LDR attenuator with tube cathode follower buffer?

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tortugaranger

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I've received inquiries regarding the possibility of combining our LDRx attenuator/preamp-controller with a tubed based buffer. Of course with a buffer....tube or SS, a passive preamp becomes an active preamp with no gain. It makes the whole passive impedance mismatch issue moot, will work with practically any source/amp combination, but may lose some of the benefits of a true LDR passive...or not....and may gain the benefit of that "tube sound".

I'd be interested in feedback from the AC community. Do you see merit in a buffered LDR attenuator and if so why?

C17FXR

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #1 on: 7 Sep 2013, 06:08 pm »
I'm already working to do just this very thing, I just haven't had the time to work out the details yet. The biggest reason as you mentioned is to eliminate the impedance mismatch issue.
That's the reason I use buffered preamp now is to drive a 27' pair of wires to my mono-blocks. I like to keep my equipment to the side of the room as it provides a better soundstage, just wish I could get rid of that center channel.  :nono:

I'm even considering your LDR3x with the Slave board. Hopefully I'll have sometime this week to work on a few of the details. But right now I have a couple of other irons in the fire that are getting pretty hot.
Plus I have to let the funding department cool down before I start another costly project,  :roll: she was pretty heated over the one I'm currently working on.

WireNut

Re: LDR attenuator with tube cathode follower buffer?
« Reply #2 on: 7 Sep 2013, 06:58 pm »
I've received inquiries regarding the possibility of combining our LDRx attenuator/preamp-controller with a tubed based buffer. Of course with a buffer....tube or SS, a passive preamp becomes an active preamp with no gain. It makes the whole passive impedance mismatch issue moot, will work with practically any source/amp combination, but may lose some of the benefits of a true LDR passive...or not....and may gain the benefit of that "tube sound".

I'd be interested in feedback from the AC community. Do you see merit in a buffered LDR attenuator and if so why?


 I’m thinking a buffered LDR attenuator may work best in my bi-amp system. I use an external crossover but only for bass frequencies. The amp used for lower frequencies differs from the amp used for the mains. Also, I need 2 separate sets of unbalanced preamp outputs so I can run one set directly from the preamp into the main amp bi-passing the external crossover. I'm thinking the 2 sets of preamp outputs should be buffered rather then wired in parallel.   

Does this sound like a situation better suited for a buffered LDR attenuator?

Thanks,
WN







tortugaranger

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #3 on: 7 Sep 2013, 07:36 pm »
That's the reason I use buffered preamp now is to drive a 27' pair of wires to my mono-blocks.

27 feet!! Great leaping capacitance batman! Yeah, a passive would likely see some serious roll off not to mention phase shifting.

Quote
Plus I have to let the funding department cool down before I start another costly project,  :roll: she was pretty heated over the one I'm currently working on.

Hard to enjoy music when you're crosswise with the funding department. No easy solutions.  :scratch:

tortugaranger

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #4 on: 7 Sep 2013, 08:03 pm »
Does this sound like a situation better suited for a buffered LDR attenuator?

I have customers who are running unbuffered parallel outputs from our LDR1/LDR6 units into both mono tube amps and also sub amps. The feedback I've received is very positive. I suspect this works because the tube amps have extremely high input impedance (typically ~1M) and the sub amps have sufficiently high input impedance such that there's adequate signal for both notwithstanding the impedance mismatach. I've personally run run parallel outputs from our LDR6 (using a splitter) into both a SS main with 47k input impedance and also a sub amp with ?? input impedance.  The main amp output sounded the same with or without the subs connected. The subs sounded a bit flabby to me and since they're servo amps (Rythmik A370-PEQ3) which should sound very tight I suspect they weren't all that happy. I no longer run subs so it's become a moot point for me personally.

So while there's no doubt that a buffered LDR preamp would work, an unbuffered passive scenario can also work but it would depend on the mix of amps you're using. That's the thing with passives. When they fit well, they're fabulous. But when they don't, they can disappoint. I believe that much of the negative commentary regarding passives is due to their misapplication which then sends a muddled message that passives aren't as good as actives.

