Does anyone care to ask an amplifier designer a technical question? My door is open.

@ramtubes. 

I’m not ready to abandon passives from one less than favorable experience. The passive I have is a Luminous Audio Axiom... one input, one output, and volume pot. I listen to vinyl 95% of the time, so I was feeding it from the output of a phono preamp. To my surprise, the output with the pot at about the 10:00 position was approximately the same level as my tubed linestage at 9:00. The sound was bright.. too bright, and the deeper bass sounded more prodigious, but a layer of midbass was missing, and female vocals, which had sounded natural minutes before (through the preamp) were almost painful to my ears. It was like someone made the old eq smiley face curve on my system. The cables I’m using are old Tara labs solid core... 1/2 m from the ‘table to the phono pre, and 1 m from the phono pre to the amplifier. The amp sensitivity is rated at 900 mv, the cartridge has an output of 2.5 mv, and the phono stage is 42 dB/47k. I was going to question you about preamps, but didn’t want to stray too far from the thread topic. However, as I have found, others may be interested in your answers. I read forum threads that interest me frequently, but rarely write in. This one caught my attention more than usual because it was started by someone who actually builds this stuff... and is known to do it successfully. No offense to anyone else! I read all opinions and add them to my mental database. One thing I’ve learned is, what appears to make sense on paper is not always the case in practice. Thank you for your time. Truly 

Ramtubes 11-27-2018:

Thanks for your measurements and math. I think the math will leave most with their head swimming. Perhaps you could add some details to the steps so that others, less math inclined, might work things out. it is nice for people to know how different speakers fall off with distance. Perhaps you could write that up for us. :)

I ask for SPL both at listening position and 1 meter for two different purposes.

I want to get to know the listener and measuring at 1 meter leaves out all those other calculations. One just works off the 1 meter speaker spec and adds or subtracts. If one is 3 db higher than the speaker spec he is at 2 watts, 6 db is 4 watts, 10 db is 10 watts, 20 db is 100 watts.

Thanks for your comments about my post, Roger.

I guess the first thing I should emphasize is that the dynamic ranges of the two recordings I referred to in my previous post, while being applicable to MY determination of minimum required amplifier power, are VASTLY greater than the dynamic ranges of most or all of the recordings most others listen to. And in fact are much greater than the dynamic ranges of most of the recordings I listen to. So my bottom line of a 32 watt minimum power capability, for my purposes, is likely to be considerably less for those who listen at comparable average volume levels (average SPLs say in the 70s at the listening position) **if comparably efficient speakers are used.**

This is consistent, btw, with statements I believe Roger made earlier in the thread to the effect that many listeners require less power than they tend to believe.

I’ll also say that the point to going through the kind of calculations I described in my previous post is of course NOT to make any kind of final determination of what amp to choose, which of course should be made by listening, assuming that is possible. The point is to narrow the field of candidates that are to be considered, and to minimize the likelihood of making an expensive mistake, that would perhaps work for some of the listener’s recordings (especially those having narrow dynamic range), but would not work (or at least would not work well) for other recordings (especially those having wide dynamic range).

Another important point to keep in mind is that speaker sensitivities are often specified as the SPL produced at 1 meter in response to an input of 2.83 volts, rather than 1 watt. For an 8 ohm speaker (that is truly 8 ohms) 2.83 volts corresponds to 1 watt, so the two kinds of specifications will be identical. But 2.83 volts into 4 ohms corresponds to 2 watts, so for example a 90 db/2.83 volt/1 meter/4 ohm speaker is really just an 87 db/1 watt/1 meter/4 ohm speaker.

Regarding measurements at 1 meter, I haven’t done that (aside from during the unique speaker calibration processes that are required to utilize the capabilities of my DEQX HDP-5), mainly because it wasn’t necessary for my purposes, and because at least in my case making the determination from the listening position and performing the necessary calculations isn’t difficult. Also, while 1 meter measurements would simplify the calculations somewhat, as Roger mentioned, one would still have to somehow address the effects on SPL of having two speakers, as well as addressing room effects in some manner.

... it is nice for people to know how different speakers fall off with distance. Perhaps you could write that up for us. :)

In general the SPL provided by a box-type dynamic speaker (meaning a speaker that is non-planar and is not a line source) will fall off at 6 db per doubling of distance, **neglecting the effects of room reflections.** (As can be seen in my previous post, I added 3 db to my calculations as a rough guess of "room gain" as perceived at the listening position). The fall-off of planar and line source speakers as distance increases will be significantly less than that, as those speakers tend to throw their sound forward. The reason being, I believe, that as listening distance increases the angle between the listener’s ears and central part of the speaker and the upper part and the lower part of planar and line source speakers decreases to a greater degree (pun intended) than in the case of typical dynamic speakers having drivers that are spaced relatively closely. Making the sound emitted by those parts of planar or line source speakers more able to contribute to the perceived SPL than at smaller distances.

