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Superlux HD668B SE classA amp

Posted 23rd May 2017 at 09:58 AM by abraxalito
Updated 8th June 2017 at 03:38 PM by abraxalito

My Superlux HD668Bs are the best value purchase I've ever made in headphone kit, far and away the best bang for the buck at their very modest price-point. If any pair of cans is worth designing a specific amp for, they are most certainly on the short-list. The amp I designed specifically for my AT MSR-7s doesn't have enough ooomph for these as the impedance is appreciably higher and the efficiency lower.

So here is my attempt at a dedicated HD668B amp with no corners cut in SQ while still being cheaper to buy the parts for than the cans themselves. I've designed it to run from a single lithium ion cell - a Panasonic 18650 would work fine and give around 60hrs of listening time. Its a single-ended classA topology driven by a single current-source loaded MOSFET. And naturally enough it sounds like no amplifier at all - a straight wire without gain, seeing as its just a buffer.

In actual fact it certainly does have gain, but only current gain. And no shortage of...
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Old

Tweeter passive XO based on a transformer

Posted 1st May 2017 at 01:04 PM by abraxalito

As avid readers of my blog over the years may well know, I've modded quite a few active speakers. Mainly Swan/Hivi D1010s and D1080s of various generations along with some 3Nods. The entry level Swans are super-cheap on Taobao nowadays with pairs of older generation D1010s going for about 375rmb.

As a result of all these mods and my learning by doing I haven't got a single pair of actives in working condition, primarily because the mods I currently want to do (including fitting transformers inside and swapping out the classAB chipamps on bass/mid to replace with classD) won't fit in the available space. All the amp and active XO parts have been hacked about almost beyond recognition and are of very dubious reliability. So I have come up with a plan - I've thrown all the electronics boards away and will convert the carcasses into passive speakers. Since I also prefer to feed speakers via step-down transformers for better dynamics, I wanted to explore the possibilities of using...
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Old

Finally - a cheap FLAC player with digital out

Posted 8th February 2017 at 12:30 PM by abraxalito
Updated 24th May 2017 at 01:05 AM by abraxalito

I've been looking for something like this on Taobao for a few years now. A cheap player which includes a digital output so I can pair it up with one of my DACs. There have been cheap players, and there have been FLAC players but never cheap FLAC players - the cheapest prior to this one being a media player around 500rmb. This one goes into the pocket change category as its just 88rmb. What's more it includes Bluetooth. Order placed - will update after I've played with it.

https://item.taobao.com/item.htm?spm...cket=18#detail

Just received this - the heart of the beast is the Actions Semi ATS2825, some information about its features is available here : http://www.actions-semi.com/en/productview.aspx?id=195.
Whereas a connector for headphones (top right) is shown on the photo, this doesn't support any analog out (my board has no connector fitted there). Its purely digital output - a header for I2S and...
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Old

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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Old

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 - http://www.diyaudio.com/forums/vendors-bazaar/283672-new-audio-op-amp-opa1622-6.html#post4868193) 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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Old

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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Old

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 - https://item.taobao.com/item.htm?spm=a230r.1.14.19.4wjcvg&id=526018865372& ns=1&abbucket=13#detailhttps://item.taobao.com/item.htm?spm=a230r.1.14.19.4wjcvg&id=526018865372& 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 - https://item.taobao.com/item.htm?spm...cket=13#detail...
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Old

Update

Posted 7th July 2016 at 11:58 PM by abraxalito
Updated 8th July 2016 at 12:01 AM by abraxalito

I've not posted for a few months here as I've been involved in moving apartment which was quite a major project given the quantities of parts and assemblies I've accumulated. Even though I've been in my new place for over two weeks now, very little has been unpacked so far, but I have just yesterday rebooted my desktop active XO/amp system which had been quite literally assembled on my desktop with no casework whatsoever Its still without casework and survived the move with only a few wires falling off but now 'installed' in a drawer (pic attached). Its being fed from my 'Domino' balanced TDA1387 DAC and Taobao TFcard player and delivering bags of emotional satisfaction through stand mounted '3Nod' two-ways.

The amps are all LM4766 bridged running from 60V total but with heroic measures to keep supply noise under control, the central PSU has a CLC configuration and ferrite input transformers are used for coupling between AXO and amps. Output transformers (ferrite for...
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Old

Transformers and crossfeed

Posted 15th April 2016 at 04:09 AM by abraxalito
Updated 25th April 2016 at 03:15 AM by abraxalito

Listening to headphones creates, on many recordings, an unnaturally wide soundstage. I decided to play with the idea of 'crossfeed' whereby some deliberate crosstalk is introduced between the two channels. With transformers its jolly easy to do - just create an extra winding with the right number of turns then put this winding in series with the main output of the opposite channel.

In the picture I've done this with some EP17 ferrite cores and added a DPDT changeover switch to A/B between the original and crossfed version. The crossfeed factor I've gone for is 25%. It works in practice in that there's a more natural presentation without any 'extreme' stereo effects (almost a kind of phasiness on some recordings). But sometimes I enjoy the 'all around' effect so switchable is the name of the game

Trafo winding details as follows - primary 500T of 0.1mm, two secondaries first 120T 0.21mm, second 40T 0.21mm.

Made a second one as a colleague liked...
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Old

Mighty mite balanced SE classA headphone amp

Posted 5th April 2016 at 09:39 AM by abraxalito
Updated 25th April 2016 at 11:06 AM by abraxalito

I was going to call this amp 'fleapowered' but after doing some searching online I found fleapowered amps in general were much higher output power than this - single digit Watts typically. Whereas this one is more than a couple of orders of magnitude lower than that (under 10mW) so deserved a different moniker. Seeing as a mite is smaller than a flea, 'mighty mite' seemed to me as good as any descriptor for it.

The mighty mite started life in my imagination after I walked into a local headphones shop and tried out some cans that the shop assistant recommended to me - AT MSR7s. I'd brought my 'Buffalito' headphone buffer with me and some step down transformers for its output - fed from my mobile phone its way too loud for my AKGs and Superluxes. However the MSR7s were too loud even with the 2:1 step down trafo so I told the assistant I'd come back and listen again once I had a more suitable trafo. I figured it needed to be 4:1 step down, so I built a pair of those (PQ32/20)...
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