Onix/Melody SP3 Upgrades and Mods - diyAudio
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Old 21st October 2009, 01:02 AM   #1
rhing is offline rhing  United States
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Default Onix/Melody SP3 Upgrades and Mods

I have an Onix SP3 vacuum tube integrated amplifier that I purchased from AV123.com a few years ago. I like this amplifier and I believe it has potential to be a lot better. It has been characterized to have a "solid state sound" and I have to agree with that description. What can be done to the circuit to expand its soundstage and "warm it up" a bit? I've rolled tubes, and some people have installed new coupling caps (I think C6R and C7R: 0.47uF) or bypassed them with 0.1uF Teflon film caps.

I've included some recently posted schematics from a WIKI for this amplifier, Onix SP3 WIKI:

Onix SP3 Amplifier Schematic.jpg
Onix/Melody SP3 Amplifier Schematic

Onix SP3 Power Supply Schematic.jpg
Onix/Melody SP3 Power Supply Schematic

I have also included a photograph of the interior of the amplifier:

Onix SP3 Interior Photo.jpg

These amps show up frequently on Audiogon in good-excellent condition for $400 to $500. I believe many have been sold, resold and traded, so I have to believe that there could be a lot of interest in modding these.

I can solder and read schematics. I'm just not knowledgeable enough to know how to modify a tube circuit to improve a design for better performance. Any help is greatly appreciated.
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Last edited by rhing; 21st October 2009 at 01:19 AM.
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Old 21st October 2009, 06:03 AM   #2
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I'd love to hear some thoughts here too. I've thought about trying to do something like this from scratch, but I think I'd get much better bang for the buck upgrading something like this.
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Old 21st October 2009, 11:28 AM   #3
rhing is offline rhing  United States
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Here are a couple links to some upgrade efforts on the Onix SP3 found at the AV123.com forums:

http://forums.av123.com/showpost.php...44&postcount=1

http://forums.av123.com/showthread.php?t=13266
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Old 22nd October 2009, 01:36 PM   #4
tyu is offline tyu  United States
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Default Tube to be characterized to have a "solid state sound"

Yes i ran in to this in the 90s what i came up with an the one thing,not wire not caps,not new tubes,
Look at the 5881,6l6 cathiodes thay can not move with the sound! run them togather an put 10-15ohm 5watt to earth the tube bias well be diff. but the amp well putout more power an sound like a Tubeamp,look at the Dynaco MK3 ,best Mod i have ever done to a TubeAmp Goodluck
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Old 25th October 2009, 04:10 PM   #5
pjanda1 is offline pjanda1  United States
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I'm disappointed more folks haven't commented, as I love this little amp. I think the price might be worth it for just the iron and chassis! I wouldn't spend too much $$ on coupling caps. Whether you are bypassing or replacing, look into Russian military surplus. The FT-3 Teflon's would be something entirely different, though not necessarily tube-y-er. The russian stuff is a much better deal than boutique names. That isn't much of a B+ supply. I'd think that some quieter diodes could make a real difference.

You should give Jim Mcshane a holler. He could probably set you up with some nice parts that won't break the bank. He'd be a great source if you want to roll tubes too (those AV123 forum guys seems crazy about tube rolling, I don't know if you'll find the results worth the $$), and his advice is honest, not audio guru-ism.

It seems to me the main culprit for the "solid state sound" (be it known that I do think these amps sound really nice for the $$), is probably excessive gain and feedback. It looks like you've got too many tubes! The problem is that modifying the circuit of a PCB based amp is tough. I don't know how sturdy that board is, but you'd have easier options were the amp point to point. I am certain that some circuit mods would make a bigger difference than parts swapping. Maybe someone will speak up and suggest something easy.

No matter what you do, your power supply will be limited by space.

My thought would be play with it until you damage the PCB beyond repair. Then, strip it out and build something point-to-point and entirely different. Maybe something differential like an Allen Wright PP-1 with an outboard power supply. I believe they used the same OPT's as on their more expensive EL34 amps. This guy just has a cheaper and smaller power transformer.

have fun,

Paul
www.wildburroaudiolabs.com
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Old 25th October 2009, 06:25 PM   #6
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Looks like a nice little amp. Certainly very cheap for $500. As for the absence of "tube sound" i have never heard a tube amp with solid state rectification that i really liked. And there seems to be quite a lot of NFB. Getting it to sound more tubey won't be easy but it can certainly be improved in other ways. I would try to fit at least a regulator for the screen voltage and get the best possible parts for the bias circuit - improvements in both are very audible to me. What are the current coupling caps? Chinese? Russian? I really like the cheap K40Y9 pio caps but hardly any of the other Russian caps.
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Old 27th October 2009, 10:54 PM   #7
rhing is offline rhing  United States
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Quote:
Originally Posted by analog_sa View Post
Looks like a nice little amp. Certainly very cheap for $500. As for the absence of "tube sound" i have never heard a tube amp with solid state rectification that i really liked. And there seems to be quite a lot of NFB. Getting it to sound more tubey won't be easy but it can certainly be improved in other ways. I would try to fit at least a regulator for the screen voltage and get the best possible parts for the bias circuit - improvements in both are very audible to me. What are the current coupling caps? Chinese? Russian? I really like the cheap K40Y9 pio caps but hardly any of the other Russian caps.
Is there a way to reducing the NFB? As I understand, the coupling caps are the little Silver-colored caps on the top and bottom of the photo I posted and the 4 greenish colored Russian PIO caps bolted to the PCB.
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Old 18th December 2010, 11:32 PM   #8
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Bumping this back up. I may be getting a used Onix SP3 and my first thought is to replace those coupling caps (russian mbgo?). But I would love to know if there's a way to reduce the gain or any other mods that might improve this circuit. Anyone?
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