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Old

This vs. that.

Posted 13th January 2012 at 12:41 AM by rjm (RJM Audio Blog)
Updated 26th January 2012 at 06:42 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 01:18 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 11:47 AM by Wolfsin (SE-BRGD a bridged mono 'wire')
Updated 12th January 2012 at 11:54 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 11:16 PM by Wolfsin (SE-BRGD a bridged mono 'wire')
Updated 12th January 2012 at 11:13 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 11:41 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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Old

one small step...

Posted 8th January 2012 at 01:58 AM by cberger

... for mankind, but one giant step for me

After sucking big time in Halo, In felt like I needed some positive experience today, so I hooked up my CHR-70s to my trusted, semi-retired NAD.
Took about 1h, involving looking for my cutter to cut some old CAT5 and strip the wires, do the actual cutting and stripping, hooking up the receiver, connecting the cables and turning on the whole assembly.
And *drum roll* FM94.9 played Motorhead's "Ace Of Spades" Can it get any better???
First listening impression: sounded a bit like AM radio, but given the way the drivers are hooked up I'm not really surprised...

in other news: parts express delivered some fluff and my terminals
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Old

SouthSemi modified F2J Clone

Posted 7th January 2012 at 10:13 AM by Tea-Bag
Updated 21st November 2010 at 01:46 PM by Tea-Bag

This last weekend I replaced the IRFP240 in my F2 Clone with the R100 SouthSemi Power-JFET.

tubes-guy used psychic powers to control my mind and force me to do it.

The bias dropped off the bat. I re-biased and gave it an A-B listen to the standard channel. Seemed to be a bit 'cleaner' sounding the the default channel.

Nelson has recommended the following changes to the circuit, which I hope to implement soon. I will be re-wiring these on Peter Daniels boards.
The Peter Daniel boards are still available I believe.
http://www.diyaudio.com/forums/group...group-buy.html

Papa sayz...

The F2's that I have updated with the JFETs need the
following changes:

R6 = 47k (changed to 8.2K of FW Site)

C1 = 10 uF

Z1 removed

The potentiometer must be re-adjusted for minimum
distortion at 4.5 watts (symmetric clipping) and readjusted
...
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Old

Progress so far

Posted 6th January 2012 at 01:02 PM by rjm (RJM Audio Blog)
Updated 7th January 2012 at 03:00 AM by rjm

The first stage is getting the power supply installed and running. That's the little board by the transformer, which just has the diodes, main filter capacitors, and isolation resistors on the voltage rails. The case top lid is transparent plexiglass, so the internals are lit by three white LEDs to show off, and light the control labels, which are printed on the top lid rather than the front and back sides of the case.

You'll notice I have followed the instructions and removed the plastic covers on the electrolytic capacitors.

For the record I do think they sound better that way, I just normally can't be bothered as, for example, my phonoclone 3 boards have 28 of them and I consider the exposed metal surfaces a bit of a safety issue as well.
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Old

47 Labs 0247 unboxing, part deux

Posted 5th January 2012 at 11:29 AM by rjm (RJM Audio Blog)

My kit was missing the amplifier circuit boards! So many other neat stuff distracted me from this vital fact!

So far I've wired the AC power line components, transformer, and the power supply board. It's slow progress, mainly for wanting to be sure of getting it right the first time. It's dense and fiddly, desoldering stuff in there to fix a ****-up would not be pleasant work at all.
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Old

Five-times-bandwidth

Posted 3rd January 2012 at 05:44 PM by rjm (RJM Audio Blog)

If we accept that conventional wisdom that the audio bandwidth extends from 20 Hz to 20 kHz, a good rule of thumb for the f(-3 dB) high and low cutoff points of the frequency response of each audio circuit element is 5x the bandwidth, or 4 Hz to 100 kHz. In practice most designs tend to shift that range a little to the higher frequencies, so perhaps 5 Hz - 200 kHz, or 4 - 250 kHz.

Personally I "tune" my circuits to 4 Hz. That is, the time constants are adjusted to about 4 ms. Capacitance is usually cheap enough to go even longer, but the influence on sonics is typically net negative.

The high frequency side is more interesting, since many circuit elements naturally run into the megahertz range, the the question is do you actively try to prevent that, and if so, where and how?

The biggest issue is bypassing: a small value electrolytic (100 uF) is probably fine up to 100 kHz or so, but quite useless at 2 Mhz. The textbook solution is...
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