Cartec EQP1A

Mono Valve Equaliser

Published in SOS October 2010
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Reviews : Microphone

British 'boutique' outboard manufacturers seem to be rather thin on the ground these days, but if this Pultec clone is anything to go by, newcomers Cartec look set to make a big impression.

Hugh Robjohns

Cartec EQP1A

The original Pulse Techniques Inc (better known to all simply as Pultec) Model EQP1A programme equaliser, introduced in 1951, is something of a legend in the pro‑audio world. So much so, in fact, that pretty much all the hardware manufacturers that still work with valves — Manley, Summit Audio, Tube-Tech and so on — have a version of this classic design somewhere in their product list, and so do many of the software plug‑in makers. And for those with a soldering iron and the desire to wield it purposefully, the 'interweb' is full of schematics, kits and discussions about various DIY versions too, many of them being extremely good incarnations.

Moving things up to date, the focus of this review is the new Cartec EQP1A — a British‑made 'clone' of the Pultec, developed principally by Liam Carter. The use of the term 'clone' is always fraught, as many will argue that a clone must be built with identical components in an identical way. By that definition, the Cartec isn't a clone — it uses modern metal oxide resistors instead of the original's carbon components, and Sprauge 'orange drop' capacitors instead of the paper‑in‑oil (PIO) types that had to be used when the original Pultec was manufactured a lifetime ago. Moreover, the transformers and inductors installed in the Cartec are all bespoke designs from the Sowter company's workshop, instead of the original American Triad and Peerless models.

So, bearing all that in mind, it might be less argumentative to call this a 'modern recreation', which uses modern (but accurately designed and carefully chosen) components... But in all other respects, the Cartec EQP1A is, indeed, an extremely faithful 'recreation'. Notably, it adheres absolutely to the original's signal path, power-supply configuration, panel layout, and even the design of the 'Daka‑Ware' control knobs, and — so it is claimed — provides a very, very, very(!) close rendition of the sound character of the original, with slight, but useful, improvements to the technical specifications, thanks to the improved performance of modern components.

Overview

Internal view of the Cartec EQ showing the three transformers, one tapped inductor and the three valves, all mounted on a fibre‑glass 'circuit board' — albeit a rather unconventional one.Internal view of the Cartec EQ showing the three transformers, one tapped inductor and the three valves, all mounted on a fibre‑glass 'circuit board' — albeit a rather unconventional one.

I was supplied with a pair of Cartec EQP1As for the review, each with a lovely 3U metallic-green front panel engraved with clear yellow legends in the thick aluminium plate. The controls are all classic Pultec‑styled knobs, laid out in the original arrangement — and even the 'handbook' is an amusing homage to the original's data sheets. The unit extends 155mm behind the rack ears, and has ventilation slots on the case lid, so leaving a 1U breathing space in the rack above the equaliser would be a good idea.

At the extreme left‑hand side, a bypass toggle switch sits below the bass controls. A four‑way rotary switch determines the bass EQ's centre frequency (20, 30, 60 or 100Hz), with Boost and Attenuate rotary controls above, scaled simply 0‑10. These gain controls can be used individually or together to create a range of EQ responses (see the test plots). The attenuation slopes extend slightly higher in frequency than the boost slopes for the same frequency settings — which is a significant part of the characteristic sound of the Pultec. When the Boost and Attenuate controls are used individually, the result is a gentle shelf EQ boost (up to about 14dB) or cut (down to about ‑18dB) with something approaching a modest 3dB/octave slope. However, the fun starts when these controls are used together, as the boost and attenuate filters interact to produce a characteristic 'resonant shelf' response. The key feature of this is a mid‑frequency dip (up to 8dB deep) before the LF shelf boost starts to build (see plots again). This response characteristic helps to bring important clarity to bass instruments, and is a key part of the magic and charm of the Pultec sound. You can achieve a similar effect with conventional EQs by cutting an octave or so above the boost frequency... but it happens automatically and in a distinctive way with the Pultec.

