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Chinese class D amp modules.

Posted 25th August 2016 at 10:42 AM by Stage Sound

I have imported several class D modules from China and all but one have had RFI problems and seemed to me to unusable and probably illegal in a production item. Far to much RFI on the output and looking at the on board components they have only a single stage output filter comprising of a square grey ceramic choke and one or two mylar caps. Has anyone else found the same thing and is there a mod that is known to work? The modules are all very well made using good quality components but like behaving like a radio transmitter.
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Old

Cyrus Two Amplifier & Power Supply Upgrade

Posted 15th August 2016 at 03:28 AM by erictoucan

Cyrus Two would be the best selling and successful British Amp ever made. Definitely, it is the most important product to bring the market standing in its history.

After almost 40 years, it is still a very good amp around. Definitely, very good value for its second hand price.

Cyrus two phone section consists of super matched pair LM394 which is still the best matched pair today.

Here welcomes any idea to maintain and upgrade the Cyrus Two and its Power Supply to improve its performance to race with today's best amp.

For instance,

1. Replace the aged E-cap
2. Replace of coupling cap for better performance
3. Replace low value and better feedback resistor
with higher quality resistor.
4. Replace the important resistors in RIAA section
the best quality type
etc.

However, a lot of component and design is still very good like the Holden & Fisher...
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Old

RJM Audio discrete op amp (circuit schematic)

Posted 8th August 2016 at 02:06 AM by rjm (RJM Audio Blog)
Updated 8th August 2016 at 05:37 AM by rjm

This is my first op amp design which doesn't completely suck.

Now, it's a terrible op amp... don't misunderstand... (Nat Semi will not be making me any offers)... but it does perform the job I want it to do reasonably well: remain stable while providing 6-20 dB of line-level voltage amplification with low distortion, decent PSRR, and sufficient bandwidth.

The main limitation is the distortion at high frequencies rises to -70 dB. The circuit needs less open loop distortion, or more open loop gain above 10 kHz, or both.

You will note the circuit has no current sources. This is intentional. I wanted to see how far it was possible to get without them. Obviously headroom takes a big hit, but distortion and PSRR ended up better than I imagined.

This is a simulation. No guarantee it will work, and there are no safeties (current limiters, input voltage clamps, etc) shown.

PS. Frequency response in image is open loop, while...
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Old

Sapphire Line Preamplifier - My Build Notes

Posted 28th July 2016 at 03:15 AM by rjm (RJM Audio Blog)
Updated 5th August 2016 at 02:14 PM by rjm (add measurement data)

Introduction

This is my build log for relatively basic line preamplifier based on rev. 30f boards of the Sapphire3 headphone amplifier. I modified the circuit to run at lower currents (about 10 mA output bias) and adjusted the gain settings to 10/16 dB.

It is built in a Hammond 1550 cast auminum chassis, with an external Plitron 160VA 2x12VAC rectified power supply. The volume control is a 50k Goldpoint V24 stepped attenuator, while the RCA jacks are rhodium plated from Oyaide. The feature set is limited to two switchable line inputs and an output mute.

Chassis Layout Notes

Audio components are conventionally designed as rack-mounted equipment with all controls on the front panel and all connectors on the rear panel. To try and keep internal cabling to a minimum I'm modelling my preamp more like a recording console with both the controls and I/O on the top plate.

Build Notes

All components mounted...
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Old

Just the buffer, ma'am.

Posted 28th July 2016 at 02:05 AM by rjm (RJM Audio Blog)
Updated 21st August 2016 at 11:49 PM by rjm (added BOM)

I've had quite a few requests for the bboard buffer circuit without the built-in regulators, so here is a bboard 2.1 standalone 2-layer board, measuring 5x8 cm. Gerber files attached in zip file.

It is designed for +/-12 V rails, but the circuit will work with anything from +/-5 V to +/-18 V. A regulated power supply is recommended.

This is a line buffer. It intended to drive cables, not headphones.

Available for $15/pair shipped. Several people have asked me about kits. I figured the BOM was so basic it wouldn't be necessary but I can send you the boards with the parts to populate them for $50/shipped. You will still need to provide the power supply.

BOM attached.
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Old

Sapphire Line Preamplifier

Posted 24th July 2016 at 01:11 AM by rjm (RJM Audio Blog)
Updated 26th July 2016 at 11:45 PM by rjm

Although the original Sapphire headphone amp can be configured as a line stage, or use as-is as a line stage, I've gone ahead and made a new circuit variant with a new set of boards.

