Would the Stereomour be a good upgrade?

[Dr. Toobz]

I'm seriously considering the new Stereomour, but need some convincing. I could easily sell the MQ irons from my enhanced S.E.X. amp to offset some of the cost, along with some other things, but the question becomes one of "do I really need another tube amp?"

-The enhanced S.E.X. is already plenty good, though my quest for better damping and linearity has lead me to do all sorts of mods, and I still am not sure I've arrived at my "perfect amp" (mods have ranged from playing with CFB to different OPT's, LED's as bias resistors, etc). Maybe something with 45 tubes or 2A3's will be more linear, and give me more power all the way down to 20Hz? Is the difference between a 45/2A3 and a 6DN7 with NFB that big?

-I have 99dB/1W/m speakers surrounding a corner in small room. Being horns with sealed woofers, too loose of a damping factor makes them sound puffy in the lows and honky in the mids. So, power is not key; linearity and a quiet background are. The speakers sound very good with the cathode feedback, and not as good without it. The bass gets less distinct and puffier, and the speakers sound less clear.

-I never need more than 1/2W. So, gaining a watt or two isn't that compelling, and the 45 tubes would be about the same in terms of power output (1.75W vs. about 2W for the S.E.X.).

-I don't need a headphone output, as I already have the Crack, in which I've put some GIGANTIC 100uF caps and will soon be putting the Speedball into. I already like the Crack better than the S.E.X. for my Senns and don't use the latter for headphones anymore.

So, would there be much benefit to going to AC-heated 2A3 or 45 tubes? Would I get the linearity I crave, and the "magic" sound of DHT's I so often hear about, or is the difference so small as to not be worth parting with another $750? (Plus, I'm not sure what I'll do with the S.E.X. amp if it's decommissioned!).

[Jim R.]

Angelo,

Without hearing them both, but based on experience with other sweep tube amps and a couple of 2a3/45 amps I own/have owned, I'd say with reasonable confidence that the stereomour will give you more in every department, and for driving speakers with a more complex load/crossover such as the H3s have, you always have the option of trying the 4 ohm tap on the OPTs for better damping.  Sonically, I dont think there is anything that can compete with a DHT.

Why not sell the s.e.x as a unit and put all the money towards a stereomour if you don't have a use for them both?  Assuming that you get enough feedback from others that makes you feel comfortable with this decision.  There really is a difference in having that extra headroom even if you think you aren't using it, and the 2a3/45 is really sweet with horns.

Hopefully one of the BH guys who have heard both can give you more insights, but if it were me, I had the speakers you do, and had to choose between the s.e.x and the stereomour, I'd take the 2a3 anyday.  The 12at7 with a CCS is also a really nice combination as a driver.

HTH,

Jim

[Paul Joppa]

It's always very difficult to make a useful answer to subjective questions - for instance, the 2A3 and 45 will give you greater linearity; whether it's "the linearity [you] crave" is harder to say. The DHTs run about half the distortion at maximum power; for the 2A3 the greater headroom means even less distortion at comparable powers - perhaps 1/3 the distortion. (These are ballpark figures, not careful measurements.)

On the plus side, the 2A3 does seem to have some DHT magic. The original Paramour was conceived as a replacement for the original SEX monoblocks specifically because of that sonic advantage - I built a pair just to see how they might work, and we all heard something special that was not in the 6DN7 amp, so Doc commissioned me to help make a product out of it. It was my first full design for Bottlehead, though I had helped refine some of the earlier designs.

On the minus side, the AC-heated 2A3 will have more hum than the SEX amp - about 16dB more in fact. A good 45 will be quieter, but still 10dB above the SEX amp.

[Paul Joppa]

I forgot to mention the output transformers. The ones in the Stereomour are designed to work equally well at 4K or 8K impedance levels - I am planning to put them into a revised SEX amp eventually. That means that you can wire for a 4 ohm load and use 8 ohm speakers, which will double the damping factor. You'll lose a bit of power, but with the 2A3 you'll still have more power than a SEX amp. It's just an option.

[Dr. Toobz]

Thanks for the advice so far! I hadn't thought about selling the actual amp itself - do people really buy pre-made DIY stuff? I sure wouldn't :-) I mean, the build quality is fine - i can solder as good as anyone - but the idea of selling something homemade has never crossed my mind.

