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RJM Audio in 2017 [modular simplicity]

Posted 19th December 2016 at 12:11 AM by rjm (RJM Audio Blog)
Updated 27th December 2016 at 11:50 PM by rjm

The time of year where I like to draw up some plans for the next 12 months.

This years output looked something as follows,

At the end of all this I find myself sitting on four different voltage regulator circuits, several variants of the transistor output diamond buffer, five phono stages variants, and a couple of nebulous ideas about developing a discrete voltage gain amplifier.

I'm considering how to package this all up in such a way as to best appeal to diyaudio builders of widely varying application needs and skill levels while keeping a simple...
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CFA audio gainblock - a second take

Posted 16th December 2016 at 01:21 AM by rjm (RJM Audio Blog)
Updated 16th December 2016 at 01:28 AM by rjm

This is either an ingenious interfacing of the output buffer and current feedback amplifier by adding second arm to the central current mirrors ... or it's another really bad idea(tm).

It simulates nicely though.

[What's happening here is current output of collector Q9 is no longer being asked to bias and drive the buffer Q16. Instead Q2 and Q17 do that job. The change lowers distortion, improves bandwidth, and even raises the PSRR a little.]
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A slightly different kind of voltage amplifier

Posted 12th December 2016 at 10:44 AM by rjm (RJM Audio Blog)
Updated 21st December 2016 at 04:50 AM by rjm

(which is a current feedback amplifier)

Lately I have been looking again for a discrete transistor voltage gain amplifier for line level duty that isn't just another op amp.

I keep coming back to variations on this circuit. It's a diamond buffer input with current mirrors in the mid-section strapped around a voltage divider / feedback loop that provides the gain. This backs out into either another diamond buffer or, in the revised version below, a standard Sziklai output stage which can be more easily scaled up as needed for a headphone amp for example.

An offset bias adjust circuit could be added to trim the output offset voltage. Or use a coupling cap. It's a few hundred mV otherwise.

Circuit gain is R4/R6, approximately. The total value of R4+R6 should be kept about 20 kohms. C1 is a compensation capacitor. Circuit gain as shown is 14 dB, -3 dB at 250 kHz.

As with all CFA, the choice of the feedback resistance...
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Lynx and RME Sound card pinouts

Posted 7th December 2016 at 05:28 PM by 1audio

Attached are the pinouts for a Lynx (L2 or L22) sound card and for an RME HDSP(e) sound card. The host end is a standard Dsub connector
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RJM Audio desktop wallpaper

Posted 2nd December 2016 at 06:45 AM by rjm (RJM Audio Blog)

16:9 4K

rendered from VSPSX phono stage Eagle .brd output
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HP 339A Mods & Upgrades

Posted 29th November 2016 at 12:42 AM by SyncTronX
Updated 5th April 2017 at 04:57 AM by SyncTronX


The HP339a was a state of the art (SOTA) distortion analyzer in the 1980s.
It is a full featured all-in-one product including a volt meter,
low-distortion oscillator, distortion analyzer, AM detector, and VU meter.

There was an in-depth analysis of low distortion oscillators
on DIYAudio on the following link:

<Low-disortion Audio-range Oscillator>
This was followed by a specific HP339a thread, linked here:


This is a summary of those threads along with the links, conversations, and
posts, about the research and development that went into the suggested modifications to the venerable HP 339A.


Why HP 339A?

HP339A Post #78
Well, a stock 339A gives you a wide band ac voltmeter, an oscillator and analyzer. A stock one will do you pretty well as long as you have no interest in reading below...
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Grundig Studio RPC 350 need help

Posted 7th November 2016 at 10:01 AM by Keepsake10

Question for transistor experts.
I recovered the amplifier module from a old Grundig studio RPC 350. This amplifier is a classical concept of the time (around 70) differential input with CCS, driver, darlington complementary finals (BDW 94 / 95) It has DC transistor protection for the output and as well a kind of thermic protection switch on the heater sink (this protection cut entirely the main if heater sink get too hot).
I changed all electrolytic Cs and checked the rest of the parts. The rectifier bridge was replaced with fast recovery rectifiers and both 10.000/30V capacitors where replaced with 18.000F/40V. Measured values as shown by Grundig are ok and the offset can be done within Grundig specifications (the old 1Kohm trimmers will be as well replaced with multi-turn ones to get much easier control).
There is one issue I cant get fixed. The original schematic show the T2004 as BD517. Actually in this place was used a small Motorola in TO92 package...
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Goosebumping headphone DAC-amp

Posted 31st October 2016 at 08:42 AM by abraxalito
Updated 1st December 2016 at 03:23 AM by abraxalito

The Intersil D2 chipset based amp arrived and while it sucked pretty badly on speakers I had a hunch that like the STA333BW boards I've been playing with recently, it would acquit itself admirably on headphones, given the much lower demands on the supply.

The mods I've done to this are fairly straightforward - changing the output filter to work more optimally with headphones (via a 7:1 step-down transformer) and tweaking up the power supply a bit.

To run with cans I went for an output load impedance of 1100ohms. This was determined primarily by the choice of inductor (MSS1210 8.2mH from Coilcraft) which in turn was chosen so that the carrier frequency would be below the SRF of the coil. This choice is fairly arbitrary in that the inductor would still work fine above its SRF just it's starting to look like a capacitor. However it turned out that with 8.2mH the cap value required was fairly simple, 5nF so I stuck two 10nF NP0s in series.

Mods in the...
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Using a digital amp as a DAC

Posted 28th October 2016 at 03:19 AM by abraxalito
Updated 4th November 2016 at 03:03 AM by abraxalito

I've been mighty impressed with the STA333BW board (just 62rmb on Taobao, see an earlier post for the link) driving headphones via a transformer (ferrite cored step down 7:1, I've tried various sizes). So much so that I don't much feel the need to upgrade my headphones beyond my Superluxes (HD668B). The sound I'm getting is immensely satisfying - rich (which I got from my TDA1387-based DACs, but not from my phone with its ES9018K2M, that doesn't qualify as rich in tonality) and dynamic beyond anything I've so far heard. I noticed in a recent thread post by TNT (here - he says he doesn't much like listening on headphones due to the 'in-head' quality of the sound. Yeah I used to get that, transformers pretty much get rid of it giving an out-of-the-head-all-around-holographic kind of experience, along with stupendous dynamics. I like listening to classical piano and that's very demanding...
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When is a clock not a clock?

Posted 29th September 2016 at 04:31 PM by Tam Lin

When it goes through a logic gate.

I heard that riddle in 1978 when I worked as a programmer for a computer startup in Silicon Valley. For some reason, it stuck with me although I didnt appreciate its profundity until I started studying DAC design.
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