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Peach Audio M196sx

Stereo Valve Mic Preamp By Hugh Robjohns
Published May 2020

Peach Audio M196sx

Once in a while, a new preamp brings something genuinely worthwhile to the table...

David Peach started repairing and modifying audio gear in the '70s. Having developed a fascination with what made equipment sound good (or not!) he eventually concluded that the power-supply design, and especially the internal grounding arrangements, are critical. That has guided his designs ever since.

Engineering Elegance

The new M196sx dual-channel, all-tube mic preamp is the first of Peach's new rackmounting Savannah range (which will also include EQs and dynamics processors) and it builds on the success of two previous incarnations. Weighing 13kg and extending 430mm behind the rack ears, this 2U device is very substantial — yet it's also elegant and purposeful. Its lilac front panel is littered with silver switches and buttons (in momentary, latching and interlocked forms) but the layout doesn't feel crowded. Contributing to this elegance is that all the controls and buttons are mounted on a sub-chassis behind the front panel; there are no visible screws or fixings. The traditional, old-school engineering approach is continued inside, where the construction is heavy-duty and most impressive.

The input transformers (in mu-metal cans) sit just behind the front panel, on PCBs which host much of the input conditioning circuitry, while the output transformers are contained in a box bulging from the rear. All are made by Harbuch Transformers in Sydney. A substantial toroidal mains transformer on the right-hand side of the chassis sits on a main PCB, which occupies most of the chassis floor and carries the power-supply circuitry and both preamp sections. Each channel employs a 7025 dual-triode (low-noise 12AX7WA/ECC83 equivalent) for the front-end gain, and a 6189 dual-triode (12AU7/ECC82) to power the output, and all these valves are mounted vertically.

Under the hood: the input–conditioning circuitry and input transformers can be seen on the boards at the top, mounted on the controls sub-chassis. All the valves are mounted vertically on the motherboard, which hosts high–quality through-hole components throughout. Relays for switching the output modes and M‑S matrix can be seen on the vertical board at the bottom.Under the hood: the input–conditioning circuitry and input transformers can be seen on the boards at the top, mounted on the controls sub-chassis. All the valves are mounted vertically on the motherboard, which hosts high–quality through-hole components throughout. Relays for switching the output modes and M‑S matrix can be seen on the vertical board at the bottom.

A grounded IEC mains inlet, switchable for 120/240 V, is joined by a sextet of XLRs for the mic and line inputs, and the line outputs. A removable Option Access panel covers a port which will accommodate future plug-in modules, adding functions such as digital converters, remote-control facilities, or a multi-channel summing mixer.

Instruments can be connected via front-panel quarter-inch sockets using normal 'mono' unbalanced TS plugs. Neatly, these sockets also serve as unbalanced insert points, allowing effects pedals or other processing to be applied to mic or line input signals. To access this, a Y-cord (TRS to dual-TS) is required, and it must be used in the opposite way to most applications, as the send is necessarily on the ring and the return on the tip. The insert point is post the input source selection, input transformer, input attenuator and impedance mode switching, so is at an instrument-ish level and has a relatively high source impedance of 5kΩ. The return signal/instrument input goes straight into the first valve's grid, where it sees an input impedance of 2.7MΩ.

Switching the unit on starts a delayed power-up sequence which helps protect and preserve the valves, and avoids unwelcome thumps from the outputs. The VU meters initially light with output Mutes engaged. Five seconds later the (regulated DC) valve heater supplies ramp up, and 20s after that the 400V HT supply ramps up. After another 20s delay, when everything is stable and operational, the outputs are unmuted and the unit is ready for action. Phantom power takes 10 seconds to ramp up, too.

An 'Option Access' section will, in the near future, allow installation of various add-on modules, including an A-D converter.An 'Option Access' section will, in the near future, allow installation of various add-on modules, including an A‑D converter.

Control & Operation

The preamp's two channels operate entirely independently, the only shared controls being the power switch, the metering sensitivity selectors, and a button to engage a Mid‑Sides output matrix. The rotary controls are all high-quality switches. The top-row switches for each channel control the input gain, low- and high-pass filtering options, and output attenuation, while the lower row selects the input source, input pads, and impedance variations. Buttons above the VU meters operate the output mutes and meter sensitivity functions, while those below swap the output polarity, select output configurations, and engage the M‑S matrix.

Mic and Line inputs use the same transformer, so the line input is padded down by 17dB to avoid saturating it. The preamp gain can be adjusted from +18 to +62 dB in 4dB increments, although the maximum gain available depends on the output mode: it's 54.5dB when using the output transformer; and 68dB with the direct OTL output mode. An input attenuator provides up to -24dB in three 8dB steps, while, in deference to Spinal Tap, the output attenuator goes down to -11dB (in 1dB steps). This is useful for gain structuring with other equipment, and permits the input stages to be pushed a little harder.

