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Aiming for maximum isolation

Posted 2nd November 2014 at 03:42 AM by abraxalito

Getting isolation from the mains supply is an on-going challenge for me. Here's my latest stab at getting as close to a battery as I can currently manage - a ferrite line-output trafo soon to be pressed into service as a low-capacitance isolation transformer.

The back story is that I'd heard that split bobbin trafos were the way to go for the best isolation from the mains. Having just acquired some for chipamp-drive unit interfacing I was curious to check out their credentials. I measured around 20pF interwinding capacitance which is about the lowest of any mains trafo I've checked. Of that it seems that the majority of the capacitance is directly between the windings, a smaller amount is due to coupling via the steel core (which of course is conductive). So then I figured that even if the windings could be moved infinitely far apart the steel core would be the limiting factor in reducing the capacitance.

Air core trafos are a possibility but then they're hopeless...
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Old

My latest infatuation - transformers

Posted 25th October 2014 at 09:11 AM by abraxalito
Updated 6th November 2014 at 11:51 PM by abraxalito

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...
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Old

Taobao has a TDA1387 DAC (apparently)

Posted 23rd October 2014 at 03:33 AM by abraxalito
Updated 28th November 2014 at 01:45 PM by abraxalito

Its rather difficult to work out whether there's a line-level output here or if the phono sockets are inputs to the LM1875 amps. The TDA1387 is a plug-in module for the USB-input DAC section which might feed only the amps. Anyone who's good at reading Chinese, please contribute in the comments

http://item.taobao.com/item.htm?spm=...8-3f2abc0f8bd9

Update - seems one particular vendor has quite a range of TDA1387 DACs now, all very affordable. The cheapest is this one (48rmb kit, 88rmb built in my limited understanding) - http://item.taobao.com/item.htm?spm=...0-99c1c511877f
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Old

Denon DL-103

Posted 14th October 2014 at 01:55 PM by rjm
Updated 14th October 2014 at 01:57 PM by rjm

Eye candy.

My fourth. Each one comes packaged the same, with a serial number and individual calibration sheet.

This little guy is no. 2096, .39/.40 mV. Signed off by Mr. Nemoto (根本さん、ご苦労様!)
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Old

Repair of a vintage driver (Richard Allan)

Posted 6th October 2014 at 11:30 AM by googlyone

Over the last 6 or so months I have built a nice little sub plus satellite system that comprises:
- The "Ikea Salad Bowl Speakers", with some really nice vifa premium line drivers in them, making these 1980's vintage
- A Richard Allan subwoofer, in which I used a 1970's (I guess) HP12B driver, in a complex cubical enclosure with a corner cut off for the driver, and
- A ridiculously complex amplifier built to drive this lot, with DSP, multiple channels, optimised even to the point of addressing the fact the HP12B is 16 Ohms, and running bridged for that output.

So imagine my reaction when last night I settled back and ran the system up "properly" for maybe the second or third time, and CRACK!!!

My initial thought was that I had over "excursioned" the sub, and the voice coil was hitting the magnet backplate. I dug into the amplifier and programmed a subsonic filter, as you do, and settled back in again. CRACK!...
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Old

Phonoclone noise measurements

Posted 3rd October 2014 at 01:40 AM by rjm
Updated 3rd October 2014 at 10:27 AM by rjm

What we are looking at here is the Fast Fourrier Transform (FFT) of the line output from my b-board buffer recorded at 24 bit, 96 kHz by an Onkyo SE-200PCI sound card. Upstream from the b-board is the Phonoclone 3 MC phono stage, connected to a Denon DL-103. The tonearm is Denon DA-307, and the deck is a Denon DP-2000.

Four recordings, taken 1) with music playing, 2) with the tonearm raised 3) with the phonoclone powered off and 4) with the b-board and all upstream components powered off.

True 24/96 data was obtained, measurements out to 48 kHz are possible, with -130 dB noise floor. (I was using Digionsound 6 to do the recording as Audacity truncates 24 bit recordings to 16 bit in Windows due to licensing issues. The FFT was generated in Audacity however.)

The soundcard's line input may have an impressive-looking low noise floor, but it's still useless for measuring line level audio devices like the b-board because the noise of the preamp/ADC...
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Old

New active speaker on order from Taobao

Posted 26th September 2014 at 03:49 AM by abraxalito
Updated 6th October 2014 at 03:44 AM by abraxalito

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...
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Old

Revisiting active speakers - HiVi D1010

Posted 21st September 2014 at 01:07 AM by abraxalito
Updated 24th September 2014 at 03:51 AM by abraxalito

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...
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Old

Apt-x try-out

Posted 27th August 2014 at 08:38 AM by Jaac

In the search for a nice Bluetooth module i found a chineese ebay shop that sells TTK Technologies modules.

Sure Electronics MB-CM15115

TTK Technologies partnumber BTM98-8AA. It says it will support Apt-x codec more info

I bought the module connected the power, leds are blinking,.... no descoverable devices in the bluetooth domain.
To be able to stream music from a pc i also bought a USB dongle Azio

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Old

Retro Amp - final product

Posted 26th August 2014 at 02:18 PM by googlyone

I finally got the retro am finished. At least to the point of all the parts set to work, the case built and assembled and wired up.

The photos really don't show just how orange it is, nor does he slant in the case "pop out" as much as it does in real life.

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Yes, I did the "writing" on the front panel by hand with a paintbrush.... I came close to getting the thing CNC milled, but concluded that the hand painted approach would be (a) fun, (b) a new challenge, and (c) more in line with a retro amp

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The amplifier has:
- A digitiser on its input.

- DSP implementing crossover and...
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