Differences in sound: DAC vs. DAC + Pre-amplifier

[CG]

18 hours ago, barrows said:

Sorry, i guess I was not clear.  In no way do i mean to suggest that those who design preamps are trying to add colorations.  What I am saying is that any component addition (like a preamp) can only result in loss of fidelity; by adding a preamp, where one was not present before, the only possible result is that doing so adds more distortion and noise than there was before.  If one then prefers the sound with that preamp in place, what that person is preferring is the coloration added by that preamp's noise and distortion profile.

 

Actually, that's not entirely true.  It is in regard to thermal noise, but not necessarily with regard to the rest.

 

Distortions can cancel.  This is the basis for feedforward used in amplifiers as well as the use of predistortion.  (Too much to cover here!)

 

In addition, common mode disturbances are often defined by the size and shape of the current loops of the system configuration and the interconnections, including powering and stuff like network connections.  (Again too much for here - try searching on the term "Henry Ott")

 

And, most amplifiers are sensitive to the source impedance at their input.  That affects not only amplitude response over frequency, but also actual distortion.  This is true with opamp configurations, which are the dominant topologies in use today, whether in integrated circuit form or not, as well as open loop configurations.  (Again...  Walt Jung has authored some articles on the subject that are pretty easy to digest.  But, there's loads of others.)  

 

This isn't voodoo or any other form of magic.  

[barrows]

4 hours ago, CG said:

Distortions can cancel

Hey CG, I certainly get what you are saying, but just as easily they can by additive as well, as the operative word here is "can".  As far as the rest goes, totally agree on impedance, etc.  But there are plenty of DACs these days with the exact same type of output buffer as one would find in the output of a preamp, with no difference.  These DACs have the exact same ability to drive the input stage amplifiers.  I would suggest one is getting very, very, lucky if they have preamp which just happens to actually cancel the distortion of their DAC, I mean, are we not talking about a very small possibility here?

There are of course some very unusual exceptions, but for the most part, DACs have output z at 100 ohms and often less, and amplifiers these days are usually over 50 kOhms input impedance, so no problem.  Now one could, if they really tried hard, manage to find a DAC/Amp combination thta is problematic, but it woudl be a rare exception rather than the rule.

[CG]

It's more complicated than whether the DAC or preamp can drive the amplifier input impedance.  The source impedance is actually part of the circuit.

 

Here's just one example:

 

https://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Jung_W/ADI_1992_2002_Audio_Seminars.pdf

 

Look at page 8-70.

 

Distortion cancellation isn't really common, because of the gain structures in audio systems, but is not as rare as you might think.  It's actually possible to cancel distortions created in the original source (aka recording) and this does happen in communications systems.  The problem is that unless you carefully control the distortion cancellation process, the result will be selective and will be all over the place.  Anyway, I only brought that up as one example.

 

To me, a much bigger question is the system topology with cables of various lengths connected in various ways in all sorts of combinations, that often are additionally coupled to other nearby electronics gear.  That could be due to electromagnetic coping or plain ole direct connections.  Amazing, but true.

 

Or, you (generic YOU, not Barrows specifically) plain may not care.  that's ok, too.

 

[barrows]

Thanks CG, i realize it is "one circuit".  Point being that most DACs have enough drive to drive the inout stage of most amplifiers perfectly.  And, as I mentioned, one can find exceptions, but one does have to work a little bit to do so.

 

Here is my example, of my direct experience back when I used a preamp:

 

I had a DAC with a discrete IV stage, followed by a single small signal MOSFET for each signal leg (so four small signal MOSFETs to drive the output).  This DAC did not do well enough going amp direct (my preamp at the time was an Ayre K5-xeMP).  I noted lower noise floor and better low level detail retrieval amp direct, but there was a problem with dynamics and presence-this were a bit wimpy, if you will.  Then the designer of this DAC changed the output stage design, to where the output buffer used 3 MOSFETs per signal leg (12 in total for balanced stereo output).  When I made this change to the output stage I tested DAC direct again (amplifier at the time was a Pass Labs X150.5), with the output stage change, now the DAC direct approach was clearly better in all parameters, lower noise floor, better detail retrieval, and perfectly fine dynamically.  I tested this to be sure, over long term as well, and eventually sold the preamp.

Since then I am totally committed to amp direct and have no need of a preamp.  I just make sure my amps are not to crazy difficult to drive (easy to do these days), and make sure the output stage of any DACs I use have plenty of current capability (which is easy to do as well given the nature of current IC OPAs).

 

As mentioned, one can find combos which will have compromises, but those are pretty rare these days, requiring a DAC with a rather "weak" output stages and an amplifier which is a bit difficult to drive (less likely to find?).

