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Old 15th November 2006, 03:26 AM   #1
prorms is offline prorms  United States
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Default 100w 8ohms does not opperate

every time i try to get this amp to work it justwont run
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Old 15th November 2006, 04:05 AM   #2
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I'm no expert but...

Looks like someone forgot to update the component values to ones that will work.

Feedback is taken from the protection circuit - a connection from there to the output would be appropriate. As is you've got a comparator except that there is virtually no current in the differential because the load resistors are too high.

The 100 pf feedback cap means your low frequency limit is above the audio band. 100 K and 100K means once you get in band you'll have a gain of 2 if the feedback is properly connected.

The Vbe multiplier isn't correct - as drawn it is a diode connected transistor in series with 100K. The CCS is trying to sink 6 mA - (too low for a VAS) and the VAS Q will be saturated by the excess base voltage due to the LTP load. This means the 100K in the "Vbe multiplier has almost rail to rail voltage across it. This tries to blow the outputs. I'm not sure if the protection circuit will prevent blowing outputs or not - it is missing a reference to the output.

You'll probably want to add a predriver to ensure enough current drive capability.

35V rails are enough to get maybe 100V peaks, but this is more of a 60W size supply. One pair of MJL21194/3 will do fine unless you plan to make

Try Mike B's Symasym if you have a 35V supply.
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Old 15th November 2006, 04:45 AM   #3
Duo is offline Duo  Canada
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There is enough for adequate current drive there; but as BobEllis mentions, there are some serious problems.

The 100K resistors in the diff amp are way too big, and the one on the right hand side shouldn't even be there, it should just be tied straight to the positive rail.

I use 6mA current sources in some of my large designs as the VAS without any trouble here.

Otherwise, all of the above mentioned faults are definitely there. Having no feedback loop is definitely going to make things bad to start with... You'll likely see some mad DC at the output if you actually fire this circuit up.


BobEllis: You must mean 100W peaks and not 100V peaks?

At most you'll be getting 70 or so watts for 8 ohms... A set of 45V rails is more what you want for 100W to 8 ohms. And one pair of the specified output transistors will do it like Bob mentioned.
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Old 15th November 2006, 05:38 AM   #4
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RE: How to build an amp (that doesn't work) with only two values of resistors.
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Old 15th November 2006, 05:55 AM   #5
Duo is offline Duo  Canada
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Hahahaha, you crack me up... I was thinking something to that order myself.

Maybe I should be a bit more helpful too.... Perhaps a resistor in the 2.2K to 10K range and you'll be then in the ballpark. Or a current mirror? Those do make life easy and can have some nice benefits, but don't get carried away like I do.
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Old 15th November 2006, 07:56 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
It looks like the poster has re-drawn the circuit for our benefit and forgotten to change the components and left out a few connections.

The VAS current is OK @ 6mA. It could go even lower with all that current gain in the driver hFE~100 to140 and output ~30 to50.

The LTP could go a lot lower than 6mA try 2mA. There is nothing wrong with the resistors in BOTH collectors of the LTP, it helps keep the operating conditions for both halves the same. Keep in mind that the other half (inverting) operates as an amplifier just as often as the non-inverting side.

The IV protection circuit is missing two resistors on each half. That shown has a poor protection locus.

Add the missing resistor in the upper leg of the Vbe multiplier base side.
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Old 15th November 2006, 03:34 PM   #7
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looks a bit like :
http://www.atjoo.hostiz.com/m250.html
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Old 15th November 2006, 08:50 PM   #8
MikeB is offline MikeB  Germany
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The big question: Has provyf built his amp like shown by schematic ?
If yes, it definitely can't work. (most resistor values wrong, missing resistors, missing connections)
If no, please show correct schematic !

Mike
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Old 15th November 2006, 10:13 PM   #9
prorms is offline prorms  United States
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Default LABEL EACH PART

I DONT UNDERSTAND WHICH PARTS YOU ARE TALK ABOUT
I LABEL EACH PART R1 C1 T1 D1 .........
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Old 15th November 2006, 10:27 PM   #10
MikeB is offline MikeB  Germany
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Quite simple, R3/4/6/12 can NEVER be 100k.
You seem only to have 100ohms and 100kohms. An amp does not work that way.
Also, you are missing a resistor from T6 base to R10. You are missing a connection between output and emitters of T7/8.
The emitters of t9/10 need to be connected with a resistor.
R12 is way too big, r10 is way too small.
The whole schematic is competely flawed and can never work.

Also, it's nuts to have 3 output pairs with a supply voltage of +/- 35v. Even a single pair of MJL21194/3 will be "bored" driving a 4ohm load.

R3/4 need to be carefully calculated, 100k is way too big, driving T1/2 into reverse bias.
R6 sets the gain of the amplifier to a stunning 1:2.

Shall i continue ?

Mike
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