Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: Carbon13 on June 12, 2021, 07:59:52 AM
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Well, I finally finished my stereomour II and I have to say that it was so much fun to do this build! This kit was exactly what I was looking for in a first build, all the components were included, instructions were clear and support was available (and excellent!). As you can see by the pics, I painted the top plate a piano black and the transformers in silver. The base is a cabernet stain and finished to high gloss poly.
My only hiccup was during the voltage check I was getting a weird reading on one terminal (I forget which one off the top of my head) but then I noticed that one of the LEDs was out on the board. I had a tough time soldering that board because it's so small. Turns out that a bit of solder seeped through the back side of the board and shorted the LED on the front. I removed the access solder and voila! All good! I feel lucky I spotted the issue so quickly and was able to fix it.
So, first impressions. This amp is replacing a Fisher 500C tube amp that is about 10x the power, so keep that in mind. I really noticed the difference with this amp when I play Springsteen's Nebraska album. WOW! midrange was amazing. I could hear texture in his voice that I've never heard before. I recently have read reviews that talk about hearing "emotion". This would be an accurate description of what I was hearing on this album. Good start.
I next thought I would try stretching those 3.5 Watts with some Billie Eilish. This was a bit of a disappointment for me. My Fisher 500C driving my Klipsch Fortes usually hit me in the chest with bass with Billie. Not so with the Stereomour II. This lack of low end could be a deal breaker! But then I read on the forum that switching the 4ohm tap may improve the bottom end at the cost of some SPL. So I cracked out the soldering iron and switched over to the 4 ohm. And here comes the bass! Maybe not quite as robust as the Fisher but pretty darn close. Add in that stellar midrange and I think this amp is real winner!
I have noticed that the paint on the transformers seems to be "cracking" under the clear coat. I wonder if this just my shoddy painting or a result of heat? Anyway, not a major issue yet but may require a fix down the road.
Next steps: I'm going to live with the stock unit for a while and then do the upgrades incrementally (I bought them already so they're waiting in the wings). Once complete and I've listened for a while, I'll switch the Fisher back and in then make final decision on which to keep. But for now, the leader definitely is the Stereomour. Plus I built the damn thing so there's bragging rights too ;)
Thanks Doc and BH team!
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Very nice looking build! It's nice to have more than one amp.
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Wow great job!
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Very nice build. The finish on the base looks deep, with an almost liquid quality from the poly.
Karl
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Thanks for the kind words! Yes, I'm super happy with the amp and I'm really enjoying discovering new sounds in old records! I've already started tube rolling with a couple of NOS Mullards CV4024s. That "mullard flash" is quite the fireworks show ;) Glad I read about that here before I tried them. They sound pretty good to my ears though.
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I just built the Stereomour II earlier this year and boy talk about bang for the buck! I have the shunt regulator and DC filament upgrades which were installed during initial build so can't directly compare with and without. I'm driving a pair of Madisound BK-16 single driver kit speakers with the better Fostex driver. In a smaller room these play plenty loud and sound glorious (with good/great recordings). Very pleased indeed. NOTE: When you add the shunt regulator upgrade be very careful with your soldering especially on the larger 3-terminal transistors (center pin requires lots of heat to flow the solder properly). This integrated is a definite keeper for me.
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Thanks for the kind words!
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So I have all the upgrades installed and the amp sounds fantastic! But I still find I miss that hit you in the chest bass and midrange of the Fisher. Is this just a matter of the 3.5watts vs 30 or can I get that kind of impact from the stereomour with the right speakers (Cornwalls? Zus? Frugal horn?). Don't get me wrong, the stereomour plays loud and has better articulated bass but not the same kind of slam. Top end and upper mids is no contest, stereomour is amazing!
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Get another and make mono blocks and try some 12 inch high efficiency speakers. That should get you up to 6.5 - 7 watts, enough to get fuller/deeper bass. Room treatments also help. Do you have a preamp before the Stereomour?
