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

[craighartley]

On 2/5/2020 at 5:39 PM, Miska said:

 

Challenge with passive ones is that they drastically modify output impedance seen by the amplifier following the passive attenuator. In addition this impedance is not constant by varies depending on volume control position. This in turn increases the impact interconnect cable has. In addition power amps typically have first order low pass filter in their input that assumes low output impedance from the source. Varying output impedance means varying frequency response of this filter.

 

This is why active buffer stage (impedance converter) is usually good idea after passive type volume control, such as potentiometer. In addition, the active buffers I've been building have an additional cable capacitance compensation circuitry.

 

Additional resistance on the path also increases noise. Higher the series resistance, higher the noise. ESS Sabre chip for example has 600 ohm worth of resistance/noise. So if you want to use analog volume control that has lower self-noise than good digital one combined with a good low impedance DAC. You need to have all series resistances below 1 kOhm.

 

An alternative is a transformer based ‘passive preamp’. 

[craighartley]

20 minutes ago, barrows said:

OK, here is my take.  Please feel free to disagree with me, but do not tell me I am wrong.

 

I first tried no preamp, with a Buffalo DAC (DIY) using the first generation "Legato" I/V stage, and the preamp was a highly capable Ayre K-5xe-MP.  The results were:

 

With preamp, higher noise floor, a little less transparency, but better dynamics and fuller, more robust sound.  I preferred with preamp, but wanted to figure out DAC direct as it was clear there was an advantage in transparency with DAC direct.

 

Then, the designer of the Legato output stage changed it, tripling its output current capability.  I now tried DAC direct vs. preamp again:

 

DAC direct: lower noise floor, better detail retrieval, and now, no loss of dynamics either.  So now that the DAC was driving the input stage of the amplifier optimally, there was only loss by adding the preamp.

 

After about 3 weeks of testing, i sold my Ayre preamp, and have not looked back.

 

It is clear to me that DAC direct, for those who do not need source switching. results in the best fidelity to the source, as long as the DAC's output stage is capable of driving the amplifier input stage robustly.  Additionally, most DACs have the capability to drive most amplifier input stages entirely adequately.  Look for a DAC with low output impedance (less than 100 ohms is nice, but up to a couple hindered ohms is going to be OK for most amps) and enough output voltage.  Most IC opamps in DAC output stages can drive amps directly with no problem.  Occasionally there might be a DAC with a bit weaker output stage, but these are rare.

 

I would advise that taking advice direct from manufacturers who sell preamps might not be the best way to make decisions on this, as those manufacturers have an interest in selling preamps.  One really needs to listen in their own system, to the DAC, Preamp, and amplifier one is considering, to decide for sure.  And, definitely do not take advice from "old skool" audiophiles who just claim that any serious audio system must include a pre amp (for no good technical reason), this is a quaint and antiquated notion which has no basis in reality.

 

There is one addition i would make here:  I have heard the claim that adding a preamp can reduce the high frequency noise from the DAC, leading to "blacker" backgrounds.  This supposition relies on the idea that preamps typically will have low pass filters on their inputs, to avoid RF getting into the circuitry.  OK, this is possible, but a couple of things to consider if you think you are hearing this:  DACs all have low pass filters on their outputs, and well designed DACs should not have a lot of high frequency noise on their outputs.  Power amplifiers also usually have a low pass filter right at their input to avoid RF getting into the amp.  My conclusion is that with competently designed DACs and Amplifiers, there should not be any noise problem going DAC direct which would benefit from adding a preamp as an additional filter element between DAC and Amp.  But, there are probably cases of poorly designed DACs, and/or poorly designed amplifiers where adding a preamp actually helps because of this low pass filter effect.  If this happened to me though, i would want to determine what component was poorly designed and allowing this to be the case.

Does your DAC have a volume control, or are you using fixed output and something like HQPlayer to control volume?

[craighartley]

3 minutes ago, barrows said:

I have more than one DAC, some have volume controls, and sometimes I may control volume in software.  

 

So, on volume controls:  Digital Volume control, in software on a computer, or in in hardware on most DAC chips, is generally going to perform better than most analog volume control implementations.  As long as one does not need huge amounts of attenuation (more than -40 dB or so).  If one is listening at very low levels, small details will be below the noise floor anyway, so at high levels of attenuation one is not missing anything.  Despite many audiophile myths to the contrary, there is nothing about digital volume control when well implemented (at 32 bits or more, with a properly matched system gain structure).

