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"Activator 2" FET preamp for .. guitars and basses?

Posted 13th February 2012 at 10:48 PM by rjm
Updated 2nd April 2012 at 02:04 AM by rjm

I have been doing a web-dredge, searching for simple, single supply, transistor buffer/gain stages, preferably FET-based.

This one seems interesting.

The designer, Henry Nurdin, has kindly posted a full schematic.

Ignore the fact for a moment that it's designed as a tiny battery-operated module to retrofit into electric guitars, the basic circuit block with 5-6 dB non-inverting gain could be used as a front end for a headphone buffer stage like, I'm thinking especially, the Szekeres MOSFET buffer.

That's if the bandwidth is sufficient for high-end audio (should be!?) and, slightly more worrisome, acceptable gain matching between channels can be achieved without resorting to trim pots.

This circuit reminds me of something. ... Sziklai pair? Time to do some more dredging...

Ah, bingo!

Update:

And with some jiggery-pockery this is taking shape as a (yet another) headphone amplifier....
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Old

Converting Eagle printed circuit board layout files to Gerber format

Posted 10th February 2012 at 11:20 PM by rjm
Updated 26th November 2013 at 11:39 PM by rjm

File this under "things-I-should-have-learnt-to-do-many-years-ago-but-was-too-lazy-to-bother".

Many pcb fab outfits that do business with hobbyists and DIYers choose to accept Eagle .brd files, which means they do the conversion to Gerber output so you don't have to. I've relied on that for far too long, but when an error showed up in one the .pdf proofs on the last batch of boards I sent out for fabrication, they asked me to send the Gerber files instead. So I bit the bullet and after a couple of false starts managed to give them what they wanted. Looking back at it, it was easy and something I should have learnt, as I wrote up above, years ago, but, for posterity, here's how it's done:

Eagle 6.1~6.5 on Windows
  1. Make a working folder for the Gerber files.
  2. Copy the Eagle board (.brd) file to this directory.
  3. Open Eagle (version 6.1)
  4. Select menu item "file/open/CAM job"
  5. The open file dialog appears, defaulting to the Eagle
...
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Old

Audio op amp comparison NE5534A vs. OPA134

Posted 10th February 2012 at 10:49 PM by rjm
Updated 11th February 2012 at 02:48 AM by rjm

Finally got around to some more comparison listening with the Sapphire headphone amplifier. To recap: the circuit has an open loop diamond buffer output, so the op amp is just providing voltage gain. It configured for a non-inverting gain of 21 dB to match my 300 ohm HD600 headphones. Pretty much textbook operating conditions.

The op amp inputs are impedance balanced at about 1 kohm. This is about the crossover point where you start thinking about using FET input stages, but BJTs should still be fine.

I'm interested to see if there is a definite signature to a FET-input opamp. The original build called for an OPA134, which is a JFET input circuit. I tried the OPA27, which is a low-noise, high-input-current BJT design, and last night I tried the NE5534A, a classic general purpose audio opamp with bipolar inputs.

I've long been in agreement with Douglas Self on the NE5532/NE5534 : anyone who reports these op amps sound bad is either not using...
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Old

RJM Z-reg + Pass B1

Posted 3rd February 2012 at 11:53 PM by rjm
Updated 4th February 2012 at 12:01 AM by rjm

I was asked to suggest a voltage regulator for the First Watt (Pass DIY) B1 buffer. One thing led to another and the next thing I'd sketched up a circuit board for the buffer as well as the regulator.

It's like a B-board, but with the JFET buffer instead of the diamond buffer, and with a single supply and, hence, the coupling caps front and back. Since it's using JFETs for the buffer I used a JFET for the pass device in the regulator, too.

Full credit to Nelson Pass for his design.

Eagle files do not show 2SK170 because the package is not in Eagle. All transistors 2SK170 or equivalent. Zener is 18-22V DO35 or DO41 i.e. Vishay BZX85. V++ is 5-15 V above whatever you select the Zener reference to be.
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Old

The Z-reg

Posted 1st February 2012 at 10:58 PM by rjm
Updated 2nd April 2012 at 02:02 AM by rjm

This is a followup of sorts to the X-reg, though there is nothing original about the circuit this time around.

