I measured the performance of the Audio-GD Vacuum HE1 XLR with my Audio Precision SYS2722 system, checking some of the results with the magazine's APx500 analyzer. Although this preamplifier has both balanced and single-ended inputs and outputs, HR told me he exclusively used the single-ended input and output so he could use the same unbalanced interconnects with the HE1 XLR that he used with his other preamplifiers. I focused primarily on the preamp's single-ended performance, therefore.
The HE1 XLR preserved absolute polarity (ie, was noninverting) with all combinations of inputs and outputs. (The XLR jacks are wired with pin 2 positive.) The maximum gain was 11.05dB for the single-ended and balanced inputs and single-ended outputs, 5.05dB for the balanced inputs at both output types. The volume control operated with approximate 0.8dB steps at the top of its range, 0.5dB steps at lower settings. HR told me that the maximum volume control setting he used was "62," out of an indicated "99." In addition to testing with the volume control set to "99," I did some tests with it set to "83" (unity gain) and "60" (–26.15dB).
The single-ended input impedance was a very high 85k ohms at 20Hz and 1kHz and 32.7k ohms at 20kHz. The balanced input impedance was twice these values, as expected. The preamplifier's single-ended output impedance was 1644 ohms at 20Hz, 806 ohms at 1kHz, and 804 ohms at 20kHz. The balanced output impedance was 1806 ohms at 20Hz and 1047 ohms at 1kHz and 20kHz. The preamplifier's output at 20kHz varied with the setting of the volume control. With it set to the maximum (fig.1, cyan and magenta traces), the response was down by 5.5dB at 20kHz. With it set to "60," the 20kHz output (blue and red traces) was down by 0.9dB. These two sets of responses were taken into the high 100k ohms load; into 600 ohms (green and gray traces, offset by –1dB for clarity), the low frequencies rolled off, reaching –5dB at 20Hz. The channel matching was very close at all the volume control settings I measured.
The Audio-GD preamp's channel separation was excellent, at >95dB across the audioband (not shown). With the single-ended input shorted to ground and the volume control set to "99," the wideband, unweighted signal/noise ratio, ref. 1V output from both output types, was 64.6dB in both channels. Restricting the measurement bandwidth to the audioband didn't increase the S/N ratio, though switching an A-weighting filter into circuit improved the ratio to 78.7dB. Fig.2 shows the low-frequency spectrum ref. 1V with the volume control set to "60." While the level of low-frequency random noise is low, AC-related spuriae are present, with the highest in level, at 120Hz, close to –90dB in both channels. Absolute spuriae levels were unaffected by the volume control setting, indicating that they are introduced after the control.
Fig.3 shows how the THD+noise percentage varied with voltage with the single-ended output driving 100k ohms. The downward slope of the trace below 500mV is due to the reading being dominated by noise. Actual distortion starts to rise above the noisefloor above that voltage, and the THD+N reaches 1%-our usual definition of clipping-at 3.3V into 100k ohms. The balanced output clipped at the same voltage into 100k ohms, while the single-ended output clipped at 3.1V into 10k ohms (fig.4). The Audio-GD's maximum output into 600 ohms was just 100mV (not shown).
Fig.5 shows how the THD+N percentage changes with frequency at 1V into 100k ohms and 10k ohms. It doesn't change significantly with frequency but, as seen in fig.4, the distortion is higher into 10k ohms than into 100k ohms.
The Audio-GD's distortion signature (fig.6) is dominated by the second and third harmonics, these lying, respectively, at –72dB (0.025%) and –77dB (0.014%) ref. 1V into 100k ohms. Intermodulation distortion with an equal mix of 19kHz and 20kHz tones at a peak level of 1V into 100k ohms (fig.7) was also low in level, the second-order difference product at 1kHz lying at –73dB (0.022%).
Overall, the Audio-GD Vacuum HE1 XLR's measured performance was very good, especially for a tubed design, but it shouldn't be used with power amplifiers that have an input impedance of 10k ohms or lower. Also note that it offers its most extended high-frequency response at volume control settings of "60" and below on the front-panel display.—John Atkinson
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COMMENTS
... Falcon Acoustics LS3/5a Gold Badge speakers, HR ought to consider getting an Audio Research I/50 integrated amplifier.
The I/50 costs a mere $1,500 more than the HE1 preamp.
If JA1 still has the review unit, could he send it over to HR for a follow-up?
https://www.stereophile.com/content/audio-research-i50-integrated-amplifier
https://www.hifinews.com/content/audio-research-i50-integrated-amplifier
I think I am still in the market for the Holo Serene KTE, given its competitive performance with the Audio G-D and the Lab 12. I may lean towards the Lab 12 given it's price and value therein.
