Hi everybody, this is my first post here. I have an amplifier mod that I'm really happy with. I wanted to post about it, and this place looked like a really good fit. Hope you enjoy it!
Many years ago I bought a Groove Tubes Spacestation SFX-100 for my keyboard setup. This amp caught my interest because I was never able to pull off a useful stereo sound while playing live. It is based on the Center Point Stereo concept developed and promoted by Aspen Pittman himself. The idea is pretty ingenious. It uses a novel application of Mid-Side processing, which historically has been used mostly in recording/mixing/producing, but turning it on its ear for live playback. What they did was to output the Mid signal (Right plus Left) through the front center speaker system, and the Side signal (Right minus Left) through a side-mounted open-frame speaker aligned with the center of the Mid speaker. (To avoid confusion, note that in the context of the Mid-Side language here, "Mid" means "middle", and not "midrange".) The two signals then propagate outward where the waves interact constructively and destructively to reconstruct the Right and Left portions of the sound everywhere in the sound-field. Because the stereo information all comes from a single point, in a large room you still can hear both "sides" of the stereo, which is really hard to do with separate Right and Left speakers - usually, except for a relatively narrow "sweet spot", either the Right or Left side will inevitably dominate over the other. I have to say that the CPS effect is pretty dramatic, even impressing this crusty "experienced" live performer/engineer. You can actually walk around and hear stereo just about everywhere in the room (or at least a useful approximation of it), both on and off the stage.
Now, my SFX-100 is from an early design, and it is clear that they were still ironing out some wrinkles. Here are some pics off the web of what the stock unit looks like (mine was the same):
The pic on the left is the whole cabinet, with the bottom being the open frame for the Side speaker mounting. The pic on the right is looking straight into the bottom left of the cabinet, showing the back of the Side speaker.
The Mid speaker system (meaning "Middle" here and not "midrange" - just for clarity) is pointed straight out the front, and has a 10 inch Eminence driver with a coaxial compression horn. The horn does well with the "bright" portion of the highs, but falls a bit short for the "crisp" part. The designers addressed this by tacking on a super tweeter, which appears to be a piezo type connected in parallel with the horn. When listening close to the piezo, it does put out some crispy highs, but at a much weaker level than the main drivers, and has that "splatty" quality so often associated with piezos. Unfortunately, piezos do get a bad rap because of a huge misunderstanding: even though they don't technically need a crossover to function without damage, they still react badly to huge low-frequency transients, leading to the splatty sound. In fact, a properly designed crossover not only smooths out the sound of a piezo beautifully, but it can also greatly increase the operating life. (More on this topic as we get deeper into the mod.)
The side speaker is a 6 inch coaxial. The front-wave of the Side speaker shoots the R-L signal to the right, and the open-frame design ingeniously allows the back-wave of the Side speaker to shoot the L-R signal to the left. The hole in the middle of the magnet is to allow the back-wave of the coaxial horn to also shoot some highs out of the back of the Side speaker (which, again, is goes out the left side of the cabinet). Note that in this design, there is no tweeter for the side signal. In the wider sound-field, starting about 3-6 feet away from the speaker, the three waves combine to create what your ears perceive as Left toward the left, and Right toward the right. At least that's the general idea, and it actually holds up pretty well.
What I found is that the piezo on the front did more harm than good to the Mid sound (to my taste), and the total lack of the tweeter on the Side speaker left the Side portion of the sound very muddy and boxy. After some thought, I decided that adding the right choice of high-output tweeters for both the Mid and Side drivers, along with nixing the wimpy piezo, would really go a long way to filling in what was missing.
I'll go into the choice of tweeters and the crossover design in the next post...
