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Old 15th May 2008, 07:56 PM   #1
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Default 300 Watt Amp

Hey audioholics!

I am working on my first discrete project here and could use some help.

Goal of this project is to achieve 300W into 8ohm load. Why so much? Because it's interesting Main use will be subwoofer amplification, but it can also

be used as full range amp.

Power

Schematics

I plan to use 2 toroidal transformers (2*54V 500VA and 2*15V 50VA). Smaller one is for preamp, soft start and protection circuits. I have separated the

main supply into two: low current one for input and VAS stage and other for output stage. The goal is to minimize noise from class B currents. Secondary

supply is for preamp and is precisely adjustable.

Questions:
* Is the divided supply idea worth using?

Soft Start

Schematics

Soft start is using secondary transformer for power.

Questions:
*Is it a good idea to fuse second transormer's primary and secondary? When one of these fuses blows, soft start will be without a supply and main

transformer is powered through resistors, that will overheat very quickly.

Preamp

Schematics

Input is obtained from a balanced line through unified XLR and phone plug connector. Input stage is from Jensen transformers. It is followed by phase

inverter and phase controller (can be bypassed). Then comes adjustable low pass filter (can be bypassed). Last stage is voltage amplification to create

suitable voltage for main amp.

Questions:
*Are the values for input filtering suitable?
*Should i create separate ground for signal and power. This way the signal ground should unite with main ground in the main amp. Otherwise there will be

a ground loop (preamp - (signal ground) - main amp (ground) - power). Is it worth the concern?

Main amp

Schematics

Main amp is a compose of mirrored differential input and mirrors VAS and output triple. On the same board also resides speaker protection.

Questions:
* Is the concept ok?
* Can a MJE15032(MJE15033) in TO220 casing dissipate 1.5W without heatsink (used as current source for input)?
* Should I mount Vbe multiplier onto main heatsink or on a separate one with predrivers?

That's it for now
Thanks in advance!

Aq
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Old 16th May 2008, 03:14 AM   #2
Leolabs is offline Leolabs  Malaysia
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500VA for one or both channel??Are you planning to drive anything below 8 ohms???
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Old 16th May 2008, 04:44 AM   #3
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I just looked the mje15032 up and the data on it says
2w max at 25c ambient without a sink 1.5w max 55c ambient
without a sink. I believe Nelson Pass said in the old A40 article
that designs with a 2 to 1 safety margin generaly blow up!
So I darn well wouldn't use it without a sink.

If you don't mount the vbe multiplier on the sink what are you
going to use to stabalize the bias to prevent thermal runaway.

I also see that in the preamp power supply cap c11 and c12
are tantalums. You probably don't want to use tantalums I
believe they sound realy harsh.

One other thing in the Hafler 200 thread there is mention of
a new drive board sutiable for an amp like this with realy outstanding specifications
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Old 16th May 2008, 07:42 AM   #4
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just to butt in:

I could see Randy Slone's name under the main amp page......

is this Randy Slone's new design?........from which edition?

thanks
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Old 16th May 2008, 08:48 AM   #5
AndrewT is online now AndrewT  Scotland
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I think C11 & C12 are in the wrong place. adj to common?
I don't like your choice of input HP filter and NFB HP filter.
How about adding the extra diode + R from protection base to ground for two slope characteristic?
Have you deliberately chosen transistors with a similar fT? why?
Output Re=0r1 seems a bit low. Optimum output bias for ClassAB is around 800mA to 1000mA giving around 150W of quiescent dissipation. Using Cordell's rule for heatsinking stability, these may need to be raised.

4pair off +-75Vdc supplies is asking a lot. 4ohm is not on.
6ohm might be OK. With 8ohm, the sink must be kept below 30degC, how with 150W of dissipation?
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Old 16th May 2008, 10:21 AM   #6
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Hey!

Thanks for your replies.

I have updated the schematics (see first post).

