Musmy a75 clone (not a review)

[Malohm]

Thanks RickS -

I'm interested in collecting more (objective) data. Is anybody here with a suitable audio analyzer and location somewhere in the east of the Netherlands or western part of NRW in Germany with me?

 

[Malohm]

Alternatively, pls feel free to bring your amp for comparison to my listening room. AB switch and coffee are ready ...

 

[dlaloum]

True, but, at the same time, people mostly don't realise how much power is actually needed to represent transients. Which can become a quite hairy affair into reactive loads.

Roberto

In the realm of home theatre, the recordings are based on a peak headroom of 20db - so one can play safe by providing 25db of headroom - but that still results in a peak SPL of only 100db for a typical domestic environment average SPL of 75db....

With an 86db @1Wm speaker - that means circa 0.25W for 74db (@3m) continuous, and 128W for 101db (with excess headroom of 5db) - actual peak would be 95db which is achieved at 32W

So in theory, assuming that you listen at 75db average (from a single speaker - I have not calculated the impact of multiple simultaneous speakers - which substantially reduces the required power per speaker in actual use!- and note that my typical average SPL is 72db - which halves the power requirements!)
What you actually need is an amp capable of circa 1W into whatever load your speaker provides, and 32W peak / dynamic capability...
If you want additional margin ... 128W peak provides for 25db dynamic peaks.

Having said that, some of the more excessive "bass heads" in our community target 130db SPL peaks for their bass... and it is worth noting that on an 86db/wm speaker, 130db would require circa 65,000W - so obviously if such SPL's are desired, it will require subs with substantially higher efficiencies and / or a multitude of subs to aggregate to the required SPL's.

From my own experience and my own power utilisation measurements... I don't believe my setup has ever used more than 16W peak

note: my speakers are reactive with a capacitive tweeter, and impedance dropping down to 1.63ohm - amps that are unstable into that kind of load are audibly impacted, and immediately identifiable... the amps need not be overly powerful - although I have used Crown XLS2500's for some years - but much less powerful amps have shown themselves to be audibly identical to the very powerful crowns... such as Quad 606/707 - but those amps are "unconditionally stable into any load" as per specs!
So yes reactive loads have an impact, and powerhouse amps often can handle those reactive loads (and are specified down to 1ohm) - but there are also less powerful amps that can handle reactive loads without any trouble.

 

[Malohm]

Indeed ... and Amir's measurements here on ASR often include reactive load tests (indcuctive / resistive / capacitivy impedance). Some of these measurements are rather revealing. E.g. check out his Topping B200 results (output power varies between 140 and 210 Watts depending on load's phase.

For my situation, this is most relevant in the 100 Hz bass region where my Sonus Faber Cremona speakers drop below 3 Ohms. E.g. my home theater AVR (NAD T778) is struggling with these; bass sounds muddy even at moderate volume. Not so the Mumsys A75: output power remains constant within 0.1 dB down to 10 Hz with Cremonas connected, tested between 0.1W and 10W and measured using digital signal generator plus Fluke 8060 A RMS multimeter. A75 bass slam accuracy is also (subjectively) a lot better than that of T778. Thinking of getting rid of that ugly active 12" subwoofer I have for movies...

As per earlier quoted Amir tests, Topping B200 drops from 210W (at 4 Ohm) to 84W (at 2 Ohm) for purely resistive loads. This is about 4 dBs in logarithmic terms. I suspect this is is due to critical sizing of output stages and/or power supplies. Not a good match for my speakers, although I can see why other folks love these amps.

 

[Cobra2]

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Realizing that the amp's thermal design can easily handle higher idle currents, I have now shifted my bias from 13 mV (across the 0.22 Ohm source resistors of each of the 24 power mosfets) to 37 mV. This translates to roughly doubling the amps idle power (from 105W to 210W; slightly lower than Accuphase A75: 260W). Transistor temp when playing music loud is now around 50 degC, heatsink temp is 42 degC, top of case at 35 degC. All well within safe limits.

This effectively increases my class-A output range from 1.8W to 8W (at 4 Ohm) per channel, meaning all my listening is now class-A indeed. Transients, depth, mid-range liquidity and woofer grip have improved as expected. This is an easy (and reversible) mod which I highly recommend.

There are 2 pots on the board/pcb, is one bias/other dc-balance?

 

[Malohm]

Yes, and bias is the top one. I have measured the bias voltage between the transistor source pins (rightmost pin) and speaker out. In my case, the voltage variance between transistors was about +/-10%, i.e. the transistors are reasonably well matched. Pick the one with the highest bias to adjust. DC needed only small adjustment after increasing bias.