 

An (possibly) even better approach, which I am hoping will become more popular for consumer audio, could be to lower the amplifiers gain, and have higher voltage output from the DAC (as well as high current capability), as this could make the connection less subject to interferences (noise pick up) and cable differences.  Such is in pro audio where line level voltages are typically higher.  this approach is used by Benchmark in their gear.

[Miska]

38 minutes ago, CG said:

To me, a much bigger question is the system topology with cables of various lengths connected in various ways in all sorts of combinations, that often are additionally coupled to other nearby electronics gear.  That could be due to electromagnetic coping or plain ole direct connections.  Amazing, but true.

 

On my DAC designs, the output buffer stage has cable capacitance compensation circuits and such, to ensure that for example square wave outputs don't end up having overshoot or other unwanted effects. This also applies to design of the analog reconstruction filter.

 

For example 7 kHz square wave at DSD128 on DSC1:

 

And with same input on a typical DAC:

[CG]

23 minutes ago, Miska said:

On my DAC designs, the output buffer stage has cable capacitance compensation circuits and such, to ensure that for example square wave outputs don't end up having overshoot or other unwanted effects. This also applies to design of the analog reconstruction filter.

 

Very good!

 

Too bad you can't do that for the rest of the system...

 

Most people don't realize that the input and output impedances affect the performance.  Basically, almost every amplification device is unstable at some point.  Fundamentally, that's because the gain drops over frequency for a bunch of reasons.

 

Dennis Feucht is no quack:

 

http://audioworkshop.org/downloads/AMPLIFIERS_OSCILLATION_BJT_CIRCUITS.pdf

[barrows]

@CG, thanks for the links.  I do not have time now, but will read up later.  I try to keep my system as simple as possible, and use just a 1 m pair of interconnects (Iconoclast with low capacitance and inductance) between DAC and Amp.

 

My main point being, though, that these same problems you mention can occur between a preamp and amp as well.  Having a preamp in the system is no guarantee of avoiding what you are mentioning.  On most DACs and preamps I look at these days, the output buffer is often the same (an IC OPA of some variety, with a lowish value series resistor).  Given that the output stages of either DAC or preamp are often the same, the notion that there is actually a "problem" going DAC direct to amp is just not valid (given certain rare exceptions).

 

I guess the real "problem" here is that there really are no standards for home audio in terms of output to input of various components, impedances vary quite a bit from product to product, as do voltages, as do the stabilities of various circuit topologies.  And audiophiles are constantly mixing and matching various components and brands.  

[Miska]

27 minutes ago, CG said:

Most people don't realize that the input and output impedances affect the performance.  Basically, almost every amplification device is unstable at some point.  Fundamentally, that's because the gain drops over frequency for a bunch of reasons.

 

And this is related to what? And relation to this topic is?

 

Preamplifiers essentially look exactly the same input and output wise as any other line level device. Or these days you should actually call it preattenuator, because it provides only negative gain.

 

[sandyk]

Re my post #112.

 The Amplifier measurements that I showed were of a published amplifier design from Silicon Chip magazine  with typical Distortion measurements as shown on the front page of the magazine.

 All of the DIY Audio members participating in the threads were hearing an improvement in measured distortion below the typical .002% quoted by the magazine as they were experienced DIY constructors , some of who even used closely matched semiconductors for a further improvement .

It should have been obvious from the posted measurements that the distortion figures were already very low.

[sandyk]

46 minutes ago, barrows said:

 On most DACs and preamps I look at these days, the output buffer is often the same (an IC OPA of some variety, with a lowish value series resistor).  

 

 That has been my experience as well, as many opamps (LME49720/LM4562 etc.) do not like to directly drive capacitive loads of >100pF.

A typical series resistor value appears to be 100 ohms.

[CG]

2 hours ago, Miska said:

 

And this is related to what? And relation to this topic is?

 

Preamplifiers essentially look exactly the same input and output wise as any other line level device. Or these days you should actually call it preattenuator, because it provides only negative gain.

 

 

I guess the situation is already perfect then!  Great!  

 

No point in even discussing the matter.  Farewell, then.

[sandyk]

4 hours ago, barrows said:

There are of course some very unusual exceptions, but for the most part, DACs have output z at 100 ohms and often less, and amplifiers these days are usually over 50 kOhms input impedance, so no problem.

 

 Actually, many Power Amplifiers these days have an Input Impedance of 10K to 20K which usually results in improved S/N as well as contributing to a lower DC offset.  e.g. Audio Power Amplifier Design Handbook 5th ed - D. Self.

[Kimo]

11 hours ago, sandyk said:

 

 Actually, many Power Amplifiers these days have an Input Impedance of 10K to 20K which usually results in improved S/N as well as contributing to a lower DC offset.  e.g. Audio Power Amplifier Design Handbook 5th ed - D. Self.

