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Old 1st October 2005, 05:10 PM   #1
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Cool My first amplifier design, what do you think?

Allright i think iv'e almost finished drawing on my amplifier after reading Audio Power Amplifier Design Handbook (D. Self)
Most of it is quite conventional so it's nothing special really but still

I added an current overload protection which enters around something like 5.5A output
Im hoping to take out something like 50-80W from this little beast but please tell me if im a bit too positive

Please comment anything on this scheme, if any values look strange or so. I'm a little bit unsure of the power-rating for the 10R output resistor, shouldn't have to be greater than a couple of watts?


Ok here's the circuit:
http://upl.silentwhisper.net/uplfold...d5/ampproj.GIF

Thanks to you
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Old 1st October 2005, 06:36 PM   #2
MikeB is offline MikeB  Germany
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Hi !

Looks fine to me, have you verified stability in sims ?
I can't see any problems at once, except you might consider an inputfilter ?
That would be a ~1k resistor in series with the inputcap and a ~330pf
in paralell with the 10k.
This would protect the amp from incoming RF and add some extra stability.
You are biasing the bc556 in the ccs for LTP with ~4.5ma, you should
check if it does not get too hot.
The mje340/350 in vas are biased with ~12ma, that gives 0.4watts,
you might consider some minor heatsinks for them. (try out !)

I would expect ~50watts into 8ohms.

On the other hand, just go and build it, learning by doing !

Mike
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Old 1st October 2005, 07:04 PM   #3
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"Most of it is quite conventional so it's nothing special really but still"

Ohhhh... you are very wrong! It is special... it is your first amp design and you should be proud for that, because for a first design it is more than good. Congratulations!

"... 50-80W from this little beast but please tell me if im a bit too positive"

Maybe just a little bit less then 50W, if you are planning to use 8 ohms load. +/-36V would be much more on the safe side for firm and "healthy" 50W.

"Please comment anything on this scheme, if any values look strange or so. I'm a little bit unsure of the power-rating for the 10R output resistor, shouldn't have to be greater than a couple of watts?"

I will suggest 3-5uH for inductor and not more than 3,3 ohms 1-2W in parallel.
Of course, that's just my point of view!
Negative pole of the 1000uF and bottom wire of the input 10k resistor and CCS 5k6 resistor should go together on the so called "quiet" or "signal" ground which is separate from "power" or "noisy" ground. Put about 10-47 ohms between those two grounds.
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Old 1st October 2005, 07:34 PM   #4
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Hi thank you for your replies!!

I've simulated it in Multisim and it acually works pretty fine!
Yes the Vas ccs and vas might get a little warm and if im thinking of putting a small piece of metal to them.
And grounding the input signal, feedback ground and ccs through a 10R to main ground sounds like a good idea. Douglas Self talks about the importance of not mixing "Psu" ground and signal ground together.
I had in mind to take a bit of metal to put the to the two reservoirs together with and put all main grounds there and draw a small piece of metal out of it and use like signal starpoint ground.

Maybe i could take a 10R from the centre of reservoir caps and use as starpoint for all signal earths (including preamp) ?


Anyway here's an update of the scheme:
-added an input filter
- decreased input pair ccs to something like 3mA instead of 4.5mA. I don't think it will make a much different in sound/thd but the transistors will get less hot (thanks for pointing this out) 4.5mA input current is actually quite large

here you go

http://upl.silentwhisper.net/uplfold...projscheme.GIF

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Old 1st October 2005, 08:14 PM   #5
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Hi!

Nice schematic! The 10 ohms / 5W is more than enough. This R-C is used for stability, normally therev is almost NO voltage across the resistor. I use 1W here with no problems. Unless you have oscillations, but this R-C is there to prevent it.

You should be carefull with that 5.5amps current limit. I don't think the output transistors can support 5.5 amps for all output signal levels and real speaker loads. For instance, if you have a reactive load like a speaker with a xover filter, the output may need to supply 4 amps at a Vce across the output transistor of 50V. You would think that this 200W would be safe for the output transistor, but that is not true. The max Ic for a bipolar transistor falls dramatically at Vce above a few 10-s of volts, see the data sheet graph for Safe Operating Area. At Vce of 50V, Ic max is probably (I haven't checked, this is an educated guess) not more than 2 amps or so. I'm sure D Self has a lot to say on this and offers even current limiting circuits that limit current depending on Vce that are really safe!

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Old 1st October 2005, 08:34 PM   #6
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Hi Fritzell,

While you have it in sim, you may care to check how much you will be listening to the power supply - by inserting a sig gen alternately in series with the supplies and taking a bode plot (magnitude in dB). This will show you the strength/weakness of your LED current source,miller lagged driver, and overall topology.

See attached a diff'l input with Vas and EF output with very little comp taken on the Vas side.

Cheers,
Greg
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Old 1st October 2005, 09:03 PM   #7
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Hi thanks for your input!

janneman:
Thanks,
I decreased resistor to 3.3R/2W from suggestion from boraomega. Also i agree that the current limiter can be made much better and Douglas Self presents a limiter that also takes the Vce into account but it tended to be a little bit more complicated. And looking in the datasheets for MJ15003/2 their SOA for 5A is Vce~50V (250W) and in this circuit there can never be more that 33V Vce. But as you said with inductive loads things get more complicated...

But i don't think i will ever go so far, i only think that it is good to protect the expensive output transistors. 5.5A limiting is more better than no limiting at all if you by accident short the output
But just to be a little more safe I'll put a 1k-1k divider instead for the current limiters.

amplifierguru:

Good point, i'll try check that. I know Led current source is not as good as the "feedback current source" with two transistors, but it looks cool with an onboard Led Maybe it's better to put the "feedback current source" to the input pair which is more sensitive and a Led for the Vas?

/Fritzell
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Old 10th October 2005, 02:33 PM   #8
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Default Now with PCB

Hi i'm back with a PCB for my amplifier now
Here it is:
http://files.upl.silentwhisper.net/u...lfifierpcb.GIF

The thing in the middle is a small heatsink for the drivers, and output transistors are mounted off-board

Updated schematic:
http://files.upl.silentwhisper.net/u...hematicnew.GIF

Please comment if i could do something to make it better.

Thanks
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Old 10th October 2005, 11:58 PM   #9
gerhard is offline gerhard  Germany
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Fritzell wrote:
> And looking in the datasheets for MJ15003/2 their SOA for 5A is
> Vce~50V (250W) and in this circuit there can never be more that
> 33V Vce. But as you said with inductive loads things get more
> complicated...

If you drive the output close to the negative rail, the upper
transistor (15003) will see a Vce of 66V minus 2 Re drops
minus Vce sat of the lower transistor (15004) which could be
more than 63 V, or am I missing something?

btw is it 15003/2 as in the text above or 15003/4 as in the
schematic?

best regards,

Gerhard

(my first post on diyAudio!)
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Old 11th October 2005, 07:43 AM   #10
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Hi gerhard!

It is true that the upper transistor can get 63V of Vce, but when the signal goes under 0V the lower transistor will take over and just the bias current will flow through the upper so the power dissipation can never get higher than ~33Vce * I (or am i totally lost here?? )

It should me MJ15003/4 and not 2/3, must have written a little wrong there. Anyway thanks for your reply
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