The Pioneer Hpm-100. For many audiophiles, the name alone conjures images of 70s hi-fi and a sound that defined an era. These iconic speakers, born from Pioneer’s ambition to challenge JBL’s dominance, are more than just vintage audio equipment; they’re a piece of audio history. Legend has it that Bart Locanthi, formerly head of engineering at JBL and the mastermind behind the legendary JBL L-100 Century, joined Pioneer in 1975. His mission? To create the HPM series as direct competitors to JBL monitors. And they succeeded, capturing market share and etching their name in audio history.
Alt text: Pioneer HPM-100 speaker brochure showcasing the iconic 1970s design.
Alt text: Pioneer HPM-100 brochure highlighting Bart Locanthi’s influence on the speaker design.
These speakers exude quality from the moment you encounter them. Weighing in at a substantial 26.7kg (almost 60 lbs), their heft is immediately noticeable. In an age of lighter, less robust Japanese speakers, the HPM-100 stood out with its remarkable craftsmanship, especially considering its mass-production scale. The impressive 50mm thick faceplate is just one example of the robust build quality. The JBL influence is palpable throughout the HPM-100’s design, and we’ll delve into driver similarities and crossover performance later in this exploration.
The Patient Arrives: Assessing a Vintage Pioneer HPM-100 Pair
Despite their legendary status, time takes its toll. When a friend brought me a pair of Pioneer HPM-100 speakers for restoration, I eagerly accepted the challenge. While acquired at a reasonable price, they had clearly seen better days. Unfortunately, I didn’t capture high-resolution “before” photos, but the primary issues were evident:
- Cosmetics: The speaker cabinets bore the scars of time and misuse. Water rings and various marks marred all surfaces, indicating they had been used as makeshift tables. The black faceplates also showed scratches.
- Damaged Midrange Driver: After decades of use, the midrange driver was in poor condition. It’s not uncommon for vintage speakers to suffer damage over 25 years, whether from accidents or simply wear and tear. In this case, the midrange had clearly endured some kind of impact.
- Crackling Bass: Distorted sounds emanated from the bass units, particularly at higher sound pressure levels (SPLs). Initially, the source was unclear, but closer inspection revealed a hole in one of the woofer surrounds.
- Listening Fatigue (“Headaches”): The owner reported experiencing headaches after prolonged listening sessions with the Pioneer HPM-100s. While subjective, this complaint suggested potential sonic imbalances or distortions.
Alt text: Close-up of cosmetic damage on a Pioneer HPM-100 speaker cabinet showing scratches and marks.
Alt text: Damaged midrange driver of a Pioneer HPM-100 speaker with a broken dust cap and cone deformation.
Alt text: Hole in the surround of a Pioneer HPM-100 woofer, causing air leaks and distorted bass.
Upon initial listening, it was immediately clear that something was amiss. The sound was a “mess,” as the owner described. Headaches after listening? Definitely plausible. It was time for objective analysis using speaker measurements to diagnose the issues.
Time to Measure: Unveiling the Sonic Truth of the HPM-100
Before diving into measurements, a crucial disclaimer: measurements are tools, not absolute judgments. They provide valuable context but don’t capture the entire subjective listening experience. Over-reliance on graphs can be misleading. Measurements alone can’t determine if a speaker sounds “good” or “bad” unless severe flaws exist. However, for speakers, measurements are invaluable for verifying fundamental aspects like tonal balance and identifying driver or crossover problems. Distortions, cancellations, and unusual frequency response curves immediately signal potential issues. Box simulations and in-room measurements inform us about bass performance. Sensitivity and impedance plots indicate amplifier compatibility. A clean bill of health from measurements is a prerequisite for meaningful subjective evaluation.
However, measurements don’t reveal everything. Subtleties like micro-detail retrieval, transparency, soundstage depth, and the impact of premium crossover components remain outside the scope of typical graphs. A capacitor, for instance, is not just a capacitor; quality matters. In-depth psychoacoustic understanding is needed to extract deeper meaning from raw data – a long-term goal of mine.
Now, let’s examine the initial SPL (Sound Pressure Level) measurements of these vintage Pioneer HPM-100 speakers.