When I say "linearity," what I mean is an even frequency response, with no one region more emphasized than another. Yet, I don't want the amp to sound cold and overdamped, either - and the line between the two is a fine one. I can come very close to that ideal with a bit of cathode feedback on the power tubes and CCS-loads on the drivers, but I've found that you're especially dependent on the transformers that way. The little Edcors I bought for this purpose don't have the balls to give good bass, even if they shine elsewhere. Bigger irons are going to cost a whole lot more and not fit on the S.E.X. chassis plate, so I might be better off getting the flatter, more refined response from a better tube (i.e., a DHT like the 2A3) instead of going through all that trouble to make a TV tube (6DN7) behave more like a DHT.

The fact that there's a 4-ohm tap on the Stereomour is something I hadn't considered, but a definite plus. My Heresys really seem to "wake up" when the Z-out goes below 2 ohms. Will the irons designed for the amp be pretty good at not losing much when using the 4-ohm taps? I know the Specos on the stock S.E.X. amp seemed to roll off a lot sooner in the HF on the 4 ohm taps, due to what I have assumed to be leakage inductance.

EDIT: One thing I should mention relates to hum: with the Edcors, which are open-framed, and the CFB, the S.E.X. amp is not a quiet as normal. I "sense" (if not really hear) hum, whereas there is just no hum whatsoever with the amp wired in autoformer, grounded-cathode mode. Over speakers, this little bit of hum isn't really hearable, and certainly not from more than a foot away. So, I would be just fine with a millivolt or two of hum from a DHT - I doubt I could hear it to begin with. And it's not like I'd want to use headphones with the Stereomour, as I already have the Crack for that.

[Doc B.]

I can offer my experience from a purely subjective standpoint - take that only for what it is worth. I have used a S.E.X. amp with MQ iron for several years in my home theater, on Lowther America Alerions and also with Straight 8s. When we built the Stereomour, that went into the system instead. I prefer it. It has a better sense of resolution and definitely a little more horsepower. Those speakers are more like 97 and 96dB respectively so while the Stereomour  sounds very quiet on either speaker I can't absolutely guarantee that it will have the same perceived background with 99dB speakers. But it probably will be plenty quiet.

[Paul Joppa]

I'm going to have to think about these questions for a while before I can offer any useful technical answers.

The capacitance and leakage inductance of the Speco transformer do not vary much with the secondary tap. And the Speco's high frequency response is excellent - the cutoff due to leakage inductance is 35kHz with a matched load, and 70kHz with an 8 ohm load on the 4 ohm tap. The cutoff due to capacitance is even higher, 120 or 60kHz respectively. I have not yet plotted the combined effects, but the resonance is around 65kHz and varying the Q of that resonance won't make much difference in the audio band. These numbers are a good part of why the Speco sounds way out of it's class; it's limitations are in the bass.

It will take me a while to dig out the data on the new OPTs; the project took a couple years and my notes are scattered. Most of the effort went into getting the LF power handling to a place more comparable to good transformers such as the Magnequest units. Subjectively I am very happy with the new iron, it beats the crap out of the old Paramour iron - which had a great reputation for many years.

That said, there is much more to a good transformer than the easily measured specifications. In the Paramount, the Stereomour, and the eventual SEX amp upgrade, I have tried to get the measurable specs more in line with MQ parameters - I'm close but not quite there. The more subtle audible effects have to do with materials and build quality, and there's no way we can get an inexpensive commercial winding house to take care of those things the way that a boutique like Magnequest does. (In fact, so far I have not found any boutique house "like" Magnequest - Mike does some things that nobody else does.)

[Dr. Toobz]

I can offer my experience from a purely subjective standpoint - take that only for what it is worth. I have used a S.E.X. amp with MQ iron for several years in my home theater, on Lowther America Alerions and also with Straight 8s. When we built the Stereomour, that went into the system instead. I prefer it. It has a better sense of resolution and definitely a little more horsepower. Those speakers are more like 97 and 96dB respectively so while the Stereomour  sounds very quiet on either speaker I can't absolutely guarantee that it will have the same perceived background with 99dB speakers. But it probably will be plenty quiet.

I'm really impressed to hear that you had the enhanced S.E.X. in your own HT rig. If it's good enough for the Doc himself....

