Mono Switch

One of the many reasons I've been drawn to the Accession (or Reflex) is the mono switch capability.  I know its purpose but not the actual way in which it works.  How does the mono switch eliminate the unwanted vertical output of a stereo cartridge when playing a mono LP?

Hi Graham - I was not inferring that you said the operation of a mono switch is as I stated in the post.  I apologize if you thought so.  As I stated in the Grado thread a few days ago, I've just jumped back into vinyl after 40 or so years.  So my memory is a little cloudy of where I've read, heard, etc., about the operation of a mono switch.

A+B/2 (should read (a+b)/2) takes L and R groove output (usually after amplification and equalisation) and adds it. Replacing variables with 1's we get 1+1 which is 2, but we want 1, so we divide by 2. Hence the mono switch function of A+B/2 which keeps the loudness constant, but only if monaural to start with.

The V-groove places difference signals at 0.7 or -3dB and sum signals at 1 or 0dB, and in-fact, all stereo sources have it this way.

But by making one of the variables (a or b) zero, we obtain 1+0/2 which equals 1/2 or -6dB.

Therefore stereo cannot be made mono.

To obtain true mono would require one channel being at 90 degrees to the other, but all we can achieve in "flat" amplification is 0 or 180 degrees.

Therefore to obtain true mono from stereo we have to use all-pass filters which shift phase such that at any point the phase shift on one channel is 90 degrees different to the other channel.

This means the difference signals are reproduced at the correct relative level.

In comparing radio broadcast output from the AM days, the difference could be heard, with the commercial UK stations having a harder sound, which was simply down to the A+B/2 summing. The state broadcasters often used true mono converters. Such AM modulated phase effects might be heard by an off-tune station.

The reason for me knowing all this useless stuff is because I was given an old Studer catalogue and told I had to design everything they were doing, so my employer could sell the same gear. After studying all this, the other directors overruled the first director, and so all I'd learnt was in vain.

Or, to put it another way, if any of the above can be proven wrong, I'm blaming Studer!

patientot wrote: RussL wrote: Hi Graham - I was not inferring that you said the operation of a mono switch is as I stated in the post.  I apologize if you thought so.  As I stated in the Grado thread a few days ago, I've just jumped back into vinyl after 40 or so years.  So my memory is a little cloudy of where I've read, heard, etc., about the operation of a mono switch. So if I understand your response, the mono switch is simply there to select (mono) when playing a mono record when using a stereo cartridge, to reduce the output level by half. Makes sense (now) to me. I have a large collection of mono LPs as I got into audio my first year in college in 1960. I've also recently purchased some new mono reissues.  Guess I'm going to need a mono cartridge to hear them at their best.  Russ My selection of mono records is fairly small, probably less than 15% of my collection. For that reason, I like the mono switch. If you have a lot of mono records then a mono cartridge could be worth investing in, especially for a TT with a removable headshell. IIRC after a certain year in the early 60s most mono LPs were actually cut on a stereo cutter head with a mono signal. That is certainly true of most mono reissues coming out now. Some reissue programs will use a mono cutter head, but that is rare. Classic Records used to do that, but they had a host of other problems and eventually closed.