Michael Cooper Recording
By Michael Cooper
A fat-sounding, high-quality tube mic preamp at an affordable price.
James Demeter of Demeter Amplification has been designing and building quality tube gear for over two decades. In 1980 he launched the first commercially available tube DI box, the venerable Demeter VTDB-2b Tube Direct, and it's still selling strong today. (For more information about the VTDB-2b, see "Direct Action" in the November 2001 EM.)
I've always been impressed with whatever Demeter cooks up, so I was eager to take the affordable new HM-1 Tube Mic Pre-Amplifier for a test drive. Belonging to the company's H Series product line, which is comprised of hybrid signal processors (those containing both tube and solid-state components) offered at attractive prices, the HM-1 is a dual-channel, 1U, rackmountable unit employing quality components throughout, including Jensen input transformers, 12AX7EH tubes (for amplification duties), and solid-state line drivers (to electronically balance the transformerless outputs). The HM-1 boasts a remarkably flat frequency response less than 0.1 dB fluctuation from 20 Hz to 20 kHz.
Skin I'm In
The HM-1's sturdy steel and aluminum chassis is ventilated on its top and bottom plates and gets only slightly warm when cooking up your signals. Rear panel I/O consists of balanced XLR and TRS 1/4-inch jacks for each channel, wired in parallel so you can use any combination of jacks for input and output. The TRS jacks are not cross-coupled, so you can safely patch unbalanced gear to the HM-1's I/O with standard guitar cords. As with other equipment that uses this ubiquitous plug-and-play I/O topology, you'll lose 6 dB of signal level at each jack that you use in unbalanced mode. The rear panel also provides a standard IEC connector with a detachable, three-prong AC cord.
Each channel also provides an unbalanced, 1/4-inch DI jack on the front panel for direct-instrument input. These DI inputs access the HM-1's circuitry after the input transformer but before the tube gain stage. Both front and rear input jacks can accept either mic or line-level signals.
Each HM-1 channel sports the same generous allotment of front-panel controls. Four button switches access, from left to right, a low-cut filter, 20 dB mic pad, 48V phantom power (with associated status LED), and polarity inverse. The low-cut switch rolls off lows below 200 Hz with a gentle 6 dB per octave slope, resulting in a 12 dB cut at 40 Hz.
The 20 dB mic pad is placed in the circuit before the input transformer, so it doesn't work for DI inputs (which enter the box after the transformer). Musical instruments put out relatively weak signals, however, so there's usually no need to pad them. And giving DI inputs a transformerless entry into the HM-1 better preserves an instrument's original tonal balance. Demeter's design makes good sense.
Each channel also has a continuously variable, rotary gain control that provides up to 30 dB of additional tube amplification gain. Of course, the minimum and maximum values vary depending on which input (mic or DI) you're using and whether or not the pad is switched in for mic input. With the gain knob set at hard left (fully counterclockwise), a DI input will receive 12 dB of gain; with the knob turned hard right, the 30 dB boost results in a total 42 dB of added gain for a DI input. That should be enough to attain close to a 0 dBFS level on most digital recorders having balanced inputs.
A hard left gain-knob setting will provide 30 dB of gain for mic input signals (with the pad switched out); switch in the pad and the level drops to 10 dB of gain. Setting the gain knob hard right gives you 30 dB more gain, or 60 dB total with the pad switched out and 40 dB total with the pad switched in. The HM-1's maximum gain of 60 dB should be adequate for recording with most condenser and dynamic mics. The unit can handle a hefty +29 dBm maximum output level in balanced mode, which is more than sufficient for most professional applications.
A front-panel overload LED lights to warn you that the signal level is within 6 dB of overloading the tube-amplification circuit. If you have the gain control set to minimum for a mic signal and the overload light is still on, switching in the mic pad should drop the level enough to preclude distortion.
Each channel also provides a continuously variable rotary volume knob, which you can use to smoothly fade your output level from unity down to silence. As this attenuator occurs after the tube amp, however, it will do nothing to prevent overload caused by blazing levels occurring before the tube amp. That's the mic pad's job.