Alt text: Initial SPL measurements of both Pioneer HPM-100 speakers, showing a significant dip in the crossover region due to reversed midrange polarity.
dBSPL @1 Meter on Tweeter axis, 1/12 octave smoothing, 4ms gate window. Blue – right, red – left speaker. Both attenuators at 0dB.
The initial frequency response was alarming. For anyone familiar with crossover design, the deep dip in the midrange region was a telltale sign. This “litmus test” strongly suggests driver polarity issues at the crossover point. Reversing a driver’s polarity and observing the depth of the resulting dip is a standard technique to assess driver integration at crossover frequencies. The measurements clearly indicated that someone had reversed the polarity of both midrange drivers – likely during a previous repair attempt. But why?
Alt text: Close-up of the HPM-100 internal wiring, showing potential for misconnection if not carefully traced.
Alt text: Wiring diagram illustrating the color-coding for Pioneer HPM-100 speaker drivers, highlighting potential polarity confusion.
It’s surprisingly easy to miswire the midrange drivers during reconnection. The white wire connects to the positive terminal on the midrange, whereas on the tweeter and woofer, it connects to the negative terminals. This unusual wiring scheme can easily lead to errors. Correcting the midrange polarity yielded a dramatically improved frequency response.
Alt text: SPL measurements after correcting midrange polarity, showing improved frequency response and driver integration.
dBSPL @1 Meter on Tweeter axis, no smoothing, 4ms gate window. Blue – right, red – left speaker. Both attenuators at 0dB.
Much better! One can only wonder how long someone listened to these Pioneer HPM-100 speakers with reversed midrange polarity – perhaps long enough to decide to sell them. With corrected polarity, they finally sounded like a pair of speakers, not audio sirens. The headache complaint might be resolved simply by this correction. However, issues remained, particularly around the woofer-midrange crossover and the high-frequency response. But for now, a significant improvement had been achieved.
Alt text: Full-range SPL measurements of the Pioneer HPM-100 with corrected midrange polarity, showing overall frequency response.
dBSPL @1 Meter. 1/12 octave smoothing. 4ms gate window. Tweeter axis. Mid attenuator at +3dB, High +2dB. Near field stitch @200Hz.
Alt text: Frequency response graph from a 1977 Pioneer HPM-100 promotional brochure, for comparison with measured results.
From a 1977 US promotional brochure. No details provided.
Comparing our measurements to the frequency response curve published in a 1977 Pioneer HPM-100 brochure revealed a reasonable resemblance. Let’s temporarily disregard the dip around 1kHz, which we’ll investigate later. It’s hard to imagine anyone enjoying these speakers with the attenuators set to “+3dB Mid, +2dB High” as shown in some settings. Even a brief sweep tone at those levels became ear-splittingly harsh.
The Whistling Woofer: Addressing Bass Distortion
The unpleasant whistling sound emanating from the woofer was a clear indication of a problem. Upon closer examination, the issue became apparent: air tightness was compromised (pun intended). The textile surround, heavily treated with a viscous goop (likely phenolic resin-based) intended to seal and damp it, had deteriorated over time.
Alt text: Close-up of the Pioneer HPM-100 woofer surround showing the hardened and accumulated viscous goop, causing surround stiffness and air leaks.
Over the years, this goop had flowed downwards, accumulating in the lower portion of the surround and stiffening the material. This restricted cone movement in that area, eventually causing the textile to tear and create air leaks. Fortunately, the repair was relatively straightforward.
Alt text: Repair process of the Pioneer HPM-100 woofer surround involving removing excess goop and applying speaker repair adhesive.
The first step was to remove the excess hardened impregnation. A hairdryer and fine tweezers proved effective for carefully softening and extracting the goop, being cautious not to further damage the surround. Next, several thin coats of rubberized PVA adhesive, commonly sold online as “speaker repair glue,” were applied to reseal the surround. While standard PVA glue could also be used, its lack of flexibility when fully cured makes it less ideal for long-term durability.
The Midrange Crisis: Finding a Replacement Driver
The damaged midrange driver presented both technical and aesthetic challenges.
