SEX as preamp? Running out of steam on speakers.

[Dr. Toobz]

I know this question has probably been asked before on the old forum, but I'm wondering about the best way to use the SEX amp as a pre-amp. For headphones, the SEX is wonderful, but I've run into problems with speakers. I've yet to hear a horn-loaded or single-driver speaker I actually like, and the typical off-the-shelf multi-driver offerings from the likes of B&W, KEF, Paradigm, etc. seem to be a) too insensitive to get decent volumes or dynamics from 2 watts, and b) often have DC resistances and impedance dips WAY below their specified 8 ohm nominal impedance, which leads to odd-sounding bass and tipped-up treble and mids. 4-ohm taps only help a little. SET's therefore seem to be very difficult to match to speakers, and I'm not sure that going towards something like a 300B arrangement would make much difference (still single-ended, still transformer coupled, and 8W isn't drastically louder than 2 - an 87dB/W/m speaker on 8W would yield 96dB at 4 feet, but still not enough at my typical 10-foot listening distance).  I've therefore been toying around with the idea of a hybrid arrangement, taking the SEX output into the input of a vintage McIntosh or Marantz solid-state amp, which would have far more power and better damping, allowing for the use of practically any speaker (while still keeping the Bottlehead sound in the signal path). In that case, would it be preferable to a) take the signal right from the driver section of the 6DN7, after the coupling cap, and run it to the pre-in of the solid-state amp, or b) take the signal off the headphone jack, following the extra "baggage" of another stage, coupling cap, and transformer? My logic would say "a," but I'm not sure of a) the output impedance on the little driver half of the 6DN7, or b) the voltage and current. My idea would be to take the output from the little driver triodes and run it to a set of RCA outs. If the impedance is high, would I run into problems with cable length?

[Grainger49]

All SET amplifiers have limited power and require high sensitivity speakers.  The SEX amp is the lowest powered least expensive kit Bottlehead sells.  I would expect the selection to be pretty small because of those reasons.  With an an 87dBW sensitive speaker you are going to need push pull or a 300B in a very small room, much less the open loft you have.  There are hundreds of speakers out there with sensitivity higher than 97dBW, which is what is suggested on the SEX page here.

IIRC, on the old site (still there but the product pages aren't) there were suggested speakers for SEX, there is a list on the SEX page now.  But from my experience I wouldn't expect a B&W or KEF to be sensitive enough.  It isn't the nature of those beasts.

Many of the speakers suggested are horns and single drivers, that is to be expected.  Possibly there are some that are not horns or single drivers.

I have the 2A3 Paramours and they drive my 94dBW Triangle speakers well in my fairly good sized room.  The Triangle speakers are neither single driver nor horn.  So there are designs out there that should fit your bill.  But remember, I have a whopping 3.5WPC, you have 2WPC.  And, believe it or not, that makes a difference.  The same thing goes for me stepping up to a 300B amp.  That jump from 3.5WPC to 8WPC is noticeable with appropriate speakers.

Hang in there there are answers!

[JC]

I would certainly be tempted to just try taking it out of the headphone jack for starters.  Driving the input impedance of an amp with that output should pose no problem that I can see, and it can be done with an adapter and no other modifications to your headphone listening set-up.

Just a guess, but I would almost think you might like a Quickie better for preamp duty than the first stage of the S.E.X., but I have not tried any of this.

[Dr. Toobz]

I would certainly be tempted to just try taking it out of the headphone jack for starters.  Driving the input impedance of an amp with that output should pose no problem that I can see, and it can be done with an adapter and no other modifications to your headphone listening set-up.

Just a guess, but I would almost think you might like a Quickie better for preamp duty than the first stage of the S.E.X., but I have not tried any of this.

True - though I would think you'd have to have the volume down pretty low to avoid overloading the input stage of the solid-state amp. Even with the 120ohm resistors in the circuit, the headphone output from the SEX is pretty beefy! That's why I figured going from the driver stage might be better, but who knows....