This chart demonstrates how quiet and clean the Cartec is, with a mild tendency towards third-harmonic distortion.This chart demonstrates how quiet and clean the Cartec is, with a mild tendency towards third-harmonic distortion.The right-hand side of the control panel deals with the high-frequency EQ response options, and although the controls look very similar, they actually work rather differently. The HF Boost control operates in the same way as the LF boost control, with a seven‑way rotary switch to select the centre frequency (3, 4, 5, 8, 10, 12, or 16kHz), and a rotary control to adjust the boost amount (up to 18dB). However, the adjacent HF attenuate control works differently to the bass section. Rather than operating at the same frequency settings as the boost control, it operates instead as a shelf‑cut equaliser. It provides up to 16dB of cut at one of three frequencies, which is selected on the adjacent three‑way rotary switch (5, 10 or 20kHz). As before, the boost and attenuate sections can be used individually or in concert, the latter producing some spectacular dip‑before‑peak responses (see plots yet again).

The final front‑panel control is a rotary mains power on‑off switch, accompanied by a jewelled power light. The rear panel carries just a pair of clearly labelled XLRs for the line-level input and output, and the usual IEC mains connector with integral fuse holder and a separate mains-voltage selector (115/230V AC).

Construction

The maximum HF boost responses at each different centre-frequency option, and with both the sharp (higher) and broad (lower) bandwidth settings.The maximum HF boost responses at each different centre-frequency option, and with both the sharp (higher) and broad (lower) bandwidth settings.

The Cartec EQP1A is a very expensive equaliser by anyone's standards, but the external finish and detailing of the latest production batch is superb, and the internal construction is neat and to high standards, despite this being a hand‑made, small‑volume product (it is CE certified, of course). On removing the lid, it's obvious that the EQP1A is assembled with care and attention to detail, as demonstrated by the fact that the screens (pin 1) of both XLR connectors are linked together and directly to the chassis. In other words, the cable ground goes nowhere near the audio circuitry and, not surprisingly, the Cartec passes the 'Windt Hummer test' with flying colours! (see box below for more on this).

A lot of the marketing and press releases for this product claim 'point to point' wiring in the Cartec — and this is the typical approach in vintage and vintage‑styled valve equipment. The 'points' are usually isolated metal 'tags' on paxolin strips, bolted to the internal chassis, where a number of components are soldered together to form a circuit connection. However, the current production versions of the Cartec don't employ this old‑school construction technique at all, and a cursory inspection of the internals would suggest a more modern PCB is employed instead... casting some doubt on the point‑to‑point wiring proclaimed in the publicity bumf.

A family of bass-response curves at maximum settings for each available frequency.A family of bass-response curves at maximum settings for each available frequency.However, closer inspection reveals that the single, large PCB that carries most of the components and valves is not a PCB in the conventional modern sense at all. Although the fibreglass board is clad in copper on both sides, there are no conventional circuit tracks as such. Instead, the components are actually connected together in point‑to-point fashion, via isolated islands of copper on the underside of the fibreglass board — which is why the component layout appears rather haphazard and untidy compared with a modern PCB design. There are no copper tracks linking physically spaced components together (as there would be on a conventional PCB design), and large areas of the PCB are employed for ground plane screening. Clearly, this arrangement works well at a practical manufacturing level, and Cartec claim that it also brings benefits as far as the technical specifications are concerned.

The rotary switches on the front panel are high‑quality Grayhill types, and the continuous rotary controls are Precision Electronics Corporation (PEC) devices, all wired back to the circuit board with individual wires, loosely loomed together where appropriate. PEC was previously known as Allen & Bradley, and was an original supplier to Pultec.

