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Why not eliminate the input buffer? (And a big thanks to March Audio!)

dortreo

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Hello all,

First time post here but long time lurker. I've been following the conversation on Hypex and Purifi amps as closely as I can considering that I have no background in EE whatsoever. With that caveat, it seems that there are benefits to eliminating the input buffer stage as seen in the Amir's Purifi review or in the low gain mode measurements for the Benchmark AHB2 amp. It also seems that there's little difference between discrete "audiophile" op amps and certainly there's debate about whether or not these op amps present any value over stock chips. So, would there be a benefit to eliminating the input buffer if you had a pre-amp that had an appropriately high output voltage? I'm fortunate enough to own one, the Benchmark DAC3, which by default outputs 24 dBu (12 volts RMS).

With that in mind, I asked @Alan March if he could install a switch or jumper to bypass the input buffer on the NC1200. Here's what he wrote:

"It is possible to configure the module to bypass the input buffer however it is done by configuring soldered links on the board. So yes we can supply an amp configured that way but it is not a simple switch operation to revert it. However thinking about it we could fit jumpers on the board to make it easier.

"Buffered the amp gain is 27.8dB which requires 2.2v RMS input to reach full power output into 4 ohms. Unbuffered the amp gain Is only 11.6dB so will require an input voltage of 14.8 volts RMS to reach full power output. The DAC 3 at +28dBu is 19.4 volts RMS so yes this would work fine. This would indeed provide lower noise and distortion."

And here's what he installed: A push switch (in blue) to bypass the input buffer on the NC1200. Brilliant!

P701 switch.png


From what the NC1200 datasheet states, that should improve the SNR for 1W at 8 ohms from 103 to 112 dB. The Benchmark DAC3 is putting out more than enough voltage to drive the amp and speakers to ear splitting volumes. And the sound is smile-inducing. :):):):):)

So, hats off to Alan for building and modifying a terrific amp. I realize that eliminating the input buffer isn't practical in most cases, but if you have the potential to do so, why not? What's the downside?
 
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ppataki

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This is really cool!
I have NC250MPs and according to the manual it is not possible to bypass the input buffer....does anybody know if that is really the case?
I would love to bypass it
Thank you
 

CDMC

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dortreo

dortreo

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The Benchmark article begs the question why we’re not seeing more studio-grade equipment in audiophile circles. But maybe all the cryo treatments, directional fuses, etc. soak up the costs necessary to make this possible. It’s interesting that the equipment that does well in Amir’s reviews come predictably from brands with strong backgrounds in research (Harman/Revel) or studio work (RME, Genelec, Benchmark).
 

CDMC

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The Benchmark article begs the question why we’re not seeing more studio-grade equipment in audiophile circles. But maybe all the cryo treatments, directional fuses, etc. soak up the costs necessary to make this possible. It’s interesting that the equipment that does well in Amir’s reviews come predictably from brands with strong backgrounds in research (Harman/Revel) or studio work (RME, Genelec, Benchmark).

There are a few reasons I see:

1) Years of conditioning of "Audiophiles" that studio equipment is "cold", "sterile", "lifeless".
2) Most systems use RCA, not XLR. It is only in the last few years we are starting to see a lot of components come with true differential balanced connectors.
3) You still get over 100 db sn ratios from RCA connectors which far exceeds most audiophile components.
4) The noise issues in studio components are more important due to them 1) operating in noisier electrical environments, 2) longer runs, and 3) the long component chains from the initial recording from the mics to the final mix, which may result in the signal going through dozens of devices. Each device degrades the sounds slightly, so the more you can prevent this, the better the final mix. In a home environment, you are going through only a few devices. This is also the reason that is makes sense to do all mixing at a 24bit depth, but for the final product distribute in a 16 bit format.
5) Most "Audiophiles" aren't really concerned about taking a systematic approach to improving the sound they hear. They want the mystic and magic of tweaks here and there and the "changes" that component swapping give to tune the system. Buying good equipment, speakers that work for the room, measuring and using DSP, and treating the room as needed is comparatively boring. If you work in pro sound your goal is different, you want a reliable, repeatable systems. Time spent tweaking and changing things out is money lost.
 
