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The Not Passive Pre

Posted 16th February 2012 at 07:31 AM by hbc

Had this running for a few weeks now, and the sound coming from the Sonys altogether better, now just a bit bass heavy rather than treble shy. The speakers are mounted right next to the ceiling in the room corners, so not really a surprise.

I have simulated the crossover and equalisation circuit of the preamp section, and realised that I can add a variable resistor to adjust the amount of bass boost the circuit gives, not a horrible mod, just one pot, and a new cap, just need some time to do it.


The not passive pre has no muting circuit in at present, so power sequencing is required. I intend to make it direct coupled at the output. To do this I am planing to do a micro controller servo loop. When I made the Class A digital amplifier, mentioned earlier I used an opamp servo, and I can state that this circuit does definitely have an effect on the sound.


In the interests of minimalism the Not Passive Pre II is already being...
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Old

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

Posted 13th February 2012 at 11:48 PM by rjm (RJM Audio Blog)
Updated 2nd April 2012 at 03:04 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
Rating: 2 votes, 5.00 average.

Converting Eagle printed circuit board layout files to Gerber format

Posted 11th February 2012 at 12:20 AM by rjm (RJM Audio Blog)
Updated 20th December 2016 at 12:49 AM 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 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 CAM
...
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Old

Audio op amp comparison NE5534A vs. OPA134

Posted 10th February 2012 at 11:49 PM by rjm (RJM Audio Blog)
Updated 11th February 2012 at 03:48 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 4th February 2012 at 12:53 AM by rjm (RJM Audio Blog)
Updated 4th February 2012 at 01:01 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 11:58 PM by rjm (RJM Audio Blog)
Updated 2nd April 2012 at 03:02 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 06:15 AM by rjm (RJM Audio Blog)
Updated 31st January 2012 at 06:22 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

Protoamp project.

Posted 29th January 2012 at 08:52 PM by DigitalJunkie

So,I've been pondering this for a while,and I'm going to finally build it.

Basic idea- Fairly simple Push-Pull amplifier,that will deliver ~50Wpc,Using common/available tubes.
I decided to go with the 'usual' octals,since they are all a similar basing/pinout(7ac?).
This leaves us open to use 6L6,EL34,6550,KTxx,etc. for output tubes.
We'll have to have a -Vg1 supply with a wide voltage range to cover the needs of all those tubes,shouldn't be a problem.

The power transformer is a big 1kva/500W beast,used in the Tektronix 545A. (P/N 120-120)
Pete Millett was kind enough to post some data on this transformer,and a few others here:
Tek power transformers

The output transformers are rated ~100W,they came from a pair of old RCA MI-12188A amps.
They are ~3.5-4.0K (p-p) primary impedance.
There is some information on these amplifiers/transformers here:
The Shrine of the Ancient and Most Excellent RCA MI-12188A Power Amplifier...
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Old

F5

Posted 28th January 2012 at 06:39 PM by alexcp (My DIY projects)
Updated 11th April 2016 at 09:27 PM by alexcp (Added photos)

Just finished fine tuning my First Watt F5 clone.

I built it about a year ago out of parts that I had on hand back then, including a power supply reused from another project. The resulting sound was rather disappointing. Now I spent time matching transistors and tuning the feedback network. The amplifier now is very close to the performance described in the original service manual.

I am listening to F5 (the music is Tchaikovsky performed by Wiener Philharmoniker and von Karajan) and realizing that it is so good that if this was my first amplifier, I might not be interested in building anything else.

Update: after a year, this is still the best amplifier.

Update: I purchased the official F5 PCBs from the diyAudio store. I will rebuild the amplifier, hopefully with better FETs and other parts.
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Old

NOS TDA1541A double crown

Posted 27th January 2012 at 05:54 PM by dvb projekt
Updated 30th November 2016 at 09:23 AM by dvb projekt

I am a lucky man!

Ive got two NOS TDA1541A double crown DACs from a friend



Click the image to open in full size.

He bought the last from Philips a long time ago...


Just now i swapped them on my Red Baron modules...

Click the image to open in full size.

What could i say.... They will never leaving the DAC modules again !!!




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