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Evaluating Potential Setup, Solid State Pre-Amps, buffers, XLR Cable Length

Hi everyone,

I am in the process of completing my audio chain setup. I've recently purchased a pair of Genelec 8040B studio monitors a few months ago and recently decided to upgrade my basic audio interface to a well regarded DAC. I've purchased the denafrips ARES DAC which is on the way and I'm in the amidst of researching how I can match all components including purchasing new cables.

Questions at the bottom of the post.

Due to the fact that the ARES has no built in pre-amp or attenuator I am in the hunt for most likely a passive Preamp with balanced Input/output placed immediately after the DAC. However given the technical specifications below particularly on impedance matching and the idea that the ARES outputs a relatively high output impendence, I am also open on the possibility of adding an active solid state pre-amp. Finally, from the research I've done, it may be that a passive preamp with a buffer integrated or added right before it may be the component I would need to convert a higher impedance signal and output a lower impedance signal. This is because I may have some impedance mismatching involved however I would like some feedback on that.

The reason why I am slightly concerned is the general rule from what I've read across the web states that the input impedance of the power amplifier (in this case the built in amplifier in the studio monitors) should be roughly 10 times the output impedance of the source to ensure no roll-off in high frequencies or loss of dynamics. In the case of my active studio monitors, "the audio input is via a 10 kOhm balanced XLR connector", so I'm assuming that is of course the input impedance required to be matched right before the amplifier section.

Here is my proposed chain in simplified terms:

Razer Blade Laptop >> USB cable >> ARES DAC >> Solid State Buffer? >> Passive Pre-amp (balanced Input/Output) >> Genelec Active Studio Monitors

From https://www.genelec.com/answers-library#qid-926
"I have no preamp outputs on my equipment. Can I connect my monitors to the speaker outputs of my power amplifier?"

It seems the circuitry to the Genelec's amplifier is quite non-trivial as compared to a individual amplifier. In the answer, they advise to never connect a power amplifier output to the monitor as the input stage would be damaged, however it seems a typical passive pre-amp with an attenuator would be fine.

Finally here are the specifications I am working with to ensure proper matching:

ARES DAC Specs:

Analog Output: XLR at 4.4Vrms +-10% (Peak voltage calculated to be 6.22 volts)
Output Impedance of 1250 Ohms

Genelec 8040B Spec:
Crossover Section:
10 kOhm input impedance
Input level for maximum short term output of 100 cB SPL @ 1 M: Adjustable +6 to -6 dBu

Mains voltage:? 100,120, 220, or 230 V according to region - not exactly sure what this is.

According to this paper: http://www.decware.com/paper55.htm
In order to select an ideal preamp I must know

1. The output voltage of the source (DAC) which is 4.4Vrms
2. The input sensitivity of my amplifier - I am not sure what this is number is.

From basic electronics, it seems like my DAC has adequate line stage voltage (should be at least 2V), and need to find he input sensitivity of the Genelecs expressed in volt, the Tortuga website mentions most amps have sufficient input sensitivity, so it maybe I don't need to amplify the signal twice.

What Tortuga does mention is regards to problems with passive pre-amp is this: https://www.tortugaaudio.com/what-is-a-passive-preamplifier/

"Using a passive preamp between a high impendence source (ARES-1250 Ohms) and a low impedance source (Genelec - 10,000 Ohms) may overwhelm the ability of the source to drive audio signal without negative effects (dynamics and punch)."

Lastly, they mention high capacitance of cables (in my case all XLR) is highly detrimental and can add considerable impedance to the audio signal so as to act as a low pass filter, rolling off higher frequencies which I don't want either. A high capacitance cable is of the order of 50 pF/ft while a low one is of 20 pF/ft.

Questions I have are

1. Given, the parameters, what kind of attenuating section should I invest in, an active pre-amp or a passive pre-amp? Do the input and output impedances matter, and if they do, what ranges on input and output impedances do you recommend for the pre-amp.

2. How short do I need the upstream XLR cables to be, how short should the downstream cables be (running between the pre-amp and the speakers).

3. What are some good XLR cables with capacitance as low as 20 pF/ft you can suggest?

4. What are some good passive pre-amps if I need that type?

5. Do I need to invest in a buffer to ensure proper matching, what does the buffer look like and where can I purchase one?