Many years ago I bought a Groove Tubes Spacestation SFX-100 for my keyboard setup. This amp caught my interest because I was never able to pull off a useful stereo sound while playing live. It is based on the Center Point Stereo concept developed and promoted by Aspen Pittman himself. The idea is pretty ingenious. It uses a novel application of Mid-Side processing, which historically has been used mostly in recording/mixing/producing, but turning it on its ear for live playback. What they did was to output the Mid signal (Right plus Left) through the front center speaker system, and the Side signal (Right minus Left) through a side-mounted open-frame speaker aligned with the center of the Mid speaker. (To avoid confusion, note that in the context of the Mid-Side language here, "Mid" means "middle", and not "midrange".) The two signals then propagate outward where the waves interact constructively and destructively to reconstruct the Right and Left portions of the sound everywhere in the sound-field. Because the stereo information all comes from a single point, in a large room you still can hear both "sides" of the stereo, which is really hard to do with separate Right and Left speakers - usually, except for a relatively narrow "sweet spot", either the Right or Left side will inevitably dominate over the other. I have to say that the CPS effect is pretty dramatic, even impressing this crusty "experienced" live performer/engineer. You can actually walk around and hear stereo just about everywhere in the room (or at least a useful approximation of it), both on and off the stage.
Now, my SFX-100 is from an early design, and it is clear that they were still ironing out some wrinkles. Here are some pics off the web of what the stock unit looks like (mine was the same):
The pic on the left is the whole cabinet, with the bottom being the open frame for the Side speaker mounting. The pic on the right is looking straight into the bottom left of the cabinet, showing the back of the Side speaker.
The Mid speaker system (meaning "Middle" here and not "midrange" - just for clarity) is pointed straight out the front, and has a 10 inch Eminence driver with a coaxial compression horn. The horn does well with the "bright" portion of the highs, but falls a bit short for the "crisp" part. The designers addressed this by tacking on a super tweeter, which appears to be a piezo type connected in parallel with the horn. When listening close to the piezo, it does put out some crispy highs, but at a much weaker level than the main drivers, and has that "splatty" quality so often associated with piezos. Unfortunately, piezos do get a bad rap because of a huge misunderstanding: even though they don't technically need a crossover to function without damage, they still react badly to huge low-frequency transients, leading to the splatty sound. In fact, a properly designed crossover not only smooths out the sound of a piezo beautifully, but it can also greatly increase the operating life. (More on this topic as we get deeper into the mod.)
The side speaker is a 6 inch coaxial. The front-wave of the Side speaker shoots the R-L signal to the right, and the open-frame design ingeniously allows the back-wave of the Side speaker to shoot the L-R signal to the left. The hole in the middle of the magnet is to allow the back-wave of the coaxial horn to also shoot some highs out of the back of the Side speaker (which, again, is goes out the left side of the cabinet). Note that in this design, there is no tweeter for the side signal. In the wider sound-field, starting about 3-6 feet away from the speaker, the three waves combine to create what your ears perceive as Left toward the left, and Right toward the right. At least that's the general idea, and it actually holds up pretty well.
What I found is that the piezo on the front did more harm than good to the Mid sound (to my taste), and the total lack of the tweeter on the Side speaker left the Side portion of the sound very muddy and boxy. After some thought, I decided that adding the right choice of high-output tweeters for both the Mid and Side drivers, along with nixing the wimpy piezo, would really go a long way to filling in what was missing.
I'll go into the choice of tweeters and the crossover design in the next post...
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Correction on the above post: I believe that the Side speaker for this unit is actually a full-range speaker, as there is no crossover for it. The hole in the magnet to let the high frequencies through the back would still would work the same way, though. The Mid speaker is indeed coaxial as there is a crossover for the coaxial horn.
