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...

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 - https://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...

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...

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...

My experiments last autumn with putting a transformer between a chipamp and a drive unit left me with one nagging question. Why was the sound so much clearer with the trafo than without, given that the trafo's voltage ratio was relatively low (1.5:1)? Even Frank was surprised at so much change and suggested some other effect was in play (amp instability). I've not ruled out amp instability but I have made some progress on understanding chipamp PSRR.

The first trafo experiment was done with the TDA7265 - I've since tried trafos on two other chipamps - TDA8947 and LM4766 with similar subjective results - a much clearer sound, more depth to the soundstage and so many tiny details on recordings becoming clearer that the experience has become akin to headphone listening. The trafo ratio has increased - 2.5:1 for the TDA8947 (21V rails, bridged) and 5:1 in the case of the LM4766 (again bridged), running on 62V total supply.

My current listening system is LM4766 with...

I built this up to see how the TDA8946AJ DC-controlled volume element sounds. According to the datasheet and comparing the figures with those for the same amp minus the volume control (TDA8946J) there's some additional THD so I was curious to hear how transparent it is.

There are transformers both on the input and the output of this. On the input to match levels (4:1 step down) and to do unbal-bal conversion. On the output again for the same reasons, except bal-unbal to drive my phones which I've not modified for 4 wire operation. The trafos are ferrite (=very cheap) RM10 for the input (under the board) and RM12 for output (behind the board). In order to be able to match a variety of phones, I've wound 4 identical secondaries so they can be combined in series or parallel.

From the pic you'll see the customary array of caps - I need to order up some lowESR (preferably NCC) 1800uF caps to do better justice to the rails. Last night I experimented with some ceramics...

I've found that some of the el-cheapo trafos (18rmb each) at a shop at the local electronics market are of split bobbin construction. This makes bodging up an audio OPT from two mains trafos a fairly straightforward matter.

I bought some with 9-0-9V and others with 0-12V secondaries. Then I disassembled them (fortunately they're not varnish dipped) and swapped out the 220V primary bobbin for the secondary of the other one. This gives me a trafo with 18V on the primary and 12V on the secondary, a step down of 1.5:1, impedance ratio of 2.25:1. So it makes a 4R drive unit appear as 9ohms to the chipamp.

And when I applied this to the output of the bass/mid of my chipamp (residing in the Phenix active speakers, its a TDA7265), apart from it sounding quieter I suddenly realized how much power supply noise I was still listening to. Incredible

So if you want to know if your chipamp PSU decoupling is really up to snuff, see how much difference a 1.5:1...

Very interesting paper which I'd not come across before which raises about as many questions as it answers. Thanks to Bob Cordell mentioning it in his book, here's the link :

https://www.eetasia.com/STATIC/PDF/20...URCES=DOWNLOAD

Compare the FFT in fig3 (closed loop) with fig4 (open loop) in the text - which do you think will sound better?

Here's my take on what the paper is claiming, FWIW. That PSRR measurements when the amp has a BTL (balanced) output aren't appropriate because there is output stage cancelling of them. My first head-scratch moment came because this is true of other amp topologies too, not just classD as BTL outputs are very much alive and well in NXP's range of chipamps for automotive use. So why restrict this to just classD?

Secondly what's the argument that PSRR in BTL amps isn't appropriate? They seem to be saying (though they don't express this very clearly) that whilst output stage...

One channel is completed - barring the off-board hexacap shunt - second channel under construction. Then for a comparison with the TDA1521s

Update1 - both channels running now, 160,000uF on each. Bass is tuneful but LF colourations noticeable on piano (Ashkenazy Diabelli Variations). Solution to this is simple - more caps

Update2 - an extra ring of caps around the original hexacaps now, so we're around 250,000uF. Bass is so much better than from the TPA3116 but the HF is more ragged, less transparency overall than the TPA3116. So I'll next try an outrageously decoupled implementation just to check I'm not limiting the chip with poor PSU hygiene....

Update3 - I have over 30 1206 10uF caps per side now as local decoupling - HF problems are slight. Makes me wonder if I use it only on the bass/mid and a TDA1521 for the tweeter duty we might reach audio nirvana.... Looking at the DS its easy to see why the HF's a bit of a mess - the PSRR is about...

I've only simmed this so far, no listening tests. The aim here is to have a passive filter which works into a line level impedance (i.e. 10k or greater) and which gives a decent amount of RF rejection without compromising SQ. I plan to incorporate it behind the RCA input jacks of my chipamp.

Design-wise, it started life as a simple 2nd order filter with single series L and shunt C. But when it came to wind the inductor I didn't have a wire diameter small enough to make the required value (44mH). So I split up the inductor into 2 * 22mH and then couldn't resist hanging a cap off the centre tap. The result has turned more into an over-damped elliptic with a higher corner frequency than the original, but with an impeccable phase response to 20kHz. The elliptic-type plot comes about because of the SRF of the two inductors (around 350kHz).

Since fas42 remarked on the phase performance, I've added the group delay plot for the passband. Almost pure time delay of...