2v or 6v into a tube pre-amp? what would you do (and why?)

[Bill Brown]

@sandyk is giving good advice.  Getting good gain structure/gainstaging (what you are working on) is very worthwhile.

 

One rule of thumb would be to minimize any gain that you subsequently attenuate.  This maintains S/N ratio.

 

Generally the least gain required to get the maximal speaker output that you want is best.

 

The above specification 900mv balanced/450mv unbalanced, means that those are the voltages at which the preamp will amplify it to its maximum output.  Anything sent to the preamp higher than that must be attenuated in the preamp (which it will do) or it will drive the preamp into clipping.  Tube preamps are traditionally high gain, though not always these days.

 

From the specs above, your gain on the high level inputs is 26.9 db:

 

gain = 20(logVo/Vin) db

 

Most amps will be driven to clipping with 2-4V (thought that, too, has changed some).  To get 4 V out of your preamp you need to put in only 181mv (0.18V)........

 

If you knew the line level input sensitivity of your amp you could calculate the whole gain structure with precision.

 

People don't realize how much gain we discard since the development of the CD with its 2V standard

 

Hopefully I didn't screw up any of the calculations!  :)

 

If I had your system I would decrease the output of the DAC as much as possible.  My goal would be to have my typical max volume settings for listening as close to 0.0 db on the digital side if using that for volume, or the knob on the preamp as close to wide open as possible (3 o'clock or greater with old analog pots).

 

Objectively, I believe the above will give you the best measured dynamic range.  Re. subjective "dynamics" I am not sure where the notion you described comes from (not that I don't believe in the concept of dynamics).  I wonder if it relates to our prior experiences with old high gain equipmentwhere by 9 o'clock on the knob it was blasting.

 

Bill

 

 

[Bill Brown]

My pleasure.  I find these calculations sort of fun.

 

I looked for the input sensitivity on your amp.  It wasn't on the main web page, but I find it in the owner's manual:

 

4.2V in balanced will drive the amplifier to its rated output (half that for unbalanced), so that is the most your preamp will have to put out.

 

I was pretty close with my 4V calculation above :).

 

For 4.2V out your preamp needs 0.189779V :).

 

I like to use digital attenuation so as not to have to buy a preamp as I think small amounts of digital attenuation (possible with good gain staging) are transparent.  If I had that preamp though, I would use it.  I would set the DAC output at 0.2V and listen.  If I needed more volume than I could get with the knob all the way up I would go to 0.6V (though see below re. the autoformer taps), and then up again if needed.  The choices of voltage outputs on your DAC are great in this regard.  At 0.2 or 0.6V your gain arrangement will be fantastic and you will get the full S/N ratio out of your system.  The key is the lowest setting at which you get full power output with the least attenuation with your volume knob.

 

There is one more choice you are going to have to make....which autoformer tap you are going to hook your speakers to, and the gain of the amp depends on that.  With tube amps I like to use an output that is always at or below the speaker's impedance.  JA's measurements also suggested that with this amp setting lower rather than higher was better from a distortion perspective (though you get more power from higher ohm taps relative to the load).  You can't break anything by hooking to any of them.  Your speakers are rated at 8 ohms, but that is always nominal, there may be dips here or there.  I would probably use the 4 ohm outputs, there is still plenty of power from that tap.

 

I am sure you have seen these:

 

https://www.stereophile.com/content/mcintosh-laboratory-mc462-power-amplifier-measurements

 

No matter where you try the voltage/which output tap, you won't break anything, so just try different settings and enjoy.  I have never lived with Mac components, but I strongly suspect you are in for quite a treat!

 

Bill

[Bill Brown]

I realized I may have implied your amp had a tube output stage, but I know it is SS.  The commonality it has with tube outputs is the transformer output (actually an autoformer with Mac, unique in that regard and a neat concept).

 

Bill

[Bill Brown]

14 hours ago, sandyk said:

 0.2V suggests that they may have already digitally (most likely) attenuated it already.

 

I would be very surprised if that is the case.  I would make a huge bet it is in the analog stages.  An interesting question to me would be the raw voltage coming off the DAC chips as to whether they are voltage or current output.  If the latter it would require a I/V stage and gain.  Hard to know whether the gain stage/output driving stage is optimized at 0.2, 0.6, 2, etc. volts.

 

I am sure it is implemented very well regardless making the latter consideration moot, and the S/N issues described above remain.  The DAC is certainly not operating in isolation.

 

With wide dynamic range material (very soft sections not boosted by compression) the need for more gain might be seen (heard).  That is where I would start- a wide dynamic range classical recording, setting the V out so those were at the max level I needed.

 

Bill

[Bill Brown]

Truthfully, though, this is all sort of "how many angels can dance on the head of a pin" kind of stuff anyway.  It is a beautiful system that will sound great no matter what you do.  I would just play with it, pick a setting, and forget it/listen to music.

 

Bill

[Bill Brown]

No worries. :)

 

Most of us have been there (I know I have)!

 

Bill

[Bill Brown]

5 hours ago, Summit said:

 

Truthfully, why this type of comment to a serious question? I do not believe it’s stupid to try to get the best SQ out of one’s system and discuss which “things” and implementations that does.  

 

"Stupid" isn't a word I used.  I would ask you to not put words into my mouth.  Also, why would I write so much above about a subject that I think is "stupid?"  Did you read near the top where I emphasized the importance of gain staging in some detail?

