Look for help reviving Foreplay

[Grainger49]

Grainger, Paul,

As always thank you very much for the helpful discussion.

After reading Grainger's post this morning I ended up sketching out some volume/balance designs while riding the BART (publish transit this morning). It's definitely a tricky problem, I ended up with a solution that uses two dual pots but with the downside that instead of the balance progressively muting one channel while not effecting the other instead it would decrease the signal on one channel while actually increasing the signal on the other. This is not the desired effect.    .  .  .  .   

That is exactly what I was alluding to.  And I know you don't want one now.  But here is the key.  A balance pot is not a Volume pot.  The balance pot at 12:00 doesn't attenuate either channel.  When rotated CCW from the 12:00 position it attenuates only the right channel and has no effect on the left channel.  When rotated CW from the 12:00 position it attenuates only the left channel and has no effect on the right channel.  What compounds the confusion is that the balance acts on the opposite channel that you are using the balance to fade into.  (and now I'm confused too!)

This is what gave me a headache when putting in one in the last couple of years.  I had no problem with the one I put in during the 90s.  I'm getting older. 

The really odd one is in my first home made, not by me, tube preamp where the builder used a volume pot and it did exactly what you describe.  You tweaked the balance then tweaked the volume, or just ignored it.

[slomatt]

Paul, thank you for pointing out your s/n article I found it very interesting. Here's a link if others are interested.

http://www.bottlehead.com/loosep/signals.htm

It describes my case perfectly, the Foreplay's line level output is much higher than either my NAD amplifier or LM3886 chipamp were designed for which requires me to use the "last resort" solution of turning down the preamp volume to a very low level. Because of the large gain of the amplifiers the ~0.3mv of hum from the Foreplay is greatly amplified and very audible. Conveniently I have a source of pink noise and a SPL meter so I'm going to measure the actual sensitivity of the amp/speakers using your procedure and then look into building an appropriate attenuator.

For now the foreplay revitalization continues. It sounds like the PEC 100k pots I linked to below are the easiest solution. I like the idea of stepped attenuators but the effort involved in calculating the resistor values, soldering everything together, and physically fitting them into my enclosure is more than I want to take on at the moment.

That said... I was searching through my parts box this morning and found two different multi-pole switches that would work so I might try out the stepped attenuators at some point in the future. Here are some pictures just for fun:

Here's a shot showing the new heater wiring (green wires). The other green wire with an alligator clip is a temporary short between the signal ground and the chassis ground.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_2748.jpg&hash=c258db070e3b613da1e3105012e0ba843af580b0)

In my junk box I had a 2-pole 12-position mil spec selector switch that I picked up somewhere.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_2751.jpg&hash=c239557acdab1659d518d6ba364aa3f51dbfb45a)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_2753.jpg&hash=685eab0c89c389c2f3b681f55cf3f45a047841ae)

I also have two 2-pole multi-position selector switches from a past BurningAmp event.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_2754.jpg&hash=50ef6b599ae6b57eafddcf650c2e1aafa4f3f59b)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_2754.jpg&hash=50ef6b599ae6b57eafddcf650c2e1aafa4f3f59b)

Neither of these switches will fit easily in my Foreplay case, so the PEC pots are the solution I'm going with for now.

- Matt

[slomatt]

(Note: Since audioasylum is being shut down I'm going to be overly verbose here to add to the content base.)

The 100k PEC pots arrived yesterday and the build quality seems to be quite nice. The diameter of the threaded section is wider requiring larger holes in the chassis, and the threaded section is taller which will make the pot stick up more above the plate. Last night I removed the stock pots, hopefully I'll have time in the next few days to put in the new ones.

I also purchased some 100k resistors so I can try out a shut-mode attenuator. The idea is to replace the "top half" (signal to wiper) of the potentiometer with a high quality resistor which should provide better sound quality than using the potentiometer's internal resistance. The resistance of the "bottom half" (wiper to ground) of the potentiometer adjusts the voltage divider and controls the volume. This changes the input impedance from a fixed 100k to instead vary from 100k to 200k. More details can be found at the following link.

http://www.siteswithstyle.com/VoltSecond/12_posistion_shunt/12_Position_Pure_Shunt.html

When I re-ran my heater wires last week I set them up so that they are connected to the transformer using a terminal strip. This way I can easily change my heater power supply without having to re-solder any components. I suspect my hum is caused by the AC heaters, so I plan to try out DC heaters using a wallwart. The tubes require 6.3v @ 300ma for the heaters and many people have successfully used a 6vdc 800ma+ wallwart to reduce hum. I have a 12vdc wallwart and a LM317 1.5A adjustable voltage regulator sitting around so I'm going to setup the regulator for 6.3v to see if that reduces my level of hum.

