. I believe the RM-10 to be the first commercial amplifier to employ this technique.

The EL-84 filaments are connected in series to allow a 12.6 volt winding to provide both filament power and bias voltage, thus eliminating the fragile, low current bias winding and its delicate fusing.

Unlike other low cost amplifiers, individual bias pots are provided for each channel along with test points to check each tube individually. All test points and adjustments are on top for ease of checking and adjustment. The test points were thoughtfully placed outside the optional tube cage and right up front.

Because I firmly believe in matched pair output tubes, the individual tubes are not adjustable; I know the pitfalls in that road. Tubes are computer selected and RAM TUBE WORKS can match replacements with the data shipped with your amp. Individual test points are provided so you can periodically check that both tubes are up to snuff. Output tubes are fused to save them in case of occasional lint shorts or other overloads. The power transformer primary is also fused for protection.

In the power supply, you may note that the customary equalizing resistors have been eliminated from the two series-connected electrolytics feeding the driver stage. The "conventional wisdom" of using equalizing resistors makes no sense on close examination; because electrolytics have a smooth leakage current mechanism, they will simply equalize themselves. In fact, the equalizing resistors often drift with age and cause more harm than good. To my knowledge, you have the first amplifier in history with these resistors intentionally missing. I want you to know about this and other unusual aspects of your amplifier so you can have some fun with your technical friends who may raise questions when they study the schematic included in your manual.

Noise is very low in the RM-10, which makes it especially attractive for sensitive speakers like my original QUAD ESLs, the Klilpschorns, and the high efficiency speakers I am developing to add to the Music Reference line. Owners of these speakers often have been frustrated when an expensive, respected amplifier has too much noise. (Note that noise figures given in technical reviews are often in error, reporting the amplifier to be as much as 30 dB quieter than it is in actuality.)

For sensitive speakers, 35 watts is more than enough. Owners of such speakers can rest more easily with a small amplifier that is far less likely to damage sensitive speakers. Knowing what I do about the Quads, I was always a little nervous with the RM-9 connected to them. What if a cable came loose, or someone played the system too loud? A properly sized amplifier is the best insurance for your speakers.

The styling of your RM-10 resembles the RM-9, yet this design was reached the long way around. Although it is obvious to have it resemble its bigger brother, I was not initially able to achieve the look at the price I desired. In its development period of over 4 years, the RM-10 went from a breadboard to an enclosed broad-fronted chassis, to a narrow deep chassis, to a chassis with a shelf for the tubes, and finally to the form you see now. Although a complete circuit board was developed and prototyped, the final design went to hand wiring for its sonic superiority, ease of service and long life. I expect that most circuit board tube amplifiers, especially the ones with hot power tubes on the board, will crumble in 20 years. Rest assured that your RM-10 will last far longer.

I want you and your audio friends to know that I did not follow "conventional wisdom" when I found it to be unwise. They may also pull out tube manuals and argue with some of my choices. Tell them they are free to write me with their concerns. For you, I trust that the sound and the reliability I have achieved in the RM-10 is proof enough.

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