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SAA7220 filter replacement project

Posted 20th September 2016 at 12:24 PM by abraxalito

Some years ago I reverse engineered the coefficients of the Philips SAA7220 filter, just for fun. At that time the microcontroller I implemented the filter on (LPC1114) didn't have a fully implemented I2S interface so I needed additional logic to get the data in (an array of 74HC595s) and I used the SSP to output a serial stream, albeit not reliably as I didn't implement a foolproof method of generating WS. Hence the filter wasn't ready for primetime.

Now though STM has introduced a Cortex M0 MCU which does have the dedicated I2S ports to realize a pukka emulation of the SAA7220. At least that's what I'm hoping - I know the code worked on the LPC1114 but I seem to recall I needed a slight overclock - there is a report that the STM32F072 can be overclocked but I shall need to check this out for myself. One major advantage this replacement will have is going to be lower power consumption - something of the order of 100mW or under.

In order to kick this project off,...
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Taobao shopping for 'digital' amps

Posted 18th September 2016 at 02:45 AM by abraxalito
Updated 30th November 2016 at 01:32 AM by abraxalito

A number of interesting amps have caught my eye on Taobao of late, they (except for the last) are true digital amps in the sense that they're 'power DACs'. The first is using ST's STA333W which is a phenomenally cheap fully-integrated digital amp chip, hence the very low price of this board - ns=1&abbucket=13#detail ns=1&abbucket=13#detail USB input, speaker out but needs an external power brick of some sort, up to 20V.

The second one's a boxed product, albeit with an external power supply. Its using Intersil's D2-audio chipset (D2-41051), the first implementation I've noticed on Taobao. It uses discrete MOSFETs in the output stage. The enclosure and controls look to be in the PS Audio territory, a cut above the usual Taobao cheap stuff -
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Not Another Gosh-darn EL84 Amplifier!

Posted 17th September 2016 at 06:32 PM by ballpencil
Updated 26th March 2017 at 06:01 AM by ballpencil

.. or NAGA84 for short.

That familiar feeling.. All tube enthusiasts will have a hard time resisting the urge to build an EL84/6BQ5 push pull amplifier, at least to hear for themselves what makes this tube so well-known (and overpriced).

Lurkers of the tube section most probably have already seen the Baby Huey wiki and thread. With such unique local feedback scheme, it's unsurprising that it garners some avid follower. As amazing as the feedback scheme may seem, there are few things i prefer to do differently on that amp. The most prominent thing would be NOT to use a triode for the input/driver stage as the rp of a triode will change with the signal swing and cause inconsistent feedback ratio. That, and the fact that it doesn't measure really well under simulation, motivated me to do my own version of the EL84 push pull amp.

To remove the triode from the feedback equation, there are two options i can think of:
1. Use pentodes, obviously....
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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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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

However, a lot of component and design is still very good like the Holden & Fisher...
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RJM Audio discrete op amp (circuit schematic)

Posted 8th August 2016 at 02:06 AM by rjm (RJM Audio Blog)
Updated 22nd November 2016 at 10:29 PM 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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Posted in The Lab
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Sapphire Line Preamplifier - My Build Notes

Posted 28th July 2016 at 03:15 AM by rjm (RJM Audio Blog)
Updated 4th December 2016 at 10:55 PM by rjm (add measurement data)


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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Just the buffer, ma'am.

Posted 28th July 2016 at 02:05 AM by rjm (RJM Audio Blog)
Updated 8th April 2017 at 05:37 AM 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. (updated to 21f4)
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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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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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