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WLIO TV Guy 13th February 2020 11:00 PM

ART Tube Opto 8
 
Does anyone know where I can get a schematic for an ART Tube Opto 8? I've seen this model mentioned here....tube subs and mods....but schematic? I contacted ART and they say its their policy NOT to provide schematics for current production units. My Opto 8 has some crackling in one of the mic preamps but the PCB layout doesn't lend itself to easily understanding the circuit. I started drawing a schematic myself (after finding none on the web)....but the ckt layout is screwy,the PCB is double sided, there's a mix of SMD and through hole parts, and a number of the traces are obscured by the components. After a few weeks, I made it thru the preamp and line level audio ckts...but lost the signal path somewhere in the digital stuff. I tried working backwards from the output jacks...and drew schematic of the analog output ckt.

I probably know enough about the ckt to to fix the noise problem....but having put so much time into this thing...it'd be nice to have a full schematic.

I looked at all the ART schematics I could find online....and I see a number of similarities to the Opto 8.

Incidentally, my Opto 8 has several obviously bad caps (bulging tops) and a number (like 20) of high ESR (5 to 50 Ohms) caps. These aren't the cause of the noise, but I wouldn't be surprised if the THD is worse than spec'ed.

BTW, anyone have a preamp noise problem with an Opto 8. What was the cause? The IC, a transistor, resistor...etc.

If no one has a schematic and wants one, I can email what I've figured out so far...

WLIO TV Guy 9th March 2020 03:46 PM

ART Tube Opto 8 Schematic
 
8 Attachment(s)
I didn't get any replies to my request for a schematic for the Tube Opto 8....so I made a schematic myself. Its not a complete schematic. There are 8 channels, and I only diagrammed 2 of them, one with an Instrument Input (CH 1) and one of the regular channels (CH3). I didn't diagram the digital circuits either....I was just interested in the analog signal path.

The plates are operating at about 25V. Other voltages are pretty much as one would expect.

Enjoy!

MediaGary 9th March 2020 03:59 PM

I'm glad you've done this work!

I no longer have my TubeOpto 8, but I do have a TubeFire (firewire version with same analog) languishing in my closet. It has suffered some horrors of my attempted fixes when I had trouble with the firewire circuit, but its analog section has never been touched. If you're interested in paying the shipping for it, PM me, and it's yours.

WLIO TV Guy 10th March 2020 04:38 PM

ART Tube Opto 8 Schematic REVISION
 
2 Attachment(s)
I noticed a mistake in the PREAMP schematic. The actual circuit drawing is correct, but I got dyslexic with one of the part numbers. The parts identified as 2SA790's are actually 2SA970's. This makes a big difference, because '790' is an actual part number....and they definitely won't work! They're Darlingtons...and the 970's are regular bipolars.

My noise problem, the reason I started this thread, was caused by these transistors. I replaced them with transistors I had in stock, some 2N5087's. The 5087's work fine and the repaired channel is just as low noise as the other channels.

I've attached the affected revised schematics. Just download the new files or mark the previous ones with the correct part number.