Alt text: Side-by-side comparison of a damaged and a good Pioneer HPM-100 midrange driver, highlighting cone wrinkles and dust cap damage.
While the dust cap could be replaced using a salvaged part from a similar driver, the cone damage was more severe. Significant wrinkles on one side compromised cone rigidity and concentricity. This eccentricity caused the voice coil to rub against the magnetic structure at larger excursions, misaligning it within the magnetic gap.
Here’s a challenge: can you identify the damaged driver based solely on the SPL graphs below, without looking at the descriptions?
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Alt text: SPL graphs comparing a good (red) and damaged (blue) Pioneer HPM-100 midrange driver.
dbSPL@1m, 4ms window, good driver (red), damaged driver (blue).
Counterintuitively, the blue graph (damaged driver) appears smoother. This is because the broken dust cap no longer produces the high-frequency breakups in the 10-20kHz range, and the non-uniform cone rigidity results in a more controlled breakup around 5-10kHz.
Alt text: Total Harmonic Distortion (THD) measurements comparing a good and damaged Pioneer HPM-100 midrange driver, revealing higher distortion in the damaged unit.
THD@1Vrms. Black – Total THD, Brown – Fundamental, Red – 2nd, Light Brown – 3rd, rest – higher order harmonics.
However, the distortion measurements tell a different story. The damaged driver exhibits significantly higher higher-order harmonics in the critical 1-3kHz range. This distortion is definitely audible. The THD hump around 300Hz is expected, corresponding to the driver’s resonant frequency (fs).
Alt text: Replacement used Pioneer HPM-100 midrange drivers sourced for the restoration project.
Therefore, a search for replacement Pioneer 10-721* midrange drivers began. After some searching, a used pair in good condition was located and acquired. While not inexpensive, having a viable replacement was essential. The Pioneer HPM-100’s popularity, both then and now, meant that parts, though sometimes pricey, were still obtainable, resolving the midrange crisis.
The Makeover: Veneering and Cabinet Refinishing
With the functional issues addressed, the cosmetic condition remained a significant concern. As the saying goes, “ugly speakers don’t sound good!” – psychology plays a significant role in our perception of audio quality. Since visual appeal is important to most listeners, addressing the worn cabinets was crucial.
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Alt text: Comparison of American dark walnut and straight grain walnut veneer samples for Pioneer HPM-100 cabinet refinishing.
Replacing the cabinets entirely was not feasible, so revenering was chosen. Walnut veneer was selected to restore the classic aesthetic. However, the variety of walnut veneers available was surprising. After considering options, a lighter, straight-grain walnut veneer was chosen to provide contrast with the black faceplate. This lighter veneer would darken to a richer tone after applying a semi-matte lacquer.
Alt text: Pioneer HPM-100 cabinets after applying straight grain walnut veneer, ready for final finishing.
While not a woodworking enthusiast myself, having done my share of woodworking projects, I didn’t document the veneering process in detail. It’s not overly complex. Oversized veneer pieces are cut, and corner over-cuts are planned. Veneer pattern orientation is crucial. PVA wood glue is applied to both the veneer and cabinet surfaces, and allowed to cure slightly. Then, a standard clothes iron (dedicated for this purpose only) is heated to around 200°C (392°F) and used to press the veneer onto the cabinet, activating the glue. Practicing on scrap pieces beforehand is recommended to get a feel for the process.
Corner trimming is the trickiest part. A sharp office knife with high-quality blades is essential. Make the final over-cut slightly beyond the edge, and finish with fine-grit sandpaper. The resulting bond is incredibly strong; removing the veneer afterward would likely tear off MDF board layers with it – testament to the effectiveness of this method.
Alt text: Close-up of a veneered Pioneer HPM-100 cabinet corner, showing a clean and seamless joint.
Alt text: Pioneer HPM-100 cabinets after veneering and repainting the black faceplate, showing the rejuvenated appearance.
With careful work, the corners should appear as shown. Three coats of TIKKURILA Paneeli–Ässä water-based semi-matte lacquer were applied to the new veneer. The front panels received a fresh coat of general black-matte spray paint. At this stage, the Pioneer HPM-100 speakers were beginning to look promising once again.
