James Demeter of Demeter Amplification has been designing and building quality tube gear for more than two decades. In 1980 he launched the first commercially available tube direct injection (DI) box, the venerable Demeter VTDB-2b Tube Direct, and it's still selling strong. (For more information about the VTDB-2b, see “Direct Action” in the November 2001 issue.)
I've always been impressed with what Demeter cooks up, so I eagerly took the affordable HM-1 tube mic preamp (see Fig. 1) for a test-drive. The unit belongs to the company's H-Series product line, which is composed 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 ¼-inch TRS jacks for each channel (see Fig. 2), 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 patch unbalanced gear safely 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, 6 dB of signal level is lost 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 ¼-inch DI jack on the front panel for direct-instrument input. The 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 mic or line-level signals.
Each HM-1 channel sports the same generous allotment of front-panel controls. Four buttons 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 you usually don't need to pad them. 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 as much as 30 dB of additional tube-amplification gain. The minimum and maximum values vary depending on which input (mic or DI) you are using and whether 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 with 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. You get 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 up 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 microphone 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. Because the attenuator occurs after the tube amp, however, it will do nothing to prevent overload caused by blazing levels occurring before the tube amplifier. That's the microphone pad's job.
Each channel contains a tricolor, ten-segment LED output meter — an upscale feature that is not found on most cost-effective microphone preamps. The LED is referenced to a +3 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 0 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 HM1's red LEDs. Plenty of headroom was still available (both on the HM-1 and the 02R) beyond the top of the meter's range, but the meter was useless for gauging hotter levels. I would prefer the HM-1's meters to be calibrated so that the top LED is referenced to 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 that's 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.
It's hard to judge the subtle sonic signature of a mic preamp in isolation, so I set 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 preamp and a Pendulum Audio MDP-1 all-tube mic preamp. I then recorded some acoustic-guitar tracks through each unit, with a spaced pair of DPA 4011 mics.
The HM-1 sounded more present in the upper mids than the HV-3 did, but not as full in the bass (though it produced more bass than the MDP-1 did). 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 compete with the HV-3 or the ultradetailed MDP-1. As for noise levels, with 50 dB of gain applied, the HM-1 sounded about as quiet as the HV-3.
The HM-1 sounded great on male and female vocals, both recorded with an AKG C 414 B-TL II solid-state microphone. The sound was present but not harsh. Male vocals cut easily through a pop-rock mix and positively brimmed 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 examine the HM-1's DI function, I plugged in my '62 Strat through the front-panel ¼-inch input jack and laid down a track. The results were absolutely awesome. The track sounded ultrapristine and present, sprinkled richly with tube harmonics, yet warm and round. The crystalline high-end detail was beautifully married to a tight bottom end.
In an A/B comparison 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 to drive the tube gain stage harder. That 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 with the VTDB-2b on direct injected 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.
The Demeter HM-1 is a solidly built, generously featured, versatile, and all-around great-sounding tube mic preamp that costs considerably less than you 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 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 are 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 definitely thank you.
tube mic preamp
FEATURES5.0EASE OF USE4.5AUDIO QUALITY4.0VALUE4.5
RATING PRODUCTS FROM 1 TO 5
PROS: Rich tube sound. Quiet. Loaded with features. No-fuss interfacing with balanced and unbalanced gear. Easy, flexible use. Priced attractively.
CONS: Instrument DI could use a slightly deeper bottom end for recording bass instruments. Output meters not calibrated to any standard. Owner's manual inadequate.
Amplifier TypeClass AInput Connectors(2) balanced ¼"; (2) balanced XLROutput Connectors(2) balanced ¼"; (2) balanced XLRMaximum Gain60 dB (mic); 42 dB (instrument)Maximum Input Level-5 dBu (mic, pad out); +13 dBu (instrument, pad out)Maximum Output Level+29 dBmFrequency Response20 Hz-20 kHz (±0.1 dB)Dynamic Range124 dBTotal Harmonic Distortion
(@ 30 dB gain with -30 dB in)0.015% (@ 1 kHz)Signal-to-Noise Ratio-120 dB EINPhantom Power48VTubes(2) 12AX7EHFilter200 Hz highpass (6 dB/octave)Pad20 dBMetering10-step LED (input)Power Supplyinternal (IEC jack)Dimensions1U × 10.5" (D)Weight11 lb.
By the time you read this, the Demeter HM-1 will have been upgraded to the HXM-1. The new model, priced roughly $100 more than the original, will incorporate component upgrades, including improved output transistors, an additional capacitor said to extend the unit's bass response, and minor cosmetic changes (different graphics for the front panel and round silver buttons rather than the rectangular black ones).