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Designing high order active filters

Posted 20th September 2015 at 11:54 PM by abraxalito
Updated 25th September 2015 at 01:20 AM by abraxalito

I've been getting a lot of use out of Simon Bramble's webpage for designing active filters recently - Its a great resource.

Right down at the bottom of the page the last filter he shows the schematic of is a 9th order Chebyshev, 1dB ripple, with a corner frequency of 1kHz. A textbook frequency response plot is obtained using LTC6241s. I latched on to this and tried changing the corner frequency to 18kHz, wondering if I could use such a design for an anti-imaging filter for my DACs. So I divided all the capacitor values by 18 and ran the sim. Disaster! The frequency response I obtained is below - a 7dB spike at 17kHz.

The problem seems to be inadequate Q - high order filters are composed of sections which increase in Q (more positive feedback) and the chosen opamps aren't fast enough (18MHz GBW). I went to a faster opamp for the highest Q stage which brought about some improvement...
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New tested JLH 2005 class A amplifier, works excellent

Posted 15th September 2015 at 08:12 AM by siliconray

Hi, our engineers just tested JHL 2005 class A amplifier. It works excellent.
The Q1 and Q2 is 2N3055. Also you can choose 2SC3280,MJ15003 or MJ15024. any of them is ok for us.
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Taobao headphone amp is an FX box

Posted 14th September 2015 at 01:19 PM by abraxalito

I bought this amp because the case attracted me - no pics on the Taobao page were giving away anything about the insides, quite unusual. After receiving it I couldn't resist having a quick listen and it turns out its a hardwired tone control with bass and treble turned up to the max, about +16dB wrt 1.3kHz. What a surprise! - no matter as I was going to strip out the innards anyway to use as a test platform for my amp.
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Can you identify this opamp from its simplified schematic and PSRR plot?

Posted 10th September 2015 at 01:28 AM by abraxalito
Updated 21st September 2015 at 12:23 AM by abraxalito

In my search for opamps with better real-world PSRR behaviour, I came across this beauty. For now I'll just post up its simplified schematic and PSRR plot - if any of you know of it please put your deduction in the comments. I may add more clues later if nobody nails it early on.

The reason I find this part interesting is its cascoded output stage - I believe this is what leads to the 'hump' in the positive rail PSRR. I've never seen that behaviour on any other device.

Well over a hundred views now and not one single stab at the answer. Its the industrial-strength version of the now obsolete LM308, a Bob Widlar special with super-beta input stage. The output stage cascode I take it isn't primarily to improve the PSRR rather its due to the high maximum operating voltage (72V). TI does still have the DS on its website though it doesn't put in an appearance in parametric search as its been obsolete for a while. Even more interesting is its decompensated variant which...
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Getting CS5381 Distortion down!

Posted 7th September 2015 at 11:35 AM by googlyone

On thinking over the sensitivity of the CS5381 to the drive and input filtering capacitor I decided to explore the capacitors further.

I was also interested to note that a number of manufacturers seem to recommend a range of different capacitor values here.

Purely because it was handy - i.e. right in front of me on the desk - I threw an extra 2nF capacitor across the differential input of the CS5381. Boom - the distortion dropped 6dB straight off.

I muttered a few choice profanities, which made me feel an awful lot better, then arbitrarily threw a 470pf NPO ceramic cap across that lot, just to be sure. Well given I was off doing such arbitrary things - why not?

On analysis, 5nF capacitance at 20kHz is about 1.5K Ohms (reactive) which is within the capability of the op amp to drive.

The results?

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Which I found rather pleasing, as using the "default" 2n7,...
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"Nazar`s regulator", variants

Posted 6th September 2015 at 07:59 PM by Nazar_lv (Creative High end audio)

A couple of variants of this ideal regulator for audio applications.

Ideal because:

1. Stable and small output impedance in the 20-20k area and above (for 2x trans. version ~ 0.05ohm and ~0.005ohm for 3x trans. version).