Very gentle first-order (6dB/octave) low- and high-pass filters have selectable turnover frequencies (4, 8 or 16 kHz, and 20, 40, 60, 80 or 120 Hz, respectively), and the low-pass control additionally offers two extra, mysteriously labelled, options: e1 and e2. These turn out to be inverse filters (Air EQs, if you prefer), introducing a nominal +6 or +9 dB at 16kHz (although the HF boost continues on to over 70kHz where it reaches +20dB).

I say a 'nominal' boost because the HF boost depends enormously on the preamp's gain setting. The lower the preamp's gain, the greater the e1/e2 boost that's applied, presumably because the amplifier runs out of high-frequency gain capability at higher gain settings. But this unusual arrangement allows a variety of tonal effects to be introduced by juggling the input gain, input attenuator and output attenuator settings.

The four-way input-selector switch accesses the option module, the line input, or mic input with/without phantom power (a status LED is provided). Plugging a TS jack into the instrument input automatically overrides all of these sources.

At the time of writing, the user manual and website claimed that 50mA of phantom current is available. I got quite excited, thinking this was the first preamp I've reviewed that uses the new SP48 phantom power standard. (SP48, or Super Power 48V, is an enhanced variant of standard phantom power that provides much more current — up to 44mA instead of just 10mA — and that means more power to the microphone, theoretically allowing more sophisticated mics to be developed). But it turns out that the M196sx provides standard P48 phantom power after all! The 50mA references actually mean that the phantom supply is very robust, with sufficient current capacity and power rating to maintain the maximum current specified by the P48 standard from both channels indefinitely. This is important, as so many preamps, mixers, and interfaces can't actually meet the standard, and that results in increased distortion and transient clipping from the mics. It's good to know that this preamp is bomb-proof in that regard. I measured the off-load phantom voltage at 47.1V, which is well within specification, and it easily passed my full-load test.

The 'instrument' sockets also serve as unbalanced insert points, allowing effects pedals or other processing to be introduced to mic or line input signals.

There are four impedance options for the mic input, labelled A, B, C, and D: the A position impedance of around 880Ω is intended for 50Ω microphones (should you happen to have any still lying around!); the B position's 2.1kΩ is for use with standard 150-200 Ω mics; and the C and D modes employ a different input transformer configuration (paralleled windings instead of serial windings) to deliver 5dB more overall gain while presenting lower input impedances of 240Ω and 610Ω, respectively.

Engaging the input pad changes these impedances substantially and, regardless of the selected impedance mode, the actual impedance falls to about 180Ω with the 8dB pad, 500Ω for the 16dB pad, and 1.2kΩ for the 24dB pad (the values vary ±20Ω depending on the A/B or C/D configurations). The line input has an input impedance of around 600Ω which is independent of the impedance switch and varies only slightly with the pad setting.

If this all sounds confusing, don't worry. Simply use whatever mode sounds nicest! The character of most dynamic mics and some capacitor mics with output transformers is likely to change with different impedances to some extent, but it's a largely unpredictable effect. Mics with transformerless outputs will usually remain unaffected unless the impedance is so low it becomes difficult to drive properly. Choosing different impedances won't damage any mic, though, so you can treat the options as a 'lucky dip' tone control!

Two output configurations are available; the XFMR mode uses an output transformer to provide a symmetrically balanced output, while the OTL mode provides an unbalanced output AC-coupled directly from the 6189 valve's anode. In this latter mode the signal appears on the XLR pin 2, while pin 3 is grounded.

The direct OTL mode provides a maximum output level of +26dBu, but requires a high-impedance destination (a line input, for example) of over 10kΩ. In contrast, engaging the output transformer loses around 10-12 dB of signal level (depending on destination impedance) and so delivers a maximum output level of +18dBu, although it is able to drive any destination impedance down to 600Ω.

This massive 12dB change of level between the output modes could easily catch out the unwary in an unpleasant way, but at least the two VU meters monitor the actual signal level at the XLRs. Usefully, three buttons allow the meters' 0VU mark to be calibrated to +4, +14 or +24 dBu — a helpful option for those who like to fill the digital meters of their DAW when tracking.

As a devotee of Mid‑Sides recording, I was pleased to see an M‑S matrix. The user manual refers to it only as a means of encoding the output to Mid‑Sides, but it can be used equally well for decoding an M‑S mic array to left-right stereo. Unusually, rather than perform this matrixing within the active electronics, Peach chose to achieve it passively by interconnecting the output transformers' secondaries via relays. This means the function isn't available when the OTL output mode is selected. The matrixing introduces around 6dB of attenuation in each output, which is helpful in maintaining headroom.

All Good?

I found the M196sx a joy to use. The knobs and buttons all feel solid and reliable, and there are lots of options for optimising the gain structure and sound character. That said, I do have a couple of tiny criticisms. When viewed directly on axis the output Mute and phantom power status LEDs are visible, but even a small off-axis viewing angle (vertically or horizontally) renders them invisible since they're set back in the thick front panel. Also, given the confusion that could occur if the M‑S matrix is engaged accidentally, I'd like to see a status LED for that button, too.