This is the tweeter that I chose:
This is the tweeter that I chose:
Goldwood GT-1025 3" x 7" Wide Dispersion Piezo Horn Mid/Tweeter
Based upon tests, it appeared to be a good match to what was missing on the high end for both the Mid and Sides. It is a piezo horn similar to the Motorola KSN-1025A, which is one of the larger horns having better dispersion and a much more robust driver element than the typical piezos out there. The horns also fit neatly into the open spaces already nicely cut out on the cabinet (with just a little cutting needed for the front - more on that in a following post). These horns obviously provide no back-wave, so to maintain balance on the Side output, two horns need to be mounted symmetrically facing out on opposite sides of the cabinet. As the wave emanating out of the right side of the cabinet is exactly out of phase with that of the left, to maintain proper M-S phase alignment the horns must also be wired out of phase with each other, with the one facing to the right wired in-phase with the Side speaker. On the front of the cabinet, two more horns are placed symmetrically close to the center, wired in parallel and in-phase with the Mid signal.Attachments
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Here is the schematic for the crossover and connections:
The Mid tweeters (designated FRONT) are on top of the schematic, and the Side tweeters are on the bottom. Note that the left side tweeter is wired out-of-phase. The tweeter elements are driven by identical crossover circuits. The series capacitor (C1 / C2) work with the resistors to create a 2kHz highpass response with slight attenuation to match sensitivities. As piezos look like a capacitive load (nominally 0.125uF each for this model), the series resistor results in a 26kHz lowpass response, which smooths out the extreme high end and decouples the amp up at ultrasonic frequencies. Here is the modeled frequency response of the circuit:
And in case you're interested, here's the Matlab code to do the modeling and generate the response plot (you should be able to run it in Octave, which is a free open-source version of Matlab):
The Mid tweeters (designated FRONT) are on top of the schematic, and the Side tweeters are on the bottom. Note that the left side tweeter is wired out-of-phase. The tweeter elements are driven by identical crossover circuits. The series capacitor (C1 / C2) work with the resistors to create a 2kHz highpass response with slight attenuation to match sensitivities. As piezos look like a capacitive load (nominally 0.125uF each for this model), the series resistor results in a 26kHz lowpass response, which smooths out the extreme high end and decouples the amp up at ultrasonic frequencies. Here is the modeled frequency response of the circuit:
And in case you're interested, here's the Matlab code to do the modeling and generate the response plot (you should be able to run it in Octave, which is a free open-source version of Matlab):
Attachments
On the schematic, the crossover input terminals (J1,J2 and J12,J11) connect across the Mid and Side speaker terminals on the Spacestation. Note that the Mid (FRONT) input terminals (J1, J2) connect across the horn terminals of the Mid speaker on the SFX-100. That's because the coax element already has a crossover - and if you connect to the woofer terminals, you won't get much output from the piezos! The SIDE input terminals (J12,J11) connect across the Side speaker terminals. The polarities of the connections on the schematic should match the polarities of the speaker terminals. Now for the fun part... pictures of the mod!
1. First is taking off the front grill. 2. Now you can see the front with the grill removed. 3. Carefully removing the Mid speaker.
4. Wires from the Mid speaker to the Mid amps. Note two pairs, one for the woofer and one for the coaxial horn. On my unit, two of the wires were soldered on, which to remove the speaker meant cutting the wire and adding inline terminals.
4. Wires from the Mid speaker to the Mid amps. Note two pairs, one for the woofer and one for the coaxial horn. On my unit, two of the wires were soldered on, which to remove the speaker meant cutting the wire and adding inline terminals.
4. (continued from above) Wires from the Mid speaker to the Mid amps. There is a nice tie-down in the middle of the cabinet to hold the wire bundle all together. The wimpy black wire goes to the wimpy piezo supertweeter. The red/black wire pair at the bottom head down through a hole to the Side speaker. Note two pairs of wires going to the Mid element, one for the woofer and one for the coaxial horn. On my unit, two of the wires were soldered on, which to remove the speaker meant cutting the wire and adding inline spade lug terminals.
5. (below) Mid speaker fully removed with male spade lugs added. I instead could have unsoldered and cleaned the terminals, but this was much simpler.
6. Labeling the speaker wires to keep them straight. Then, tapping and soldering additional pigtails on the Mid horn wires and the Side speaker wires to go to the crossover. These were also terminated with spade lugs. (Rather than totally removing the little piezo super tweeter, I just left it disconnected and tucked its wires into the harness.)
7. Close up of the tapping/soldering of crossover pigtails to the Side speaker wires.
5. (below) Mid speaker fully removed with male spade lugs added. I instead could have unsoldered and cleaned the terminals, but this was much simpler.