 

My point above is not that it isn't worth thinking about and that it wouldn't be fun to experiment, but that in the end, his equipment is so well-engineered that any combination of settings will work fantastically (a point I have repeatedly emphasized).

 

I would also encourage you to read a bit on the effect of S/N measurements after gain and subsequent attenuation:

 

"But if we look back at the system, we see that for most situations, we have more gain than we need. Maybe 15 to 20dB (10X) more gain than we really need. And that can lead to noise. Why? Because any noise that occurs after the volume control does not get attenuated. In fact it gets amplified. You've cut down the signal from the CD player by 17dB, so now it's 17dB closer to the noise in every circuit that follows. Any noise from the preamp, the cables, bad connections, etc. will be also be amplified 30X by the power amp. You took a medium level signal of 320mV (0.32 volts) and divided it down to 44mV so its now much closer in level to all the noise living in the bottom of the system."

https://www.diyaudio.com/archive/articles/160464-de-lite-amplifier-next-thread.html

 

"One consequence of this is that the more you attenuate the signal the more you reduce the SNR (since the noise is fixed). So gain management consists of adjusting and/or selecting a chain of components that will amplify the signal enough to listen to and not much more."  @Kal Rubinson in Stereophile

 

"The maximum gain for both balanced and unbalanced inputs to balanced outputs was just 0.2dB. This is lower than the norm, but it's something I welcome because preamplifiers in general are used to attenuate, not amplify, the levels of source signals" JA in Stereophile measuring an integrated amp

 

• Low Gain – The AHB2 has a low gain setting that optimizes the gain structure of professional monitoring systems. Maximum rated output is reached at an input signal level of 22 dBu. This places the upstream equipment in an ideal operating range to maximize the SNR of the monitoring chain. Most power amplifiers have far too much gain, and this degrades noise performance of the overall system. The AHB2 has an ultra low-noise input amplifier with two gain boost settings that can be enabled to allow direct interfacing with Hi-Fi components that usually operate at significantly lower signal output levels.

https://benchmarkmedia.com/blogs/application_notes/14680625-the-ahb2-a-radical-approach-to-audio-power-amplification

 

"The DAC1, DAC2 and DAC3 all have internal pads on the XLR outputs that should be used to set the output levels to the range that is required by the downstream device. This will allow direct interfacing to virtually any pro or consumer product.

Turning down the volume control is not a substitute for selecting the proper pad setting!

Excessive use of the volume control skews the measurements by adding excessive noise. This in turn impacts the linearity measurements, which at their low end are really SNR measurements. The tests should be run at or near maximum volume (or the calibrated switch on the back of the DAC1 should be enabled)."

 

Jon Siau, Benchmark designer

 

"The wideband S/N ratio, measured in the high-gain, stereo condition with the input shorted to ground and ref. 1W into 8 ohms, was very high, at 89.3dB. Reducing the measurement bandwidth to 22Hz–22kHz increased the ratio to 106dB, while switching an A-weighting filter into circuit increased it further, to 108.5dB. Referenced to the AHB2's clipping power of 100W, this is equivalent to a dynamic range of 128.5dB, which is close to the specified 132dB. The S/N ratio did increase by around 3dB in the lowest-gain condition, implying that the specified ratio was measured in that mode."

 

https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements#HkauPY4Q0eXu8lOd.99

 

"To maintain the full dynamic range within the best operating level, RME's ADI-2 FS includes electronic switches, which give control over input and output level via two keys from the front plate, for a perfect adaptation to the levels +4 dBu, +13 dBu and +19 dBu. Two compact level meters provide 6 LEDs each. Multiple brightness stages and peak hold functionality make read- ing and adjusting input levels easy and convenient."

 

0. Mode ANA (Analog In to Analog Out)

   •  Signal to Noise ratio (SNR) @ +19 dBu: 123 dB RMS unweighted, 126 dBA

   •  Signal to Noise ratio (SNR) @ +13 dBu: 119 dB RMS unweighted, 122 dBA

   •  Signal to Noise ratio (SNR) @ +4 dBu: 112 dB RMS unweighted, 115 dBA

https://www.rme-audio.de/downloads/adi2fs_e.pdf

 

The last shows the best dynamic range with the least attenuation (the closest to wide open, i.e.the least attenuation of preceding gain).

 

I'll quit.  The Liverpool game is about to start. :)

 

My implication throughout this thread is that there are things worth considering, but should be approached as fun.

 

Bill

 

[Bill Brown]

Thank you.  Yes, I have the input sensitivity at the the lowest setting on my Benchmark amp and with the analog/automatic gain switching of the RME DAC usually listen at -0 to -6db of digital attenuation (with higher quality recordings).  I think it is a good way to go.  As long as the output stage of a DAC is well-designed and the volume control is implemented well (and it has one, not all do, of course), I don't see the need for a separate preamplifier.  It is nice to avoid the expense (the good ones are $$) of a separate preamplifier.

 

Again, this isn't to say that @992Sam won't enjoy the heck out the sound he gets from the Macintosh preamp.  He may fall in love with its tube gain stage, there are certainly many that do.

 

Bill

[Bill Brown]

Ah yes, I just remembered as well that @992Sam will be running an SACD player and an LP system, where a preamp is mandatory (I have converted to digital files exclusively).  I expect the Macintosh to be a wonderful performer for him.  As the common pathway for all of the sources a great preamp is considered by many to be the heart of a system.

 

Bill