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An Innocent $99 Preamp Kit... called 'Foreplay'

Part 2; in which the protagonist learns to his dismay that tubes are cool and he really can build his own preamp...

I must have one of the most understanding 'spouses' around for this sort of thing. My partner Blithe is incredibly tolerant of speakers in the middle of the floor and tweaky looking cables running everywhere. After I found the Foreplay kit online, she bought it for me as a Birthday present back in September, though with the house renovation and upgrade parts ordering it was 2 months before I was able to sit down to start assembling it.

This was my first foray into the world of DIY electronics; while I was with SpectraLogic we built a lot of cool microelectronic systems and even debugged some RF problems some of our consulting clients were experiencing, but I was totally dependent on the engineering staff to keep me out of trouble. I had a great time learning the practical theory and techniques to build this preamp, and would highly recommend it as a starter project to anyone who is interested in trying something like this. Doc and the folks at bottlehead.com are really helpful and supportive of 'tube newbies' and have put together a great kit here with clear instructions and lotsa room for growth.

Jonathan Kranz' Bone Stock Foreplay

Despite the fact that this was a first project, ambition got the better of me and I ended up changing quite a bit in my design from the original stock Foreplay shown at right. But this also gave me a chance to learn many new things, and everything came out just fine. That's probably more to do with the excellent design Dan Schmalle (AKA 'Doc Bottlehead') has provided than with my own blind gropings with the technology. The Foreplay has a really simple and direct circuit, so it's possible to experiment easily with it and not get in too deep.

I'm sure that Doc would want me to point out that his advice on making gradual changes is really sound; don't do as I did (repeatedly) and make too many alterations at once, else it gets to be very difficult to hunt down problems that do crop up. It's always easier to evaluate a single change than a whole chain of things.

Disclaimer:
Tubes mean big voltages. Big voltages can stop your heart and kill you. Be careful! Follow all the suggestions of folks who are wiser than me and don't even try this if you don't know what you are attempting. Be responsible and don't sue me, OK?

In this page:

Oooo, the Preamp.
Top view: Electronic Tonalities 'Foreplay' preamp, pRC signature model.

Inside View, lotsa discrete electronics.
Bottom view:
Yup, nothing between you
and all those pretty parts.
Be careful!

Bypass caps in the high voltage power supply section.
Detail: Bypass caps in the high voltage power supply section.
(Snubber on Rectifier input, Dale wirewound resistors, Teflon wire insulation, Star Ground at negative of 3rd capacitor, Ferrite beads on filament supply, Sorbothane layer backing top plate)

 

So. What's the big deal?top arrow

1) It sounds great
2) I built it myself!

Seriously, there is great satisfaction in actually feeling like you have mastered some small piece of the technology that surrounds us. While I cannot/will not say that I've mastered the art of preamps, I do have a fantastic sounding addition to the 'audiophile' system in our house, and (I think) a much better grasp of what it takes to make a good-sounding design.

One of the benefits of tube designs is that they are simple. If great truths are simple, then the Triode has got to be one of the coolest and greatest truths ever made by humans. The simplicity of the triode makes it much easier for a beginner to build a successful circuit, and to take faltering steps towards making it better.

I won't try to explain the basic design as I'm sure others, Like Doc, can do a better job, but if you are interested, you can read more about the changes I made below.

 

Hrm. What did I change?top arrow

Vibration Control
With a background in industrial design, this was one part of the modifications that I felt really comfortable altering from the very beginning of the project...

Front Volume Controls
Eventually a turntable will be used on top of my stand, so I needed a way to adjust volume from the front of case. This was a pretty easy feature to add and also improved the rigidity of the chassis plate. The volume pots are the same distance from the tube sockets, so the wiring lengths and routing are the same as the top mounting. Also makes it easier to operate, since the original design has the knobs really close to each other to reduce signal paths to a minimum. Be careful when tightening the bolts if you use a dampening layer, you'll see the chassis plate start to warp when they are too tight...

Upgraded Parts
I added a range of 'boutique' parts to improve the sound of the kit. This is an easy, yet sorta lame way to improve the sound by basically throwing money at it. The metal film resistors that Doc supplies with the kit are pretty good actually, but the coupling caps and connectors can be improved a lot. This circuit is so simple that it really isn't very much money to go whole hog on the resistors too.

Re-routed Ground Bus
Doc admits that the ground scheme for the Foreplay is designed for easy construction and can be improved; there have been several discussions on this on the Bottlehead Forum. I went to a hybrid Star Ground method, isolating all of the grounds from the chassis except at a single filtered contact point. Input and output and volume pots all get separate ground wires; C4S and high voltage grounds are run to separate tabs. All 'front' grounds are then tied together and a twisted pair of 20 ga copper is run to the PS ground bus to connect at the negative of the third PS capacitor. The ground bus is tied to chassis at this point with a paralleled RC network. Also added a nice ground post to the chassis for tying in a future turntable.

