Foreplay III Line Stage Kit

Our most popular kit for ten years was the original Bottlehead Foreplay stereo tube line stage. Thousands of Foreplays are making sweet music in the homes of audiophiles who otherwise might never have experienced the delight of making one's own tube audio gear. The outrageous success of the Foreplay allowed it to stay relatively unchanged for ten years, with only a small upgrade to Foreplay version 2.0 few years ago. But we're experimenters, and we've come up with so many cool ideas since the original Foreplay that we just had to incorporate them!  Enter Foreplay III -

photo of prototype - production version may appear slightly different

The circuit

The classic Foreplay circuit has been favorably compared to much more expensive circuits. The new design reaches an even higher level of sonic pleasure and quiet operation. The original Foreplay's 12AU7 operating points were somewhat limited by the small power transformer used. The new Foreplay III uses 12AU7s running at optimized operating points for lower distortion and beefier output drive - 10V rms before clipping! DC is now supplied to the heaters for super quiet operation. And the most visible change is the addition of a OD3 gaseous shunt regulator tube. This shunt regulator gives a more quiet background and a sense of extended response at both ends of the frequency band. The combination of these new features yields a line level zero feedback all triode tube preamp with a 90dB signal to noise ratio at 2 volts output. It's an excellent choice for use with our Seduction Phono Preamplifier and our Paramour and Paramount amplifier kits.

Wiring is all point to point with Teflon insulated wire and solid core shielded twisted pair cable. The high voltage power supply is an efficient solid state ultrafast soft recovery voltage doubler feeding a C-R-C-R-C filter, which in turn feeds a 0D3 shunt regulator in parallel with the preamplifier circuit. The DC heater supply is a full wave bridge of super quiet Schottky rectifiers feeding a resistor input cap filter that can be adjusted for the heater current draw of various preamplifier tubes. The preamplifier circuit is 1/2 of a 12AU7 voltage amplifier direct coupled to 1/2 of a 12AU7 as a cathode follower output. Only one output coupling capacitor is in the signal path of each channel - a metallized polypropylene type. Resistors are a combination of high quality metal film, carbon film and carbon composition resistors, each chosen for optimal performance in the intended application. Filter capacitors are overrated for long life. Switches are enclosed for protection from contact corrosion. Tube sockets are high quality ceramic. Knobs are phenolic, with the feel of classic studio console knobs of the last century.

The input circuitry has been improved as well. Three gold plated RCA inputs are available, and both the hot and ground connections are switched for quiet operation. The stock dual mono attenuators are now our tried and true Bottlehead Sweetest Whispers 11 position stepped shunt attenuators, which come with padding resistors to allow you to get all mismatched source levels nearer to the same volume settings. There are two sets of gold plated RCA outputs, to allow one to feed a subwoofer without the need for a loop through the sub amp to feed your speaker amps, or to allow one to set up a tape monitor loop.  

Maximum output is 10V rms (28V pk-pk) before clipping. Gain is 10 dB. Signal to noise ratio is 90 dB at 2V rms output (a typical CD player max output level), and 114 dB at 10V rms output. Input impedance is 33K ohms-48K ohms (variable shunt attenuator).  Output impedance is 700 ohms - recommended output load is 10K ohms or greater. Frequency response is +/- 0.5 dB from 2Hz to 50kHz. Phase is inverted.

We consider this a Skill Level 2 kit - assembly can be accomplished in a few evenings, and is tailored for the builder with maybe a kit under their belt or for a first timer who is looking for a bit more challenging project. A very thorough assembly manual is included in printable Adobe PDF format on CD ROM. It contains step by step instructions you check off as you go, detailed photographs of the assembly that can be blown up for great detail, and a detailed checkout procedure, along with tips on soldering and tool selection. All the builder needs to supply is solder. You will need a copy of the free Adobe Reader software to use the manual. 

Extended Foreplay Upgrade Kit

Everyone knows that Foreplay is best when given your most lavish and prolonged attention, yes? Here's our our our recipe for a most delightful Foreplay experience. The Extended Foreplay upgrade kit takes your Foreplay III to new levels of ecstasy, with the addition of the following new toys:

Dual mono power supplies fed from separate high voltage Bottlehead PT-3 power transformer secondaries, ultrafast soft start full wave bridge rectification and Panasonic CRCRC filtration

Paul Joppa designed dual hybrid 12AU7 tube/TL431 shunt regulators with Camille Cascode Constant Current Source active loads

Camille Cascode Constant Current Sources (C4S) on each stage of the direct coupled, buffered output preamplifier circuit

Auricap 2.0 uF polypropylene film and foil output capacitors. 