The Sapphire Line (in development) combines the shunt-series regulator, bboard 2.0 buffer and an op amp voltage gain stage. Same basic idea as the Sapphire of course, but with a much less beefy output stage so the low noise regulator can be added and everything still fits on the board.

rev 10e - now with support for 2520 op amp modules
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Old

Multi-Channel, High Sample Rate PCM

Posted 18th July 2016 at 01:36 PM by Tam Lin

Here is a multichannel implementation that continues the theme of high sample rate PCM with no logic chips in between the oscillator and the DACs. There are two variations: seven and fifteen channels using USB2 bulk mode transfers up to 1.536M samples per second. More channels and higher rates require USB3.

What I find appealing about this design is that it is easily bread boarded. For USB, use a Cypress FX2 evaluation board. It has a built-in quantum FIFO and packet buffering. For the oscillator, use a Si570 evaluation board. With it, you can easily switch between the 7 and 15 channel versions, 44.1K and 48K based sample rates, 16x and 32x oversampling, and 24- and 32-bit sample frames. For the DACs, use Twisted Pear CODs with SCK = BCK and the format switches set for DF bypass.
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Old

Quick note on line level output currents

Posted 18th July 2016 at 03:58 AM by rjm (RJM Audio Blog)
Updated 22nd July 2016 at 10:41 PM by rjm

Consumer audio standard line level output is -10 dB, 0.316 V rms [dB = 20 * log (V/1V)]. Some devices like computer sound cards can boost that at the max volume settings, my Asus Xonar can do 6 dB or 2 V rms. Quite a lot of digital audio produces 2 V rms output, DACs and CD players and not just computer sound cards.

The amount of output current required by the line driver is the signal level divided by the load impedance, so to estimate the worst case scenario we have to consider the smallest practical load and the largest likely signal. The input impedance of consumer audio is typically 10k to 100k. 10k is the lowest design point, but sometimes people do strange things like drive two components at once which halves the value, or headphones, or pro audio gear with 600 ohm inputs.

The long and short of it, though, is that consumer audio inputs are never normally going to draw more than 1 mA. For pro audio the maximum is meanwhile 3 mA. 5 mA bias current through...
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Old

A Rant on Why Passive Preamps are Totally Stupid

Posted 16th July 2016 at 02:04 AM by rjm (RJM Audio Blog)
Updated 20th July 2016 at 09:57 PM by rjm (corrected attenuator output impedance in attached diagram)

A [just my opinion, bro] post...

I actually had occasion to try this the other week. I had a box with a volume control followed by the bboard unity gain buffer and in preparation for replacing it with a similar buffer with voltage gain (a power-derated Sapphire 3) I removed the buffer and briefly used the box as passive preamp, i.e. just the 47k stepped attuator, with 1 m interconnects to the amp and 2 m interconnects back to the phono stage. Sure enough the system noise increased, depending on the position of the volume control, with some nasty low level buzzing interference.

Why does this happen? It's pretty simple really. Noise is usually induced as a current, and the larger the resistance (impedance) this noise current is forced to flow through to reach circuit common, the larger the noise voltage since by Ohm's Law, V=IR. Noise induced between the volume control and the amp is faced with the high impedance of the amp (47k) or the output impedance of the...
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Old

RC filter coupling capacitor calculator

Posted 16th July 2016 at 12:54 AM by rjm (RJM Audio Blog)
Updated 16th July 2016 at 01:10 AM by rjm

When I need to pick the right capacitor for a coupling capacitor, rather than working out the time constant or 3dB cutoff I just remember the mnemonic "0.1-220" (meaning 0.1 uF and 220 kohms) and adjust the ratio up/down for the resistance I happen to be looking at: 0.22-100, 1-22, 0.47-47.

This amounts to a time constant (t=RC) of 20 ms, and 3 dB cutoff of 7 Hz. The bass attenuation at 20 Hz is half a dB.

If there are several stages the attenuation of all these filters add up, so it can be a good idea to make the capacitance about twice as large. There is rarely any advantage making it much larger still.

Excel worksheet attached. It spits out all the numbers so you don't have to guess.

* calculating the attenuation involves complex numbers. Zr=R, Zc=-i/(2 pi f RC), attenuation (high pass) = | Zr / (Zr+Zc) |. In excel you can use IMSUM, IMDIV, and IMABS to do the complex math.
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