6. Is the setup workable in the end without compromising my listening experience?

The post is a little overboard with content, however I would like to be thorough for myself as well as others if they come across this. It would preferable in the end to not introduce any colorization (like tube pre-amps or buffers) and to keep it simple so as to pretend the original musical signal from the DAC is unaltered.

Any technical input or subjective experience for a suggestion of components would be most helpful.

Thanks.
reggy
10 responses Add your response
reggy OP
56 posts
 
@carlosaudio51  Thanks for the post. Been working just fine with the Genelec, just had to adjust the sensitivity of the speaker a couple years ago if I recall at the back. Nothing different sounding and the volume is good. Thanks for your help. I think that passive did fine with the speakers and Ares DAC.

I can tell a difference between the Focusrite and the Ares I too, I favorably like the Ares, not saying it is more transparent or detailed than the focusrite, but it definitely has character and sparkle in the treble compared to the focus rite which is probably still a good interface for the price.
Overall a pretty good setup and better when the bass is Eq'd down in Foobar. I do say having a bit extra bass can be a plus with the Ares, I would not consider it to be perfectly flat but a nice DAC to complement the Genelec.
Its been a while and I remember someone not recommending the DAC to match the Genelec, for studio monitoring I see their view but for now I think it fits well for my ears.

carlosaudio51
9 posts
 
Hey Reggy,

Thanks for the reply, even after the thread has been dormant for more than a year. I had a similar situation as you. Like you, I ended up with a 10 kOhms-potentiometer-based passive pre-amp as the solution (in my case, a Schiit Sys).

What I found was that the output impedance of the pre-amp depends on the position of the volume knob and, in general, that the max output impedance is about a 1/4 of the nominal potentiometer resistance (not exactly but close to 1/4 because it varies slightly depending on the DAC's output impedance and amp/active speakers' input impedance).

See link:
https://www.superbestaudiofriends.org/index.php?threads/passive-volume-control-for-dacs-sources.860/...

So in your case, the max output impedance of the pre-amp is ~2.5 kOhms. And that's going into your Genelac's 10 kOhms input impedance. That puts the damping factor at ~4. So it seems you still have the impedance compatibility issues based Almarg's thorough reply earlier on this thread. With my system, I didn't have as much an impedance compatibility issue as my Schiit Sys is connected to an amp with 47 kOhms input impedance (so damping factor of ~18.8).

Granted, take this with a grain of salt. I do not know if/how this will affect the sound quality of the system. And if so, whether the sound quality will be positively/negatively affected or just sound "different." You can't really tell unless you do an A/B comparison with your current pre-amp vs an active SS pre-amp similar to what Almarg suggested in his last reply.

Sorry to throw a wrench at you but I thought I should reply to your post with my opinion since you took the time to reply after the thread was dormant.

reggy OP
56 posts
 
@carloaudio51.   I chose on eof the preamps Alvin recommended. Perfectly fine for the price ($50CAD). The white one has a nice finish:  https://www.ebay.com/p/Jemmy-Audio-Volume1-Full-Balanced-Passive-Preamplifier-Preamp-Volume-Controller/27005767250
carlosaudio51
9 posts
 
Hey Reggy,

What pre-amp did you decide to go with? Can you share your shortlist of pre-amps before you decided on the one to buy? I'm interested in it because I want to get a Denafrips Ares II in the future.
reggy OP
56 posts
 
That's great to know! I will certainly take my time evaluating my options for preamps. There sure doesn't seem to be a lot of focus on those in any forum.

If anyone wouldn't mind sharing a list or what they have in their setup feel free to let me know. I will start my scrupulous search now.
almarg's avatar
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almarg
9,670 posts
 
In the end would you be saying it may still be possible to use my DAC if I use a solid state preamp in series whose input impedance is ~47 kOhms and output impedance is 200 ohms?
Hi Reggy,

Yes, absolutely. A solid state preamp providing balanced inputs having an input impedance of 47K ohms or more, and providing balanced outputs having an output impedance of 200 ohms or less, would be a very safe choice in terms of impedance compatibility.

I’ll add one other criterion, though, which is that it would be preferable to avoid preamps providing particularly high gain, e.g., more than 10 db or thereabouts. Given the output voltage of your DAC and the sensitivity of your speakers (even if the speakers are set to their minimum sensitivity setting of +6 dBu) a high gain preamp might result in your having to use the preamp’s volume control at undesirably low settings.