If we do produce a buffered LDRx preamp, the initial product the buffer stage will be integrated into the preamp. We'd likely follow that with a stand alone buffer product that could be used with any combination of passive attenuator and amp(s).

barrows

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #5 on: 7 Sep 2013, 11:37 pm »
I think there are many system applications where a unity gain buffered output would be preferred.  Of course, the buffer stage needs to be really, really great to keep the (claimed/apparent) advantages of the LDR VC…  So, I would only recommend building buffered versions if you are capable of designing a great sounding, low noise, low distortion discrete buffer-preferably with no global feedback…
I think, then making a separate buffer module available as well for the DIY folks would be appreciated as well, as would a true balanced version.

tortugaranger

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #6 on: 8 Sep 2013, 12:10 am »
I think there are many system applications where a unity gain buffered output would be preferred.  Of course, the buffer stage needs to be really, really great to keep the (claimed/apparent) advantages of the LDR VC…  So, I would only recommend building buffered versions if you are capable of designing a great sounding, low noise, low distortion discrete buffer-preferably with no global feedback…
I think, then making a separate buffer module available as well for the DIY folks would be appreciated as well, as would a true balanced version.

I built a very clean and quiet SS buffer prototype a couple of years ago but put it aside to concentrate on the LDR passive. More recently I've done some early stage work on a cathode follower design using a split voltage approach such that the follower output voltage is close to zero. Kind of like a rubber band held at two ends where the middle hardly moves. Should be very quiet with a high PSRR.

You referenced discrete. Any view on SS vs. tube or is that opening too big a can of worms?

WireNut

Re: LDR attenuator with tube cathode follower buffer?
« Reply #7 on: 8 Sep 2013, 12:22 am »

Any view on SS vs. tube or is that opening too big a can of worms?


I'd like to see a SS unit and keep all tubes at the amplifier end if so inclined.

















poseidonsvoice

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #8 on: 8 Sep 2013, 02:10 am »
I built a very clean and quiet SS buffer prototype a couple of years ago but put it aside to concentrate on the LDR passive. More recently I've done some early stage work on a cathode follower design using a split voltage approach such that the follower output voltage is close to zero. Kind of like a rubber band held at two ends where the middle hardly moves. Should be very quiet with a high PSRR.

You referenced discrete. Any view on SS vs. tube or is that opening too big a can of worms?

Probably similar to what I would do:

For SS buffer use a DCB1 designed by Salas & Nelson Pass. Need to use Vishay Zfoils here and lots of shunt current like the 600mA version.

Cathode followers have to be designed very carefully. Bootstrapping the triode in cascode configuration can work well. For tube buffer use the latest version of the late Allen Wrights SLCF but forgoeing the gain stage of course. The supply for this unit needs to be amazing, i.e. shunt regulator like Allen Wright's Super Reg or Sjostrom's super reg or Belleson's regulators, etc...

The tube implementation is much more expensive.

Whatever you do, you will need to do a blind test and see if you can hear a difference between using your VC with a buffer or not. If you do, then it's back to the drawing board :-(

Good luck Morten.

Best,
Anand.

gregfisk

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #9 on: 18 Sep 2013, 03:44 am »
I recently purchased a LDR kit and was looking at a Dodd tube buffer to go with it. I would be replacing a tube pre and I'm running TBI class D amps at the moment. My vote would be for a tube buffer if you plan on building one.

Greg

tortugaranger

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Re: LDR attenuator with tube cathode follower buffer?
« Reply #10 on: 18 Sep 2013, 11:59 am »
I recently purchased a LDR kit and was looking at a Dodd tube buffer to go with it. I would be replacing a tube pre and I'm running TBI class D amps at the moment. My vote would be for a tube buffer if you plan on building one.

Greg

From the research I've done it appears that those with tube amps prefer a colorless solid state buffer and those with SS amps prefer a bit of tube sound in their preamp/buffer. Without a doubt, SS buffer would be easier to design and be less expensive. From a marketing perspective, a well executed tube buffer would likely be the more popular choice.