6. Labeling the speaker wires to keep them straight. Then, tapping and soldering additional pigtails on the Mid horn wires and the Side speaker wires to go to the crossover. These were also terminated with spade lugs. (Rather than totally removing the little piezo super tweeter, I just left it disconnected and tucked its wires into the harness.)
7. Close up of the tapping/soldering of crossover pigtails to the Side speaker wires.
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8. Assembling the crossover board. The resistors are non-inductive and the caps are non-polarized. (If you need a parts reference, I could post one., but the brand on the capacitors should give a big hint!)
9. Showing the wire connections. Inputs for Mid and Side on the left, and outputs on the right. Note that the Mid output and Side output each have two pairs of wires, as there are two piezo horns for each.
10. Crossover installed in the cabinet. A hole is drilled (on the left of the picture) to feed wires for the Mid piezo horn and R-L Side piezo horn on the right- side of the cabinet. The wires for the Side speaker and left-side Mid and L-R Side piezo horns feed through the existing hole for the Side speaker.
9. Showing the wire connections. Inputs for Mid and Side on the left, and outputs on the right. Note that the Mid output and Side output each have two pairs of wires, as there are two piezo horns for each.
10. Crossover installed in the cabinet. A hole is drilled (on the left of the picture) to feed wires for the Mid piezo horn and R-L Side piezo horn on the right- side of the cabinet. The wires for the Side speaker and left-side Mid and L-R Side piezo horns feed through the existing hole for the Side speaker.
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11. Placing the Mid piezo horns into the front openings. The desire is to mount them symmetrically as close as possible to the center while still clearing the Side speaker. So, some cutouts will be needed to enlarge the holes for them to fit properly.
12. Piezo horns measured, and construction lines drawn for the expanded holes. The bevels at the top and bottom edges are filed to match the sloped back on the horns, and as a plus, they allow us to barely miss the bottom board of the Mid enclosure. Hitting that board would have been a problem!
13, 14. Two views of the finished cutouts. The cuts were made with a coping saw, and the bevels were made with a hand file. (Kudos to my very skilled and benevolent father-in-law who helped me through this portion.) The Side speaker should be removed or protected during the cutting, which generates a lot of particles and dust. Rather than remove the Side speaker, I carefully covered both the back and front grille with masking tape, which worked just fine.
12. Piezo horns measured, and construction lines drawn for the expanded holes. The bevels at the top and bottom edges are filed to match the sloped back on the horns, and as a plus, they allow us to barely miss the bottom board of the Mid enclosure. Hitting that board would have been a problem!
13, 14. Two views of the finished cutouts. The cuts were made with a coping saw, and the bevels were made with a hand file. (Kudos to my very skilled and benevolent father-in-law who helped me through this portion.) The Side speaker should be removed or protected during the cutting, which generates a lot of particles and dust. Rather than remove the Side speaker, I carefully covered both the back and front grille with masking tape, which worked just fine.
15. Masking tape off, all speakers and piezo horns connected and mounted. The piezos are connected using female spade lugs. The blue power LED on the front panel had to be moved to make room for the right Mid piezo horn. Now ready to place the grille (lower left of the picture).
16, 17. The finished product, happily back in its favorite habitat. It is sitting on top of a 12" Alto powered subwoofer that complements the SFX-100 perfectly. (Bonus: I really like how the centered power LED now lines up with the one on the Alto!) On top of the SFX sits a Bluetooth receiver for testing.
The verdict: The horns breathe new life into this amplifier. No more muddy midrange-heavy sound, neither in the Mid or the Side. Even knowing the design improvements on the latest models, I feel that this mod really makes the SFX-100 a worthy contender. I have to say, this thing is fun to listen to with just about any listening material, and as a keyboard amp, it's almost surreal!
P.S. I think guitar players would really enjoy a CPS amp as well - acoustic guitar players for sure with stereo pickup gear, and electric guitar players too if they treat this as a special kind of FRFR setup. I bet that stereo effects would come alive on this amp.