Listening

This plot shows the responses for flat, maximum bass-boost and maximum attenuation (at 100Hz). The fourth line is the result of having both maximum boost and maximum attenuation selected at the same time, and it reveals the characteristic dip before the peak.This plot shows the responses for flat, maximum bass-boost and maximum attenuation (at 100Hz). The fourth line is the result of having both maximum boost and maximum attenuation selected at the same time, and it reveals the characteristic dip before the peak.

I realise that the internal construction of a product is of little interest to some people, and for many only the sound is important — so I'm pleased to report that the Cartec EQP1A sounds unequivocally fantastic. A lot of complex and interacting factors determine the sound quality and character of a design like this — the quality of the three audio transformers, the HF inductors, the capacitors, the way everything is laid out, the power-supply design... Clearly Liam Carter has put in a huge amount of meticulous development time to ensure this product is as faithful to the original sound as it possibly can be, while modernising and improving the physical construction, with consequential benefits in noise performance. As far as I'm concerned, the latter is a bonus and detracts nothing from the former. This is more than just an 'homage' to the original: I don't think it is overstating the case to suggest that this is what the original would look and sound like if it were still being made today — which is about as flattering a critique as I could give any 'clone' product.

This plot shows some of the HF response curves achievable at extremes of frequency and gain/attenuation settings.This plot shows some of the HF response curves achievable at extremes of frequency and gain/attenuation settings.The overall sound character of the Cartec EQP1A is full and slightly warm, in a musically flattering and supportive way, with a wide, open bandwidth, which lets it sound natural and open at the high end, with only the most subtle hint of extreme HF roll‑off. The noise floor is commendably low and entirely hum free, and the EQP1A only starts to sound a little crunchy and dynamically compressed when driven with exceptionally high input levels — such as when feeding it directly from maxed D‑A outputs running in excess of +24dBu peaks.

The bottom end is smooth and rich, and with the combined boost/attenuate controls it is simple to add weight and scale to the music — whether on single source or across an entire mix — but without muddiness or veiling. In fact, Cartec produce the EQP1A in matched stereo pairs specifically for mix‑bus applications. At the high end, the two higher-frequency boost settings bring a classic‑sounding brilliant sheen to a mix, which quickly becomes totally addictive. Like the original equalisers, the boost and cut controls become progressively more effective as they are increased in a kind of logarithmic way, rather than behaving entirely linearly. The narrower HF bandwidth settings need to be handled with some caution, as they become biting and quite aggressive‑sounding pretty quickly, but where you need to add some impact and edge to an individual source within the mix, this is a good tool to deliver that result without unpleasant side‑effects. The high‑shelf cut section is a really useful tool for taming and smoothing slightly harsh, rough or aggressive sounds too, without drawing attention to the fact.

Even Better Than The Real Thing?

The original Pultec earned its reputation because of the way it delivered a desirable sound: a full but musical bottom end, with a sweet and open high end, while providing creative, natural-sounding tonal shaping. It has a character, even with the controls set flat — and the Cartec EQP1A delivers exactly the same qualities, character and overall tonality, but without the control crackles and inconsistencies of a vintage unit. Furthermore, it does all of this with a quieter and smoother background noise floor. Yes, it's an expensive product, but there's no doubt you get what you're paying for, and the price is still very comparable with the best of the alternatives.  .

Alternatives

No one hides the Pultec influence from their equalisers, and the closest alternatives that spring immediately to mind include the Manley EQP1A, the Summit Audio EQP200B, and the Tube Tech PE1C — although there are numerous others that bring variations to the same general theme.

The Windt Hummer Test

Ground loops are an inherent part of any audio installation: the loop is formed by the mains safety ground of one device, the earth wiring of the building, the mains safety ground of a second device, and the screen of the audio cable(s) passing audio from one unit to the other. A small difference in the voltage potential of the mains safety earths (which can often be a few volts in a typical building) will inevitably cause a small current to flow around this loop, and if that current finds its way into the audio circuitry it will manifest as an audible hum or buzz. Thankfully, in a well‑designed product such ground currents aren't able to get into the audio circuitry, and the Windt Hummer test is a very simple way of checking whether a product is susceptible to ground currents or not.