Of course, I also have been looking at the Klipsch Heresy III, which supposedly sounds smoother and more balanced on tubes than their "modern" (i.e., non-Heritage) offerings. Their 99dB sensitivity would probably give me about 95dB at 10 feet with a full 2W output, which would negate the need for another amp. There's just nowhere close to audition them, so I'd be plunking down a lot of dough for something I may hate - and the Heresy I and II's are supposedly not as good (worse bass and less controlled sounding), so I wouldn't bother with either of those on eBay.

SO, do I go Heresy and SEX - or SEX, Marantz, and a more "traditional" speaker? A tough call, though a vintage amp and decent pair of non-Klipsch, mid-sensitivity speakers would still likely be less than just the Heresy III's on their own.

[Grainger49]

You said:  .  .  .  "I've yet to hear a horn-loaded or single-driver speaker I actually like" . . .  and "Of course, I also have been looking at the Klipsch Heresy III  .  .  .  .  "

You seem conflicted.  To ferret out the answer you might post in the General forum and ask for a Bottlehead near you who would let you come listen to their horns/single drivers/conventional speakers that work well with SET amplifiers.  Let them know in the post that this is what you are looking for.

If you are determined to use the SEX as a preamp and using the driver stage be sure that you tap in after the 0.1uF interstage coupling capacitor.

And like you say, and I said, a more appropriate speaker would make a large change for the good.

[Paul Joppa]

?? something ate my post ??

I'd go with the speaker or headphone output. The driver output has too high an impedance (14K) to drive more than a foot of low-capacitance cable (IMHO) and the signal voltage is quite high, around 15 volts rms.

[Dr. Toobz]

You said:  .  .  .  "I've yet to hear a horn-loaded or single-driver speaker I actually like" . . .  and "Of course, I also have been looking at the Klipsch Heresy III  .  .  .  .  "

You seem conflicted. 

Not so much - what I mean is that to date, I've not heard a horn-based speaker or single-driver that I've liked, which has included modern Klipsch offerings, Fostex, and Lowthers. I've not heard any Klipsch heritage stuff, however, which is supposed to sound better than their Reference or Icon stuff, especially on tubes. Honestly, I may not like the Heresy, either, but with a 99dB sensitivity, it certainly is worth ruling out. If I were to dislike those too, then I'm stuck with going towards a higher-wattage amp (either SS or push-pull tube), as I'm not aware of any 97dB+, tube-friendly multi-driver products that don't cost more than $1.5k. It's almost like there's a gap in speaker offerings - either stuff is really tube friendly or really not friendly, and the "really friendly" stuff (i.e., horns and single-drivers) has some pretty big shortcomings that I find hard to ignore.

Life is easier with headphones!

[Dr. Toobz]

?? something ate my post ??

I'd go with the speaker or headphone output. The driver output has too high an impedance (14K) to drive more than a foot of low-capacitance cable (IMHO) and the signal voltage is quite high, around 15 volts rms.

Thank you - that's exactly the info I was looking for. If I do go the pre-amp route, I'll try the headphone output, as I would imagine the speaker outs would be WAY too loud and allow for no play whatsoever on the volume pot.

[Grainger49]

Dr. Toobz you have a PM.

[JC]

Well, again, the headphone out is the easy one to try.  A lot will depend on the sensitivity of the power amp you decide to feed, as you have noted.

But, again just a guess, the output stage probably does not add very much in the way of voltage gain to the chain; output stages usually tend toward  providing current gain for the job of driving the speaker motors.

The added bonus is that the relatively low output impedance, even from the headphone jack, will generally drive quite a bit of cable.

[Paul Joppa]

...Thank you - that's exactly the info I was looking for. If I do go the pre-amp route, I'll try the headphone output, as I would imagine the speaker outs would be WAY too loud and allow for no play whatsoever on the volume pot.