The real cause of audible hum in my system is that my solid-state amplifiers have much more gain than the Foreplay is designed for. In my case the Foreplay puts out ~0.3mv (rms) of hum on each channel, with a lower gain amp this would not be a problem but with a high gain amp it is very audible. My hope is that the shunt-mode attenuator will decrease how "hair trigger" the volume controls are and that the DC heaters will remove the hum. Another potential solution is to decrease the gain of my chipamp which can be easily done by changing a resistor in the circuit.

- Matt

[JC]

IIRC, .3 mVac is about as low as the stock FP2 with AC heaters gets, so switching to DC for the heaters is probably going to improve things for you.  I have my FP2 mated with a stock Stereo 70, so the mechanical hum from the power transformer  dominates!

Having said that, if reducing the gain of your amp is simple enough, that can only help both hum and gain issues, as you indicated.

Finally, I always save extra nuts and tooth washers off of controls so I have a selection available to "adjust" the position of pots with long threaded sections; do you have any lying around that fit your new pots, perchance?

[Grainger49]

I hope you do get the noise reduction you are looking for.  If not a voltage divider on the input of the amplifier, as JC is suggesting, will give you some assistance.

[Mike F1]

I had a similar problem with my Foreplay (re: too much gain for my EL84 amp)

I did this and it did the trick!

http://www.goldpt.com/mods.html

M

[slomatt]

Success! Tonight I wired up a 12vdc wall wart and a LM317 adjustable regulator to form a 6.3v supply (ok so it actually put out 5.8v due to resistor variances but it was close enough for an experiment). Using this to power the heaters drastically reduced my hum, all I hear now is a soft hissing that is only audible within 1-2 feet of the speakers. The speakers I'm using are an older set so I have no impression of any sound quality changes yet, but my initial listening seems to show a quieter sound stage which you would suspect and possibly tighter bass than I remember from the last time the preamp worked.

Unfortunately regulating a 12v supply down to 6v and then drawing 300ma results in ~1.8w dissipated by the LM317 which leads to extremely high temperatures even with a heat sink. I'm going to look for a more appropriate wall wart.

Again, thanks to everyone here for their input and help during this project.

- Matt

[Grainger49]

Matt,

The soft hiss might even go away with some tube rolling.  Tube rolling is one of the most fun things you can do by yourself.  I bought all kinds of 12AU7 variants just to play.  Toshiba and Mazda branded tubes were interesting but not the best I heard.

You can use the straight 12V DC if you use pins 4 to 5.  But that takes rewiring.  Otherwise you can find a 6V wall wart.

[slomatt]

I definitely plan to try out some other tube options.

Are there any downsides/benefits to running 12v heaters as opposed to 6v heaters? My 12v wallwart puts out about 13-14v so it would be perfect to regulate down to 12v.

- Matt

[JC]

The tube was designed to run on either Voltage, and I certainly have never heard of any advantage/disadvantage to running it on either.

You will need to rewire, of course, because 12 Volts will need to be applied across pins 4 & 5, with pin 9 left unconnected.  At 12 Volts, the heater draws 150 mA, so two of them will need 300 mA.  If this is an older FP with a DC bias Voltage applied to the heaters, then it will be good to maintain that.  Simply leave the heater supply floating; in other words, do not connect the heater supply to any ground.  Then, you can apply the bias Voltage to Pin 9.

[slomatt]

Small update... I bought a 6vdc 800ma wall wart at Radio Shack. They are usually $20 but luckily they are clearing out the Radio Shack branded ones for $5 so I bought one of those. Step 1 was to cut off the prongs so it would fit in the case.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_0086.jpg&hash=e290749ccfa86cf3141874dca4f4b025d5dffc6f)

I'm using a terminal strip so I can go back and forth between AC and DC heaters. The wall wart fits nicely into the corner of the enclosure and is wired to the stock fuse and power switch.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Fforeplay_preamp%2FIMG_0087.jpg&hash=cd85eb6461e378d1738cac69ef40060ebee2215c)

Under load it puts out 5.8v which is within the 6.3v +/- 10% rating of the tube, and in fact the slightly lower voltage could prolong tube life. The hum is decreased to a level where I can live with it. Next up I need to lower the gain on my chipamp and I might try out some more tubes.

Here's the chipamp in case anybody is interested.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.blankwhitepage.com%2Fgallery%2Falbums%2Flm3886_chipamp%2FIMG_2734.jpg&hash=84c539eed21fbf19cc1c9a886ba3e88d913ffc9e)

Thanks again for the input on this project!

- Matt