 

What a lot of people seem to ignore in the discussion of volume controls is the analog volume controls are far from "perfect", they add noise and distortion to the signal of their own.  Digital volume control is actually more transparent than analog within it s limits:

 

Consider a 32 bit volume control in the digital domain, and a 24 bit source file:

 

32-24= 8 bits, so with a 32 bit volume control one has 8 spare bits to use with no impact on resolution.

 

8*6=48, so one has -48 dB of volume reduction before any resolution (of our 24 bit source file) has been lost, this means there is no problem here!

 

Then consider that most volume controls in software are running at 64 bits!

 

Then consider that actual i room dynamic range of a system is never better than 16 bits...  So in our above example there are actually 16 bits of resolution we can lose without any audible consequences, so we really have 16*6=96, -96 dB of reductio possible with our 32 bit volume control without any impact on real world resolution in room.

 

One can see that there is no "problem" with digital volume control. 

Thanks for this, which is very interesting. I'd like to use digital volume control, but the problem I've found is that software (eg Roon with HQPlayer) limiting of maximum volume is unreliable, so my speakers are laid vulnerable to potential damage.

[craighartley]

13 minutes ago, barrows said:

I have not found that to be the case with a dedicated machine as the server (not on Windows though, some have reported that Windows can be a problem for this).  With ROON running on Mac OSX or Linux I have never had a volume control "incident".  I have only used HQPlayer briefly, so cannot reliably comment on that, but Jussi (miska) seems to believe HQP volume control is stable on Linux.

If one is using a computer for general purpose and running a software player on it at the same time, that could result in problems I guess...

Thanks for this. I do have a dedicated server (Roon on a Mac Mini streaming to HQPLayer on a dedicated machine running the HQPlayer OS), and I'll have another try. My problem occurred when I changed some settings in HQPlayer and (without me noticing) this wiped the volume limit I had set in Roon.

[craighartley]

35 minutes ago, Miska said:

 

When you use HQPlayer, it is better to set volume limit in HQPlayer. If you enable "Direct SDM", volume control becomes disabled because you are asking bit-perfect pass-through for DSD data, but this is indicated in the main window because the volume knob becomes disabled as result too.

 

I can’t see where to set a volume limit in HQP Embedded; only where to set startup volume. 

[craighartley]

46 minutes ago, Miska said:

 

On Embedded volume configuration needs manual adjustment of the configuration file as there are some extra parameters that can be set to deal with software volume and software + hardware combination volume.

 

There are "volume_min" and "volume_max" attributes in the configuration file under "engine" element. Default values are min=-60 and max=0.

 

Okay. The trouble is that I’m using the bootable image so that I can leave the server headless (and inaccessible) and don’t have to deal with altering it other than via the web interface. 

[craighartley]

52 minutes ago, craighartley said:

Okay. The trouble is that I’m using the bootable image so that I can leave the server headless (and inaccessible) and don’t have to deal with altering it other than via the web interface. 

I’ve now realised I can set the volume limit by editing the xml file produced by Backup and then ‘restoring’ it. I don’t need to access the machine directly. 

[craighartley]

On 3/1/2020 at 7:11 PM, barrows said:

I have more than one DAC, some have volume controls, and sometimes I may control volume in software.  

 

So, on volume controls:  Digital Volume control, in software on a computer, or in in hardware on most DAC chips, is generally going to perform better than most analog volume control implementations.  As long as one does not need huge amounts of attenuation (more than -40 dB or so).  If one is listening at very low levels, small details will be below the noise floor anyway, so at high levels of attenuation one is not missing anything.  Despite many audiophile myths to the contrary, there is nothing about digital volume control when well implemented (at 32 bits or more, with a properly matched system gain structure).

 

What a lot of people seem to ignore in the discussion of volume controls is the analog volume controls are far from "perfect", they add noise and distortion to the signal of their own.  Digital volume control is actually more transparent than analog within it s limits:

 

Consider a 32 bit volume control in the digital domain, and a 24 bit source file:

 

32-24= 8 bits, so with a 32 bit volume control one has 8 spare bits to use with no impact on resolution.

