I've had many requests for a preamp to replace the hard-to-find RM-5. Now that phono is back and at a new level of performance, a new way of dealing with that subject is necessary. As many of you know the RM-4 tube headamp launched Music Reference and we sold an astounding number of these. I still have a few left from the last production run.
However, the RM-1 preamp and RM-2 power supply that ran it were way before their time. This unit had 12 6DJ8/6922 tubes and 6 regulated power supplies. The power supply weighed about 35 pounds and the preamp about 15 so that both together packed was just under the 60 pound UPS weight limit in 1978. A limited quality of these are available from me in a MK II version that sells for $8,000. This preamp has 2 phono inputs with individual resistive and capacitive loading for each. There are 2 high level inputs and 2 tape loops and a processor loop. This was the first commercially available preamp to use the now ubiquitous 6DJ8. There is a paper by me on this tube that was published in Glass Audio some 20 years ago. Here is the link to that article:
http://tubeaudiostore.com/suitof6dfora.htmlHere are my current thoughts on what a preamp should be: When I make reference to "both" preamps I am referring to the RM-1 and the RM-5 which have many similarities. When I designed the RM-5 I did everything possible to get the RM-1 performance with its 12 tubes into the RM-5 with just 3 and all in one chassis.
The line stage: Both preamps had internally adjustable gain in the line section. This is essential to get low noise performance from systems with high sensitivity speakers. It also allows the volume control to be used in its upper range where it is most linear, has best tracking and is most gradual in adjustment. If your system plays LOUD at 9-10 o'clock you have too much line gain.
The Phono stage: The RM-1 has plenty of gain for MC cartridges. The RM-5 does also in many systems if the power amp is of standard or high gain, the speakers are sensitive (90 dB or better) and the listener listens under 100 dB SPL. If one uses a head amp (thus the RM-4) with either you will have a better match between phono and CD levels on the volume control. Without the headamp, the volume setting for phono will be rather high and the CD rather low. Many modern preamps have this problem, though I would certainly correct this so the volume control is more for how loud you want to listen, not how loud your source is.
For those interested in the numbers a .5 mV cartridge needs 60 dB of phono gain to get close to the average CD. The old standard for phono stages is 40 dB. A good headamp can provide the extra gain and most RM-4s were shipped with this amount of gain.
Given the fact that there are now very good 6922's available (Try out our SLN in the holiday special post). Though I design and wind very nice output transformers I feel a MC input transformer negates much of the virtue of the MC cartridge which is to get the iron out of the system with its Hysteresis problems and energy losses. Even a well designed MC transformer is going to take some of the energy to run the flux up and down in the core (hysteresis) have winding resistance losses and actual (milli)watt losses.
The extra gain can be supplied by a build-in head amp or the more elegant solution of being integrated into a variable gain phono stage. This will cover all cartridges MC and MM. including those high output MC that live in the in-between land of output and loading.
Line stages need almost no gain at all which is why I made "Pot in A Box" which works fine with low capacitance cables and people who know that the best place to run a volume control is at the top of its range, not the bottom. There are people who believe that its better to have the volume control run low as this gives headroom. This is simply untrue.
However, a modern preamp should address balanced lines and have a balanced output as this is the best way to have all components grounded (for safety) and avoid ground loops and other noise sources. Of course an unbalanced output is simple to provide as well as an absolute phase switch (as in the RM-1)
Tape loops are no longer necessary which simplifies the signal path. The electronics for the absolute phase switch are already there with the balanced line amp and this only adds one switch in the path. All switches are gold plated as in both the RM-1 and RM-5. We have very good panel mount jacks (we didn't then) which will be employed throughout.
Your comments and votes will be appreciated.
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