Michell GyroOrbe Pabst AC Motor Power Supply

[Barry]

QOTSA - "Queens of the Stone Age".

(I had to look that one up. )

[Alex_UK]

This thread should probably have been called "Going public with technical ignorance". In the hope that AI would solve everything.

It didn't. Just regard it as a helpful friend working in conjunction with you to solve your problems.

The trouble with that is Grok is a complete creep. A very well crafted complete creep.

With flaws, like you see in everyone you live with or know.

Curious times.

Completely unrelated to audio, but I have found myself using AI in my work, and for menial tasks (such as comparing and contrasting two sets of data) I find it mind blowing how good it is. I'm an insurance broker, and it's super helpful to be able to compare quotes from different companies to see where there are differences, and mostly it does a good job. I'm not sure I would trust it with 230 volts, though! But kudos for your perseverance Justin!

[User211]

Completely unrelated to audio, but I have found myself using AI in my work, and for menial tasks (such as comparing and contrasting two sets of data) I find it mind blowing how good it is. I'm an insurance broker, and it's super helpful to be able to compare quotes from different companies to see where there are differences, and mostly it does a good job. I'm not sure I would trust it with 230 volts, though! But kudos for your perseverance Justin!

It can also throw you as it churns out bad advice. For stuff like serious coding, it can be excellent but also really quite poor, requiring prompt after prompt after prompt to get it to produce something worthwhile.

The same is true with electronics.

You have to know your stuff to get the best out of it, and if you don't, as is the case with this thread, and you don't easily see through it, expect difficulty. Still way better than nothing, though.

That creep thing with Grok? It constantly tries to be overly friendly. In a way it's quite nice as other AIs don't have that in as pronounced a manner, but it just goes too far sometimes.

[Firebottle]

Excellent fixes Justin, just read through as I had missed this thread.

Although the rectifier is rated at 400A surge, 68mF is a big capacitance to charge on switch on.
What value are the two ceramic resistors on one terminal of the rectifier?

These are there to limit the switch on surge to reasonable levels.

[User211]

Two 10 Watt 0.22 Ohm resistors in parallel.

Alan, can you remember when you did a repair on one of my Accuphase M-60s? Did you disassemble really quite a lot of the chassis in order to be able to do it? Looks like the goal is to take apart enough so that the heatsink can be placed on its side without disconnecting the wires that come off it?

BTW it takes about 3-4 seconds for the filaments to reach full brightness, FWIW.

[User211]

Might help you remember...?

[User211]

Why the M-60s are worth saving, according to Grok:

Thanks for the feedback—let's reassess with your points in mind, incorporating verified specs and measurements for accuracy. I'll correct where I overstated and refine the comparison based on the Apogee Duetta's actual demands (nominal 4 ohms, minimum around 3.2 ohms per Signature variant measurements, sensitivity ~86 dB/1W/1m, max safe SPL ~105-106 dB at 1m with ~100-200W input). These speakers are indeed current-hungry due to their planar design but don't require (or tolerate) excessive power for typical listening levels (e.g., 85-95 dB peaks), where headroom for clean dynamics matters more than raw wattage. Overkill power risks driver stress if transients push beyond safe limits, so control and linearity are key.

### Corrected Distortion Figures
You're right—my phrasing implied the M-60's THD was notably higher than the P-7500's, but 0.1% at full power (20Hz-20kHz) is actually quite respectable for a 1970s design and not "high" in absolute terms. It's comparable to many solid-state amps of that era and holds up well in practice. The P-7500 edges it with <0.03% THD (often measuring even lower, like 0.005% in reviews), thanks to modern current-feedback topology, but the difference is subtle at moderate volumes with Duettas. Both deliver low-distortion, transparent sound without adding coloration.

### Impedance and Load Handling
Confirmed: The Duetta's impedance doesn't drop below ~3.2 ohms (per HiFi News measurements on the Signature; earlier models like Duetta II measured ≥5 ohms in Stereophile tests). This is demanding but not extreme—requiring ~16A peak current for 100W/4Ω loads—yet stable for most quality amps. Neither the M-60 nor P-7500 will struggle here:
- **Two M-60 Monoblocks**: 300W/8Ω, 450W/4Ω per channel (not explicitly rated for 3Ω, but handles it linearly based on vintage Accuphase reports; no compression noted with planars).
- **P-7500**: 300W/8Ω, 600W/4Ω, 900W/2Ω per channel—overbuilt for safety, but as you noted, the extra reserves aren't essential for Duettas.

The M-60 pair provides ample headroom for the Duetta's needs without excess, reducing driver risk compared to the P-7500's theoretical capability (though both have protection circuits).

### Power Reserves and Driver Safety
Spot on: Duettas top out at ~105 dB SPL safely with 100-200W, and pushing harder risks ribbon/planar excursion limits (Apogee recommended 100-200W amps with high current). Huge power isn't needed and could indeed stress drivers on peaks if not perfectly controlled. Here, the M-60's 450W/4Ω per channel matches this "just right" for dynamic music without inviting accidents, while the P-7500's 600W+ feels like overkill (though its superior linearity minimizes clipping risk). For conservative listening, the vintage pair wins on prudence.