Box Analysis: Simulating Bass Performance
Speaker “in-room” performance is a complex topic deserving separate exploration. Speaker cabinet design and room placement significantly impact perceived bass response. For now, let’s simulate the Pioneer HPM-100 cabinet to understand its expected bass behavior in a typical setting.
Alt text: Thiele/Small parameters of the Pioneer 30-733A-1 woofer used in box simulation software.
The Thiele/Small (TS) parameters of the Pioneer 30-733A-1 woofer were measured using the added mass method. Inputting these parameters into LspCAD simulation software yielded the following results.
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Alt text: Simulated half-space (2Pi) SPL response of the Pioneer HPM-100 woofer in its factory cabinet.
Half space (2Pi) SPL at 1m distance.
The simulation in the factory 68-liter cabinet predicts a 3dB peak around 65Hz. This is corroborated by near-field measurements and even hinted at in the original promotional brochure. This peak indicates that corner placement of these speakers should be avoided. In corners, with full room gain, this peak could translate to a substantial +7dB boost in the 50-100Hz range, leading to a boomy, unbalanced sound. Positioning them on stands at least 50cm (20 inches) from the floor and away from walls is recommended to allow them to breathe. Additional cabinet stuffing with damping material can also help to tame the bass peak. Experimenting with a small amount of foam in the vents is another option.
Alt text: Adding damping material (box stuffing) to the Pioneer HPM-100 cabinet to optimize bass performance.
Alt text: Close-up of the massive faceplate of the Pioneer HPM-100 cabinet, highlighting the robust construction.
Aside from the bass resonance, the cabinet design itself is impressive for a mass-produced speaker. The substantial faceplate alone speaks volumes about the build quality.
Original Crossover Analysis: A Glimpse into 70s Design
Earlier, I mentioned the potential JBL influence on the Pioneer HPM-100. This becomes particularly evident when examining the crossover networks of both speakers.
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Alt text: Side-by-side comparison of the original Pioneer HPM-100 crossover (left) and the JBL L100 Century crossover (right), highlighting design similarities.
HPM-100 original cross-over (left), JBL L100 century original cross-over (right).
Designing your own drivers offers the luxury of crossover simplicity. Pioneer, unlike JBL, opted for a first-order filter on the woofer (a 0.6mH inductor). However, they mirrored JBL’s approach with the midrange, employing only a capacitor for high-pass filtering. Both speakers utilize a second-order high-pass filter on the tweeter. I’ve always had reservations about such rudimentary networks, especially in 3-way speaker designs. While it can work for carefully designed drivers in a 2-way system, a 3-way speaker demands more sophistication.
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Alt text: Driver comparison: Pioneer 30-733D woofer (left) and JBL 123A woofer (right), showing visual similarities.
Woofers: Pioneer 30-733D (left), JBL 123A (right). Tweeters: Pioneer 45-711C (left), JBL LE5-2 (right).
The similarities extend to the driver units themselves. Looking at the woofers and tweeters side-by-side, the resemblance to JBL drivers is undeniable. While direct comparison measurements with original JBL drivers would be needed for definitive confirmation, the visual similarities are striking.
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Alt text: Midrange driver comparison: Pioneer 10-721A (left), JBL LE5-2 Alnico (center), JBL LE5-12 (right).
Mid-range: Pioneer 10-721A (left), JBL LE5-2 Alnico (center), JBL LE5-12 (right)
The midrange driver comparison reveals some divergence. The original JBL L100 Century used the highly regarded LE5-2 with an Alnico magnet and superior construction compared to the Pioneer 10-721A. The JBL LE5-12, a later, more cost-effective version of the LE5-2 with a ceramic magnet, is likely the basis for the Pioneer 10-721A design.
Alt text: Measured SPL response of the Pioneer HPM-100 with the original crossover, revealing a significant dip in the critical midrange frequencies.
dBSPL@1m, 4ms window, no smoothing. Black – total, Green – woofer, Red – midrange, Blue – tweeter. Attenuators +0dB.
