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Old

Sony SA H7900, motional feedback active speakers

Posted 21st January 2012 at 10:42 AM by hbc

Following my "conversion" that valves really do sound better than transistors:

http://www.diyaudio.com/forums/solid...c-project.html

My home stereo, which is compromised by having to be small person resistant, has been in a state of flux. I worked out that the most economical tube fix was to by a receiver from ebay, so I got a 1967 ish one, 17 tubes, push pull EL84s etc, it is original and it works, but the RIAA is way, out the FM stereo decoder needs re alignment, and all the output tubes share a common cathode resistor, 2 ECC83s in series.... so it went on the bench and has stagnated... also most of my speakers don't really suit it.


So to fill the gap I thought I would drag out a pair of Sony SA H7900 active speakers I acquired about 8 years ago, not just active, motional feedback woofers and electrostatic tweeters no less, and bass reflex(?) what could possibly go wrong?


When...
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Old

47 Labs Treasure 0347 : trials and tribulations

Posted 20th January 2012 at 11:48 AM by rjm (RJM Audio Blog)

And not even sure what a tribulation is, but I'm sure I've had at least one building this amp.

Like the 0247 it's not so much difficult as it is painstaking hard work. The output transistors have to be mounted "just so" so the boards screw to the case properly, the wiring of the thermal protection transistors and gluing them to the output devices is tricky and easy to mess up.

I triple-checked and was rewarded by, it seems, error free boards. I messed up and wired the mute switch incorrectly though (the photos show it wrong, in case you come across this blog post looking for help) and I think the switch has a fault somehow as L and R channels appear to me mixed together. Taming the offsets gave me considerable grief but I think its all OK now.

Tomorrow I'll hook it up to the speakers and see what happens.
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Old

Measuring Noise

Posted 19th January 2012 at 03:28 PM by klewis
Updated 19th January 2012 at 03:32 PM by klewis

For LED's - Christer's thread http://www.diyaudio.com/forums/parts...er-diodes.html
An interesting method of grading zeners for noise, that I have used for years, is to monitor the noise across the devices with a scope while using my HP bench supply to bias them. The noise spectrum changes quite dramatically as you come into breakdown and then drops to it's minimum at it's prefered current flow. I note the current and voltage after it has settled down. I choose the ones to use by voltage match and bias them accordingly.

One other trick, if you want to know how quiet your supply is, choose a suitible coupling cap and hook the output of it up to a line input on your preamp and listen to it. Try this with and without a load and during normal operation of the supply. A good supply is dead quiet. author: MikeBettinger
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Old

A different design aesthetic.

Posted 17th January 2012 at 10:52 AM by rjm (RJM Audio Blog)
Updated 25th January 2012 at 10:21 PM by rjm

I like the concept of appearance in high-end audio. I mean, where the look of the equipment makes a statement about what it's all about inside.

The 0347 is proportioned like a vintage tube receiver, but in miniature. It even has the lit, transparent front panel where the radio tuning strip would have resided. The heavy grey hammertone and very simple, basic hardware are reminiscent of Heathkit or Dynaco. In short, the appearance evokes the spirit of a simpler, more innocent age of vintage hifi without being a slavish replica.
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Old

Another package in the mail...

Posted 14th January 2012 at 10:30 AM by rjm (RJM Audio Blog)

The 47 Labs "Treasure" stereo amplifier kit model 0367 has just arrived here at RJM Audio central command.

61,500 yen delivered, a tad more than the model 0247 headphone amplifier / DAC. Basically lose the volume control and DAC board, gain a bigger transformer and a thick aluminum bottom plate that serves as the heatsink... the amplifier circuit itself is almost identical, just with a slightly bulked-up output stage and the addition of certain safety considerations such as fine tune-able bias current and thermal protection for the output transistors.

Which makes an unusual circuit for a power amplifier: the output stage is a unity gain current buffer running open loop. For a headphone amp that's justafiable because the bias current is such that the amp will almost always be running in class A. For a power amp rated 40 W into 4 ohms, it's class AB.

Mr. Tsukahara seems pretty pleased with the result though, and I am anxious to try it...
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Old

This vs. that.

Posted 12th January 2012 at 11:41 PM by rjm (RJM Audio Blog)
Updated 26th January 2012 at 05:42 AM by rjm

47 Labs 0247 vs. RJM Audio Sapphire

I wasn't even aware that the folks at 47 Labs were developing a headphone amplifier until after the Sapphire was finished, and I'm sure they didn't know what I was working on, so it's all the more remarkable just how similar the two designs are. Two separate answers from two separate people sharing the same general design philosophy.

Similarities:

- Solid state op amp voltage stage front end and solid state push-pull buffer running open loop for the output stage.
- Gain of 5-6x (14-16 dB)
- 20-30 mA bias current in the output stage.
- All BJT circuit** (see footnote)
- Use of "diamond buffer" circuit element. (Albeit in very different ways)
- 10 V voltage rails, split supply.

Differences:

- My voltage stage is an OPA134, or any 8 pin DIP op amp IC. 47 Labs voltage stage is a fully discrete current feedback op amp with a diamond buffer...
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Old

47 labs 0247 : done, tested, working

Posted 12th January 2012 at 12:18 PM by rjm (RJM Audio Blog)

A few bumps along the way:

1. Soldered one of the caps in backwards. Removed and fixed, fortunately before power was applied to the board.

2. Soldered one of the transistors into the wrong place. Removed and fixed.

3. Dead channel on power up. Bad solder joint on the emitter of one the output transistors. Eyesight not what it used to be. Found on second inspection, fixed.

4. DAC board not level with cutout on front panel, had to use washers to raise it to the correct height.

Other than that, things went smoothly enough, though I spent more time with tweezers routing lead wires than is ideal for a commercially released audio kit. It's not so much difficult as it is painstaking - and I have quite a lot of experience doing these things so I imagine I had an easier time of it on the whole than most will.

The final result however is aesthetically quite pleasing, well worth the effort and expense. It is...
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Old

Deeper into variant one

Posted 12th January 2012 at 10:47 AM by Wolfsin (SE-BRGD a bridged mono 'wire')
Updated 12th January 2012 at 10:54 AM by Wolfsin

Will be fleshed out with image of reversed wiring for inverted channel
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Old

Mutants

Posted 11th January 2012 at 10:16 PM by Wolfsin (SE-BRGD a bridged mono 'wire')
Updated 12th January 2012 at 10:13 PM by Wolfsin (rename second variant)

When asked where frankenwires were to be filed, qusp replied

Quote:
Originally Posted by qusp View Post
file it under mutants...

...and their amplifiers
There are two variants of what I originally called the SE-BRGD frankenwire. Both start as rework boards. One gets a pair of LME49860 to replace the LME49990 using the simple surgery below. The + and - inputs to the second 49860 are reversed and resistors adjusted so the gain is equal on both sides. The effect of that reversal is to invert the signal on the second leg so it becomes a mono amp, whose differential outputs are ready to bridge one can.

The second variant, more correctly referred to as the BAL-BALM, requires no rework surgery (so could just as well not even be a rework pcb), just the addition of a tiny adapter to mount an opa1632 and leave off the 49990s. Power connection is not as clean as the first variant. It is effectively half a BAL-BAL, perfect for the audiophile with only one ear.

...
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Old

The Brush Development Co. BL-905. What is it?

Posted 9th January 2012 at 10:41 AM by tonekwest

I have what looks to be an amplifier of some sort and was wondering if any of the forum members may be familiar with what I actually have. So far, my search results have come up with nothing.
It is from the Brush Development Company. Type BL-905. Will try to post list of tubes in the unit tonight.
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