A fall-off of 6 db per doubling of distance can be extrapolated to any combination of distances based on the formula 20 x log(D1/D2), where “log” is the base 10 logarithm. If D1 is greater than D2 the answer will be a positive number; if D1 is less than D2 the answer will be the same number except with a minus sign in front of it. One can perform this calculation with a scientific calculator, or with the calculator that is built into Windows if it is set to scientific mode, or with various online calculators. In my case my listening distance is 12 feet. 12 feet is 144 inches; 1 meter is 39.37 inches; 144/39.37 = 3.66 meters. 20 x log(3.66/1) = 11 db, rounded off. So the SPL produced by a single box-type dynamic speaker at 12 feet will be about 11 db less than at 1 meter, neglecting room effects.

The conversion of dbW to watts that I showed in my previous post is based on the ratio of two power levels, expressed in db, being 10 x log(P1/P2), where “log” is again the base 10 logarithm. If P1 is greater than P2 the answer will be a positive number; if P1 is less than P2 the answer will be the same number except with a minus sign in front of it. So it can be calculated that 32 watts is 10 x log(32/1) = 15 db greater than 1 watt, which can be expressed as 15 dbW. Converting in the opposite direction (15 dbW to 32 watts) is a bit trickier mathematically, but no doubt there are online calculators which can facilitate that.

Best regards,

-- Al

@maplegrovemusic  Roger - What brand and model subwoofer do you use ?

Its of my own design. It is very smal,l less than 1/3 cu ft. Therefore its corner frequency is 100 Hz and falls off at 12dB/octave. In the crossover I EQ it up at 12dB/octave so it comes out flat. I can start the bottom EQ wherever I want. Typically 30 Hz. It could be 20 Hz, Works the same way. 

This may be hard to understand because its no longer a mass loaded driver. It is operating in the region where regular woofers do not.  It is now an air spring drive and I call it the "Air Spring Woofer". It is not longer working under mass load physics. It is working under spring physics.

The amp is any old SS 100+ watt amplifier. Currently a nothing special Denon. The woofer level control is on the crossover.

This solves a lot of problems with powered subs. Unless they have a remote (do they now?) it is difficult to adjust the level for different CDs, I find when the woofer control is right next to the volume why not trim it. I  use a range of about 3 db for myself but always bump it up depending on what I think the listener might like. Generally Im playing to a under 35 croud and they like how the bass sounds .. and more of it, sometimes I give them 6-10 dB more. The bass control is marked in dB so I know where I am. 

Plate amps have short life if played loud. The solder joints just fall apart from vibration. Some just have a short life!

In my system the only cable going to the woofer is a speaker cable, shamefully 16 ga lamp cord. The woofer is already well damped and above the range it plays. The amplifier sits on the rack with everything else.

Do you know that most people do not hook up subs properly? I can sympathize, to do it right takes miles of wire. You have to run the preamp to the woofer and back to the main amp which may be far from the woofer. Theres cable capacitance issues, potential noise pickup, grounding, and the incredible cost of premium long cables. I presume 4 meter cables cost heaps more than half meter.

In either case, if you dont use the crossover you do not get the benefit of taking the bass workload off the satellites. I bet 90% of the subs out there do not use the crossover for that and the reason audiophiles dont trust putting their sound through the XO and rightly so. The electronics in most subs is not great.

Why dont sub makers put the amp and xo into a nice chassis to put on your rack? If this is such a great idea why doesnt everyone do it?  Two good questions for sub makers :)

I'd like to echo Roger's comment regarding the use of the term "hybrid" in reference to the RM-200. I also once thought of this amp in the same manner, but given my access to Roger I have the opportunity to not only listen to a lot of his designs but understand how certain designs came to be. I also have the benefit of being the one who tests repaired amps and new designs in my system.

So I was able to borrow an RM-200 for a while and at first I can't say the amp thrilled me, but over time it did grow on me (I prefer 6550s to KT-88s). I certainly don't think of the circuit as hybrid any longer, in fact to me it's a tube amp, and a cleverly designed one at that. As previously mentioned it may be the only amp with a tube output that puts out more power into lower loads. In addition, as I think about it, those amps referred to as hybrids that I have heard of use tubes on the input and transistors on the output. I have not heard of any others with the opposite design configuration.

When it comes to MR amplifiers the RM-10 MkII is my all time favorite. My old friend Paul Rosenthal (RIP Pubul57) and I ran similar systems at one time. An RM-10 with Lightspeed Attenuator. We always commented to each other how such big sound could come from such a small package. As Roger mentioned one of the things he designs for is reliability. I have owned mine for 12 years now, and other than replacing the power tubes have had zero issues with it.