The performance of the original crossover, as measured, is far from ideal. The critical 1-3kHz region shows a significant dip. As we saw earlier, reversing the midrange polarity exacerbates this issue. And the “super-tweeter”? It’s largely for show. Measuring approximately 10dB lower in output and out of phase with the other drivers, it contributes minimally to the overall sound, becoming active only above 12kHz – inaudible to most and likely even to dogs. Disconnecting it is a reasonable step for improved sonic coherence.
Alt text: Individual driver SPL responses in the Pioneer HPM-100 cabinet, showing driver behavior and crossover points.
dBSPL@1m, Tweeter axis, 4ms window, no smoothing. Green – woofer, Red – midrange, Blue – tweeter.
Examining the individual driver responses reveals further insights. The woofer’s high-frequency rolloff is not smooth, exhibiting breakup modes above 5kHz. The tweeter struggles to extend much below 4kHz, while the midrange performs reasonably well within its intended band. Let’s delve deeper using simulation.
70s Design vs. Modern Simulation: Uncovering Crossover Flaws
In the 1970s, sophisticated speaker crossover simulation tools were not readily available. Today, we have powerful software like LspCAD to analyze and optimize designs. Let’s use it to examine the original HPM-100 crossover more closely.
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Alt text: Schematic diagram of the Pioneer HPM-100 original crossover network, as implemented in LspCAD simulation software.
HPM-100 original cross-over filter.
In LspCAD, I modeled the original crossover circuit, replacing the front panel attenuators with equivalent fixed resistors to simulate the “0” dB settings. Matching the simulated response to the measured SPL proved challenging.
Alt text: Simulated SPL and phase responses of the Pioneer HPM-100 original crossover with correct driver polarity.
dBSPL and Phases @1 Meter. Blue – woofer, Red – midrange, Green – tweeter.
The simulated frequency response initially didn’t align with the measured results. Unless… could another driver polarity be incorrect? To complicate matters further, Pioneer neglected to mark tweeter polarity on the terminals.
Alt text: Simulated impulse responses of the Pioneer HPM-100 tweeter with inverted (green) and correct (blue) polarity.
Impulse responses of inverted (green) and correctly connected (blue) tweeter.
Individual driver measurements were imported into LspCAD with their correct polarities, determined by impulse response checks during measurement. However, when analyzing the original crossover measurements, polarity wasn’t re-verified. What were the odds of both midrange and tweeter polarities being reversed from the factory? Surprisingly high, it turned out.
Alt text: Simulated SPL and phase responses of the Pioneer HPM-100 original crossover with inverted tweeter polarity.
dBSPL and Phases @1 Meter. Inverted tweeter. Blue – woofer, Red – midrange, Green – tweeter.
With the tweeter polarity inverted in the simulation, the simulated SPL response now closely matched the measured response. This raised a perplexing question: how did these speakers leave the factory with potentially incorrect wiring?
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Alt text: Pioneer HPM-100 wiring diagram showing potentially reversed polarity for midrange and tweeter drivers at the crossover board.
According to the wiring diagram, the positive tweeter terminal (red wire) becomes white and connects to the negative terminal on the crossover board. Similarly, the green wire from the positive midrange terminal goes to the negative terminal on the crossover board. This suggests both midrange and tweeter were intended to be wired inverted, contradicting the schematic diagram which depicts all drivers in phase. Confusion reigns.
Update: A reader provided a schematic confirming that their original HPM-100 units indeed had both midrange and tweeter drivers wired inverted. This means the initial SPL measurement with reversed midrange polarity was actually correct to the original factory wiring! The service manual schematic appears to be incorrect. Incredible.
Alt text: Schematic diagram from a Pioneer HPM-100 owner confirming factory-inverted polarity for midrange and tweeter drivers, contradicting service manual.
Let’s revisit the SPL graphs, considering both wiring configurations.
Alt text: Animated GIF comparing simulated SPL responses of the Pioneer HPM-100 original crossover with inverted and correct tweeter polarity.
dBSPL@1Meter. Inverted & correct tweeter. Blue – woofer, Red – midrange, Green – tweeter.