 

My DAC has a fully balanced output of 300 ohm per phase and my amp has a fully balanced input of 94K.  

 

Do you see any problem with these numbers?

[barrows]

16 hours ago, sandyk said:

 

 Actually, many Power Amplifiers these days have an Input Impedance of 10K to 20K which usually results in improved S/N as well as contributing to a lower DC offset.  e.g. Audio Power Amplifier Design Handbook 5th ed - D. Self.

I am not aware of "many" high end power amps with input impedances of 10-20 K?  Maybe a few, but not many, and certainly a minority.  A quick perusal of amps measured by Stereophile, taken at random showed this:

 

Parasound: 90K

CH Precision: 90K

McIntosh: 40K

Luxman: 300K

Constellation: 20K (specified as 200K)

Sim Audio: 32K

Pass Labs: 100K

PS Audio: 96K

Bricasti: 170K

Ayre > 1M (JA has difficulties measuring input impedances above 1M)

 

All are for balanced connection, which I consider the norm these days, as most DACs we are talking about here have balanced outputs.  So we see a couple of amps with lower numbers, but most are up there.  Certainly even the Constellation (20K measured) is easy enough to drive by a typical DAC with 100 ohms output impedance, as long as the output stage has plenty of current capability.  Most of my DACs have output impedances of around 33-100 ohms (per phase that is).

[sandyk]

9 hours ago, Kimo said:

 

My DAC has a fully balanced output of 300 ohm per phase and my amp has a fully balanced input of 94K.  

 

Do you see any problem with these numbers?

 

 I don't see any problems provided that the interconnects aren't too long.

[sandyk]

4 hours ago, barrows said:

I am not aware of "many" high end power amps with input impedances of 10-20 K?

 

Hi Barrows

These are more recent higher performance designs as per the research by Douglas Self, and variations used in the published designs from Silicon Chip (Au.) magazine etc. , which has published designs with a typical distortion of .0006% ,and a further improvement when using matched devices in the front end. The lower input impedances also markedly reduce the DC offset at the output due to the bias currents of the LTP ,  meaning that no adjustment is normally needed to reduce this provided that the LTP ( Differential Pair) devices are reasonably close in HFE. 

Please check your PMs.

Regards

Alex

[barrows]

1 hour ago, sandyk said:

 

Hi Barrows

These are more recent higher performance designs as per the research by Douglas Self, and variations used in the published designs from Silicon Chip (Au.) magazine etc. , which has published designs with a typical distortion of .0006% ,and a further improvement when using matched devices in the front end. The lower input impedances also markedly reduce the DC offset at the output due to the bias currents of the LTP ,  meaning that no adjustment is normally needed to reduce this provided that the LTP ( Differential Pair) devices are reasonably close in HFE. 

Please check your PMs.

Regards

Alex

Sure, but is not this discussion about real world amplifiers people generally own and can purchase, not theoretical designs, or obscure DIY efforts only available to a few hobbyists?

[sandyk]

10 minutes ago, barrows said:

Sure, but is not this discussion about real world amplifiers people generally own and can purchase, not theoretical designs, or obscure DIY efforts only available to a few hobbyists?

 

Douglas Self's designs are commercially available from The Signal Transfer Company.

 You will also undoubtedly see the use of his published findings has trickled down into other less well known commercial amplifiers.

[barrows]

@sandyk,

 

Yes, but that does not make this statement any closer to being accurate:

 

"many Power Amplifiers these days have an Input Impedance of 10K to 20K"

 

I would also suggest that for the purpose of this discussion, even an amp with 10K input impedance can be driven properly by many, many DACs available these days.

[sandyk]

45 minutes ago, barrows said:

@sandyk,

 

Yes, but that does not make this statement any closer to being accurate:

 

"many Power Amplifiers these days have an Input Impedance of 10K to 20K"

 

I would also suggest that for the purpose of this discussion, even an amp with 10K input impedance can be driven properly by many, many DACs available these days.

 

Yes, a DAC with a 100 ohm output impedance can successfully drive an amplifier  with a 10K input impedance, but a Power Amplifier with a 100K Input Impedance will have more HF rolloff with longer interconnects . These days, most power amplifiers with such a high input impedance use Vacuum tubes .Neither is it good practice these days to use such high impedances with solid state amplifiers due to Johnson noise. It is non intuitive to use a high resolution 24 bit DAC  with a Power Amplifier which isn't way quieter than the requirements needed for Red Book CD. 

[sandyk]

7 hours ago, barrows said:

All are for balanced connection, which I consider the norm these days, as most DACs we are talking about here have balanced outputs.

