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Author Topic: Preamp? Thoughts on my system? IMF speakers.  (Read 203 times)

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Offline jbrahen

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Preamp? Thoughts on my system? IMF speakers.
« on: October 20, 2021, 09:29:01 PM »
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?
« Last Edit: October 20, 2021, 09:40:12 PM by jbrahen »
Josh B.

Offline Paul Joppa

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #1 on: October 21, 2021, 05:30:27 AM »
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.
Paul Joppa

Offline Jay

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #2 on: October 21, 2021, 07:04:42 AM »
There is mention on these forums of using a stereomour as a preamp.
Jay L.

Online Doc B.

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #3 on: October 21, 2021, 07:18:56 AM »
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.
Dan "Doc B." Schmalle
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Offline jbrahen

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #4 on: October 21, 2021, 09:09:49 AM »
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.
Josh B.

Offline Paul Birkeland

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #5 on: October 21, 2021, 01:05:03 PM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #6 on: October 21, 2021, 01:21:41 PM »
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.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline 2wo

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Re: Preamp? Thoughts on my system? IMF speakers.
« Reply #7 on: October 21, 2021, 05:33:53 PM »
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
John Scanlon