The key takeaway is that regardless of tweeter polarity, the original crossover design is flawed. The midrange is tasked with extending too high, overlapping excessively with the tweeter. This leads to cancellations and a non-coherent soundstage. To compensate, the woofer is forced to extend too high, up to 3kHz, creating a sonic mess.
Alt text: Animated GIF comparing simulated phase responses of the Pioneer HPM-100 original crossover with inverted and correct tweeter polarity.
Phase@1Meter. Inverted&correct tweeter. Blue – woofer, Red – midrange, Green – tweeter.
Phase plots reinforce this conclusion. In both wiring configurations, the drivers operate largely independently, lacking phase coherence, particularly between the tweeter and midrange. This lack of phase alignment likely explains why Pioneer didn’t bother marking tweeter polarity in the first place – the crossover design itself is fundamentally flawed in terms of driver integration.
The Sound: Original vs. Potential
Before any measurements, I always prefer to listen to speakers to form an initial, unbiased impression. However, the “midrange crisis” in this case necessitated measurements first. After replacing the damaged midrange and connecting it “correctly” (in phase according to standard convention), I conducted an initial listening session. I was underwhelmed, suspecting something was still wrong, which prompted the crossover analysis.
After analyzing the original crossover and listening again with the factory-intended wiring (inverted midrange and tweeter), my initial impressions were confirmed. While the Pioneer HPM-100 possesses certain appealing qualities, significant sonic shortcomings are evident.
On the positive side, the HPM-100 exhibits impressive dynamics. Running the midrange with a single capacitor contributes to a certain liveliness and immediacy reminiscent of full-range drivers. The LE25-clone tweeter, despite its measured imperfections, performs surprisingly well, delivering smooth upper treble without harshness. The 12-inch woofer extending into the midrange and lower treble imparts a unique character that must be experienced to fully appreciate. The bass is ample and, with proper placement, avoids boominess. This combination creates an initially engaging and “fun” sonic presentation that is undeniably appealing for casual listening.
However, extended listening reveals significant flaws. The soundstage is flat and dimensionless, lacking instrument separation and depth. Classical music, in particular, sounds congested and distorted. Regardless of front panel attenuator settings, a proper midrange-high frequency balance is unattainable; the sound is perpetually stuck in a “loudness” mode. Vocals, crucially, simply don’t sound natural.
In summary, the original Pioneer HPM-100 is suitable for parties and background music but falls short for critical listening.
New Crossover Design: Unlocking the HPM-100’s Potential
Convinced that the original crossover was severely limiting the HPM-100’s potential, I embarked on designing a new network. The goal was not simply to “fix” the flaws but to unleash the inherent capabilities of the drivers themselves.
After extensive simulation and listening tests, the following crossover design emerged.
2024 Update: Based on user feedback from builders of this crossover, the midrange capacitor value has been updated from 4.7uF to 3.9uF. While I no longer own HPM-100s to personally evaluate this change, user consensus suggests it is a preferred modification. As noted below, midrange level can always be fine-tuned using resistor R2011.
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Alt text: Schematic diagram of the redesigned Pioneer HPM-100 crossover network (version 1.2).
New HPM-100 cross-over network (v1.2).
A connection diagram is provided for builders unfamiliar with schematic reading. A spreadsheet containing component lists and Parts Express order links is available for download: HPM_NEW_Cross_Parts_2024.pdf.
Alt text: Connection diagram for the redesigned Pioneer HPM-100 crossover network (version 1.2), simplifying component wiring.
Connection diagram for new HPM-100 cross-over network (v1.2).
The new design retains a first-order filter on the woofer, but with impedance correction to improve phase matching with the midrange. The woofer still extends into the lower midrange, respecting the original design intent. The midrange utilizes a capacitor for high-pass and a second-order filter for low-pass. An RCL circuit aids phase alignment with the tweeter, which employs a simple second-order high-pass filter. While a more complex tweeter filter could further flatten the top-octave response, sonic testing suggested a simple second-order filter was sufficient.
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Alt text: Diagram illustrating the influence of resistors R3021 and R2011 on tweeter and midrange levels in the new HPM-100 crossover.