The standard Foreplay III kit with it's gaseous shunt regulator is generally recognized by the Bottlehead community to be a significant improvement over the most heavily modified of the earlier versions of Foreplay. We feel that this new dual mono hybrid shunt regulator and constant current source upgrade takes the Foreplay III to an even higher level of resolution and sonic realism. The extensive upgrade package sonically equals or exceeds every previous line stage design we have published. The audible improvements over the stock Foreplay III include increased resolution; more articulate and dynamic bass performance; a sense of greater extension at both ends of the audio frequency spectrum; and a sense of a complete lack of electronic artifacts during quiet passages, often called a "black background" in audiophilespeak.

The kit is installed after the stock power supply, shunt regulator, and plate and cathode load components are removed from the standard Foreplay III. Two new power supply filters are installed, a new tube socket is installed for the dual triode shunt regulator, and three circuit boards are populated and installed, which contain the constant current source and hybrid shunt regulator components. 

Queen Eileen's personal Extended Foreplay III, showing the centrally mounted dual triode shunt regulator. Chassis plate is finished in Charcoal Hammerite spray and power transformer bell end has been polished and blued with Birchwood Casey Super Blue. 

(Yes, she built this herself! We welcome women and men alike to the Bottlehead DIY audio community!)

We consider the Extended Foreplay kit to be a Skill Level 3 kit - it has a lot of parts, requires some disassmebly of the existing circuit, and you have to stuff several PC boards. It's perhaps not for the first time builder, but eminently approachable by a builder with a kit or two under their belt, including the stock Foreplay III. Detailed instructions with high resolution photos will be included in Adobe Acrobat .PDF format, on CD-ROM. 

Price of ther Extended upgrade kit alone is $349 plus shipping. The discounted package price for the standard Foreplay III kit with the Extended Foreplay upgrade kit together is $754 plus shipping, a savings of $84 over the kits purchased separately.

Is Foreplay III really good enough for your system?

If your system is more exotic and complex than the one below, give us a call and we can discuss compatibility. Otherwise, we're pretty sure Foreplay III will fill the bill.

Photo of a biamped surround sound system showing three of five TAD Reference One loudspeakers driven by five Pass Labs amps and four Bottlehead Extended Foreplay IIIs, fed by three synced MSB DACs for digital sources and a Tape Project reel to reel player for analog, at CES 2007. 

Foreplay III vs. earlier versions

Some commentary by Bottlehead's circuit designer, Paul Joppa-

Posted by Paul Joppa on April 28, 2005 at 11:22:35

In Reply to: Foreplay 1/3 posted by naimman on April 28, 2005 at 10:20:16:

How good is the ForeplayIII over Foreplay1 or 2?

I think it is a significant improvement, as it incorporates many of the community's "lessons learned" over the 10 years the previous version was in production. The improvements are incremental - it's still the same classic topology and tube.

 ...can they stand-up on their own without "field modifications"- those not sold by the manufacturer?

Certainly. Many of the custom modifications are simply parts replacements; boutique parts can make a difference but it is not a deal-breaker. Often it is a matter of tuning the sonics to one's personal taste, like tube rolling. (I know I'll get some argument here!) The Foreplay, by virtue of its simplicity and low price, has always attracted extensive modifications; Bottlehead has generally supported those we thought most worthwhile.

What is the purpose of the OD3 gaseous shunt regulator?

The Foreplay uses a cathode follower output stage, like many tube preamps. In a cathode follower, all of the output signal current flows through the power supply. The shunt regulator is a superior power supply, more stable (especially at lower frequencies) and more linear. It contributes probably the largest part of the improvement over the previous versions. Better shunt regulators are possible, but this one is inexpensive, reliable, and gets the job done.

Also seen owners posting problems with hum - is this a design or construction problem?

Hum and buzz make a never-ending battle, which is especially difficult in the absence of negative feedback. The original Foreplay (versions I and II) have been perfectly quiet for a large majority of users, but it did not have a large performance margin - it originally sold for $99, after all! Given the large number sold, there were inevitably a number of situations where the hum or buzz was audible, and those are extensively discussed in the archives. It does lead to an unbalanced perspective! These are typically cases where the following amplifier/speaker is especially sensitive, or the environment has an unusually large amount of radio-frequency interference, or the associated equipment induces a ground loop, or the particular tube is prone to heater-cathode hum coupling.