 

 I would put it to you that the amplifiers that are in common use by the vast majority of consumers do not use balanced topology, as do the vast majority of affordable commercial DACs, some of which MAY provide this facility , but is often not used by typical consumers.

You appear to be taking an elitist position here, as there is very little, if any, advantage to Balanced Topology in most domestic situations,where relatively short interconnects and speaker leads are used.

 

[photonman]

7 minutes ago, sandyk said:

there is very little, if any, advantage to Balanced Topology in most domestic situations,

What if we are using a class D amp, still no advantage?  Just curious as I am following this conversation as I am using Class D with a DAC with volume control.

[barrows]

6 minutes ago, sandyk said:

 

 I would put it to you that the amplifiers that are in common use by the vast majority of consumers do not use balanced topology, as do the vast majority of affordable commercial DACs, some of which MAY provide this facility , but is often not used by typical consumers.

You appear to be taking an elitist position here, as there is very little, if any, advantage to Balanced Topology in most domestic situations,where relatively short interconnects and speaker leads are used.

 

Alex, I just did not understand what you keep posting these things?  I mean, the "vast majority" of consumers are probably not using a Doug Self designed amp.  But why even mention the average "consumer", this is an Audiophile Forum, read and contributed to by Audiophiles; I do not think the "vast majority" of consumers are reading this, and most of them are listening to cheapo bluetooth speaker via Alexa and are not concerned whether they should use a preamp or not...  There are many, many, many amplifiers which do feature balanced input stage topology, it is not uncommon at all.  Now balanced all the way through to the output stage is less so, but that has nothing to do with the DAC/Amp interface anyway.  Contemporary dual IC opamps also make implementing a balanced input stage relative easy and affordably. 

As for DACs, again, there are tons of truly balanced DACs out there, and not at high prices either.  For just one example, I am playing around with a Topping D-90 right now, it is well under $1K USD.  There are many other DACs <$1K with true balanced outputs.  Especially with a DAC it is important to have balanced outputs, as almost all DAC chips used these days have balanced outputs, and it actually takes more circuitry (if one is going to do it right via balanced to single ended conversion stage) to convert the output to single ended than it does to have balanced output.

Nothing elitist about this, balanced topology is relatively common place, and easily produces real benefits for the output stage of a DAC (a simple example of this advantage is the cancellation of power supply noise).

 

Anyway, not really sure what these posts have to do with the interfacing between DAC-Amp and/or preamp?

[sandyk]

31 minutes ago, barrows said:

Alex, I just did not understand what you keep posting these things?

 You appear to be under the impression that the majority of members are using balanced topology.

 I would suggest that they are only owned by a relatively small percentage , probably no more than 20% of our members and readers, as they are simply not necessary in a typical domestic situation.

 My posts have everything to do with improving the performance of the analogue area using a Preamplifier etc. vs. using Digital Attenuation, the quality of which will vary depending on the way it is implemented.

Until there is standardisation on the output levels of DACs ,and the Input levels and gain of various Power Amplifiers, and no longer the need to listen to other legacy formats such as Vinyl. FM Stereo etc. , a direct connection between the DAC and the Power amplifier is not always the best way to go for many members, and may even lead to degradation in a few cases .

 

Perhaps we should simply agree to disagree on this issue ?

[barrows]

3 hours ago, sandyk said:

 You appear to be under the impression that the majority of members are using balanced topology.

Nope, I would never assume anything like this.

 

3 hours ago, sandyk said:

3 hours ago, sandyk said:

I would suggest that they are only owned by a relatively small percentage , probably no more than 20% of our members and readers, as they are simply not necessary in a typical domestic situation.

 

And such a suggestion just reflects more about what you think and your own approach, rather than anyone else's.

 

3 hours ago, sandyk said:

Digital Attenuation, the quality of which will vary depending on the way it is implemented

Nope, any notion of digital attenuation being a "problem" only exists in the past.  With the possible exception of a poorly put together system with absolutely terrible gain matching-point being that in any system with bad gain matching, there are considerable benefits to fixing the real problem (the poor gain structure) regardless if one uses analog or digital attenuation.  Higher levels of analog attenuation come at a price, higher noise levels due to resistor noise.

 

In my opinion, you are living in the long past: single ended DAC outputs are a really poor idea in this time of balanced output DAC chips, and compromise the possible performance of any DAC.  And, digital volume control is not compromised in its current implementation in any contemporary DAC of which I am aware (32 bit volume control and higher is transparent).

 

If one is interested in high fidelity to the source recording, the only valid reason for having a preamp is if one has multiple sources, including analog ones, otherwise going DAC direct has a distinct advantage in fidelity to the source.