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dortreo

dortreo

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There are a few reasons I see:

1) Years of conditioning of "Audiophiles" that studio equipment is "cold", "sterile", "lifeless".
2) Most systems use RCA, not XLR. It is only in the last few years we are starting to see a lot of components come with true differential balanced connectors.
3) You still get over 100 db sn ratios from RCA connectors which far exceeds most audiophile components.
4) The noise issues in studio components are more important due to them 1) operating in noisier electrical environments, 2) longer runs, and 3) the long component chains from the initial recording from the mics to the final mix, which may result in the signal going through dozens of devices. Each device degrades the sounds slightly, so the more you can prevent this, the better the final mix. In a home environment, you are going through only a few devices. This is also the reason that is makes sense to do all mixing at a 24bit depth, but for the final product distribute in a 16 bit format.
5) Most "Audiophiles" aren't really concerned about taking a systematic approach to improving the sound they hear. They want the mystic and magic of tweaks here and there and the "changes" that component swapping give to tune the system. Buying good equipment, speakers that work for the room, measuring and using DSP, and treating the room as needed is comparatively boring. If you work in pro sound your goal is different, you want a reliable, repeatable systems. Time spent tweaking and changing things out is money lost.

Thanks for the considered reply. Your last point is especially spot on. Ever since discovering this forum, I've been thinking about deliberate goal-setting when it comes to system upgrades and improvements. The little things, like speaker/sub placement and DSP, do indeed make a disproportionate difference to sound quality. It's more geeky than boring, but there's the potential for getting some exercise when doing it.
 

Digital Mastering System

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Balanced XLR connections are important in a studio with a patch bay and tens if not hundreds of pieces of gear. But a balanced input is difficult to get as quiet as a single ended RCA input. Doug Self has a long discussion in Small Signal Audio Design about the difficulty of getting a balance input to be as quiet as a simple single ended input. One opamp can make a good single ended input. Making an equivalently quiet balanced input can cost 10 times as much and require many more parts (Self has an example of a low noise balanced input stage using 12 opamps nearly achieving the noise of a single ended input).
For the home, especially with a two channel stereo, unbalanced connection are actually better, unless extraordinary efforts are made to make the balanced inputs ultra high performance.
 

mkt

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So, hats off to Alan for building and modifying a terrific amp. I realize that eliminating the input buffer isn't practical in most cases, but if you have the potential to do so, why not? What's the downside?
If you have the potential :)
 

CDMC

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Thanks for the considered reply. Your last point is especially spot on. Ever since discovering this forum, I've been thinking about deliberate goal-setting when it comes to system upgrades and improvements. The little things, like speaker/sub placement and DSP, do indeed make a disproportionate difference to sound quality. It's more geeky than boring, but there's the potential for getting some exercise when doing it.

I think for a lot of people, they want to believe the self professed experts. Taking the mystique away ruins it for many people. Also, many don't want to become informed, it is easier to throw components at it than be methodical, informed, and boring. Testing for me revealed why my main listening room never sounded right. 700ms decay times. Without fixing that, nothing will ever truly be right. It was for that reason when I bough new speakers, I bought less expensive ones and allocated a portion of my budget to treating the room.
 
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dortreo

dortreo

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Balanced XLR connections are important in a studio with a patch bay and tens if not hundreds of pieces of gear. But a balanced input is difficult to get as quiet as a single ended RCA input. Doug Self has a long discussion in Small Signal Audio Design about the difficulty of getting a balance input to be as quiet as a simple single ended input. One opamp can make a good single ended input. Making an equivalently quiet balanced input can cost 10 times as much and require many more parts (Self has an example of a low noise balanced input stage using 12 opamps nearly achieving the noise of a single ended input).
For the home, especially with a two channel stereo, unbalanced connection are actually better, unless extraordinary efforts are made to make the balanced inputs ultra high performance.

I got the impression from the Benchmark article that balanced had a higher SNR than single ended primarily due to higher signal. Hadn't thought about the noise portion of the ratio, but I'm a newbie at this. In an environment rife with $10K+ a piece audiophile gear, there has to be room in labor and material costs to include components that will actually make a discernible difference in measurable parameters. Of course, this is only after you've properly installed power crystals throughout your components.
 

razamanaz

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If you buy Eval1 1et400a, you can turn off the preamp gain by simply switching the jumpers
 

March Audio

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Balanced XLR connections are important in a studio with a patch bay and tens if not hundreds of pieces of gear. But a balanced input is difficult to get as quiet as a single ended RCA input. Doug Self has a long discussion in Small Signal Audio Design about the difficulty of getting a balance input to be as quiet as a simple single ended input. One opamp can make a good single ended input. Making an equivalently quiet balanced input can cost 10 times as much and require many more parts (Self has an example of a low noise balanced input stage using 12 opamps nearly achieving the noise of a single ended input).
For the home, especially with a two channel stereo, unbalanced connection are actually better, unless extraordinary efforts are made to make the balanced inputs ultra high performance.

Whilst you are mostly correct (yes I have Dougs book and know the chapter :) ) single ended is not better. It has a fundamental flaw. That of ground loops. You will get noise currents flowing in the shield which will spoil any slight inherent noise advantage they have. Whilst this isnt always an overt problem it is interesting to see the number of threads on here talking about "computer noises".

