Preamp? Thoughts on my system? IMF speakers.

[jbrahen]

Just finished piecing together a stereomour II, my first SET amp and my new reason to never leave the house
Doc and people at Bottlehead - thank you!

The amp was intended to drive Forte II's since they're my most efficient speakers. I thought I'd finally love those speakers, but ehhh maybe I'm not a Klipsch guy (also I never upgraded the 40 year old caps so maybe I'll blame that). I'll never sell them because of that stereotyped live rock concert feel- but I find something lacking when I play other genres. Yes, I'll upgrade the caps soon, chill dear friends.

I was lucky enough to snag and refurbish some lesser known IMF RSPM mkIVs (basically a huge transmission line 4-way speaker with complicated crossover) They've been in my main system, driving them with the preamp section of a Peachtree Nova feeding a dynaco ST70 I upgraded with the Bob Latino mod.

The RSPMs are reportedly 96db sensitive at 1meter with 40 watts(??) So, I was hopeful but not optimistic the stereomour II (with 8 ohm tap) would drive them well. To my surprise, they sound gooood. They present the famed SET mids very well, and the transmission line bass is oh-so present. Comfortable listening levels occur with the volume pot at about 60-80%. At 100% they sound loud-ish.... and the truth is, although the bass I will admit sounds more controlled with the stereomour than with the dynaco, it does lack that slam. I mean, Charlie Haden's bass is in the room with me, but Cardi B's WAP isn't calling attention to itself as it should.

So obvious next step is see what the preamp section of the Peachtree can add. I hooked it up and immediately got more than enough volume, and house shaking slam. There was also sense of lush presence and guts that just wasn't there before. Problem solved? Not really. Mids lacked that articulation, imaging less delicate and that SET magic was less palpable.

What do you guys think? Is the preamp section of the peachtree just not up to par? Is it a bad match?
Would a Moreplay or a BeePre add the necessary gain without obscuring the gorgeous SET mids and highs? That could be my next step, I'm pretty hooked on Bottlehead right now. (although you guys, we need to talk about adding a remote for us lazies)

Kaiju is probably the ultimate answer for the speakers, but then I'd need a preamp anyhow. Plus I just put so much love (and money) into this stereomour, and hey, I bet I can find the right preamp to make this perfect

Bottlehead people - thank you. Thank you for making kits cool again, for encouraging people to get involved in stuff and for being gentle when we're not immediately proficient. And thanks for this community!



Finally-
I've included some photos because I'm super proud. I had some discipline with the paint job which I think paid off. Also - I read all this stuff about "bluing" for the PT bell. I didn't have any bluing solution, but I did have acetone and a torch - got a gorgeous rainbow effect which I preserved with some clear enamel rustoleum. I definitely recommend trying that out - just be careful the thin metal doesn't warp. I wonder if that would work on the chassis plate?

[Paul Joppa]

Beautiful work!  And also, thanks for the kind words!!

That is a peculiar efficiency spec all right. I looked up the woofer (KEF B139, a real classic) and I see it's rated 11 volts RMS for 96dB SPL at 1 meter, which is 15 watts into 8 ohms. That translates to a more standard spec of 84dB/2.83v/1m. I think you need more power, not just a better-matched preamp, if you want to get both bass slam AND articulate mids. At present, our highest power kit would be a pair of Kaijus, each bridged as a monobloc. That would get you 16 watts, 3dB less than the ST-70.

It's possible that a Moreplay preamp into the Peachtree power section would provide some tubey goodness while retaining high power - it was designed with that possibility in mind. But no promises; I don't yet have enough listening experience with it to make any claim.

My two cents. I admit, I do tend to be conservative about power requirements.

[Jay]

There is mention on these forums of using a stereomour as a preamp.

[Doc B.]

The flame bluing is very nice. Never thought to do that on a transformer bell, but I have done watch hands that way. To answer your question - nope, the aluminum chassis panel will not blue with heat.

[jbrahen]

Wow, thank you everyone for responding so quickly.

Paul- A pair of Kaijus bridged as monoblocks seems like the holy grail for me, but alas, that can't happen until bitcoin is well over 100K, so I will continue to hunt for alternatives, plus it's so fun to tweak stuff and see what happens
Good call checking on the sensitivity of the KEF B139, 84db sensitivity makes much more sense. All seasoned audio people I've spoken to about these IMF speakers have told me to use a high output solid state amp. I must say though, the stereomour is, without a doubt, driving these speakers to sound better than I've ever heard them sound.


Jay- I saw those posts and laughed at the picture of the banana plugs to female RCA- but you've got a point. And Paul, for some reason I never thought to use the peachtree's power section, I've only thought about it as a preamp.... very good thought. I guess I like tubes...

Do you think I could experiment with using the stereomour as a preamp with both the peachtree and the ST-70? Really the question is, will I hurt anything? That'll be fun to try.

Doc- thanks for the kind words about the flame bluing and good idea for watch hands.. haha thanks for pointing out the chassis panel is aluminum, good thing I didn't try it

Thanks for the input everyone. Please let me know if I'll blow anything up using the stereomour into the ST70 or peachtree - if not, I'll report back.