The speaker and headphone output are actually the same, except for a 120 ohm series resistor. Feeding a 10K or higher amp input, that resistor will provide almost no attenuation. (less than 0.1dB!)

I'll take this opportunity to expand on voltage and impedance levels, in the hopes this will be useful to others and might even be found in a search. Anyone who is tired of me pontificating, just skip the rest of this post!

=======================

For the SEX amp, at maximum output:

Input is 0.62 volt RMS, input impedance is 100K (the volume control) That's about 0.000004 watts.

Driver output is 12.1 volts RMS, driver source impedance is about 14.3K ohms (the plate resistance at this operating point). Input resistance of the power triode is 249K (Rg) Power dissipated in Rg is about 0.0006 watts

Power triode output is about 130 volts RMS, with a source impedance of about 2500 ohms (the plate resistance at this operating point). The load impedance is about 8K ohms, for a power of 2 watts.

Transformer output on the 8 ohm tap is 4 volts RMS (which is 2 watts into an 8 ohm load), due to the transformer step-down voltage ratio. Apparent source impedance is 3 ohms (2.5 ohms from the power triode, plus a bit from the transformer wire resistance).

Voltage at the headphone jack is also 4 volts RMS, but with a source resistance of 123 ohms.

=======================

Having made those observations, I see that the speaker or headphone output, at 4 volts maximum, might be higher than what you would like for direct connection to a power amp, especially one with a high sensitivity - something that is all too common. And you might want to adjust the output so that headphones are equally loud as speakers, without having to make drastic changes in the volume control. If you are lucky, the amplifier you choose might have its own input volume control. Otherwise, the simplest way would be to install an 8-ohm speaker L-pad at the SEX amp's speaker terminals, with the output going to an interconnect. Actually, any potentiometer of 8 ohms or greater will do, just don't exceed 1000 ohms so you keep the driving impedance low. I think Radio Shack still carries a nice 25-ohm wirewound rheostat which ought to sound very good in this application.

For even more information on level-matching in audio systems, you can look up my "Signals and Noise" page at Bottlehead/Community.

[Paul Birkeland]

That's why I figured going from the driver stage might be better, but who knows....

Doing that, you will loose the step-down of the output transformer, and you will have a ton of gain (and high output impedance).  I would switch the SEX amp for 4 ohm operation and use the headphone jack, that will give you a linestage that can drive 100 feet of cable, and the gain shouldn't be too bad. 

[Dr. Toobz]

Wow, lots of great info here! If I fail in my quest to find acceptable speakers for my S.E.X. amp, it's nice to know that I can still keep it in the chain - and taking things off the headphone output means no work.

Bottlehead really needs to offer a p-p or tube/SS hybrid someday! 2W was fine for near-field listening in a small apartment, even with 89dB speakers, but the huge open spaces of my new condo aren't working out so well - esp. since the three models of Klispch speakers I've tried haven't seemed to play nice with SET amps and their high Z-out. More sensitive doesn't always equal "better," as I've learned from this exercise. I may just have to go the p-p or SS route in order to drive "typical" speaker offerings to reasonable levels in my living room while having enough headroom for peaks without clipping. Unless Bottlehead offers a huge transmitting triode/ "cookie jar tube" amp, that is :-)

[tsingle999]

I had a hard time with Klipsch Forte's and SET's. My Fostex 206 in a passive radiator box is currently my fave speaker with the SEX amp. I think you could get good volume out of them in a large room.

[corndog71]

If you want a high efficiency, full range speaker then check out this kit:

http://www.audiocircle.com/index.php?topic=72093.0

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.gr-research.com%2Fmis%2FSV2.jpg&hash=1227befe0d36a9d328b4cb08fd12d120e5bc6ad7)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.gr-research.com%2Fmis%2FSV5.jpg&hash=f245ac955e85d67bbff2fcc06ae8212ed6968d88)