I've been asked the question of "Why did Sansui put blend resistors into the VM decoder?" so many times, I thought I'd post an answer here for everyone:
As to why Sansui put them in there.......two reasons;
1) the blend resistor between the left (total) input and the right (total) input helps to insure that the decoder gets a balanced input. Decoding ability is very dependent on the input being balanced......the better the balance, the better the decoding ability.
This was an issue in the '70s......turntables, 8track decks, cheap RCA cables, etc.....all could have 2 channel outputs that weren't as "balanced" as they could be. The blend resistor insured that the decoder would never see just a left or right input......always both channels.
However, the decoder operates best the higher the separation is between the inputs, as this gives it more information to deal with. And, with todays digital sources (CD, etc,).....they have "infinite" separation on their
output.......and a perfectly balanced output too!
So, get rid of the blend resistor and performance (and output separation increases dramatically! From the stock rating of 20db between the channels......to over 40db and technically only limited by the source material.
2) There are two more blend resistors between the LF and LR, and RF and RR signals. These were put in to insure that there was always "something" in the rear channels. Even if the decoder didn't pick up any "out of phase" information for the rear channels......you'd still have sound there at a
lower level dependent on the value of the blend resistor.
In other words.......no matter what, all four channels would have output. This is true even if there is NO out of phase information in the source signal. You can easily check this yourself........play something that is "mono" into the decoder.
Based upon the accuracy and capability of the Sansui QS
chips.....you should have nothing in the rear channels at all (there is no out of phase info available). But, you will notice that you still have sound in the rear channels.......basically the same as the front channels only at a lower level......... you are hearing the "bleeding" that the blend resistors introduce.
Remove them and the decoder is totally controlling the
rear channel output.........you will ONLY hear out of phase information and/or the "horseshoe" output of the "surround" mode where the out of phase info is combined with the Hard Left/Hard Right signals moved to Rear Left and Rear Right as designed. Again, you've got dramatically increased
separation in the rear channels........depending on the source material, you may have little or no output in the rear channels now.........but the rear channels are much more discrete, and much more exciting when they are getting the signals from the chips!
At the time Sansui designed the QSD-1/QSD-2 and 001 receivers VM pcbs.......the above reasons were why they put the blend resistors in. And, they thought limiting the separation to 20db was still better than any other
decoder on the market!
It wasn't until the Audionics S&IC and TATE II that they realized that we wanted ALL the separation we could get!! <G> It is notable that they did away with the LT/RT blend resistor in the QSD-1000 that was produced in 1982. By then they knew that the best decoding was the most discrete decoding that you could obtain! Although they still put blend resistors between LF/LR and RF/RR to still keep all four channels active. But, just the removal of the LT/RT blend resistor raised its separation rating to 25db in the QSD-1000.
So, if you really want to compare how QS decoded through any of the VM decoders compares to SQ through a TATE or S&IC, you should remove the blend resistors, recalibrate the boards......then you've got an Apples to Apples comparison!
QuadBob
As to why Sansui put them in there.......two reasons;
1) the blend resistor between the left (total) input and the right (total) input helps to insure that the decoder gets a balanced input. Decoding ability is very dependent on the input being balanced......the better the balance, the better the decoding ability.
This was an issue in the '70s......turntables, 8track decks, cheap RCA cables, etc.....all could have 2 channel outputs that weren't as "balanced" as they could be. The blend resistor insured that the decoder would never see just a left or right input......always both channels.
However, the decoder operates best the higher the separation is between the inputs, as this gives it more information to deal with. And, with todays digital sources (CD, etc,).....they have "infinite" separation on their
output.......and a perfectly balanced output too!
So, get rid of the blend resistor and performance (and output separation increases dramatically! From the stock rating of 20db between the channels......to over 40db and technically only limited by the source material.
2) There are two more blend resistors between the LF and LR, and RF and RR signals. These were put in to insure that there was always "something" in the rear channels. Even if the decoder didn't pick up any "out of phase" information for the rear channels......you'd still have sound there at a
lower level dependent on the value of the blend resistor.
In other words.......no matter what, all four channels would have output. This is true even if there is NO out of phase information in the source signal. You can easily check this yourself........play something that is "mono" into the decoder.
Based upon the accuracy and capability of the Sansui QS
chips.....you should have nothing in the rear channels at all (there is no out of phase info available). But, you will notice that you still have sound in the rear channels.......basically the same as the front channels only at a lower level......... you are hearing the "bleeding" that the blend resistors introduce.
Remove them and the decoder is totally controlling the
rear channel output.........you will ONLY hear out of phase information and/or the "horseshoe" output of the "surround" mode where the out of phase info is combined with the Hard Left/Hard Right signals moved to Rear Left and Rear Right as designed. Again, you've got dramatically increased
separation in the rear channels........depending on the source material, you may have little or no output in the rear channels now.........but the rear channels are much more discrete, and much more exciting when they are getting the signals from the chips!
At the time Sansui designed the QSD-1/QSD-2 and 001 receivers VM pcbs.......the above reasons were why they put the blend resistors in. And, they thought limiting the separation to 20db was still better than any other
decoder on the market!
It wasn't until the Audionics S&IC and TATE II that they realized that we wanted ALL the separation we could get!! <G> It is notable that they did away with the LT/RT blend resistor in the QSD-1000 that was produced in 1982. By then they knew that the best decoding was the most discrete decoding that you could obtain! Although they still put blend resistors between LF/LR and RF/RR to still keep all four channels active. But, just the removal of the LT/RT blend resistor raised its separation rating to 25db in the QSD-1000.
So, if you really want to compare how QS decoded through any of the VM decoders compares to SQ through a TATE or S&IC, you should remove the blend resistors, recalibrate the boards......then you've got an Apples to Apples comparison!
QuadBob
: 
