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Hello! Been lurking around here forever!
I've built the S.E.X. amp (w/ shorting jack and soon-to-be C4S boards) and a Seduction (w/C4S boards also being installed this week) during this past year. With an AT120E cartridge and .5 meter interconnect to the input of the S.E.X. amp, the Seduction's gain is rather low for my tastes, and I suspect that the extra 2-3dB gain from the better loading from the C4S boards will only help a little bit. I may build a Foreplay, but not in the near future (due to cost). In the meantime, I was thinking of sticking a little board in the Seduction to raise the gain prior to sending the output to the S.E.X. One idea would be something like these: http://diyaudioprojects.com/Tubes/6418-Tube-Preamp-Headphone-Kit/, which use tiny, 6418 sub-mini twin triodes for voltage gain and op-amps as output buffers. The current draw on the little board I've linked to is only about 12mA! Does the Seduction's power trans have enough headroom on the heater taps (6.3V) to rig in either a voltage regulator or some sort of bridge rectifier/voltage doubler type of arrangement to get 9VDC/20mA? And not effect the filaments on the 6922s? Or a I better off not bothering and just sticking to a 9V battery?
Thanks in advance - I'll be interested to see what these little boards can do once they arrive. Probably would make a good iPod headphone amp....
I just stuffed and installed the Seduction C4S board, and the gain has indeed increased significantly - I can decrease the SEX amp by about a quarter turn of the volume pot (i.e., 12 o'clock is now as loud as 3 o'clock prior to the boards). That leaves me with more headroom. The sound is also definitely more weighty and controlled. The background is no more or less quiet - I have to turn the SEX volume all the way up to hear the faintest of hiss or hum. I will start stuffing the SEX C4S boards tonight, but won't likely install them until tomorrow.Best $75 I've spent in a while! I can hardly wait to see what the boards do in the SEX. I would never have thought of using transistors this way, but it makes a lot of sense after thinking about it.
Edits: 09/08/09
Possible lack of clarity here.
The high voltage supply puts out 130vDC at 20mA maximum; the Seduction circuit uses pretty much all of that.
The heater supply puts out 5.7 to 6.0 volts DC at 600mA for 6922s, up to 730mA for 6DJ8s. It will give up 800mA if you need it. (The higher currents may need some adjustment of the 1.2 ohm resistor to get the voltage right.) That supply is not very "clean"; the ripple is low enough to eliminate it as a hum source but probably not enough to power an amp directly. And the voltage is low for this project.
It is probably possible to make up a voltage doubler add-on, but I don't know a circuit offhand that will do it. There are chip voltage doublers with DC in and out which would probably also make 12 volts, then a 9v regulator chip could bring the ripple down to a workable amount. So I'm pretty sure this is possible, one way or another - but I don't have a specific circuit in mind.
It does sound like cheap fun - you might start with 9-v batteries and research power supply circuits for use later if you like the results.
Incidentally, there is no rule against turning the knob all the way up. If it's loud enough at 3:00 then it's loud enough.
Finally, yes the stock iron will overload in the deep bass, and more available midrange power will not change the deep bass distortion or increase the deep bass power. The 6DN7 and 6EM7 have about the same total gain; the EM7 just operates at a somewhat higher efficiency operating point so more power is available. My notes say the MQ upgrade will do nearly one octave deeper bass at full power before saturating, compared to the stock outut transformer.
Thanks for the replies! I didn't realize that the transformer put out such little current - maybe I was confusing it with the power trans in the SEX amp or something. Definitely not a good idea, then - maybe I'll just wait and see what the C4S boards in both amps do for gain before playing around with pre-amps and such.
As for the gain issue, I think part of the problem is that my DAC and CD player are just so darn loud, which is a good thing - the Marantz probably hits 5.0V, and is significantly louder than any of my other components. In comparison, the Seduction then sounds downright weak, with the SEX needing the volume pot at the 3 o'clock position for a lot of records over my cans! Louder records can get by with a lot less, but I still need the pot at least half open. Extra volume would help, since I'm driving Beyerdynamic DT880 Pro's, which are 250ohms impedance and not terribly sensitive to begin with. Speakers are two Klipsch Icon W-series bookshelves, which are probably over-rated at 92dB/w/m. I may eventually build a pair of Voight pipes, but I haven't the tools or facilities in my new, smallish condo. (The other issue is the fact that the room they're in has 10 foot vaulted ceilings and oak flooring.) Everything of > 95dB efficiency seems to be very expensive for me, a person just out of a PhD program and paying a new mortgage and student loans!
Also, I've thought about swapping out the resistors in the SEX (and putting in higher voltage caps) to suit 6EM7's, but I've read a lot of mixed things here. Some say they don't have the "magic" of the 6DN7s, which as I recall, Doc B. thought may be due to the wimpier triode halves (compared to the 6DN7s) used for voltage gain. Also, I have not upgraded to the MQ irons, though I'm debating it. With the stock irons and chokes, wouldn't I run into some problems with the core saturating by 2W RMS? It already seems like the audio compresses at close to full output, with a noticeable flattening of dynamics and decrease in low bass. The 6EM7's seem like they should be able to put out about 3W in the SEX with better irons, but the stock ones seem like they would be a limiting factor. Even still, another watt would only be what, a 1.5dB increase in volume, though?
My C4S boards came today, so I'll report back my findings once they're in! Perhaps a "quickie" is in my future after all...though I'd rather save the dough for the Stereomour, personally......
You are quite correct about the stock iron. I run my SEX amp with EM7 equivalents, and I feel that when they are properly optimized with C4S loads, they do quite well. In the stock circuit, with the single resistor change, you do lose a bit of performance.The Speco does indeed run out of gas around 2 watts, the MQ iron offers a significant increase in headroom.
Edits: 09/08/09
I would also suggest, as Caucasian Blackplate did, that you install the C4S and then reevaluate the necessity of more gain.
Mine works so well that I'm seriously considering a 2.5mv HOMC cartridge to try with the Seduction feeding my Cayin A-88T amp. Granted, my situation is quite different than yours in that the A-88T is 22w triode but I'm currently using an Audio Technica AT-150MLX with the C4S Seduction and have plenty of volume in my smallish room at only 9-9:30 on the volume pot of the A-88T.
"I'll play it and tell you what it is later..." - Miles Davis
12ma is quite a bit in a circuit that draws ~18mA total. The transformer is rated at 20mA, so I would say you are outside the margin by a bit.
The C4S boards in the SEX and the Seduction will compound to make quite a bit more gain, but you can get more if you need it. I suggest doing all the C4S mods before anything, but if that isn't enough, go ahead and modify the SEX amp for 6EM7's. (See the manual for the resistor change, and up the 1000uf 35v cap to 1000uf 50v, then change the R1 on the SEX C4S boards to 1k). The EM7 has loads more gain, and it may just get you there.
If all of the above doesn't work, there is always the Quickie!
-Paul
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