16, 17. The finished product, happily back in its favorite habitat. It is sitting on top of a 12" Alto powered subwoofer that complements the SFX-100 perfectly. (Bonus: I really like how the centered power LED now lines up with the one on the Alto!) On top of the SFX sits a Bluetooth receiver for testing.
The verdict: The horns breathe new life into this amplifier. No more muddy midrange-heavy sound, neither in the Mid or the Side. Even knowing the design improvements on the latest models, I feel that this mod really makes the SFX-100 a worthy contender. I have to say, this thing is fun to listen to with just about any listening material, and as a keyboard amp, it's almost surreal!
P.S. I think guitar players would really enjoy a CPS amp as well - acoustic guitar players for sure with stereo pickup gear, and electric guitar players too if they treat this as a special kind of FRFR setup. I bet that stereo effects would come alive on this amp.
Thanks so much for this! I had one years ago. Just picked up another on the cheap for rehearsals. Forgot how muddy they sounded. Can’t wait to try your mod.
Hey I'm gratified that this thread could help! Yeah, the latest models solve a lot of the issues, but they sure are way expensive.
After reading back through, I realized that where the wires were tapped and soldered, it could have been more easily done with 1/4" terminals hooked up like a Y-adapter (or maybe just tapped off at the connector itself). I guess I was so busy soldering up the piezo crossover that I didn't mind soldering a few more wires too. Anyway, hindsight is always 20/20. Godspeed with your mod!
Thank you for this! : ) I've an early edition of the 100 and use the Radial JDI passive stereo direct box between my keyboard (Korg Stage Vintage) and the amp. Makes a big sonic difference! I met the late Mr. Pittman a good while ago and thanked him for his design. : )
Thanks for posting this. Can you please supply a parts list with quantities? I did see the link for the tweeter. Is it four tweeter total?
@Dreamery, hi and apologies for such a slow response - I did not have notification set up for this forum, and have not checked back (obviously). Also, my dear father-in-law passed away last November, so we've been rather preoccupied this year. (God rest his soul, he is the very one who I mention in the above posts, who had helped me with the precision cutouts.) This project was one of "those projects" that sat on my to-do pile for many years until I finally got the time to complete it, so I had to do some digging to find my parts order - turns out that it was from 10 years ago! The supplier (Parts Express) has discontinued stocking the resistors, but I was able to find them at another supplier. As of this post, Parts Express still does stock the caps and the horns, though. Here are the links:
Bare minimum, you would need 2 each of the resistors and 2 of the capacitors, and yes, four piezo horns. (But note that for one-off projects like this I usually order 1-2 extra of every part, just in case I get a bad one or I mess up somehow.)
| Mills 82 Ohm 12W Non-Inductive Resistor https://www.soniccraft.com/product_...ra-12-p-697?osCsid=8clj4oc2rffgch182iglmtss33 |
| Mills 47 Ohm 12W Non-Inductive Resistor https://www.soniccraft.com/product_...a-10-p-3672?osCsid=8clj4oc2rffgch182iglmtss33 (Note this is actually 51 Ohms but close enough in value to be pretty much equivalent. The 47 Ohm seems to be N/A) |
| Dayton Audio PMPC-0.47 0.47uF 250V Precision Audio Capacitor Part # 027-206 https://www.parts-express.com/Dayto...-Precision-Audio-Capacitor-027-206?quantity=1 |
| Goldwood 3" x 7" Wide Dispersion Piezo Horn Mid/Tweeter Part # 280-062 https://www.parts-express.com/Goldw...ion-Piezo-Horn-Mid-Tweeter-280-062?quantity=1 |
Bare minimum, you would need 2 each of the resistors and 2 of the capacitors, and yes, four piezo horns. (But note that for one-off projects like this I usually order 1-2 extra of every part, just in case I get a bad one or I mess up somehow.)
