Croft Series 5 valve power amplifier

This is a budget audiophile valve amplifier from Glenn Croft, who is better known for his Futterman-inspired OTL amps and his cigar-box valve preamps from the 80's.

The styling of the Series 5 has improved hugely since its early days - my ten-year-old pair are from the original generation of industrial black bricks, but these days the amp (now Mark III) comes in a more domestically acceptable finish. It also comes with a proper mesh enclosure for the valves, which probably brings it almost up to EU safety regulations¹.

Back in the early nineties, the Series 5, along with the more wacky Audio Innovations amps, formed the entry level to valve equipment in the UK. Croft's designs offered exceptional value for money, with the Series 5 having four EL84s per channel and a fully regulated power supply with a 6080 regulator, giving a clean 25-30W per channel for just £500.

Here is the circuit, with the component values from the first one I bought (SN. 142).

The Sound

The Series 5 replaced a Quad 306, which was a solidly-designed and good-sounding amplifying brick, and which I was partnering with a Croft Super Micro preamplifier. I'd already heard, and been very impressed by, an Audio Innovations valve system. Although I had no particular complaint about the sound of the combination of Croft and Quad - in fact it could sound extremely enjoyable - I couldn't help wondering what an all-Croft system would sound like.

In KJ West One the junior Croft power amp, the Series 5, sounded very lush, with an expansive soundstage, and generally very seductive compared with the competent but slightly flat sound from the Quad. The only weak point of the valve amplifier was a slight lack of dynamics at higher volumes - I guessed that this was a consequence of the largish demonstration room, and reckoning it ought't be a problem in my living room at home made the decision to buy one. Indeed, I was very happy with the sound in my own system. In some ways I am an objectivist in audio matters - any audible difference must be at least in principle measurable, and any amplifier whose output deviates from an exact copy of the input is by definition inaccurate. I guess the little Quad amp was in most technical senses more "accurate" than the Croft. However, music reproduction at home is above all a matter of personal taste, and for me a system that makes music unsatisfying or even unpleasant to listen to (and I've heard plenty) is unwelcome in my living room. Not that the Quad was unpleasant, I have to say - just that it sounded a little bland and next to the Croft. The Series 5 is one of those components that always makes listening a pleasure, having sins mainly of omission.

After I'd had my first Series 5 for a couple of years, I decided to buy a second so I could passively bi-amp my speakers. At the time I was using a pair of Shan Shimnas, a clever and original design by a now-defunct company based near Belfast which used a non-rectangular cabinet made of a concrete/resin composite. These were limited in their ultimate performance by the limited power handling and lack of deep bass, as well as by the mediocre drivers, although they were still excellent value for money and sounded remarkably un-boxy. These were quite insensitive at 84dB /1W /1m, and the Crofts were audibly straining at higher volumes. Adding a second was quite a success, as it led to a much more open and effortless sound.

Modifications

Soon after I bought the second amp I thought I'd check inside the amps to see if they were the same - this seemed to be a priority if I was going to use one for each stereo channel. To my surprise, despite the similar serial numbers, I noticed that each amplifier had different values for the the coupling capacitors, though the major hardware such as the transformers seemed to be identical. I took the opportunity at this point to rebuild both amps, upgrading most of the resistors with Holco H2 and H8, and the polypropylene capacitors with Kimber Kaps. I also bypassed all the electrolytics with polypropylenes.

I carried out the modifications on one amp at a time so I would be able to hear the difference. It turned out that the modified Series 5 sounded much sharper and cleaner, with the standard one being to me a little too warm and muffled in comparison, although an audiophile friend whose opinions I trust still preferred the original. The amps are now definitely more revealing and transparent, although not inconveniently so. Moving up from the Shans to ProAc Studio Ones gave a big increase in scale and bass, and this turned out to be a combination I was happy with for several years and which saw me through several improvements to preamplifier and source components.

The one drawback to the Series 5 is its unusually high sensitivity: I measured its voltage gain to be about 63, or 35dB. My guess is that this is because Croft preamps tend to have unity gain in their line stages, but many preamps have a line stage gain of between 10 and 20dB. This is isn't a problem in itself, but used with a preamp with higher gain, and especially one with a linear volume control, the usable portion of the volume control will be pretty limited.

Here is Ken Kessler's enthusisatic review of the Series 5.

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¹Safety note: my Series 5s are powered from a two-core mains lead, and so rely for their safety earth on the connection to the preamplifier. Clearly the amp isn't intended to be used with nothing connected to its inputs, but I'm sure this practice contravenes several electrical regulations. You can get away without an earth wire for a plastic-bodied vacuum cleaner or a table lamp, but an un-earthed steel box with 240V AC, 350C DC and a handful of glass bottles on top does make me nervous. I assume that more recent Croft designs are up to conventional electrical safety standards.