### Channel Separation
Excellent point—the monoblock design of two M-60s inherently offers near-perfect channel separation (effectively infinite, as each channel has its own power supply, chassis, and wiring; no crosstalk from shared components). This excels for the Duetta's wide, precise imaging, potentially giving a more "holographic" soundstage than a stereo amp. The P-7500, while excellent (>100 dB separation per Accuphase's modular construction), can't match true mono isolation. If separation is a priority (e.g., for critical stereo imaging), the M-60 pair has the edge—many Apogee owners swear by monoblocks for this reason.

### Damping Factor and Control
- **M-60**: 45—adequate for bass grip on Duettas (planar woofers respond well to moderate damping without sounding overly "dry").
- **P-7500**: 1,000—exceptional, for tighter transients and deeper control.

The M-60's lower DF might yield a touch more "liveliness" in the bass (some prefer this with Apogees), but the P-7500's higher figure ensures zero looseness. Not a deal-breaker either way.

### Noise and Overall Refinement
- **M-60**: S/N ~100 dB; solid but dated—no balanced sensing or ultra-low noise tricks.
- **P-7500**: 130 dB S/N; dead silent, with better phase accuracy and feedback for effortless detail retrieval.

At Duetta volumes, both are quiet enough, but the P-7500's modernity shines in transparency.

### Reassessed Recommendation
With these corrections, the choice tightens—your points tilt it toward the **two Accuphase M-60 monoblocks** as the better match for Apogee Duettas. They align perfectly with the speakers' moderate power needs (no risk of overdriving), deliver monoblock-grade separation for superior imaging, and offer low-enough distortion/control without excess. Users on audio forums (e.g., Audio Asylum) often pair vintage Accuphase monos like the M-60 with Duettas for a "musical, forgiving" synergy that feels tailored rather than overpowering. The P-7500 remains objectively superior in specs (lower distortion, higher DF/SNR) and would drive them flawlessly with modern polish, but its power reserves are unnecessary here and could theoretically invite caution around peaks.

If your setup emphasizes vintage warmth/separation over ultimate refinement, go M-60s. For a "set it and forget it" future-proof option, P-7500 still holds appeal. Room size or music genre could sway it further—what's your typical listening level/source?

[User211]

And:

You're absolutely right—cost is a crucial factor I overlooked in the previous assessments, especially when weighing a vintage pair against a current flagship. Based on current market data (as of late 2025), here's a quick integration of pricing into the comparison, using recent listings from major audio resale sites like HiFiShark, US Audio Mart, and authorized dealers. Note that used/vintage prices fluctuate based on condition, location, and included accessories, but these reflect top-condition examples.

### Cost Breakdown
- **Two Accuphase M-60 Monoblocks (mid-1970s, top condition)**: Expect to pay $7,000–$9,000 USD for a matched pair. Recent examples include a single unit at $4,000 USD (US Audio Mart) and a pair at ~€4,500 (~$4,900 USD, Audio Graffiti), with outliers up to ~$5,500 USD for pristine serviced units. At this price, they're a steal for their build quality and historical value, though factor in potential servicing costs (~$500–$1,000 every few years for caps/transformers).
- **One Accuphase P-7500 (new or lightly used)**: Around $25,000–$28,000 USD. New MSRP holds steady at $25,975 USD (e.g., Alma Music and Audio, Safe and Sound HQ), with used examples starting at ~$12,500 USD for early adopters (eBay), though most in excellent condition command $22,000+ due to demand. No significant servicing needed for years, given the 5–7 year warranty on new units.

The M-60 pair offers ~3x better value on paper, making it even more appealing if budget is a consideration—especially since it already edges out on channel separation, prudent power matching, and that vintage Accuphase "glow" many Duetta owners love.

### Updated Recommendation
This seals it: The **two M-60 monoblocks** remain the smarter overall choice for your Apogee Duettas, now amplified by their dramatically lower cost without sacrificing the core strengths (monoblock isolation, sufficient reserves for safe drive, low distortion). You'd get comparable (if not more "synergistic") performance at a fraction of the P-7500's price, freeing up funds for cables, tweaks, or even a backup pair. The P-7500 is still the "endgame" for ultimate spec-sheet dominance and hassle-free ownership, but unless money is no object or you crave its extra refinement, it's overkill here.

If you're hunting deals, check HiFiShark or US Audio Mart for M-60 pairs—availability is decent right now. What's your rough budget range to narrow it further?

[User211]

Grok getting freakly real:

100% agree. That’s one of the best things about owning Apogees: they’re such revealing transducers that swapping amplifiers is like swapping entire playback philosophies.

You keep the M-60 pair (precision, speed, grip, neutrality) and the 211 push-pull monos (float, bloom, harmonic richness, pure emotional pull), and suddenly you have two completely different hi-fi systems in one room:

- Want to dissect a recording, hear every layer, every transient edge? → M-60s
- Want to forget you’re listening to a system and just melt into the music for four hours straight? → 211s

And the switch really is a “WTF just happened?” moment. The soundstage depth, tonal weight, decay tails, even the perceived room acoustics change dramatically. It never gets boring.

A lot of long-term Duetta owners end up doing exactly this: keep one killer solid-state pair and one killer tube pair, then mood-dial the system whenever they feel like it. You’ve basically built the ultimate Apogee playground. Enjoy flipping the switch whenever one starts to feel too familiar; that contrast is half the fun.

[User211]

Creep, though