Compatability with Class D

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LarryD56

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Compatability with Class D
« on: 27 Sep 2014, 10:13 pm »
I'm wondering if the new balanced preamp would work well with my system. I'd like the benefits of LDR technology, but I'm not sure if it would be compatible with what I have. I'm using very inefficient Infinity RS-2.5s as my speakers and from what I can find they have an 83db SPL rating. The amp is a class D Cherry Ultra which puts out 675 WPC into a 4 ohm load. I believe that the input sensitivity is 2.10V @ 300 watts into 8 ohms. 

If it would not work for me, would it be possible to install the boards in an Acoustic Research LS2B MKII balanced preamp? A lot of this stuff is new to me so please excuse me if I do not make myself clear.

Larry D.

tortugaranger

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Re: Compatability with Class D
« Reply #1 on: 28 Sep 2014, 02:39 pm »
I'm wondering if the new balanced preamp would work well with my system. I'd like the benefits of LDR technology, but I'm not sure if it would be compatible with what I have. I'm using very inefficient Infinity RS-2.5s as my speakers and from what I can find they have an 83db SPL rating. The amp is a class D Cherry Ultra which puts out 675 WPC into a 4 ohm load. I believe that the input sensitivity is 2.10V @ 300 watts into 8 ohms. 

If it would not work for me, would it be possible to install the boards in an Acoustic Research LS2B MKII balanced preamp? A lot of this stuff is new to me so please excuse me if I do not make myself clear.

Larry D.

Hi Larry,

While I haven't run the numbers on your setup I suspect your amps have more than sufficient power to drive your speakers without needing any additional gain from your preamps. With that assumption your system should work with passive attenuation provided your amp's input impedance is high enough which I suspect it is give that it's class D.

When it comes to passive resistive attenuators a key consideration is the impedance bridging ratio between your source and your amp. The bridging ratio is the ratio of your destination device's input impedance to your source's output impedance. With active preamps a 10:1 ratio is usually sufficient for good performance. With passive preamps, things get a bit more complicated because you have to consider the effective parallel impedance of the preamp together with the amp. I suggest a 50:1 ratio between your amp and source for passives but this is guidance only and not a rule. Most class D amps have fairly high input impedance (50k or higher) since their input stages are basically opamps. The class D amp I'm familiar with has a 100k input impedance. These amps should work well with most any source which usually have 1k or less output impedance. I couldn't find an input impedance spec on the Cherry Ultra and while it's probably high enough that's something that would need to be confirmed. Bottom line is, subject to some further verification, I'm reasonably confident that our LDRxB would be a great fit with your system.

While retrofitting our LDR3x.V2 preamp controller boards into existing active preamps can (and has been) done, a key challenge is finding sufficient space to fit them into the existing enclosure given all the stuff that's already in there. Of course that can be worked around by putting the board in an external enclosure. The interconnects gets messier but it's doable. However, this would be a special project better suited to a DIY project.

Best,
Morten

srb

Re: Compatability with Class D
« Reply #2 on: 28 Sep 2014, 04:04 pm »
Most class D amps have fairly high input impedance (50k or higher) since their input stages are basically opamps. The class D amp I'm familiar with has a 100k input impedance.

I was wondering about the potential match with the Class D Audio Inc. amplifiers.  They (the CDA and SDS series) are a popular low cost Class D amplifier on this forum based on the International Rectifier IRS 2092 chip (182 page topic in the Cheap and Cheerful HiFi circle).  Although the SDS series has a 47K ohm input impedance, the CDA series only has a 7K ohm input impedance.

Steve

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Re: Compatability with Class D
« Reply #3 on: 28 Sep 2014, 08:52 pm »
I was wondering about the potential match with the Class D Audio Inc. amplifiers.  They (the CDA and SDS series) are a popular low cost Class D amplifier on this forum based on the International Rectifier IRS 2092 chip (182 page topic in the Cheap and Cheerful HiFi circle).  Although the SDS series has a 47K ohm input impedance, the CDA series only has a 7K ohm input impedance.
Steve

I know the Hypex class D amps are up around 100k which is more than enough - and they work well with our LDR preamps. I would think that the SDS 47k version of the IRS 2093 would work as well. I have my doubts about CDS at 7k. While I'm no amp design maven, I find it odd that anyone would design a contemporary amp with anything less than 10-20k input impedance. Especially if it's AC coupled it shouldn't be difficult to change the input C&R values to a higher effective impedance. And aren't the input stage to class D's basically a high impedance op amp?