R3021 and R2011 influence for new HPM-100 cross-over network.
The overall voicing is slightly bright, so increasing R3021 can tailor the high-frequency balance to personal preference. Midrange level can be further adjusted by increasing R2011 to 14, 16, or even 18Ω. All inductors except L1101 should be air-core with minimal DC resistance. Capacitors should be audio-grade polypropylene (PP), with Jantzen Standard Z-Caps (C1021 can be a Cross-Cap) as a recommended minimum.
Alt text: Simulated SPL response of the redesigned Pioneer HPM-100 crossover, showing improved driver integration and smoother frequency response.
dBSPL@1Meter. Blue – woofer, Red – midrange, Green – tweeter.
The SPL graphs reveal significant improvements. Driver integration is vastly improved, with smooth transitions and no major dips or peaks. The woofer now exhibits a clean second-order rolloff from a 1kHz crossover point, eliminating its previous high-frequency breakup issues. The midrange no longer attempts to reproduce high frequencies, and the tweeter is smoothly integrated with a 4kHz crossover frequency and second-order rolloff.
Alt text: Simulated phase response of the redesigned Pioneer HPM-100 crossover, demonstrating excellent phase coherence across the audio spectrum.
Phase@2Meters. Blue – woofer, Red – midrange, Green – tweeter.
Crucially, the phase response shows excellent coherence across almost the entire audio spectrum. Phase coherence is paramount for accurate soundstage reproduction and imaging. Perfect phase alignment is targeted at the listening distance rather than the standard 1-meter measurement distance, a subtle but potentially audible refinement.
Alt text: Simulated impedance curve of the Pioneer HPM-100 with the redesigned crossover, maintaining a minimum impedance of 5 ohms.
Speaker impedance – 5Ω minimum.
The impedance curve remains above 5Ω, similar to the original design, ensuring compatibility with a wide range of amplifiers, including low-power single-ended tube amplifiers.
Alt text: Measured SPL response of the Pioneer HPM-100 with the redesigned crossover, closely matching the simulated response.
Measured dBSPL @1Meter. 1/12 octave smoothing. 4ms gate window. Tweeter axis.
The measured SPL response of the prototype crossover closely matches the simulation. The slight hump around 1kHz in the measurement was due to using a 1mH inductor for the woofer instead of the specified 1.2mH during prototyping. With the correct 1.2mH inductor, this hump disappears.
Update: To retain the original front panel attenuators, the following circuit modification can be implemented.
*However, using 45-year-old potentiometers is strongly discouraged unless they are in pristine condition or replaced with new, high-quality attenuators. Crackling or inconsistent potentiometers in the signal path negate the benefits of upgraded crossover components.
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Alt text: Schematic diagram of the redesigned Pioneer HPM-100 crossover network incorporating the original passive attenuators.
New HPM-100 cross-over network with original passive attenuators.
Minor component value adjustments to C2011 and R3021 are needed to maintain adjustment headroom. Minimum impedance remains above 4Ω even at maximum attenuation. However, front panel markings will no longer be accurate. “Zero” will not be truly neutral, and “+3” or “-3” will not have their original meaning. Pioneer used ~30Ω attenuators (marked as 8Ω), so small rotations produce noticeable SPL changes, potentially making precise tonal balance challenging. However, for those who enjoy tweaking settings, this option retains the original adjustability.
Alt text: Connection diagram for the redesigned Pioneer HPM-100 crossover network with original passive attenuators, detailing wiring modifications.
Connection diagram for new HPM-100 cross-over network with original passive attenuators.
Listening Impressions: Transformation and Revelation
The Pioneer HPM-100, with the new crossover, is fundamentally transformed. It’s no longer the same speaker. Those accustomed to its exaggerated, “in-your-face” sonic character might be initially disappointed. Indeed, anyone resistant to change in general should avoid this modification. But for those seeking genuine sonic improvement, the transformation is remarkable.
After listening to the new crossover for a week, even with prototype-grade components, I was captivated. Despite using basic polypropylene capacitors and cored inductors from my “crossover prototyping” parts bin, the sound was compelling enough to warrant publishing the design with my endorsement.