 

8*6=48, so one has -48 dB of volume reduction before any resolution (of our 24 bit source file) has been lost, this means there is no problem here!

 

Then consider that most volume controls in software are running at 64 bits!

 

Then consider that actual i room dynamic range of a system is never better than 16 bits...  So in our above example there are actually 16 bits of resolution we can lose without any audible consequences, so we really have 16*6=96, -96 dB of reductio possible with our 32 bit volume control without any impact on real world resolution in room.

 

One can see that there is no "problem" with digital volume control. 

I so want to take my expensive transformer-based ‘pre-amp’ out of my system, but if I do that I get a prominent click out of the speakers on the start of music playing, which is negligible with the ’pre’ inserted. Miska has suggested that this may be due to DC offset from the DAC analogue output, but it disappears if I stream from Roon direct (via MicroRendu in RoonReady mode) rather than Roon>HQPlayer>NAA. 
I know my amps have too much gain (39db) for the software volume solution to be ideal, but it’s the click that prevents me pursuing it. 

[craighartley]

1 minute ago, barrows said:

Wow Craig, 39 dB is really high and quite unusual gain for a power amplifier.  Have you ever contacted the manufacturer and asked if the gain could be lowered?  That much gain is probably also make the amp(s) noisier than they could be.

 

Of course your "click" problem is something other than preamp related.  If i had that "click" problem i would track down the ideal solution for the problem, rather than covering it up by adding a transformer based pre/volume control.

There’s a switch to reduce the gain by 6dB when using the amps in monobloc mode, but I’m already doing that. I have a pair of these:

https://www.leema-acoustics.com/product.html?prid=59#spec

I may send the T+A DAC8 DSD back to T+A to check if there’s a fault, but that means sending it to Germany (I’m in the UK) and I might wait until things are a bit calmer (I mean in relation to Covid-19, not irritating clicks from my speakers). 

[craighartley]

1 hour ago, barrows said:

@craighartley, cool looking amps.  but I ma confused, looks like the Leema is an integrated with its own volume control?  Why not just use the amp's VC?

No, the Hydra is not an integrated amp, it’s just a power amp. If it’s used with a Leema pre-amp (Pyxis) or integrated (Tucana) you can use a data cable which controls the gain in the Hydra, but it has no volume control of its own. The circular illuminated display on the front simply shows the volume when controlled by a Leema pre-amp or Integrated using the datalink.  When used with other amps or direct from a DAC the gain is fixed and that display remains centred.

[craighartley]

On 3/19/2020 at 8:07 PM, barrows said:

Wow Craig, 39 dB is really high and quite unusual gain for a power amplifier.  Have you ever contacted the manufacturer and asked if the gain could be lowered?  That much gain is probably also make the amp(s) noisier than they could be.

 

The amplifier manufacturer suggested trying these:

 

https://www.amazon.co.uk/Loops-20db-Balanced-Attenuator-Adapter/dp/B0060GDZTG

[craighartley]

On 3/19/2020 at 8:07 PM, barrows said:

Wow Craig, 39 dB is really high and quite unusual gain for a power amplifier.  Have you ever contacted the manufacturer and asked if the gain could be lowered?  That much gain is probably also make the amp(s) noisier than they could be.

 

Of course your "click" problem is something other than preamp related.  If i had that "click" problem i would track down the ideal solution for the problem, rather than covering it up by adding a transformer based pre/volume control.

Barrows, I wonder if you could help me please? If I am in right in thinking that you use Sonore Rendu (linux) native DSD, to a DSC-2 DIY DAC, with Amanero firmware CPLD_1081 slave and 2006be11, can you please confirm you have no clicks (muting problems) when playing direct to power amp with software volume control?

 

I am told that that firmware is likely to be the source of the clicks I have using the T+A DAC 8 DSD (streaming DSD) direct to power amps. 

[craighartley]

1 hour ago, matthias said:

 

It is easy, if it sounds better it is better.

The technical explanation why it is better sometimes comes much later.

 

Matt

 

Er, no. It just means you prefer it. 

[craighartley]

1 hour ago, matthias said:

 

It sounds better to you and so it is better for you.

 

Matt

And presumably not relevant to anyone else.