LarryD56

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Re: Compatability with Class D
« Reply #4 on: 29 Sep 2014, 12:57 am »
   I'm just waiting to hear back from Tommy O'Brien (owner/designer of Digital Amplifier Company) concerning the input impedance. I wonder if the LDR preamp will sound really good with the Cherry Ultra, since the amp is so clear, precise, and full (yet not harsh). Its the best amp I've ever had, sound-wise. I wouldn't want to make it unlistenable, but I don't believe that would be the case. It is nice to be able to do a lot more low level listening and having it sound so good and detailed. I used to have to really turn up the volume to be able to hear details, but not with this new amp. Even at high volumes it still sounds very good and listenable.

   The Ultra has balanced inputs, but I have adapters on it to hook up to my unbalanced preamp. I was looking at a balanced AR LS2B Mk II preamp, but if I can avoid it I'd rather go passive. I may go and build a balanced kit pre with both balanced and unbalanced inputs since all of my inputs at this time are not balanced. 'Or', maybe I should just build an unbalanced one to try out your preamp. I wish I'd caught your sale, but I'm about a month too late.

   One way or another, I will get your product and try it out. I'm looking forward to hearing even better sound out of my system.

     Larry D.

BTW,  the Infinity RS 2.5s are supposed to have around an 87db SPL, not 83db.

poseidonsvoice

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Re: Compatability with Class D
« Reply #5 on: 29 Sep 2014, 01:50 am »
 :o

Larry,

Start reading here about what your amp input impedance is per Tommy Obrien:

http://www.audiocircle.com/index.php?topic=78263.msg743813#msg743813

An old thread but it might help you/Morten.

Best,
Anand.

tortugaranger

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Re: Compatability with Class D
« Reply #6 on: 29 Sep 2014, 01:26 pm »
:o

Larry,

Start reading here about what your amp input impedance is per Tommy Obrien:

http://www.audiocircle.com/index.php?topic=78263.msg743813#msg743813

An old thread but it might help you/Morten.

Best,
Anand.

Good input. Surprised about the 10k impedance on the Cherry class D amps especially in combination with the 100:1 bridging ratio guidance which means sources with 100R or less output impedance. That certainly leaves out a lot of source components.

When I get a moment to stop and put my thoughts together I will lay out the numbers/calcs behind the whole bridging impedance guidance for passive preamps so it'll be easier to understand what's going on and why. 

forkliftHIFI

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Re: Compatability with Class D
« Reply #7 on: 29 Sep 2014, 03:59 pm »
I know the Hypex class D amps are up around 100k which is more than enough - and they work well with our LDR preamps.

I am interested in how the tortuga pairs with the nc400 if anyone can tell me more about their experience?  Right now I use a tube preamp with single ended to XLR adaptor cables.  Does the balanced tortuga convert SE to balanced and back for volume control only or could I run SE into the tortuga and then XLR out to my hypex? 

poseidonsvoice

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Re: Compatability with Class D
« Reply #8 on: 29 Sep 2014, 05:02 pm »
I am interested in how the tortuga pairs with the nc400 if anyone can tell me more about their experience?  Right now I use a tube preamp with single ended to XLR adaptor cables.  Does the balanced tortuga convert SE to balanced and back for volume control only or could I run SE into the tortuga and then XLR out to my hypex?

Read my review elsewhere in the Tortuga Audio forum. I directly replaced my Poseidon's Voice Lazarus (balanced Bent Audio TAPX build) with the unbalanced Tortuga unit that Morten had sent on tour. I was pleased with the Tortuga and I use the NC400's as my main amplifiers. Although I still use the TAPX, I am currently building a tubed differential/balanced preamp using the Tortuga balanced modules just for kicks! :thumb:

Best,

Anand.

LarryD56

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Re: Compatability with Class D
« Reply #9 on: 29 Sep 2014, 11:41 pm »
Thanks for the reply Anand. At about the same time you posted about the old thread Tommy sent me this:

 "Glad you're happy with your Cherry ULTRA. 
Cherry ULTRA input impedance is 10k balanced.  Don't worry, though, if this sounds low for a passive preamp.  Additional resistive load on a passive preamp's volume control only causes linear attenuation.  In otherwise, you might need to turn it up a tiny bit more for a given volume, but no distortion is added due to input impedance.  Many of our customers use passive preamps."