Also a differential input using a modern op amp like the opa1612 is spectacularly quiet.
 

chips666

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Hello,
I bypassed the input opamps of my TPA3255EVM board and i like what i am hearing using a RME Adi-2 dac via xlr outputs
Do not know if this actually measures better having no equipment
Enjoy
 
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dortreo

dortreo

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If you buy Eval1 1et400a, you can turn off the preamp gain by simply switching the jumpers
I think it was a bit more complicated for Alan for the NC1200. You have to do some desoldering to be able to put in jumpers or a switch.

I’m wondering if bypassing the input buffers on the NC1200 puts it in Purifi territory as far as SNR performance is concerned. I would have gone the Purifi monoblock route except I have fairly inefficient speakers.
 

Matias

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I think it was a bit more complicated for Alan for the NC1200. You have to do some desoldering to be able to put in jumpers or a switch.

I’m wondering if bypassing the input buffers on the NC1200 puts it in Purifi territory as far as SNR performance is concerned. I would have gone the Purifi monoblock route except I have fairly inefficient speakers.
Purifi is developing higher power amplifiers, so like me, you may wait until they are released.
 

Digital Mastering System

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Whilst you are mostly correct (yes I have Dougs book and know the chapter :) ) single ended is not better. It has a fundamental flaw. That of ground loops. You will get noise currents flowing in the shield which will spoil any slight inherent noise advantage they have. Whilst this isnt always an overt problem it is interesting to see the number of threads on here talking about "computer noises".

Also a differential input using a modern op amp like the opa1612 is spectacularly quiet.

A good single ended DAC like a Topping 50s has GND isolation from the PC using the USB or optical ports. In a small stereo set up you should be able to find and eliminate the ground loops if there are any. Plug everything into the same power strip for example.
I'm just saying that most of the time, using the same opamps, you'll get better performance with RCA SE over balanced.
And we haven't even started taking about how much money it takes to get decent CMRR. The typical 'HiFi' transformerless balanced inputs I've seen are 40db CMRR if that. That's pretty poor. A good transformer is the classic balanced input solution. 80db CMRR is doable. Problem: LF extension and distortion. Only solution is lots of money, like $100 Jensen line transformers. Fine, but $250 for an input stage is a lot of money fixing a problem you may not have. Pros can't be bothered and maybe can't fix the ground loops and prefer transformers everywhere.
I actually have Jensen line transformers with RCA In / Out in a separate box for fixing ground loops in specific problem locations.
 

March Audio

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A good single ended DAC like a Topping 50s has GND isolation from the PC using the USB or optical ports. In a small stereo set up you should be able to find and eliminate the ground loops if there are any. Plug everything into the same power strip for example.
I'm just saying that most of the time, using the same opamps, you'll get better performance with RCA SE over balanced.
And we haven't even started taking about how much money it takes to get decent CMRR. The typical 'HiFi' transformerless balanced inputs I've seen are 40db CMRR if that. That's pretty poor. A good transformer is the classic balanced input solution. 80db CMRR is doable. Problem: LF extension and distortion. Only solution is lots of money, like $100 Jensen line transformers. Fine, but $250 for an input stage is a lot of money fixing a problem you may not have. Pros can't be bothered and maybe can't fix the ground loops and prefer transformers everywhere.
I actually have Jensen line transformers with RCA In / Out in a separate box for fixing ground loops in specific problem locations.


It's rarely that simple. Also made worse these days by the leakage currents from smps that components are using. You can't eliminate ground loops in SE unless you use transformers. Also re "a problem you may not have" actually translates into "a problem that isn't overtly audible". A subtle but important distinction, especially if you are talking about which system is ultimately quieter.

Real world usage and application, even in the domestic environment, means that balanced will almost way's beat SE. If you don't believe me just take a look at Amirs DAC and amp tests to see which ones top the SINAD scores. All balanced.

CMRR isn't the issue, although whatever it is differential beats SE ;)

The issue is using a signal wire as the shield. Any voltage differential between components makes a current flow in it. It's a really dumb idea for an intercomponent connection method.

There is no real world disadvantage to using balanced, only benefits, which is why I would always choose it over SE if available. It's not expensive to implement.
 
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audiopile

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DMS -thank you for the devil's advocate position on the balanced v. single ended issue. Personally - roughly 55 years of dealing with RCA connectors has made me hate ,loath and despise them. But you make a pretty good case for it being the CONNECTORS that are the practical issue and NOT the cable configuration. I have circled around and sniffed the idea of converting my main system to BNC's or Camacs/Limo connectors for decades -a project-but much less of a epic than converting my mostly analog LP based stereo system to all balanced. I still think that from a purely mechanical point of view -XLR's pretty much smoke RCA's.
 
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