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There seem to be several versions of the Forte, but the Forte 3 review I looked at with an impedance plot looked difficult to drive, with a minimum impedance of 3.65 ohms and some pretty wild phase angles. That would explain the improvement when you shifted from 8 ohms to 4 ohms. The sensitivity was measured as 95.2 dB at 2.83v. which at 4 ohms is 2 watts so the real measure is 92.2dB/watt/meter. By my rule of thumb, you'd want a minimum of 10 watts to be "loud enough for most audiophiles most of the time.
(Note this varies a lot with type of music, room size and absorption, and personal preference. Good "slam" will likely take more than the minimum power.)
The bass extension and power bandwidth are above average in Steremour, so I'd be surprised if that was a factor. My best guess is that it's the power limitation that makes it struggle with "slam."
I believe the Forte is bi-ampable; you might give it a try using the Fisher on the bass ... that's another can of worms, of course!
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Unfortunately the forte 1s aren't biampable. But I did get a suggestion to try adding in my powered subs. I have 2 nearby that I use for HT so I will give it a try.
Your explanation about the forte validates my thoughts as well. Although rated as high sensitivity, I don't think they easy to drive per se.
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UPDATE:
I tried connecting my subs and that definitely improved the bass but I still wasn't happy with the integration. I know how good the fortes can sound on their own so this was still frustrating me. And it's not like the stereomour wasn't playing loud, it certainly can do that but just didn't have the low end grunt.
So, that got me thinking that maybe a preamp with (gasp) tone controls or an equalizer may be worth a try. I bought this cheap little preamp to test the theory.
https://www.amazon.ca/gp/product/B07GXBF5JS/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
To my surprise (and delight!) it worked! This little preamp in front of the stereomour sounds terrific with the fortes! Lots of bass grunt that I was missing. And still has that great detail and soundstage that I loved with this amp.
So here's my question: Is it just the voicing of the stereomour that wasn't to my taste (or maybe a match with the fortes)? If so, are there options to tune the voicing of the amp (ie. different cap values?)?
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The Stereomour is now delivering the tonal balance that you like, from the signal that is being fed into it. So it seems like it's not the weak point. It sounds more like your source needs the help of the preamp to drive the Stereomour the way you like.
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Hey Doc,
Not sure I understand but would definitely be willing to learn. The source is the exact same with my Fisher 500c, which has tone controls and lots of bottom end oomph. Keeping everything exactly the same with the bottlehead, there is more detail but less oomph. That's why I suggested maybe it's the way the amp is voiced, understanding everyone has individual preferences and unique pairings to sources and speakers. For me, I need a bit more boost on the bottom which the little preamp provides. I just wonder if I could somehow fine tune the stereomours low end response to eliminate the need for the preamp?
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If the Forte 1 has the same impedance plot as the Forte 3 (as measured by Stereophile) the region around 100-200Hz is down 3-6dB relative to the midrange; that's probably the "slam" range. So a little bass boost would likely restore the tonal balance. You could probably implement a passive shelving filter at the input (a few resistors and capacitors) to get that equalization. Subtle "voicing" mods are unlikely to be sufficient by themselves.
[Edit: The 3-6dB reduced bass that I calculated above is caused by the low damping factor of the SET amp - I wasn't clear about that.]
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which has tone controls
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Yes, and I agree that is the difference. Tone controls.
When I first looked at the stereomour, I thought it would be a great match with the fortes given they are relatively high sensitivity. When I found the match a bit thin on the bottom, I initially thought it was because it's a low power amp. But it played plenty loud. So that's why got the preamp with tone controls and yes, that seems to work. Only downside is it limits my number of inputs and kinda defeats the purpose of an integrated.
Doc - it's no fault of the stereomour! It's a fantastic amp and I absolutely love how it sounds. It's just been a bit of a mismatch I think with these speakers that requires some additional fine tuning to dial in the sound I'm looking for. If anything, I think people should be reassured that it's plenty of power to drive these large speakers to ear bleeding levels ;)
Paul - The passive shelf filter sounds like an interesting option but I will admit that I'm outta my league on designing something like that. I'll have to research it and see if it makes sense. Alternatively, I may have to try some new speakers ;)
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I'll work up a circuit for this - it should be possible to build it into the Stereomour.