I may try out an unbalanced LDR passive first and if I don't like it with my already very revealing digital amp, then I can use it with my tube amps and cause them to sound better. I have a feeling that the Ultra and the LDR passive will go together just fine, but I'll take this one step at a time.

Larry D.

tortugaranger

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Re: Compatability with Class D
« Reply #10 on: 30 Sep 2014, 12:38 pm »
"Cherry ULTRA input impedance is 10k balanced.  Don't worry, though, if this sounds low for a passive preamp.  Additional resistive load on a passive preamp's volume control only causes linear attenuation.  In otherwise, you might need to turn it up a tiny bit more for a given volume, but no distortion is added due to input impedance.  Many of our customers use passive preamps."

As Tommy points out in the link Anand provided earlier, what ultimately matters here is energy transference from the source to the amp. The whole point of having sufficiently high bridging impedance ratio (ratio of amp input impedance to source output impedance) is to make it easy for the source to convey the signal energy to the amp and not drain too much of that energy to ground in the process. All things being equal, amps with lower input impedances will drain more of that energy to ground than higher input impedance amps. At what point does that become a problem is the key issue. And by "problem" that usually means a loss of dynamics/slam and increasing flabbiness in the sound.

With passives you have the parallel impedance of the resistive preamp in front of the amp. If the preamp has a nominal 20k impedance and the amp has a 10k impedance then the effective impedance is Rtot = (R1 x R2)/(R1 + R2) = (10x20)/(30) = 6.7k. Note that effective impedance will always be less than the lower of the 2 values so even with an amp with 100k impedance the effective impedance would be only be 16.7k. 

Now if we use the traditional 10:1 bridging ratio guidance then the source should have an output impedance of no more than 670 ohms for reasonably good audio signal energy transference.

How good this sounds will ultimately depend on the robustness of the output stage of the source. A strong source capable of delivering more current while maintaining line stage voltage levels will do better (sound better) than a weak source. The higher the bridging ratio the less important the strength of the source becomes. The more robust the source, the less important the impedance ratio becomes. The same logic applies to the input stage of the amp.

Tommy's comment about customers having good results with passives with his 10k amp are no doubt true but you can be sure there are higher impedance sources that would not do so well through no fault of the Cherry amps. This is why the topic of impedance matching with passives can only be general guidance and not a hard and fast binary rule.

Mate a robust source with a low output impedance (< 500R) with most amps having 10k or greater input impedance and chances are using our 20k input impedance LDR passives will yield satisfying results (but the devil's in the details - always!)

Best,
Morten

tortugaranger

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Re: Compatability with Class D
« Reply #11 on: 30 Sep 2014, 02:15 pm »
I am interested in how the tortuga pairs with the nc400 if anyone can tell me more about their experience?  Right now I use a tube preamp with single ended to XLR adaptor cables.  Does the balanced tortuga convert SE to balanced and back for volume control only or could I run SE into the tortuga and then XLR out to my hypex?

The LDRxB (balanced) has 3 balanced XLR inputs plus an independent 4th unbalanced RCA input that gets converted to balanced internally via a transformer. All attenuation is done with balanced signals. The output has 2 balanced XLR outputs and also one unbalanced RCA output all outputting the same signal.

LarryD56

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Re: Compatability with Class D
« Reply #12 on: 30 Sep 2014, 11:45 pm »
   I will have to study all the info a bit more before I decide what to do. I may go with an active balanced preamp and then install your boards and PS, just to be sure I have what I need for great playback. Then again, with all the clarity I could be hearing I may not want the volume turned up much at all and won't need an active. I've already cut down my listening levels because with the new amp I don't need the volume turned up to enjoy it like I used to (and it doesn't get harsh at all). So, like I said, time to read some more and look up some specs.

Morten, thank you for looking into this and helping me to understand the situation a bit better. I'll let you know what I decide to do when I order......

Larry D.

SCM

Re: Compatability with Class D
« Reply #13 on: 8 Oct 2014, 09:03 pm »
I`m using an LDR1 driving a Dayton 1000 sub amp and a Class D Audio 470C.....SOUNDS AWESOME  :D