2. Load current flows locally through the regulator`s output transistor only.

3. Excellent step response.

4. Little capacity electrolytic cap on regulator output or no electrolytic capacitor at all.


5. High PSRR (for 2x trans. version 100db and 120db for 3x trans. version).

6. Noise < 1u (20-20000hz).

7. Simple and reliable.

In addition to the classical 2x transistor "nazar`s regulator" also described 3x transistor version characterized by much better output impedance and PSRR.

Also described a filter/voltage follower that makes better use of supply voltage (Uout...
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DIY Audio Analyser using CS4398 / CS5381 - The Limits of These IC's

Posted 6th September 2015 at 11:47 AM by googlyone
Updated 6th September 2015 at 11:56 AM by googlyone

I have had the time to play with the MiniDSP Streamer and my ADC and DACs now.

Initial results were disappointing - and led to me looking very closely at the ADC drive, and particularly the single ended to differential part of the circuit.

Given I pinched this circuit from an application note (and embarrassingly did not question it adequately) gives me little solace that I built, and used this!

Interestingly, Creative Labs did exactly the same thing on the sound blaster that I had so much trouble improving - the single ended conversion is just wrong. This solves the "Puzzle" that I noted in a blog a few months ago - now I know why I just couldn't get better performance out of that CS5381 drop in to the Sound blaster box.

In the process of improving the ADC drive, and to allow experimentation with the input op amps, I built two versions of the ADC drive, one using non inverting buffers and a second that uses inverting...
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DIY Audio Analyser using CS4398 / CS5381 (CS5361)

Posted 31st August 2015 at 12:50 PM by googlyone

I recently set to work the guts of the audio analyser:
- MiniDSP USB Streamer
- Interface card

The ADC and DAC were from an old project. the tests I had been able to do on these were done using simple test gear, I knew the distortion was fairly low - but had been unable to really bottom out the level.

I did know the DAC was down in the 0.001-3% level or below.

Imagine my chargin when I fired the system up, got all the bits running and measured something closer to 0.01% distortion at -1dBc!

I was initially convinced that there was something wrong. After cranking the level down to -10dB, the distortion dropped to 0.001%. Further tesing (I ran the DAC at -1dBc and attenuated the output) proved that it was the ADC dominating the measurements.

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Much fiddling led to me concluding that the ADC buffers were the culprit. I would like to blame...
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Schiit Audio using AD5791 (modern R2R/multibit)

Posted 30th August 2015 at 10:45 AM by hollowman
Updated 30th August 2015 at 03:22 PM by hollowman

Schiit's $2300 Yggdrasil D/A processor ...
link's what Schiit claims on their product page:

21 Bits, No Guessing: Mission-Critical D/A Technology
When doctors are trying to diagnose whether you have gas or cancer from MRI results, or when the military is trying to ensure a missile hits an ammo dump and not a nunnery next door, they don’t use “24 bit” or “32 bit” delta-sigma D/A converters. Instead, they rely on precision, multibit ladder DACs, like the Analog Devices AD5791. This allows them the bit-perfect precision they need for critical applications, rather than the guesswork of a delta-sigma. We chose this same critical technology for Yggdrasil. Following these unique D/A converters are sophisticated discrete JFET buffers and summers.
To their credit, Schiit also uses the same AD DAC in their $600 "gungnir-multibit" D/A processor:

Just in terms of DigiKey pricing, the DAC chip is not cheap ($15...
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Headphone amps designed for PSRR

Posted 28th August 2015 at 04:38 AM by abraxalito
Updated 6th September 2015 at 10:57 PM by abraxalito

Since acquiring and modding my Taobao headphone amp I've been enamoured of creating a much more portable headphone solution to deliver aural nirvana but on the move. Whereas transformers are a very practical solution for a desktop amp, steel and copper is not only bulky but also jolly heavy and hence a no-no for anything pocket-sized.

How else to get the dynamics I'm seeking though? For these amps I'm toying with different solutions to getting better PSRR, particularly in the all-important bass region which tends to suffer in commercial portable amps. The OPS (output stage) is what needs most attention in any classAB amp - the signal stages can all be classA but for efficiency (and hence battery life) the output stage can't be conducting all the time.

A fully discrete output stage where the output devices are cascoded looks to be one solution but initially I'm looking for a simpler, more cost-effective solution with lower overhead on the supplies if possible....
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