Good luck. Regards,
-- Al
reggy OP
56 posts
 

Hi Al,

First of all, thank you very much for your eloquent response, I think your proper explanation on the specific aspect that the output impedance of an amp varies over a wide range of frequencies is definitely something to know, or at which the highest output impedance occurs, and for that reason it make sense to ensure the 10X ratio is increased by a wider margin. 

I looked over your post and most of it I could follow.

In the end would you be saying it may still be possible to use my DAC if I use a solid state preamp in series whose input impedance is ~47 kOhms and output impedance is 200 ohms?

It would be nice to use that DAC in my Ares setup, however worst thing is I use it in another setup or sell it.

Thanks
almarg's avatar
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almarg
9,670 posts
 
Correction to my previous post. When I said:
If as is often the case ... [the output impedance at the highest audible frequency] is not known...

I meant to say:
If as is often the case ... [the output impedance at the audible frequency for which the output impedance is highest] is not known...

Regards,
--Al
timlub
2,040 posts
 
Schiit Freya will do a good job for you gives you balanced out and a choice of passive, active or tubes.  Decent sounding unit too. 
almarg's avatar
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almarg
9,670 posts
 
Welcome to Audiogon, Reggy.

First, some comments on points you raised prior to the six questions at the end of your post:

There are a number of online calculators which can be found that will provide dBu to voltage conversions. 0 dBu corresponds to 0.775 volts. As you realize, your speaker provides adjustable sensitivity covering a range of -6 to +6 dBu for 100 dB SPL at 1 meter. -6 dBu corresponds to 0.775/2 = 0.39 volts. +6 dBu corresponds to 0.775 x 2 = 1.55 volts.

At the most sensitive setting (-6 dBu), **if** the speaker were able to handle an input of 4.4 volts in a reasonably linear manner and without its self-protection circuitry kicking in (and it apparently would kick in in this case), the result would be an SPL at 1 meter of:

100 dB + 20 x log(4.4/0.39) = 121 db,
where "log" is the base 10 logarithm.

So your DAC certainly provides enough voltage to drive the speaker, without the additional gain of a preamp.

However, as you realize sonics may be adversely affected by impedance issues in this case. I’ll first say that the 10x rule of thumb guideline is commonly stated in an oversimplified manner. IMO a proper statement of it is as follows (I’m quoting from a post I made in an earlier thread):

To assure impedance compatibility the 10x rule of thumb guideline should be applied at the frequency for which the output impedance of the component providing the signal is highest. Most impedances are specified at a mid-range frequency such as 1 kHz.....

If as is often the case ... [the output impedance at the highest audible frequency] is not known, and is not indicated in published measurements (such as Stereophile often provides), then to be safe a considerably higher ratio than 10x should be used, something like 50x or 75x IMO. Especially if the component is tube-based and is likely to have a coupling capacitor at its output.

Also, to clarify a common misconception I should add that failing to meet that guideline does not necessarily mean that there will be an impedance compatibility problem. It depends on how much **variation** there is in the output impedance over the frequency range. But meeting that guideline (at all audible frequencies) assures that there won’t be an impedance compatibility problem.

In the case of your DAC my guess is that there isn’t a great deal of variation of the output impedance over the frequency range, but nevertheless the impedances that are involved IMO due not appear to be optimal or even suitable for use with a passive preamp, whether it be resistance-based or transformer-based. And cable capacitance, which is proportional to length, would also be a potential issue with a passive preamp, as you mentioned.

Mains voltage:? 100,120, 220, or 230 V according to region - not exactly sure what this is.
This simply means that the speaker is designed to be operated at the AC line voltage that is used in the country to which the speaker is supplied. 120 volts in the case of speakers supplied to the USA, of course.

Regarding your questions, I don’t see the combination of a passive preamp and a buffer as being an attractive solution, given that there are impedance concerns involving both the source and destination components. There are countless numbers of solid state preamps, on the other hand, that will be able to drive 10K balanced inputs with no problem (as I mentioned earlier that can be verified by looking at Stereophile’s measurements, if they have reviewed the particular component, or alternatively if the component’s specified nominal output impedance is say 200 ohms or less), and that will provide an input impedance of say 47K or more, which would certainly be suitable. And an input impedance considerably lower than that could very conceivably also be suitable, although I’d check with Denafrips to be sure.

Good luck. Regards,

-- Al

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