Also included on each channel is a tri-colored, 10-segment LED output meter an upscale feature not found on most "cost effective" mic preamps. The LED is referenced to either +3 dBm or 8 dBm level, depending on the setting of the reference-level switch. (Each channel has its own switch.) Three red LEDs show levels above the zero mark, but the scale of the meters is arbitrary. With one of the meters referenced to +3 dBm, I fed roughly 9 dBFS input to the A/D on my Yamaha 02R's channel insert to light all three of the HM-1's red LEDs. There was still plenty of headroom available (both on the HM-1 and 02R) beyond the top of the meter's range, but the meter was useless for gauging hotter levels. I'd prefer that the HM-1's meters be calibrated so that the top LED is referenced to either the mic preamp's maximum output level or to some digital standard such as +19, +22, or +24 dBm. As they stand now, the meters should work well with systems calibrated to 10 dBV nominal operating levels.
Demeter products tend to have a characteristic sound, which I would describe as composed of a smooth, clear upper midrange, tight bottom end, and gobs of tube richness. My first test of the HM-1 confirmed its heritage.
Of course, it's hard to judge the subtle sonic "signature" of a mic preamp in isolation. So I began by setting up a comparison test. I pitted the HM-1 against two heavy-hitters (both of which cost considerably more than the Demeter): a Millennia HV-3 solid- state mic pre and a Pendulum Audio MDP-1 all-tube mic pre. I then recorded some acoustic-guitar tracks through each preamp with a spaced pair of DPA 4011 mics.
The HM-1 sounded more present in the upper mids than the Millennia HV-3, but not as full in the bass (though it did produce more bass than the MDP-1). The HM-1 also sounded more lush and subjectively louder than the HV-3 just what you'd expect from a quality tube mic preamp. But of the three preamps, the MDP-1 was the richest and most open-sounding.
Although the HM-1 delivered respectable transient response, it could not compare to that of the HV-3 or the ultra-detailed MDP-1 in this department. As for noise levels, with 50 dB of gain applied the HM-1 sounded about as quiet as the Millennia HV-3.
The HM-1 sounded great on male and female vocals, both recorded with an AKG C 414B-TLII (solid state) mic. The sound was very present but not harsh. Male vocals cut easily through a pop-rock mix and were positively brimming with sweet overtones. The HM-1's bass and low-mid frequency response was just enough to keep male vocals sounding full and balanced, without blurriness. Female vocals sounded nicely detailed, without suffering excessive sibilance.
To test the HM-1's DI function, I plugged in my '62 Strat through the front-panel 1/4-inch input jack and laid down a track. The results were absolutely awesome. The track sounded ultra-pristine and present, richly sprinkled with tube harmonics, yet warm and round. The crystalline high-end detail was beautifully married to a tight bottom end.
In an A/B test with the Demeter VTDB-2b Tube Direct (DI box), both units initially sounded identical on electric guitar. But then I lowered the HM-1's volume control and cranked the gain so as to drive the tube gain stage harder. This rounded off the edges and added some delicate "hair" to the guitar track, making for a different sound something you can't do with the VTDB-2b.
I also compared the HM-1 to the VTDB-2b on DI'd electric bass. The HM-1 yielded a wonderfully rich and present sound, but the VTDB-2b produced a more extended bottom that gave more weight to the track.
Get In Line
To test how the HM-1 handles line-level signals, I multed a previously recorded snare track one that I really liked the sound of through one of the unit's TRS inputs. As expected, I had to switch in the mic pad to get the unit to accept line level without distorting. Once padded, however, the track sounded clean. However, to my surprise, the snare drum got brighter sounding, and the added high frequencies were too cutting for my tastes.
Fortunately, I was able to find a creative use for the new track: by delaying it 14 samples in my 02R (to adjust its phase) and combining it with the snare track's digital input signal, I got a snare sound that was like a cross between a rifle shot and a wine bottle being uncorked.
The Demeter HM-1 is a solidly built, generously featured, versatile, and all-around great-sounding tube mic preamp that is priced considerably lower than one might expect, given the unit's pedigree. Premium tube gear doesn't come cheap, but with the HM-1, Demeter is clearly doing its part to put high-quality audio within reach of those with less than world-class budgets.
The HM-1 does a really good job on a variety of instruments, and sounds downright great on vocals and as a tube DI on electric guitar. Aside from the owner's manual, which is painfully short on details, I found little to criticize. If you're looking for a fat-sounding, affordable cream machine that can handle a variety of applications, check out the Demeter HM-1. Your ears and your wallet will thank you.