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Thanks Paul! That would be amazing! I'll have a custom tuned stereomour;)
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I'll work up a circuit for this - it should be possible to build it into the Stereomour.
Tuned cathode bypass on the driver? ;)
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So I have the Forte IIs. Not sure how they compare with the Forte I you have, but mine sound best with the Stereomour set at 4ohms. However, I might still be missing out on something. Here’s the frequency response, though not sure sure to what degree this is altered by the Crites upgrades.
https://community.klipsch.com/index.php?/topic/170785-alk-xover-diy-for-forte-ii-schematic/
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**** Passive bass boosting with Stereomour II ****
Note: This is actually cutting everything above the bass by a few dB, so you will need to increase the volume setting for the same loudness.
The simplest way to do this is with a series RC network, placed across the volume control potentiometer. Connect a 15Kohm resistor to one end of the volume pot, connect 1 0.022uF capacitor to the other end of the pot, and connect the free end of the capacitor to the free end of the resistor. Insulate that connection with heat-shrink tubing or electrical tape so it can't touch the chassis or any other nearby wires or terminals! Repeat for the other channel. This will be a tight fit.
This gives about 5-6dB boost to the bass below about 350Hz. You may want to try some different component values to get the effect you want:
Increase the capacitance to get a lower transition frequency, or decrease the capacitance to get a higher transition frequency.
Increase the resistance to get a smaller boost AND a lower transition frequency. Don't decrease the resistance, because that will reduce the input impedance and sensitivity of the amp too much.
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For a much harder way, you could bypass the 365 ohm cathode resistor with a choke, cap, and resistor in series to bring up the gain in the midbass.
I would start with:
https://www.mouser.com/ProductDetail/Murata-Power-Solutions/13R336C?qs=gLyyx31KZaCXYrPIar3DqA%3D%3D (https://www.mouser.com/ProductDetail/Murata-Power-Solutions/13R336C?qs=gLyyx31KZaCXYrPIar3DqA%3D%3D)
https://www.mouser.com/ProductDetail/Nichicon/UDB1H6R8MHM?qs=kArNe9LFxXkiWy78BPgsnw%3D%3D (https://www.mouser.com/ProductDetail/Nichicon/UDB1H6R8MHM?qs=kArNe9LFxXkiWy78BPgsnw%3D%3D)
https://www.mouser.com/ProductDetail/Vishay-Dale/RN55D10R0FB14?qs=60NadEv6cQm8bIQ8le8fAg%3D%3D (https://www.mouser.com/ProductDetail/Vishay-Dale/RN55D10R0FB14?qs=60NadEv6cQm8bIQ8le8fAg%3D%3D)
Those three parts are wired in series, then tacked across each 365 ohm resistor. Reducing the resistor value increases the effect of the filter, so putting a 100 ohm pot in there is also a reasonable choice as an experiment. In fact you could probably find a 100 ohm stereo linear pot to just temporarily stick to the chassis, then wire this up from there.
The L and C can be adjusted to change the shape of the filter and/or the frequency where you get the most boost.
PJ's circuit is a lot easier to mess with though!
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Thanks guys! PJ, is this doable with the mourquiet upgrade? That's what I currently have in place.
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I just found this thread on redesigned crossover for forte 1s, maybe another option?
https://community.klipsch.com/index.php?/topic/106102-developing-a-network-for-the-forte/
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Thanks guys! PJ, is this doable with the mourquiet upgrade? That's what I currently have in place.
Alas, it is not. I'll ping PB and see if we can come up with something.
We worked on a tone-control preamp a while back. That gave us the initial design ideas for the Moreplay preamp, but the additional stage and controls added too much cost and complexity for what we expected would be a small audience.