I'm definitely going to think about the Stereomour. One thing I have also considered, and I don't know if people will agree, is that I've found the S.E.X. to be a little picky about what you put into it. It seems to do best with a really good source and a pre-amp; while my sources are fine, the only pre-amp I've used was the Quickie, whose parts have now ended up in a sort of Frankenstein project I've been building. So, there's also a good possibility that better drive and resolution would be had by putting a Foreplay III in front of the S.E.X. amp. This would mean keeping the S.E.X. and building a FP3 vs. a Stereomour. Since the MQ irons are so resolving, I think they make the amp even less forgiving than with the stock Specos, which I recently tried out again and quickly disconnected due to goofiness in the bass region (low notes actually sound as a harmonic above the fundamental vs. a solid "low" note as with the MQ irons). Might my extended S.E.X. amp be a little more "ballsy" with a FP3 between it and my sources - is this logical to think?

[jtkarma]

It's really interesting to read about the respect that people have for the SEX amp here. I have a bone stock SEX amp and still love the thing. Best piece of audio equipment I've ever bought. I'm still probably going to buy the Stereomour but it's more to play with the 45s and 2A3s than anything else. I know it won't replace the SEX amp, I'm just so in love with the SEX, weird eh?

I've thrown just about every wacky thing at the SEX amp from speakers to crazy inputs imaginable and it has been an absolute blast. I've tried SS preamps, a tube 12AU7 preamp I built from Glass Audio and passive preamps. Sometimes you hook stuff up and it's like, whoa that sounds horrible I'm not going to try that combination again or more often I'll put a combination together and be stunned by what I'm hearing.

The detail the SEX can pull out of recordings is just mind bending. I'm a classical music and voice guy and the SEX amp is always showing me something new from old recordings particularly good live recordings. Mind you this is with a two channel system and I'll be listening like the RCA dog turning my head and going, "Wow, where is that voice coming from?" Never had that from SS amps, never.

I'm curious to know if I'll get that same sense of wonder with the Stereomour. Thank you for a great thread sir and I look forward to hearing what you do with your choice.

[Dr. Toobz]

I think the respect comes from the fact that it's such a solidly-designed product, and sounds much better than it has a right to, given the Specos and triodes originally used for TV's (6DN7's), not to mention the low price tag. The stock amp sounds great as-is, and sometimes I feel bad for having done so many mods to the amp. When a product starts out so good that it has you questioning whether the upgrades are really worth it, you have a true winner. That's why it's so hard to decide whether or not I should bother with the Stereomour. Fun to build, looks great, but that much of an increase in SQ over the S.E.X.? That's the $64,000 question. And if I can squeeze another 5% out of the S.E.X. with a preamp like the Foreplay III or better DAC, that might be the way to go. It's hard to improve on perfection!

[Dr. Toobz]

Another quick question for those in the know - what is the approximate Z-out on the 4 ohm and 8 ohm taps, respectively? I ask this because my Heresies seem to really blossom once you get below 2 ohms, but sound a bit fuzzy and vague above that, and the bass isn't as tight. I've calculated a rough figure of 1.34 ohm for my heavily-modified S.E.X. with CFB and Edcor XSE-10-8-8k transformers, and the speakers seem to love it - they did not sound anywhere near as good with the S.E.X. in no-NFB mode, which I understand to have an output impedance of about 3 ohms. I don't think the NFB is what the speakers respond well to, per say - the feedback just happens lowers the output impedance into a range where the speakers don't react as much with the amp. Otherwise, the triodes are pretty linear anyway, being triodes! So, if the Stereomour output is low enough, it should fit the bill, even though it uses no feedback. But if we're talking about 3 ohms on the 4 ohm tap or something like that, it's a no-go with my current horns.

FWIW, the Heresies, being sealed, are nowhere near as bad as Klipsch's "modern" models when it comes to reactivity and finickiness with amps. The RF series sounds strange on tubes - when you look at the impedance curves and awful phase angles around the crossover frequencies, it's clear that they were designed to thrive on solid-state, or at least an amp with an output impedance of well below an ohm, resulting in a really high damping factor. At least the Heresies are happy above an ohm! I also think this is why a lot of people mistakenly think their speakers are too "bright" - it's not the horns, per say, but a poor match with the amp.

[Paul Joppa]

I calculate about 6.74 ohms wired for a 16 ohm load,  3.3 ohms wired for 8 ohms, 1.68 ohms wired for 4 ohms, and 0.81 wired for 2 ohms. About 2/3 of this is the tube's plate resistance, the rest is transformer wire resistance.