Leolabs

*There will be only one channel. 8 ohm load will be most important (my current subwoofer is based on Visaton's TIW300-8). But I'd like to drive 4 ohm loads also.

woody

*I will use separate small heatsinks for current sources.
*The question about Vbe multiplier was, that should it track the temperature of the predriver or output transistor. It's a
question of topology, predriver and driver form a CFP, which is confusing.
*I replaced tantalums in power and preamp circuits with 100u/50V electrolytics.
*Can you be more precise about this "new drive board", because the thread is quite large and it would take hours to go through it.

hienrich

*I have updated the refence on the main amp schematics. I have a copy of "High power audio amplifier construction manual. 50 to 500 Watts for the audio perfectionist" by R. Slone. Unfortunately i don't see an edition number.

AndrewT

* You are right. I'll correct that.
* Can you suggest something better?
* I will add two slope protection.
* I saw the combination of these transistors on this page http://sound.westhost.com/project68.htm. Already ordered samples from ONSemi, so I should stick with these. Is there a problem?
* 0R1 value is recommended by D. Self in his book. Also he states that Class AB is not recommended. I'm thinking of
quiesent current in range of 50 - 100 mA.
* Do you suggest adding more ouput pairs to support 4 ohm operation? Why must the sink be below 30degC?

Regards,
Aq
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Old 16th May 2008, 10:37 AM   #7
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by raitraak
* I will add two slope protection.
* I saw the combination of these transistors on this page http://sound.westhost.com/project68.htm. Already ordered samples from ONSemi, so I should stick with these. Is there a problem?
* 0R1 value is recommended by D. Self in his book. Also he states that Class AB is not recommended. I'm thinking of
quiesent current in range of 50 - 100 mA.
* Do you suggest adding more ouput pairs to support 4 ohm operation? Why must the sink be below 30degC?
D.Self's ClassB is the same as optimised ClassAB that the rest of the industry uses as a definition.
Have another look and note he recommends by setting the voltage across the output Re, not by current.
This Forum has discussed this at length and there seems to be general agreement that for an EF output stage that 15mV to 25mV across each Re maintains optimum ClassAB (=Self's ClassB). Try to stay within this range for all operating conditions and temperatures.
You adopted those transistors because you saw them on ESPs site. good enough reason, but is it well informed?

I have done a few more calculations and 4pair can do 6ohm in domestic duty for wideband non clipping music use.
You must stay within the temperature de-rated SOAR. The higher the case temperature of the devices the lower the SOAR.

I don't think you'll hit your target with +-75Vdc rails and that tiny amount of smoothing capacitance. More likely you'll need +-81Vdc and that increases your quiescent dissipation even higher (160W).

If you want 4ohm capability AND want sub capability, I recommend at least 6pair for 300W into 8ohm and use an enormous heatsink or fanned sink with temp protection/cut-off.
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Old 16th May 2008, 12:18 PM   #8
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Wouldn't it be better to go for a class D amp if the application is a sub woofer . 300 watt class D and maybe 150 watt class AB for the main speakers if a wideband amp is also required . Wouldn't that be simpler to do ? May cost a bit more I guess.
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Old 16th May 2008, 12:25 PM   #9
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Do a google for the LME49811 this driver chip has outstanding
specifations and sure would make building a 300w amp a lot easier and cheaper.
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Old 16th May 2008, 12:56 PM   #10
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Then it's a matter of defining I read books, because there is all nessecary information, to get through this forum's topics, it takes at least a full time job

At first I thought to build ESP design, but then started reading D. Self-s book and decided to create something more advanced. I already had ordered the devices, so I'm stuck with them.

I plan to use two heatsinks (1760cm2 50*191*100mm), but i'm planning to place them inside of the enclosure. If needed, I can add two thermally controlled fans. This creates strict limits on power dissipation and 150 watts for standby is clearly too much. Can you suggest "better" output topology with less standby current.

I have 5 pairs of MJL4302AG/MJL4281AG and 5 pairs of MJL21194G/MJL21193G. Can I swap driver transistors for MJL21194G/MJL21193G? This way I would have 5 output pairs.

I think I can forget 4 ohms and settle with less output power for 8 ohms. Caps are quite expensive around here, and I cannot afford more. Budget for this project is already quite big.
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