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...

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...

This one has dual TDA7265 chipamps and a nice 60VA toroidal trafo which should give better regulation than the normal EI type. Four electrolytics for the main PSU is encouraging. The opamps are socketed so opamp rolling is on the cards...

Update - received the speakers now. A quick listen showed the typical lack of dynamics opamp sound. After all they're only NJM4558s in there. So I shall re-jig the XO for TL082s by scaling up all the impedances and biassing the opamps into classA. Already all the through-hole caps have come out ready to be replaced by SMT types. The topology tends to suggest this may well be a clone of the D1010 which I'm already familiar with.

Update2 - modded the XO board but so far left the amp board 'as stock'. Mods are 11k resistors to VEE for classA bias, opamps swapped to TL082s, TL431 shunts installed to give +/-5V (from the regulated +/- 12V), plenty of 3,300uF caps across the supplies, 220uH inductors filtering from the shunts, impedances...

Back in 2010 I spent many hours tweaking the D1080s with very satisfying results. However now with my Ozone variant DACs I have a far more transparent source than I did then and would like some demo kit which does the DAC justice whilst being fairly compact, portable and not too complex to mod. In 2011 a new smaller and even cheaper model arrived - the D1010 which has undergone some very minor modifications and is now in its mark IV incarnation. Like its older and bigger brother, its also a true active - it uses LM1875 clone ICs for the 100mm bass/mid and a tiny IC amp with a clip-on heatsink powered from a separately regulated supply for the 20mm tweeter. A fairly decent foundation for some extensive hot rodding.

On first connecting a pair of these up to the original portable Ozone (that's the one in the tea canister) I was impressed by the LF soundstage bloom that was reproduced but less than overwhelmed by what happened to the HF. Ragged would be a fair approximation, rather...

I'd like to be sure that my amp and the passive XOs in my speakers aren't significantly the bottleneck in the sound I'm getting from my DAC prototypes. To get a second reference point I'm starting to build a passive line-level crossover. Here's a pic of it taking shape - its using AD605s because I have plenty to hand and they're cheap and capable of being subjectively transparent with the right power arrangements. The pot core inductors are implementing LR4 XOs in balanced. Using the AD605s with their low input resistance and noise allows manageable inductor sizes for the XO - the largest being 9mH for a 3.5kHz crossover frequency.

Ah - I forgot to mention how I propose to do the transformation from normal (2V) line levels to the 70ohm impedance/20mV paradigm at the AD605 inputs. I shall be winding a pair of ferrite-cored step down transformers with centre-tapped secondaries Lots of turns to look forward to with 0.1mm or perhaps finer wire as I'd like to get at least 5H...

Before describing the changes brought about by the mods I've described, here's a summary of what they've been:

• Crossover tweaked to flatten the lower end bass hump and knit the handover to the tweeter more tightly
  
• Power amp sensitivity reduced to allow the volume control operate closer to maximum with my Asus soundcard or a standard CD player output.
  
• Rectifier snubbers and additional filtering to the regulators fitted to reduce RF ingress on the power supplies.
  
• Power amp supply decoupling improved at both LF and HF.
  
• Signal grounds are now separate from power grounds and a star point established on the amp PCB. A ground loop between the two PCBs has been eliminated. The input signal ground is no longer connected to the tone control PCB ground - instead given its own dedicated wire to the star. Decoupling to ground on the tone control PCB has been removed.

Listening impressions

OK, so on to the part that you've been...

Having disassembled the amp board from its backplate once and got heatsink compound in various undesirable places, I was hoping that the first round of mods would also be the last. No such luck

However, first listening was extremely promising, so I was definitely up for a second round of extreme messiness to gild the lily. I did notice that my grounding changes had introduced a slight hum (50Hz fundamental only it seems) which was only audible when no music was playing and was independent of volume setting. The main gripe I had was that the stereo image was shifted over to one side - this I decided was because the volume pot was being used towards the lower end of its range where the matching is poorest. The chip amps have rather high gain (32dB in bridged) and this can't be reduced. So the solution had to be modding the resistive dividers between the XO and the amp chips - in effect reducing the power amp sensitivity. I went for about a 9dB reduction, determined by the...

Here's the schematic showing the component value tweaks I made to the tone control board. I'll list the changes in detail - only the first three are depicted in the schematic. The remainder aren't as they don't show up on the sim plots

1) The original input stage has attenuation followed by gain. I have removed both, its now a unity gain stage and also no longer rolls off the HF response.

2) The low-end high-pass filter originally had a rather too high Q for my taste. It's been tamed.

3) The bass-mid low-pass filter was rather too low Q, so I've brought this up, making the response flatter across the band - now the hand over to the tweeter is tighter. The tweeter high-pass is left unchanged.

4) The decoupling regime has been changed so as not to contaminate the local ground. Thus the original 100nF ceramics to ground from either supply rail have been moved to decouple between the positive and negative rails. Ground is thereby...

When considering how to hot rod a particular piece of electronics, my first attention always goes to power supplies and layout rather than the somewhat more popular approach of swapping out components for boutique variants. This is because so far I'm not sure that I notice the differences between audiophile grade caps (for example) and the bog standard ones. But I am sure of the differences brought about by improved grounding - to my ears these aren't subtle changes at all. So if I turn out to be dissatisfied with the sound of my layout mods, then I'll turn to tweaks on individual parts.

On examining the layouts of the two PCBs (XO layout already shown in a prior post) it turns out the amp PCB is the one with the relevant power supply components. In more detail, it has two independent supplies coming from separate windings on a standard EI-core transformer. The higher current one is unipolar, unregulated and feeding only the TDA8947s, the small-signal supply is bipolar,...

Here's a somewhat simplified schematic I entered to model the response curves in LTSpice. The volume control isn't shown (as LTSpice doesn't have the symbol) but happens before the first opamp. I've fudged up symbols for the bass and treble controls - they're simply variable resistance in the HF and LF feeds to the power amp board. LT1057 isn't actually used in this unit, its just the nearest library part to the TL084

<edit> I've added the response curves now - look and weep