The eponymously named test was described by John Windt in an AES paper over a decade ago, and basically involves deliberately injecting a modest AC current directly into the ground circuitry of the product under test. This is achieved in practice with a modified wall‑wart power supply, the output of which is connected between the equipment chassis (which is connected to the mains safety earth) and the cable screen connection of each audio connector. If there is a connection between the two, a ground current will flow — and if that current finds its way into the audio circuitry, a hum or buzz will be heard, and the measured noise performance of the equipment will be degraded.

This is a very fast, effective and simple way of checking whether a product is susceptible to ground‑loop problems without having to dismantle it and examine its internal construction — and it has been a standard element of my technical review process for some time now. For more information see www.jensen‑transformers.com/as/as032.pdf.

Cartec's Muse: The Pultec EQP1A

The original Pultec EQP1A was the result of company founders Eugene Shenk and Ollie Summerland developing a 1940s passive filter design they'd licensed from Western Electric, one of the founding fathers of professional audio. The key feature that Shenk and Summerland brought to the original design addressed the inherently 'lossy' nature of all passive equalisers (the output level is always lower than the input level): they integrated an amplifier stage after the equaliser, providing a modest 20dB or so of gain to restore unity gain when all the controls were set for a flat response. It is this that accounts for the 'A' after the EQP1. That's not exactly rocket science, I know, but it was an important step forward in studio practicality for the time, and the EQP1A remained in production for nearly 30 years. Pultec EQP1As are now very highly sought after, and they attract huge prices when they become available.

For those interested in the technology, the Pultec design employs no fewer than three transformers in the signal path: there's an input transformer to unbalance the input signal and drive the passive EQ section; there's an inter‑stage transformer to couple the unbalanced output of the passive EQ section to the balanced amplifier stage; and, finally, there's an output transformer to isolate the amplifier stage from the outside world and provide the ability to drive the constant‑impedance 600 Ohm balanced lines that were part and parcel of professional audio at the time. There's also a bespoke multi‑tapped inductor (which looks physically like another transformer), and this is used to provide the different high‑frequency EQ response options.

The simple dual‑valve amplifier stage is, unusually, fully balanced throughout, and operates in a classic push‑pull arrangement. Both valves are dual triodes, the two halves of each being used in push‑pull mode to handle each half of the balanced audio through each gain stage. The input section is built around an ECC83 (12AX7), while the output transformer is driven by an ECC82 (12AU7). Negative feedback is derived from an extra secondary winding on the output transformer — so that transformer distortion and some output loading effects are automatically compensated.

The power supply of the original design employed a 6X4 valve rectifier and delivered the best part of 350V DC from the supply's reservoir capacitors, this voltage then being reduced to about 290V for the ECC82 anodes and around 140V for the ECC83 anodes. Most of the modern Pultec recreations — both commercial and DIY designs — abandon the valve rectifier for more modern solid‑state alternatives, although many would argue that this affects the audible characteristics of the equaliser in subtle but important ways, because they don't exhibit the rail voltage droop that afflicted the original valve‑rectifier design and provided a degree of built‑in audio compression! It's also worth noting, though, that Pultec later produced a version of their equaliser with a solid‑state (operational) amplifier: the EQP1A3.

Cartec EQP1A £1995$2895
pros
Modern Pultec clones don't come any closer than this.
Beautiful to look at, and to use.
Superbly musical and involving sound quality.
Lower noise floor than the original Pultecs (and many modern clones).
Exact circuit topology, including the power supply.
cons
This level of precision, and this many transformers, make it costly.
summary
A very authentic Pultec 'clone' using modern materials but skilfully woven into an accurate circuit recreation to deliver exactly the right sound and character.
information
£1995 each including VAT.
Cartec +44 (0)161 408 2808.
$2895 each.
Cartec +44 (0)161 408 2808.

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