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Old 24th June 2010, 06:05 PM   #1
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Default Surprisingly good "amplifier" for Betsy K

I have my Betsy Ks in 6 ft^3 ported enclosures (tuned to 40 Hz). I have them in my office directly behind my desk so I can listen to them in the nearfield. I've discovered that my little Airport Express G can actually provide enough current to power them at comfortable listening levels without clipping and I dare say it even sounds quite listenable.

Bass in the 40-100 region is a little rolled off but actually sounds pretty clean. The midrange clarity of the Betsy is definitely preserved. The distortion with the itunes volume at about 75% is pretty low. At 100% distortion is more noticeable but it still isn't clipping. At equal volumes I dare say it sounds crisper and less noisy compared to buffering through my TDA7265 chipamp. This solution also provided more usable volume compared to my motorola droid, lenovo x61, and emu 0404. I know it probably blasphemous to even mention using the analog output on the Airport Express let alone to admit using it to drive speakers, but the result provided me quite the surprise.

I relate it here because the Betsy folks seems to be a budget minded crowd (no offense intended, I'm right here with you) and I think this would just be fodder anywhere else.

The Airport Express has been measured by stereophile to output 1.5 volts RMS into a 1K load so we are looking at a theoretical max of about .3 watts. This seems to be enough for a comfortable all-day-long sort of listening levels with the following caveats:
1) Betsy's in large vented enclosures (no bass eq needed)
2) In the nearfield
3) No BSC

Roger
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Old 25th June 2010, 02:25 AM   #2
pjanda1 is offline pjanda1  United States
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Roger,

That is really cool. I frequently have folks ask me how much power they need. I always respond that I and several others are running them with 3 watts, but I've often suspected many could get by with far fewer.

Almost nothing is blasphemous to me, and I'm not particularly surprised that the shorter, simpler signal path sounds superior.

What sort of cables are you using? I wonder if you are getting a low damping factor that is helping you get enough bass out of the system with no BSC and a quasi EBS tuning.

Paul
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Old 25th June 2010, 05:58 PM   #3
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Thanks for the encouragement Paul.

For speaker cabling I'm using 6 ft lengths of cat5 with two twisted pairs for positive and two twisted pairs for negative. One end has spads, and the other has some cheapo neutrik RCAs. A stereo RCA to minijack adapter mates the cables to the Airport.

The output impedance of the Airport is 15 ohms according to the Stereophile measurements so we are looking at a damping factor of ~0.5. I'm not sure how much that contributes to the bass. It certainly doesn't hurt. I'm not one to turn my nose at amps with low damping factor. The Betsy K is certainly well damped enough on its own. I think my big vented enclosures help quite a bit in getting bass out. They model with an f3 of 35Hz and an f10 of 30Hz.

I did a bit more analysis and with a source voltage (Vsource) of 1.55 volts RMS, a source impedance (Rsource) of 15R, and a load impedance (Rload) of 8R the output voltage is given by:
Vout = VSource * (Rload / (Rsource + Rload))

which works out to:
Vout = 0.539

(My cheapo DMM broke and I have yet to buy a replacement or I would just measure it).

The power delivered to the load is given by:
Pout = Vout^2 / Rload

which works out to:
Pout = 0.0363 watts

If we take that number and plug it into WinISD we get about 80dB from 1 channel. Which would equate to 86dB from 2 channels.

I thought about using autoformers to optimize the power transfer, but after running the numbers it is about a wash after factoring in the insertion loss. If 16 ohm Betsy Ks existed they would add about .6 dB per channel (if they had the same efficiency). Probably not enough to write home about.

Considering NIOSH doesn't recommend exceeding 85 dB for more than 8 hours a day this seems to be about the right level if it is going to be on all day.

Because I really like my Betsy Ks. I will plan on eventually building a proper amp designed around them (class A, low gain, something in the 3 watt range) and getting a proper DAC, but I think at $60 it is nearly impossible to beat the value of the Airport Express considering it is serving as a DAC and amp. Another advantage is that the Airport draws a little over 3 watts from the wall.

Roger
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Old 9th July 2010, 11:47 PM   #4
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My "new" DMM finally arrived (a Fluke 8050a from fleabay) and I was able to make some measurements to check my sanity. With a 1kHz 0 dbFS signal out of the airport I get 0.5574 volts RMS which is slightly better than the estimated value of 0.539. With a 2.2 kohm load I'm getting 1.51 volts.

Now I am contemplating putting together a simple mosfet power follower. I have a 14 VAC transformer that is probably good for 2 or 3 amps. Because the power is so low I'm thinking I won't need a whole lot of heat sinking. A follower should give me close to an extra 10 dB of headroom.

Roger
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Old 10th July 2010, 03:03 AM   #5
pjanda1 is offline pjanda1  United States
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I trust you'll post a schematic? Seems like something folks would be interested in!

Paul
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Old 11th July 2010, 08:09 PM   #6
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Will do.

My first step is going to put together a 14vdc regulated supply using a leftover lm338 from my regulated gainclone. I also plan on testing my Tripath 2020 with the regulated supply. Using the Betsy's nearfield made it pretty evident that my T-amp is pretty noisy.

The follower will be based off of Greg J. Szekeres headphone driver (HeadWize - Project: A Class A MOSFET Headphone Driver by Greg J. Szekeres). The FET will be DC coupled and the output cap will have to be a bit bigger to prevent bass rolloff. I plan on using a lm317 for the CCS. I'll start with a 250ma bias and try to move up a tad bit from there. The heat dissipation increases as a square of the bias current so I'm not expecting to be able to increase it by a whole lot more.

Roger
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Old 11th July 2010, 10:48 PM   #7
pjanda1 is offline pjanda1  United States
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Noise is certainly the downside to T-amps, IME. I think the little Sure board sounds decent, but it is vastly noisier than my tube gear. I have only tried the Meanwell SMPS, so maybe a nice PS would quiet them down.

Someday when I'm through a few more of the projects in my que, I'll put together some high quality SS. I've done a gain clone and the digi amp, but I haven't tried anything discreet.

Paul
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