But all else is good. This is a great-sounding preamp, with an open, fast, and appealing character that can sound either modern or vintage depending on settings. It's not the quietest or most transparent preamp on the planet, but it doesn't make such claims and it's certainly quiet enough for most applications. More importantly, it brings a very attractive warmth and musicality to all sources, especially at higher gains and when using the output transformers. It also has a vast dynamic range and headroom which seems to enhance transients very nicely, even when the gain structure is intentionally abused!

The first-order high-pass filter is useful for taming microphone proximity effects, but it's not really of practical help in removing unwanted subsonics. The HF boost is rather unusual, to say the least, but it's nice — it creates some lovely 'air' EQ effects, though these become less effective at high gains. The low-pass filter options span the gentlest taming of the high-end, to a full-on low-fi vintage dullness, which I could see being a very useful creative effect.

There's a lot to like about the M196sx. It's a classy, thoughtfully designed preamp and while it's not what you'd describe as 'inexpensive', it does offer a lot of features and facilities, as well as a wide range of characters and tonalities.

Alternatives

Narrowing it down to two-channel valve preamps, Thermionic Culture's Petrel and Earlybird 2.2 are probably the closest equivalents in terms of features and facilities, although both are slightly more expensive than the Peach. The A-Designs Audio MP2A and Summit Audio TPA-200B are both slightly less expensive alternatives but have fewer facilities. Other dual-channel tube preamps at a range of prices, again with different facilities, include the Manley Dual Mono Mic Pre, Tube–Tech MP-2A, Traverse Analogue 652, SPL Goldmike MkII, Locomotive Audio 286A, and Gyraf Audio Gyratec IX.

Tech Specs

All EQ options on the same chart: the flat response is shown by the black curves, the five hi-pass options are shown in the red curves, and the three blue curves reveal the low-pass options. Additionally, the e1/e2 HF boost options are shown by the purple curves. The top set of curves was acquired at maximum gain, where the HF boost curves only add a  decibel or so at 16kHz. In contrast, the bottom trace was acquired at minimum gain, and the e1/e2 curves reach +17 to +25 dB at 70kHz.All EQ options on the same chart: the flat response is shown by the black curves, the five hi-pass options are shown in the red curves, and the three blue curves reveal the low-pass options. Additionally, the e1/e2 HF boost options are shown by the purple curves. The top set of curves was acquired at maximum gain, where the HF boost curves only add a decibel or so at 16kHz. In contrast, the bottom trace was acquired at minimum gain, and the e1/e2 curves reach +17 to +25 dB at 70kHz.

My test bench measurements using an Audio Precision test set suggested an EIN figure of around -124dBu (A-wtd, 150Ω source, 20Hz-20kHz bandwidth) which is within a whisker of the published specification, and pretty good for a valve mic preamp. However, I couldn't get anywhere near the claimed THD figure of 0.07 percent. With minimum gain, and the appropriate input level to deliver +4dBu at the output, both mic and line inputs, and XFMR and OTL outputs gave a reasonably consistent THD figure of around 0.16 percent. And at maximum gain (with an output of +4dBu again), the THD figure increased to around a very 'analogue tape-like' 1.2 percent. All measurements produced a broad spread of both odd and even harmonics.

Noise floor at minimum and maximum gain: this Audio Precision plot shows the FFT spectrum of the residual noise floor at both minimum (red) and maximum (blue) gains. In both cases the power supply harmonics remain below -90dBu, which is impressive.Noise floor at minimum and maximum gain: this Audio Precision plot shows the FFT spectrum of the residual noise floor at both minimum (red) and maximum (blue) gains. In both cases the power supply harmonics remain below -90dBu, which is impressive.

Impressively, mains related artifacts were below -90dBu for a unity gain line input, and below -70dBu even with a maximum gain mic input, which is very good for a valve preamp. Crosstalk at 10kHz was also excellent at less than -77dB. The excellent transient response is aided by a bandwidth extending to well beyond 30kHz, reaching -3dB at about 60kHz, and the low-end response is virtually flat to 10Hz, falling to -3dB at around 7Hz.

Mic Input Spectrum: this AP plot reveals the harmonic distortion products (and mains-related harmonics) produced by the preamp. A full set of both odd and even harmonics are present.Mic Input Spectrum: this AP plot reveals the harmonic distortion products (and mains-related harmonics) produced by the preamp. A full set of both odd and even harmonics are present.

Pros

  • Plenty of gain available, with a very musical performance.
  • Useful and unusual filters.
  • Instrument input with unique insert facility.
  • Direct or transformer output modes.
  • Mid‑Sides matrix.
  • VU calibration options.
  • Input-impedance options.

Cons

  • Big level difference between transformer and direct output modes.
  • Recessed LEDs can be difficult to see.

Summary

This beautifully built dual-channel valve preamp offers comprehensive facilities and a classy sound that's full of character.

information

£2999 including VAT.

sxpro.co.uk

peachaudio.com

Published May 2020