Classical music now breathes with space and dimensionality. Instruments occupy distinct positions within a coherent soundstage. Imaging is dramatically improved. The “sparkle” and “bite” remain, but are now refined and controlled. Harshness and sibilance are eliminated. The speakers now perform “almost” everything correctly. Vocals sound natural and human – a key indicator of success.
The new sonic presentation is realistic and balanced. While it doesn’t sound like a modern, ultra-analytical speaker, the problematic sonic characteristics of the original HPM-100 are gone. The core virtues of the speaker – dynamics and engaging presentation – are retained and enhanced. With higher-quality crossover components – premium capacitors and air-core inductors – the improvement would be even more pronounced.
Circus Leaves Town: Refurbishment and Departure
Ultimately, despite the sonic transformation, the decision was made to revert to a rebuilt version of the original crossover. This was due to the owner’s desire to maximize resale value and his inability to audition the new crossover design personally, being located in a different country. I believe in letting listeners compare and decide for themselves, rather than imposing my preferences.
Alt text: Side-by-side comparison of the original Pioneer HPM-100 crossover (left) and the new crossover prototype (right).
Alt text: Rebuilt original Pioneer HPM-100 crossover using higher quality components, ready for installation.
Factory HPM-100 cross-over network remade with quality parts.
The original crossovers were rebuilt using higher-quality components. Drivers received new wiring with appropriate gauge speaker cable. New gold-plated binding posts provided a finishing touch. And with that, the restored Pioneer HPM-100 speakers were returned to their owner.
Final Thoughts: Reflections on a Vintage Icon
This Pioneer HPM-100 restoration project was a personal journey, fulfilling a long-held desire to experience these iconic speakers firsthand. It served as a reminder that childhood dreams can sometimes lead to inevitable disappointments. Managing expectations is key.
Throughout the project, the impression that the HPM-100 is essentially a JBL L100 “clone” persisted. While the original JBL L100 is not without its flaws, it possesses a more “real” and “pro-audio” feel compared to the original HPM-100.
I wouldn’t personally own HPM-100s in their original configuration. While they have their appeal and clearly resonate with many listeners, they are not to my personal taste for serious listening. However, with the new crossover, I would gladly own a pair – though not as my primary speakers. Having moved beyond the “buy & upgrade” audiophile phase into “build from the ground up,” my preferences have evolved. Yet, I believe HPM-100s with the redesigned crossover would find a place in many systems. This is a somewhat biased opinion, but one I stand by.
Alt text: Restored Pioneer HPM-100 speaker showcasing the refinished walnut veneer cabinet.
Alt text: Pioneer HPM-100 speakers after restoration, highlighting the classic 70s aesthetic.
Alt text: Angled view of the restored Pioneer HPM-100 speakers, emphasizing the craftsmanship and design.
My primary motivation for sharing this lengthy account is to offer a second chance to Pioneer HPM-100 speakers. I suspect I’m not the first, nor will I be the last, listener underwhelmed by their original sound. If you find yourself in a similar position or simply crave a change, consider the new crossover design. It might profoundly alter your perspective, as it did mine.
And so it goes. Here are more images of the final restored Pioneer HPM-100 speakers.
Alt text: Front view of the restored Pioneer HPM-100 speaker, showing the black faceplate and walnut veneer.
Alt text: Pioneer HPM-100 speaker with restored cabinet and refreshed black front panel.
Alt text: Detail shot of the Pioneer HPM-100 cabinet corner with straight grain walnut veneer.
Alt text: Restored Pioneer HPM-100 speaker grille removed, showcasing the driver array.
Alt text: Pioneer HPM-100 speaker with grille in place, maintaining the vintage aesthetic.
Alt text: Side profile of the restored Pioneer HPM-100 speaker, highlighting the cabinet depth.
Alt text: Pioneer HPM-100 speaker badge detail, a symbol of vintage audio quality.
Alt text: Rear view of the restored Pioneer HPM-100 speaker showing new binding posts.
Alt text: Full view of the pair of restored Pioneer HPM-100 speakers in a listening environment.