It's just a Zener voltage regulator with a series pass transistor. I lifted this particular configuration from the Pionner C-21 preamplifier and re-tuned it for op amp applications. My main interest here is trying to make a small and convenient board layout.

I've used this circuit block already in the Sapphire amp and come away impressed.

The output is about 1 V less than the Zener voltage, and the input voltage should be about 3-6 V above the Zener reference voltage. I'm working here with 17 V unregulated supply and 12 V Zeners, but the values can be reconfigured easily enough for any output from 4-24 V

This is a low current circuit. If you are just powering a few op amps, no heatsinks are needed. Above 25 mA small heatsinks are a good idea. The circuit is not designed for output currents above 100 mA.
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Old

Current Feedback Amplifier

Posted 31st January 2012 at 05:15 AM by rjm
Updated 31st January 2012 at 05:22 AM by rjm

Found this today: DC Coupled Amplifiers for Audio from an 11 year old web page by one Knut Harald Nygaard.

I've taken some minor liberties with the circuit, shown in the second, color image below. Replaced the current sources with resistors and moved the output buffer out of the feedback loop.

Note the resultant double symmetry of the dual-diamond buffer topology, highlighted in blue.
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Old

Whither Next for RJM Audio?

Posted 26th January 2012 at 12:22 AM by rjm
Updated 1st February 2012 at 08:20 AM by rjm

On the occasion of selling the 100th and last VSPS Stereo Kit, it's time to consider what direction(s) RJM Audio will take over the next little while.

A. A minor revision of the stereo VSPS board is planned. Just a point release to address feedback I received over the last couple of years. Point release on current boards (Phonoclone 3, VSPS 300, Sapphire) expected when I deplete existing stock and have to re-order.

B. Since my circuit designs are essentially modular (the voltage regulation of the Sapphire headphone amp could be used with the Phonoclone, the output stage of the Sapphire could be detached and used on the output buffer on the VSPS...) I'm going to work on bringing that vision to a concrete reality by developing "swap-in swap-out" circuit boards... like those toys where you can change out the head, body and legs of a character to make many variations.
  1. X-reg board
  2. Zener reg board
  3. basic op amp gain block board
  4. VSPS and
...
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Old

My system (Jan 2012)

Posted 23rd January 2012 at 12:12 PM by rjm
Updated 26th January 2012 at 02:57 AM by rjm

Biggest changeover in many years.

Denon DP-2000, DA-307, DL-103 turntable/tonearm/cartridge
Phonoclone 3 with 160 VA Plitron power supply
47 Labs "Treasure" model 0247 preamplifier
47 Labs "Treasure" model 0347 amplifier
Onkyo D-605SR speakers
Cables by oyaide, mostly.
(photo: Pentax K10D, Pentax FA 31mm F1.8 AL Limited)
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Old

Oh, MY!

Posted 23rd January 2012 at 06:51 AM by rjm

Denon DL-A100

Transparent DL-103 for Denon's 100th anniversary.
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Old

op amps for audio

Posted 23rd January 2012 at 06:36 AM by rjm

Douglas Self on op amps for audio, which I read before in slightly different form, on another web site somewhere, many years ago... today was a refresher course.

Excerpts from op amps in small signal audio design, by D. Self [Part 1, part 2, part 3, part 4].

I suppose I should get with the times and start using the LM4562 seeing as it is available in DIP8 now. (I had this memory that it was only SOIC, go figure...) Pity it's dual only. On the other hand, that leads to some neat ideas, like a compact version of the Sapphire amp for example, or the X-reg.

I am struck by how few choices we really have when it comes to practical op amps for audio in single DIP8 packages. There is the NE5534, TL071 and TL051, the OPA134, the OPA604, and OPA27, OPA627. Maybe the AD797 but I have yet to see the point. Given that the TL1071 and derivatives are outdated and replaced essentially by the OPA134, and given the excessive cost (and questionable sonic benefit)...
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