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Old

More on Doping Speakers

Posted 25th May 2013 at 11:25 AM by googlyone

I got a question or two on doping of speaker surrounds.

Here is the thing: If you buy an OEM cone kit it will either come with a pre-doped surround, or be provided with the doping material and instructions.

I have reconed a lot of drivers and until recently either used OEM kits or kits from providers who have looked after this for me.

A good example of a provider that gave doping compound was BEYMA. The instructions etc for this were idiot proof, and the material nicely packed in the kit.

To apply this I used a stiff "cleaning brush" as you would but from your local hardware store - a steel handle about 100mm (4") long crimped onto stiff bristles. Worked a treat.

The beyma doping material looked and smelt for the world like really thick PVA glue. The difference was that when dry it did not go that hard, and remained almost but not quite tacky.

The before shot is:
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Old

Headphone amplifiers: thinking aloud

Posted 20th May 2013 at 07:13 AM by rjm
Updated 21st May 2013 at 01:00 AM by rjm

The sobering fact is that the built-in headphone jack on most modern consumer electronics provides pretty decent performance. Taking that output and routing it through an external headphone amplifier rarely improves things, and frequently has a negative impact owing to increased background noise.*

[* This is a simple consequence of adding a volume control which attenuates the signal, and a gain stage which amplifies it back up. Even if the gain stage has the same noise floor as the input signal, the S/N is reduced by the amount of attenuation.]

There are specific use cases, particularly with "outlier" headphone models that require unusually high voltages or currents to drive, but in the main, for generic 16 ohm IEHs and the generic headphone ICs used in consumer electronics, I've found that external headphone amplifiers aren't worth the trouble and expense.

Instead, I've taken (I realise now) an elitist approach to focus on a desktop...
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Posted in The Lab
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Old

A catalog of headphone amplifier circuits.

Posted 19th May 2013 at 11:45 AM by rjm
Updated 21st May 2013 at 01:12 AM by rjm

Lots of circuits out there, but most are variations of a small set of archetypes. Let's see if I can put together a list:

1. Dedicated headphone amplifier IC. e.g.(lme49860)

2. Battery powered, single stage, generic audio op amp. The ever-popular mint tin cmoy.

3. Op amp + buffer (complementary transistor pair, diamond buffer, unity gain op amp, etc, in either integrated or discrete package.)
a. closed loop connection or "compound amplifier" configuration as developed by Walt Jung.
b. open loop, two stage circuit, e.g. nwavguy o2 and my sapphire amp.

4. simple 2 or 3 transistor "introduction to electronics"-style amplifier

5. The "little big amp", a scaled back version of a transistor or vacuum tube power amplifier design. (Zen, DoZ, transformer coupled SET amps)

6. The power follower. Single-ended MOSFET or BJT, with or without CCS load, voltage...
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Old

Nearly There - A Low Power Desktop Amp

Posted 17th May 2013 at 08:09 PM by BuildMeSomething

Well, this project is nearing completion. The very last pieces needed have been ordered and will be delivery early next week. The chassis and custom PCB enclosure/heat spreader were collected on Thursday morning from FedEx and UPS. Thursday night was spent drilling and tapping holes for the various parts. Now its some wire placement and waiting on the last pieces...

Problems: None really, a few sizing issues for connecting wires and the caps to the boards. Though, all are just big enough with a little patience. Certainly no issues that require a halt to the project or new PCB's.

Speakers: Decision made, Frugel-Horn Mk 3 with Mark Audio Alpair7.3 drivers... Why? I love the strong imagining and detail given by the little 3" FC full range speakers, so thought I'd give some proper full rangers a go. Should be fun trying to build these
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Old

Which soldering station should I buy?

Posted 17th May 2013 at 08:47 AM by rjm
Updated 20th May 2013 at 06:20 AM by rjm

Thinking I should get a new soldering station.

I've had my eye on a Weller station for the longest time, but put off getting one for one reason or another.

Looking at the selection, I think the choice comes down to the two following models, or, indeed the equivalent made by another company:

WTCPT (link)

WES51 (link)

The main difference is in the WTCPT the tip temperature is fixed, but rigorously controlled, while the WES51 has user adjustable power but the tip temperature is left unregulated.

I'm leaning towards the WES51 as being slightly more in line with my own style of work: I'm more likely to want higher or lower temperatures depending on the job at hand than I am to require "700F" exactly.

Update 1.

A little concerned the WES51 is only 50W max. That's less than the maximum I use now for the tough stuff.

Hakko has two models in the price...
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Old

Doping of cloth surrounds on recone of subwoofer

Posted 14th May 2013 at 01:32 PM by googlyone

Well, you learn something every day. I suppose if you don't manage to learn as you go, then you probably stuff things up every day!!!