In the new Foreplay III, we have reduced the hum and buzz considerably, probably 20dB, so that it will work well in a much wider variety of situations. The heaters are powered with DC, and the heater supply bias adjusted for optimal operation; all the signal lines are shielded; the power supply is regulated; source grounds are switched as well as signal lines; a new power transformer is used which has a form of self-shielding for external electrostatic fields and produces a smaller external magnetic field.

Any fine tuning involved?

The gain for each input in the Foreplay III can be adjusted by choosing the right resistor, so that all the various sources will use pretty much the same volume setting. Depending on the signal output of your source devices, and the sensitivity of your amplifier/speaker combination, choosing the right value may take an iteration or two. Some source devices produce enough ground loop current to create "pops" when switching due to the switched grounds; there is a modification to alleviate this problem, at the expense of slightly increasing the chance of ground loop hum. The preamp comes with serviceable old-stock tubes, but many - probably most - will try a few different brands to optimize the sonics for their system and ears. Those are the only "fine tuning" items that I am aware of.

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Having said all that, I would like to note that the Foreplay III was intended from the outset to be amenable to extensive modification and experimentation. The power supply has a very large margin so that different tubes can be easily used, and the tube sockets are designed with adapters so that larger octal sockets can be used. One of the most popular modifications is swapping output capacitors; the "capacitor of the month club" has many members! So the output capacitor and its solder lugs are readily accessible.

Foreplay III complete - WOW!

Posted by GlynnW (A) on February 04, 2007 at 19:24:47

This weekend I assembled my new Foreplay III and it is MUCH better than my Foreplay II with all the standard mods. Anyone who has been on the fence should pony up the dough and get in line now. And by the time I am used to this, my extended kit will show up and off we'll go again. Thanks to Doc and Paul for continuing to raise the bar.

 Foreplay 3 Extended Musings

Posted by John EH (A) on June 30, 2006 at 14:43:01

Well, I've built 3 Foreplay 2's, a set of Paramours, a SEX amp, and now 2 Foreplay 3's, the latest with the Extended version. Tack on the Whispers, Sweetest Whispers, Anticipation, and just about all the popular tweaks makes me not a total novice to this.

I really enjoyed building this kit. Even though it's designed to be built into an existing, built Foreplay 3, it's certainly easy enough to build it in from the beginning. It only requires the builder just looking at the "Extended" manual and highlighting the stuff in the "Regular" manual that gets torn out during the upgrade.

When you get to that step.......Just don't do it and move on. Finish the Foreplay kit. Then start with the Extended. In many respects it's easier to build it all at once as you get sweet access to the tube socket and you don't have to take apart the old power supply.

I'm an easy sell on Bottlehead gear but I'm floored with the sound of the Extended kit. Just to convince myself I've swapped the Foreplay 3 original in and out a couple times and there's almost no comparison.

The Doc has time and time and yet time again proven that GREAT sound need not come at ridiculous expense. And that boutique parts, while nice, aren't absolutely required to get some of that great sound. And he's certainly proven that tubes are nice but solid state power supplies and rectifiers and constant current sources have a place. Total tube purists are going to get an education after listening to this gear.

I think I've heard Dave Brubeck "Take Five" a thousand times but I think I just heard it for the first time here today. What an amazing preamp. Sometimes I think there's such a thing as "break in" and sometimes I think there's no such thing. All I know is that if this kit "breaks in" and gets any better then I'm in for a treat. And if it doesn't. I'm still in for a treat.

What a great bargain, what a great deal of fun, and what great sound.

John

Posted by John EH (A) on June 30, 2006 at 19:19:17

I've been running this thing non stop and have been impressed but I've got to tell you again that the improvement over the first incarnation of the Foreplay 3 is NOT SUBTLE.

In my opinion this is an incredible upgrade. I'm really impressed with all the aspects of it's performance and I hate to get too specific for fear of sounding like an audiophool but this is simply amazing and not a small improvement.

One of my favorite CD's is Bia "Sources", it's a pretty obscure CD and I'm not sure what you'd call the genre, it's a Latino gal with a silky smooth voice but I swear she's singing in French or one of the other Romance Languages, and she sings in English as well. It's a great CD for putting your gear through the paces and this Foreplay 3 Extended is passing with flying colors. I figure my LS3/5A's may have something to do with it as well but wow. Everything is in clear focus. Bia is right in the middle surrounded by instruments There's some harmonica and accordion stuff going on and it's freaky amazing with some Spanish guitar licks going on and I haven't been this excited about my gear in a while.

There's only one problem. Doc will improve this design and I'll have to spend more money.

John