What really impresses me is that these Chinese companies are using the best in audiophile parts extant and at a price point the rest of the world can't compete with. I still worry about our domestic and European counterparts though.
Kudos again HR. Your writing is always clear to me... and fun!
I regard "clear" and "fun" as my highest aspirations
I try not to waste people's time
so I am grateful for your time
peace and pumpkins
herb
Hi Herb,
A great review, thanks. I am intrigued by your number of tubes theory -- very interesting; made me want to pull out my Wyetech Coral (3 tubes) and compare it to my BAT VK52SE (8 tubes).
Most of all, I really enjoyed loading up and listening to the three records you used in this review. I was particularly taken by "The Six Seasons." Some amazing double bass work ... really exciting!
-- Monty
You know it's funny, how part of me consciously tries to write stories about records with some hardware impressions stuck in-between..
Overall, I try to present 'useful information' about listening to interesting records with interesting pieces of audio hardware.
Thank you for reading
herb
Hi Herb,
As an avid Audio-gd customer & fan of at least a decade now, it gives me great satisfaction hearing your praise for their products. I have always believed that they provide a great cost to performance ratio, with no compromise to sound quality but I’m swayed from time to time by the promise of better from more established Western brands. I run a full Audio-gd stack and you’ve given me both validation and a possible upgrade path - I happen to have bren quite curious about their vacuum preamp.
Would love to see you complete the loop and review an Audio-gd power amp too.
Hello, y'all! I'd like to add my 5 cents to the discussion.
As was pointed out, PrimaLuna sounds radically better with NOS tubes.
As expected, really. And AudioGD preamp would do as well. However, most don't realize the international compatibility of the Russian, European and American tubes, and presume that such a swap isn't possible.
6N2P (or what looks like 6H2n in Cyrillic) is a direct equivalent of 12AX7 / ECC83, but with a 6.3V heater and a different heater pinout (12AX7 has 12.6V heater with a central point). And Ebay is full of 12AX7 to 6N2P converters that allow an easy substitution of the stock signal tubes in the Audio GD preamp for, say, Telefunken, Mullard, Mazda or Brimar ECC83. And those tubes will sound considerably better than a humble 6N2P. The only catch is to make sure that the adapter you're getting is for the ECC83 tube (top) and 6N2P socket (bottom), and not vice versa, and that requires carefully studying the Ebay item description, or even contacting the seller to make sure.
Same goes about the 6U4n tube, which is in Russian is pronounced 6Z4P (and is usually cited in this transcription most everywhere). It is, in turn, a direct equivalent of 6X4W / EZ90, again with a different base. And again 6Z4P to EZ90 adapters are plentiful on Ebay, allowing to use a GEC, Mullard or Brimar EZ90 instead of the 6Z4P / 6U4n.
One could also use octal tubes with additional adapters, allowing use of 6SL7 signal tubes and 6X5GT (EZ35) tubes, but they will most likely not fit under the top plate and thus require either to run the preamp open, or ordering somewhere a custom top plate with drilled holes for the tubes (which might actually add some visual charm Cary-style).
Finally, I would add that NOS Russian (USSR, actually) special quality tubes (-EV and rarely seen -DR versions) are quite good in their own right. So it is worth changing them only to the absolute best (and very expensive) NOS European tubes. If you're not ready to throw about a grand on those 10 tubes, it's not even worth the trouble. With all due respect, most regular US tubes like good old RCA and GE will most likely provide a different flavor instead of a real quality jump.
Hope this helps.
And Herb, thanks a lot for a great review!
You really got me salivating all over this preamps virtues, so I've bit the bullet and bought me a spanking new Audio-GD Vacuum HE1 XLR. Still waiting for it, though.
I will certainly do all the experiments I described above, including stuffing it with six huge new production Shuguang WE 6SL7 Plus and four NOS 6X5GT (pity that coke bottle 6X5G will not fit as the sockets are too close to each other). Together with a custom copper top plate that thing will look to kill! And hopefully sound amazing too)))
Nice review! As an owner of the Schiit Freya+, I’m definitely interested in giving this a go.
I emailed Audio-GD to get a quote for this preamp and the price before shipping is just under $3k. The reviews says $3999 and the pictures shown in the review show two different models (the Vacuum HE1 and the Vacuum HE1 MK2).
Which unit was reviewed and what is the US price?
Would love to support this company but their website is confusing and the review here doesn’t help.
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