Ages ago I had a Hi-End AV processor EAD Theatermaster signature. It supported DTS, DD and PCM formats, no HD. Maximum supported in stereo was 24/48. I used to listen to it for several years before HD video formats had emerged. I decided to sell it and buy Onkyo 886 AV processor. Onkyo’s sound was far from perfect, but that’s another story. My friend had the same EAD, but a lower, Ovation, version. He listened to it even longer and then decided to upgrade it, but first asked me to check whether there was any point in doing that. D-to-A converters were not so bad.
PCM1702, digital filter PMD100, jitter killer available. I redrew the circuit to figure out what was going on in there. And I liked what I saw.
Filter quality is actually not very good, low-pass filter was a Chebyshev set to 40 kHz. But volume control was an interesting one – volume was regulated via i/u resistor consisting of 3 relays with 6 dB step; it switches resistor nominal from 10 kiloohm to 1.5 kiloohm; gradual adjustment in this range is effected by DF pmd100. Quite an original idea!!! But there's never too much of a good thing, so it was worth checking what will come. The potential is good. I replaced opa2134 by LME49720, power capacitors by Silmic, low-pass filter – by 30 kHz Bessel.
Now I became interested in what could be wrought from this device. I bought the same EAD for 32000 RUB. Fluorescent lighting of the liquid-crystal display was discharged – they have a small resource. I bought the same screen but with LED.
To be honest, I had to figure out how alter the mode of lighting so that it was active when needed, and not all the time. Eventually the display was replaced by a normal Futaba VFD display – I just had to change the firmware. Fortunately, I managed to find it, and the device was working really nice.
Then I came to thinking what can be done about the analog part after D-to-A converter. Two variants of boards were designed and produced. First variant:
There was a separate 170 mA trans to analog. The consumption was 70 mA. After upgrade I had to add one more the same, as the consumption increased up to 200 mA. I added 3 trigger outputs to control 2 amplifiers and one subwoofer. Only one was available in the initial modification.
Thereafter I made up my mind to make the second variant, as after my friend listened to the first one he was eager to make further upgrades, and repeating what had already been done was boring for me. This variant was an expensive but uncompromising treat. The trans was replaced again. The consumption increased to 400 mA. Double-reserve trans is installed.
I added the function of analog part power cutoff when the device is on standby. By default the analog was on all the time (typical for American hardware), but 12 parallel stabilizers get quite hot. In the standard modification, opa2134 consumed almost nothing. I added one extra relay. At present, all analog tract, including the transformer, is switched off when the processor is cutoff by remote control. In the meantime, I updated all the capacitors, replaced the D-to-A converters, instead of 1702 I installed a 3-channel 1702K.
Capacitors in D-to-A converter microchip winding were replaced: instead of the tantalum I installed 47 ufd and 0.1 ufd ceramics. On spdif I installed an uncoupling trans. All relays in volume control system replaced by new ones – the old ones were periodically clicking. I found out that some relays occasionally failed. When I disassembled one of the relays I discovered that bonding areas and magnets were worn out. In total 25 relays were replaced – all those present in the device. I also replaced diodes in the power unit.
All these manipulations resulted in wonderful sound for music and movies. That’s not something you can buy at a shop. 100% satisfaction. I don’t know the exact amount, but in total about 1000$ was spent.
Author: Timur Sheykhov