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Can you identify this opamp from its simplified schematic and PSRR plot?

Posted 10th September 2015 at 02:28 AM by abraxalito
Updated 21st September 2015 at 01:23 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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Old

Getting CS5381 Distortion down!

Posted 7th September 2015 at 12:35 PM 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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Old

"Nazar`s regulator", variants

Posted 6th September 2015 at 08:59 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.

Also

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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Old

DIY Audio Analyser using CS4398 / CS5381 - The Limits of These IC's

Posted 6th September 2015 at 12:47 PM by googlyone
Updated 6th September 2015 at 12:56 PM 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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Old

DIY Audio Analyser using CS4398 / CS5381 (CS5361)

Posted 31st August 2015 at 01:50 PM by googlyone

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

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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Old

Schiit Audio using AD5791 (modern R2R/multibit)

Posted 30th August 2015 at 11:45 AM by hollowman
Updated 30th August 2015 at 04:22 PM by hollowman

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

Quote:
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:
link

Just in terms of DigiKey pricing, the DAC chip is not cheap ($15...
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Old

Headphone amps designed for PSRR

Posted 28th August 2015 at 05:38 AM by abraxalito
Updated 6th September 2015 at 11:57 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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Old

DIY Audio Analyser using CS4398 / CS5381

Posted 21st August 2015 at 06:16 AM by googlyone

Over the years I have played with measuring - well - just about everything in audio.

distortion measurement has been one of these preoccupations, not the least because it is hard to do, and using commercial gear, expensive.

I have built analogue hear to do this, used mixes of analogue and digital and of late played with using modified commercial sound cards. My efforts on the Sound Blaster x-Fi were interesting but ultimately not rewarding enough for me to leave the mods in there.

I have also been playing with using the A/D and D/A converters from my DSP crossover for this purpose. These are modular and use a common I/O plug with power MLCK, SCLK, LRCLK and Data (alone with I2S for the CS4398 and sometimes a digital volume control).

--- notwithstanding the fact that in previous tests I have shown the digital volume control is the MAJOR source of distortion - I will plug ahead with this, and likely leave that out of the test...
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Old

an economic diy sub to compliment EV ZLX12P

Posted 17th August 2015 at 08:55 PM by colossusfan
Updated 9th September 2015 at 08:32 PM by colossusfan (Add mute function)

My first attempt at a blog so i hope i dont bore the pants off you all!!
I bought a 2 way so called 1Kw integrated speaker some time ago about which I have written various posts in the forums, management overview better value for money than DIY but vastly inflated specification as all similar units, actual measured output around 110dB max and LF rolloff starts at 80hZ!!
Soo it needs a sub!!
well if like me you like your low end forget the EV sub (ELX118p) and most of its compatriots for two reasons, one is rolloff starts at 40hZ or above, and 2nd is cost!!

Looks like i just use edit to add to the blog!

So to continue I used the UNIBOX spreadsheet (free) to model various drivers and box combinations, true to my namesake I started off with large Fane drivers but was scared of the price!! However I also discovered various nasties with ported or tuned enclosures most notably you tradeoff an extension in the 40-60hZ range for a much sharper cutoff...
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Old

Fountek (Rhyme Acoustic) active speaker A-100

Posted 24th July 2015 at 09:18 AM by panson_hk
Updated 28th July 2015 at 02:37 AM by panson_hk

I purchased a pair active speaker. This is manufactured by Fountek under the brand Rhyme Acoustic. The model number is A-100. It uses a 4" full-range driver with vented box. The electronic is contained in the right channel enclosure. The amplifier is LM4766 chipamp. The overall build quality is good. I just paid RMB 399.00 during its sales period. What a good deal!

Chinese product link is https://item.taobao.com/item.htm?spm..._u=od0rlto94d1 The frequency response shown in this product page does not match with that given the user's instruction of my pair. The curve shown in user's instruction does pretty much match with my initial listening impression: lack of treble.

I measured the speaker with my CLIO Pocket. Measurement shows left and right enclosure response are matched very well.

The last figure shows the left box (passive) driven by another amplifier. We can then see what filtering or EQ is...
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