Appearance
Again, this is something I was ready to tackle from the beginning. With the top-plate layout of the Foreplay, it already has an industrial aesthetic, I just took it further with the wire handles and finishes. I like it.

Stuff from Valve Magazine
I am amazed by the creativity and helpfulness of this publication, available in PDF form on the Valve page of the Bottlehead site. They have CDs for back issues for sale, as they only host the most recent edition online. Get 'em!
 
Snubber installation

So, what was left over from the Kit?

Inside View, lotsa discrete electronics.
Attenuators and Constant Current Sources:
Bracket for front volume adjustment, 3-D point-to-point wiring, Grayhill selector switch, Caddock resistors, and Hovland 3.0 uF coupling caps

 

What did I learn?top arrow

A lot about neatness and patience
It really pays to take your time and be obsessive about each change
- check each solder joint
- make sure that wires are routed clear of other components as much as possible
- avoid running signal and ground in parallel with high-voltage or filament lines
- be careful with the potentially lethal voltages in these units.

The fun of 'Tube Rolling'

All electronic components degrade with use, and some just degrade over time; but yet another of the beauties of tubes is that you can change out the active components easily when this happens. Even more fun is the chance to hear the difference between different tubes and manufacturers if you purchase extra sets of tubes. This shuffling of tubes is called Tube Rolling. The Foreplay uses one of the most widely available (and inexpensive) tubes, the 12AU7, a medium mu dual triode (that's 2 small triodes in a shared glass envelope with shared filament connections). There are several other ubes which are direct substitutions, including the ECC82 (euro designation for the 12AU7), the 5814 and 6189 (US Military designations), and 'Premium' versions of 12AU7 such as the 7730, the 7316, and the ECC802. A couple of companies are manufacturing new 12AU7 tubes, but the richest stock comes from NOS, or 'New Old Stock' supplies. NOS tubes are tubes that were stored back when they were manufactured in the heyday of tubes, the 50's and 60's, but never used. Most any tubehead will tell ya they don't make like they used to, so the NOS tubes are generally preferred. More info here, on the SND 12AU7 page. A set of two tubes is required to run a Foreplay, and these can vary in price from $8 a pair to more than $60 a pair for specially selected low noise specimens.

So far I've tried seven sets of tubes in this design:

The Game of Tube Matching

Tubes have a fair amount of variability, and with a parallel/sortof mono design, the Foreplay works best with matched tubes. Bill Perkins on the joelist estimated that within the same batch of tubes from the same manufacturer, values vary as much as 20%, so it really is impossible to select two tubes at random and hope that they will match. The Bottlehead C4S helps a lot with this by compensating for some variance, but if the sections are off by 20% what does that do to the sound?

Many tube vendors offer to measure and match tubes when you buy them for a small extra fee, but you can also do this yourself using the Foreplay and a multimeter as a tube tester. In fact, this is probably one of the best ways to match tubes, since they are seeing the ture load of your system. To do this, get three jumper wires with spring hook clips, and carefully attach one each to the signal ground bus, socket terminal A8, and socket terminal B8. You did unplug the unit first and take proper safety precautions, didn't you? Now attach the common lead of your multimeter to the ground jumper wire and the other lead to the wire from either A1 or B1. Keep all of the jumper wires well away from each other, and any conductive surface while you do this test. And be mindful, we are talking about VOLTAGE here, so be careful attaching and moving the meter probes!

Now insert the tubes to be tested and power up the preamp. Select Volts DC on the multimeter and turn it on. You should see a constant voltage of around 40 to 45 Volts after the amp has warmed up 30 seconds or so. This is the value you are interested in comparing. Write it down and move the meter probe to the other socket jumper and compare the values. Write down the other value. Power down the preamp and swap out the tubes, then repeat...

Ta Daa! you've just measured the output signal voltage.

Since these tubes are run very conservatively, some folks argue that measuring the transconductance (the other value used for matching tubes), is not as critical, especially since these values are closely interrelated when you are measuring in an actual circuit. Closer matching is almost always better, but then I don't have more sophisticated equipment. Most of the 'matched' tubes I've purchased and tested are off by a volt or two, but if you have enough tubes to test you can get pretty close. Of course, there is more to good sounding tubes tan just having matched gain, so you'll need to use your ears and decide (good, a subjective decision here) whether the set is a good one.

But hey, that's the fun part.

More later.

The Rollerfeet mod. Can't win 'em all.

Can't win 'em all; too sensitive to even flip switches with these DIY "Rollerfeet"".

 

Tips for those that follow...top arrow

Tools you really (really) need:

Avoiding Mistakes:

Not Required, but Nice to Have:

 

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