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Old 20th January 2012, 05:50 PM   #301
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Quote:
Originally Posted by Joshua_G View Post
However, for best separation between the 2 channels, 2 separate volume controls and input switches are recommended.
Considering how poor the separation is on even the best carts, dual mono VCs are not going to make a skerrick of difference compared to a well laid out ganged VC.
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Old 20th January 2012, 06:27 PM   #302
kenev is offline kenev  Greece
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Originally Posted by Joshua_G View Post
The convenience is an issue. However, for best separation between the 2 channels, 2 separate volume controls and input switches are recommended.
My humble perception of the issue says that, with a balanced signal, there should be no issues of channel separation. To my mind, it's just a matter of whether you prefer to do one or two hand movements in order to change volume or input.

Oh, there's also an advantage of the two separate volume controls that I forgot to mention: it offers you the ability for balance control between two channels (if ever needed - personally, I did never need to change the balance between two channels)
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Old 20th January 2012, 08:33 PM   #303
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Originally Posted by Brett View Post
Considering how poor the separation is on even the best carts, dual mono VCs are not going to make a skerrick of difference compared to a well laid out ganged VC.
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Originally Posted by kenev View Post
My humble perception of the issue says that, with a balanced signal, there should be no issues of channel separation. To my mind, it's just a matter of whether you prefer to do one or two hand movements in order to change volume or input.

Oh, there's also an advantage of the two separate volume controls that I forgot to mention: it offers you the ability for balance control between two channels (if ever needed - personally, I did never need to change the balance between two channels)
Allen used 2 separate volume controls and 2 separate input selection switches.
In his book he wrote about the importance of the best possible separation between the channels.

This is DIY and everyone is free to play with each and every aspect of the design. So, each to ones' own.

In the RTP3C I intend to build, I'm going to follow all Allen's recommendations to the letter. This is because of I trust Allen, who arrived at his findings after very many years of experience. Also, I see clearly the technical aspects that justify all of his recommendations, but I don't want to enter any argument, or try to convince others. Each and every one is free to make choices according to ones' views.
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Old 20th January 2012, 08:52 PM   #304
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Originally Posted by Joshua_G View Post
Also, I see clearly the technical aspects that justify all of his recommendations
OK, what technical aspect justifies this recommendation? Got something more than assertion to back it up?

I communicated with Allen over many years, and whilst I respect him, it doesn't mean I accept what he says all the time.
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Old 20th January 2012, 09:36 PM   #305
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OK, what technical aspect justifies this recommendation? Got something more than assertion to back it up?

I communicated with Allen over many years, and whilst I respect him, it doesn't mean I accept what he says all the time.
As I wrote before, I'm not into arguments, or convincing others.

In a nut sell, whatever restrictions are there in the source concerning channel separation, adding such restrictions in the amplifier wouldn't improve the soundstage imaging. Interactions between the 2 channels in the amplifier itself will degrade whatever coming from the source, regardless of what happens in the source itself. The source may not be very good to begin with, the question is whether we care about further degrading the soundstage imaging in the amp.
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Old 21st January 2012, 04:41 PM   #306
Alex M is offline Alex M  United Kingdom
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Originally Posted by Joshua_G View Post
As I wrote before, I'm not into arguments, or convincing others.

In a nut sell, whatever restrictions are there in the source concerning channel separation, adding such restrictions in the amplifier wouldn't improve the soundstage imaging. Interactions between the 2 channels in the amplifier itself will degrade whatever coming from the source, regardless of what happens in the source itself. The source may not be very good to begin with, the question is whether we care about further degrading the soundstage imaging in the amp.
I have to agree with Joshua on this one.

This, to me, is a very similar argument to the reason for us striving for the high-end at all. Why spend $500 for a quartet of 300Bs when most of the music we listen to was processed through a mixing desk full of cheap op-amps?

I find that the more effort I put into improving my electronics, the more I hear from my precious vinyl. Do I care about >1% of distortion? 30dB channel separation? Not really. The advantage of DIY is that it doesn't cost much more to do the job properly in the first place - this is why I use dual mono power supplies in my preamp and monoblock amps. Perhaps it's purely psychological - if I didn't go to these lengths to start with, I would probably spend far too long worrying whether I could still improve things like the channel separation. I personally am not overly bothered by dual controls (partly since every valve preamp I have owned has had them), and since there is a plausible logical argument for having them - there they are.

Alex
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Old 21st January 2012, 08:21 PM   #307
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Well said, Alex.
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Old 21st January 2012, 08:25 PM   #308
jpak is offline jpak  United States
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In my opinion you really have to use a stepped attenuator with the RTP3: the need for a 50K load for the phono stage means you can't use law-faking resistors on the pot (they would change the resistance of the pot), top-quality matched dual log pots are almost impossible to find, and linear pots have really poor control and matching at low volumes. I resorted to using 10K pots with a 40K series resistor on the input to get any real range at the lower end, but of course lost some of the upper end of the range as a result.

I have designed a 24-way stepped attenuator for my RTP3 to replace the Sfernice dual pots I have in there now. I have collected all the resistors, and am now waiting for the UK ELMA distributor to get hold of a couple of 24-way 4-pole switches for me.

I also think stepped attenuators are much better suited to dual controls than potentiometers, since you have repeatable volume settings.

Alex
I think the commercial unit is using a stereo DACT per channel. That's what it looks like to me.
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Old 22nd January 2012, 08:00 AM   #309
Alex M is offline Alex M  United Kingdom
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Originally Posted by jpak View Post
I think the commercial unit is using a stereo DACT per channel. That's what it looks like to me.
I think I would agree with that - the volume controls look like DACT, but the source select could be ELMA.

Alex
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Old 22nd January 2012, 10:37 AM   #310
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From the instructions: "This method means there is no PCB material in the space of the signal, making a cleaner sounding switch than the commercial versions made by DACT or Goldpoint."

I believe that both the selector and volume switches are from ELMA.
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