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Old 22nd March 2006, 03:23 AM   #1
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Lightbulb Studio 350

Hi All, I'm new to these forums here, but am a big DIY enthusiast.
One thing i'm dying to find out, after reading about the Studio 350- Would it be possible to increase the output power so it would run around 500 watts into 4ohms? If so... what would I need to do to achieve this? Merely add more output transistors and up the supply voltage? Or would other parts need changing?

I also have 2 ETI480 modules- same question as above for them as well... How can I soup them up a bit?

Thanks in anticipation!
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Old 22nd March 2006, 07:48 AM   #2
quasi is offline quasi  Australia
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Default 500 watt Studio 350

Assuming you do not want to run them in bridge mode then you will need to;

- Increase the supply rails to +/- 80v
- Transistors Q4,Q5 & Q6 will run hotter so I would change them to MJE340/350. Don't skimp on the heatsinks for these.
- I would add 2 MJL21194 and 2 MJL21193 to each amp module to keep within the output stage SOAR. Extending connections to these with links and emmitter resistors should be straightforward.
- Changing the 1k feedback resistor to 820 ohm will give you slightly more gain (might be needed).
- All other components should be ok as is.

Build a big power supply. For a stereo pair you will need at least a 1.5kva transformer (or 2 800va) with at least 40,000uF per rail per amp. Your AC voltages will need to be about 58 0 58 volts at 1.5kva.

Don't bother with the ETI480's. You would need to change every transistor (and most other components) to achieve significant power (single ended).

Cheers
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Old 22nd March 2006, 10:02 AM   #3
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Default studio 350

Biggest transformer i can find is 1kva with 55-0-55. Thats through Farnell. Costs a fortune though. Would give approx +-77V DC.
How would this affect desired power increase? I don't know how likely a toroid that gives 58-0-58 will be.
Might just be simpler to make them as 350w models and make more of them as +-70Vdc is easier to make than 80v. (Due to the availability of 50-0-50v toroids.

These amps are intended for use in PA style applications so headroom is of the utmost importance.

So... a 350watt model, with a 500VA toroid per amp plus say 20000 microfarads per rail? Would that be suitable?

I'm currently working on a large PA system, the plan is to make a 3 way tri amplified system. It'll have 4 x sub cabinets (size of driver undecided), 4 x mid boxes (probably 12") and 4 x high boxes (a combination of 8", 4" and piezo horns).

I already have 2 amps, 1 is 2 x 400 watt 4ohms, the other is 2 x 350 watt 4 ohms. The plan is to use these for mids and tops, what i'd like to do is build some of these studio 350 modules to run the subs and to run any further additions to this system (like foldbacks, more mid high speakers etc).
If I made sub boxes that were 4ohms each, and made 1 amp for each of these then I figure that the amps only being 350 watts will be irrelevant because there'll be 4 of them.
Any thoughts? I think i'm leaning towards the 4 smaller amps rather than the 2 really big ones, but i'm curious to hear other opinions on this. What do people think? Should i go for 1 big lot of grunt or several smaller lots...

I'm liking the look and what i've heard about the studio 350's. Can pick em up as a kit on ebay for $150, I guess what I really want to find out from people who've used them is, will they do what I want them to do?
Thanks
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Old 22nd March 2006, 11:57 AM   #4
quasi is offline quasi  Australia
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The good news is that in terms of loudness the difference between 350 watts and 500 watts is only about 2 dB. Given also that the floating voltage will be higher, the dynamic (instantaneous) power of the amp is likely to be greater than 350 watts anyway.

You should be able to buy transformers directly from here;

http://www.users.bigpond.com/harbuch...h/toroids1.htm
and
http://www.tortech.com.au/

If you can find 55 0 55 volt 500va toroids go for it. This will give you more dynamic power when you need it i.e; PA punch.

20,000 uF would be the bare minimum per rail per amp.

The Studio 350 will in my view handle your job without any problems, but note the design does not have any short cct protection so extra care will be needed in PA applications. I would still add the extra power transistors if you intend running the amp loud for a long time and add fan cooling.

Have you though about a MOSFET design? While the Studio 350 is an excellent design, a MOSFET amp might be better suited for PA work because MOSFETs are far more robust than a similararly rated transistor.

Cheers
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Old 22nd March 2006, 12:01 PM   #5
rabbitz is offline rabbitz  Australia
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Try Harbuch as you can get what ever your heart desires. Good quality and good prices.
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Old 22nd March 2006, 12:06 PM   #6
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if you really like to mess around w/transistor, MOSFETs, Capacitors, etc., consider building one per 8ohm driver (4?).

Go big (high VA rating) on your transformers and extra capacitance in the cap banks an your in business!

http://www.sound.westhost.com/project101.htm

cheers

tony
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Old 22nd March 2006, 12:42 PM   #7
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Default studio 350

Thanks everyone for the hints.
What i'm planning on doing is building a heap of these 350's as mono amps so i can customize the PA system.
So i'm intending on having the central basis of the system as the 4 mid and 4 high boxes with the 2 power amps i've got.
And having the studio 350 amps setup as mono amps, 1 for each speaker box (probably double 15" into 4ohms boxes.) So the number of subs (or other boxes) can be altered and changed to suit the type of gig.

One thing i noticed on the schematic was that there are 4 220nF 100v capacitors going from the +70 and -70v rails to ground right near the output devices. If i'm adding more transistors do I need to add more of these capacitors?

The ESP 101 does look like an interesting avenue to venture into. I might consider it sometime soon. I think i'll start off with the 1 studio 350 to start. There'd be a way to insert some sort of relay protection across the outputs wouldn't there? Because of the nature of the work they'll be doing I'd like to have some sort of protection. Is there a simple relay protection circuit i could build up on veroboard or similar? How do they work exactly? Do they cut off if the amplifier goes DC? or do they cut off when the output reaches a certain limit?

Thanks again for the quick responses!
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Old 22nd March 2006, 12:57 PM   #8
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did you notice the ESP P101 is a MOSFET design?
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Old 22nd March 2006, 06:02 PM   #9
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Default Reliability

I'm not familiar with the Studio 350 so my comments may not make any sense. But just in case...

500W average into 4 ohms requires 63V peak. I don't know what the circuit is you are intending to use but I'd have thought 70V rails would be ample...ie a 50-0-50 transformer.

The MJL21194/5 are 200W devices in TO-247 cases. You'll need to check the SOA curves to see how many of them you'll need. Otherwise you may have problems. Remember that the SOA curves in the datasheets are specified at a case temperature of 25C. This will almost certainly not be true when you are running the amp at high power in a studio. The power must be reduced by 1.43W per C above 25 for this case. I'd recommend you use transistors in a TO-3 case instead as these have better thermal handling and you should find a 250W device with less derating with Tc. If you are driving 4 ohm speakers then design for reliability into 2 ohm resistive load to be safe. You may well find you need 4 or more of each device.

If you choose mosfets you have to be careful of the Vgs drop, especially at high currents. This may cause you to have to use extra high voltage rails...thus expending more power...or to design a more complicated output stage circuit.

In any case, make sure the transistors don't get killed by a sudden short circuit of the output. Putting current limiters in place is vital. As you say it is a good idea to have a relay to disconnect the speaker in case of a transistor failure. This failure can be a dead short so a simple dc offset detector would be a good precaution.
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Old 22nd March 2006, 08:55 PM   #10
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Default studio 350

Quote:
Originally posted by gp4Jesus
did you notice the ESP P101 is a MOSFET design?

yeah i did, it does look surprisingly simple to build. but i'm not entirely confident on doing these from scratch just yet. I will dabble into it one day when finances permit
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