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Active speaker modifications Hot-rod your cheap and cheerful actives.
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Next generation active speaker electronics

Posted 3rd December 2013 at 04:12 AM by abraxalito
Updated 18th December 2013 at 01:59 PM by abraxalito

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...
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Old

D1080 MkII 08 actives - mod summary and listening

Posted 1st January 2011 at 11:41 AM by abraxalito
Updated 2nd March 2011 at 11:12 AM by abraxalito

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...
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Old

D1080 Mk II 08 actives - initial listening and some refinements

Posted 23rd December 2010 at 12:49 PM by abraxalito
Updated 2nd March 2011 at 11:14 AM by abraxalito

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...
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D1080 Mk II 08 actives - XO mods

Posted 23rd December 2010 at 11:21 AM by abraxalito
Updated 5th March 2012 at 02:53 AM by abraxalito

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...
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D1080 MkII 08 actives - power supply mod details

Posted 17th December 2010 at 03:34 AM by abraxalito
Updated 5th March 2012 at 02:54 AM by abraxalito

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,...
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D1080 MkII 08 actives - crossover schematic

Posted 15th December 2010 at 02:28 AM by abraxalito
Updated 5th March 2012 at 02:54 AM by abraxalito

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
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Old

D1080 MkII 08 actives - under the hood

Posted 14th December 2010 at 11:46 AM by abraxalito
Updated 5th March 2012 at 02:55 AM by abraxalito

I've been spending my free time these past couple of days reverse engineering these speakers, as a prelude to devising the mods. The first stage in doing that is to photograph the PCBs both sides and have both images up on the screen at once, the underside is flipped horizontally so its as though I'm looking through the board. I then annotate the components by hand with the mouse drawing tool. Sometimes I'll write the values in this way too, other times just paste text for the value next to the component. Most of the time though I'm too lazy to add any annotation.

Here's the result for the volume/tone control PCB - one of two PCBs in the design. There are two quad opamps, TL084s - they're marked up in magenta. Interestingly there's no tone control circuitry as such, the bass and treble controls just act as volume to the woofer and tweeter respectively! I've never seen tone controls implemented in this way before...
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