[Paul Birkeland]

You could try using the Stereomour as a preamp.  If you have the DC filament upgrade, it shouldn't be too noisy to operate in that capacity.

[Doc B.]

You and me both re Bitcoin. I accepted it for one of our more expensive products about a week before Musk helped nuke it last May. Looked like a bad idea until very recently. It is back on its way up and I have now made a bit of profit. FWIW, I just put the BeePre2 and Kaiju on sale this afternoon.

[2wo]

I lived with a pair of the IMF's back in the 80's . We used to drive them with a  variety of solid state amps. They like a bit of power and have a super complicated crossover that may not be the first choice for a SET amp.
 But if I still had them I would sure my Kaiju a try. Maybe someone you're way has a 300b amp you can try...John

[jbrahen]

Ok, so I tried the STII as a preamp - won't work until I do the morquiet upgrade, so that's on hold for now.

I've lived with source into just the STII into the IMF speakers and it's been really good. And by really good, I mean, basically audio bliss. The realism is stupid good, like the kind of good where Im not even in the listening position and Im cooking or something and I hear something I didn't realize was part of a song and I'm like hey whats my dog doing bumping into my drum kit - oh wait he's just laying down and there's nobody else here - that's just my awesome stereo setup!

The STII definitely opened up over the first month of listening. I'm not a person who really believes in "burn in" but no joke, over the first 2 weeks to a month of listening the experience just got better and better. People considering the Stereomour II as a purchase - this amp rules. Know that.

My original "issue" of lacking some bass slam and wanting a bit more volume with these ~84ish db sensitive speakers kind of dissolved. There are too many amazing things happening with this amp to be focused on anything that's lacking - and I think that's a high compliment to give to a piece of gear. I think eventually I will migrate to a preamp with a set of MonAmours which would be a better match... but one thing at a time. .. just ordered a Moreplay, thanks for the holiday discount!
Doc - still waiting for that 100K BTC, apparently there are some nice monoblocks on the moon



So, I popped in the first upgrade, the shunt regulator (morquiet will happen soon dont worry). I may be one of those rare people who don't notice improvement? Perhaps a blacker background, maybe appearing as slightly increased dynamics? I did notice though, that in my system the highs became less tame - something I really was enjoying from the original STII. Mids adopted some overtones that sounded muffled. Still sounding excellent  - but not as excellent as prior to the upgrade.
Importantly, I did get some imperfect readings as follows all in VDC:

IA 393 (ok)
OA 294 (slightly low)
KREG A 3.32 (ok)

IB 392 (ok)
OB 292 (slightly low)
KREG B 2.39 (quite low I think)

Are these far off enough for concern?
Yes, I triple checked all solder joints and trimmed everything as close as possible - you did mention these things just once or twice in the instructions and everywhere in the forum:).

It would be very helpful to hear from you if these readings may be causing what I am hearing with the upgrade. If this is the way it sounds, then I'll probably remove the new shunt regulator.

Anyway, thanks to everyone for reading if you've made it this far. Keep up the good work, happy holidays!

[Paul Birkeland]

Do those Kreg voltages move if you swap the driver tubes?

[jbrahen]

Paul,
It appears they do move, new readings with driver tubes swapped:

IA 392
OA 291
KREG A 3.02

IB 392
OB 290
KREG B 3.73

[Paul Birkeland]

Those are working voltages. 

You may have a flaky connection in there, so that's certainly something to keep an eye on.  2.5V or less on those Kreg terminals represents an issue. 

[jbrahen]

Paul,
Thanks so much for you help.
Do you mind explaining why you think a loose connection? I am learning.
If swapping tubes restores voltages, does it imply that the act of jostling around tubes changed a connection?

What if the circuits only measure appropriately when the correct tube is on the correct side? Would that imply that the tubes measure very differently and different sides have different margins of error?

I haven't listened yet with the tubes swapped and better measurements, excited to see if theres an audible difference.

[Paul Birkeland]

The low Kreg voltage that appeared would tend to indicate that the regulator section associated with that terminal is having to draw all the available current and the actual driver for the signal portion of that half of the amp isn't drawing the appropriate amount of current.  This could happen from something like the pin4/5 jumper not being well connected or some other wire or solder joint that's just loose enough to cause such a problem.

[jbrahen]

Thanks for the explanation. I went back and re-flowed many joints on the shunt regulator circuit board. With the driver tubes installed in the same swapped positions, everything measured properly once again. It made nice music.

Just to see, I then swapped the driver tubes once again, back to their original positions where readings were initially bad. The left channel now sounded muddy and crackly. Interestingly, I noticed that the slightest movement of the tube in the left channel causes crackling though the left speaker. This doesn't happen for the tube on the right. Swapping the tubes once again completely abolished any issue. So... it seems the left channel is just fine with one tube, but not fine with another. The right channel is fine either way. How should I interpret this, Paul?
My next step is to re-flow joints on the tube pin connectors, but I don't understand why only a specific tube would cause an issue on the one side.