Extra gratitude and thanks. Like you, I had ordered the parts and they sat on a shelf for a while and I finally did the mod over the last 2 days. Holy cow, now I can hear the upper drawbars on the Numa Compact 2x organ when I use this old used Groovetube Spacestation V2 I picked up cheap. I had to do a couple of mods. No matter how I mounted it, the front tweeter hit the magnet on the side firing speaker. So I had to remove the side speaker and remount it through the mounting hole from the other side, which fixed the problem. Just couldn't install the wire mesh grill on it but it's buried back pretty far so no worries.
So anyway, fantastic information. Thank you for the crossover design, directions, pics and parts list.
Happy Thanksgiving
John
So anyway, fantastic information. Thank you for the crossover design, directions, pics and parts list.
Happy Thanksgiving
John
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Wow Dreamery, that looks fantastic! I had to do a double-take that it wasn't mine LOL - until I saw the backdrop of your workbench, and of course, the V2 grille. So great that your keyboards sound better too. Yeah, this mode really brings this wonderful amp to a brighter new life.
One quick note - so you mounted the side speaker facing the other way? Very clever - but if so, (and just in case you didn't do this already) - make sure to reverse the (+) and (-) terminals on the side speaker, because reversing the mounting will also effectively reverse the phase of that speaker. (As designed, the side speaker faces front-ways out of the right side of the box, and when wired amp-plus-to-speaker-plus, it correctly outputs the R-L signal out of its front face. But for the side speaker facing left, you would want L-R coming out of the left side of the box, so you would wire the side speaker amp-plus-to-speaker-minus.)
One quick note - so you mounted the side speaker facing the other way? Very clever - but if so, (and just in case you didn't do this already) - make sure to reverse the (+) and (-) terminals on the side speaker, because reversing the mounting will also effectively reverse the phase of that speaker. (As designed, the side speaker faces front-ways out of the right side of the box, and when wired amp-plus-to-speaker-plus, it correctly outputs the R-L signal out of its front face. But for the side speaker facing left, you would want L-R coming out of the left side of the box, so you would wire the side speaker amp-plus-to-speaker-minus.)
Also a quick question: Your SFX MK2 has a finished hole on the front panel (to the lower left of the main front speaker). Is that a port? As you can tell from my pics, my SFX MK1 does not have one. The bass on it is pretty wimpy, but of course you can't expect much from a 10" instrument driver, either. If that is a port and it helps the bass response, that's great to know. But in any case, I highly recommend adding a sub to the setup if you need to bring out solid bass notes (I can also attest that the Alto above makes for a nice match).
Yes mine has a port, still not much bass, and today I hooked up a bass amp and adjusted it till it all sound good.
Regarding the side firing speaker, no worries, it is still facing the same direction, but not mounted on the same side of the board. I took it off and then mounted it from the other, side magnet first into the hole, so it's just moved over the thickness of the mdf board. Been looking online for a deal on an Alto sub, to bring out those low keys on the organ.
Regarding the side firing speaker, no worries, it is still facing the same direction, but not mounted on the same side of the board. I took it off and then mounted it from the other, side magnet first into the hole, so it's just moved over the thickness of the mdf board. Been looking online for a deal on an Alto sub, to bring out those low keys on the organ.
and I really get the tech behind this amp speaker combo. I am a long time recording engineer studio owner and when I read about it, I immediately thought, brilliant, Blumlein microphone technique in reverse. Decided to get the latest version for my church, which sound greats with their organ and piano. But couldn't afford (read, too cheap) to spend that much for myself, until I saw the Groovetubes V2 on guitarcenter used for a bargain price. Didn't realize it was so lacking in the treble range until I got it. Your mod saved it from gathering dust and getting sold. And I'm about to use it live for a band I just produced that wants my keys for the CD release party so they sound like the album. Fun! Thanks again!
Well, that's just great news all around! Your side-speaker placement solution intrigues me and inspires me. If I ever have to do this mod on a V2, now I'll know what to do. I'm sure that my father-in-law is looking down and smiling on us both!
P.S. ...and cheers on your church and your CD gigs!
P.S. ...and cheers on your church and your CD gigs!