About 12 months ago I came across some RCF L18/551 eighteen inch subwoofer baskets. I reconed them using aftermarket parts, the process went OK but I was kind of bemused by the untreated cloth surrounds.

I was busy, and had no immediate plans so the drivers were shelved for a rainy day.

The Thielie and Small parameters were "OK" but not exact as OEM. I measured these using the addded mass method, an important fact for later. As an aside, they would do well enough as subs, but their Fs was a bit higher than I would have hoped.

Fast forward to last week, I loaded one into a 220 litre box tuned to 34Hz, and measured the impedance.

Bugger me if the resonance showed one massive peak at pretty much the driver Fs, and a tiny lump out at 80Hz. Huh?

I swapped drivers...
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Old

Measurements with a sound card

Posted 13th May 2013 at 10:08 PM by alexcp (My DIY projects)
Updated 7th September 2013 at 10:36 AM by alexcp (Updated for ASIO4ALL)

I am using a USB sound card for measurements. It's a pain, but I have nothing better yet.

The sound card is the E-MU 0204. I've chosen it for the specs:
  • Dynamic Range (A-weighted, 1kHz, min gain): 113dB
  • Signal-to-Noise Ratio (A-weighted, min gain): 113dB
  • THD+N (1kHz at - 1dBFS, min gain): -101.9dB (.0008%)

The sound card is intefaced to the real world by Pete Millett's sound card interface.

The software is SpectraPLUS 5.0 by Pioneer Hill Software (PHS). (No link because my antivirus says their web site is infected.) The software runs on an old MacBook running Windows 7 in Boot Camp.

The results do not seem to be affected by nearby computers, flurescent lights, switching power supplies, etc.

The results are affected by:
  • Windows Mixer settings (levels and sampling rates)
  • Software settings (levels and sample rate)
  • Grounding of the sound card, the interface, and the device under test

The greatest source of pain is...
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Posted in Instrumentation
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Old

NE5532 power amplifier

Posted 12th May 2013 at 08:51 PM by alexcp (My DIY projects)

This is a project developed by Douglas Self and published in the Elektor in October and November 2010.

The idea is rather unusual: "An interesting power amplifier can be made by connecting enough 5532s in parallel, how about 32 for a start? This may sound like a radical course of action, but it actually works very well, making it possible to build a very simple amplifier that retains not only the excellent linearity but also the power-supply rejection and the inbuilt overload protection of the 5532, which reduces the external circuitry required to a minimum."

Elektor used to sell PCBs for this project, which I used. I modified the circuit slightly to incorporate an active volume control along the lines of Douglas Self's preamplifiers.

Here is the result:
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Old

Eternally Penultimate Zen - mod #4

Posted 11th May 2013 at 09:32 PM by alexcp (My DIY projects)
Updated 12th May 2013 at 05:24 PM by alexcp (Added pictures, links)

My first solid state amplifier build, in 2009, was The Penultimate Zen, aka Zen Variations Part 4, aka ZV4. It was built with the parts I could find back then, without knowing where to look. It ended up big, heavy with two 400VA toroidal transformers, ugly inside with some electrical tape here and there, without proper grounding, etc., etc. I was as disappointed with the sound as my wife was disappointed with that heavy black metal brick gathering dust.

My first mod on ZV4 came after I turned my disappointment into finding better part suppliers. I returned to my ZV4 and replaced the output caps with something marginally more decent from eBay. The sound improved a notch. Aha!

My second mod was to rebuild the power supply. I threw away one of the toroids, added a softstart circuit from Hypex, and put in a CRC filter with some Mundorf HC caps. One power supply for two single ended channels forced me to learn about grounding and add input transformers and balanced...
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Old

Audacity sucks.

Posted 9th May 2013 at 03:50 PM by rjm
Updated 10th May 2013 at 04:35 AM by rjm

Under windows at least, Audacity is unable to record audio at bitrates above 16 bit.

It will seem to, all right, but the data is quantized at 16 bit (30 microvolt LSB), regardless of the settings chosen.

The attached images show the same source, the first recording is made in Audacity, supposedly 24 bit, but actually only 16 bit, while the second is recorded with a program than actually supports 24 bit, exported, and imported into Audacity. The data is amplified +70dB in both cases to make the difference visible.

Audacity will happily manipulate and save high bit rate data, but as a result of licensing restrictions and on account of it being freeware, it does not support the actual recording of this data.

***

Any internet search will confirm that the Windows version of Audacity is limited to 16 bit recording. And yes, it's more of a limitation of Windows than it is of Audacity. My irritation, however, is chiefly with Audacity...
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