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Old 16th March 2003, 06:48 PM   #21
rickpt is offline rickpt  Portugal
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Hi

Just connect the five diodes in series and place them instead of the zener like the constant current source on the same schematic... but look at the polarities of the diodes...

About stability, didnít had any problems with this particular amplifier... in your case what I would do (and did )is to bandwidth limit the output stage because you are going to use fast transistors, RC network in front of the drivers will do the trick. A 100ohm resistor (if you look the resistor is already there) and a 100pf cap conected between the base of the transistor and the power rail.

If stability is ok on resistive load, connect a crapy loudspeaker and see what happens (nothing like a real test load )

One more time I say, oscilloscope is mandatory!!!!!

About output transistors placing... as I was modifying a commercial unit, I made an aluminum U that is attached to the heat sink and I put there the TO3 devices and connected them to the board with wires(if you are going to do that, keep your wire short...) and connected the vbe transistor to the U.


Best regards

Rcardo
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Old 17th March 2003, 11:17 PM   #22
Diode is offline Diode  United States
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OH Yeah!

Speaking of resistive loads.... I read on Anthony Holtons site that he connects a resistive load instead of the final output FET's so he can check for problems and set the bias current to an initial setting. I suppose this helps the builder to not blow up the final stage if they set the bias pot wrong. Can it be done here also? Did you use a 3/4 turn pot or a 10 turn pot for the bias adjustment pot?

Diodes..... Is the polarity the same as the zener in the original circuit? Again, the resistor needed right? Does it need to change value as the regulating diodes are different?

Thanks again for the continued information!!!

Chris
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Old 18th March 2003, 12:29 AM   #23
rickpt is offline rickpt  Portugal
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Hi Chris

When I connect an amplifier I normally donít use a load on the output stage as you already have one there... the zobel!
But you can use a resistor if you like. If you use a loudspeaker, and you have an offset problem you kill the loudspeaker

On the power up I always put a power resistor in series with the power rails to limit the current in case something goes wrong

15ohm 10watt power resistor will do the trick

Adjusting the bias... just set the pot to maximum position.
I always use multiturn pots. They are much better and easier to adjust.

About the diodes... imagine five zeners in series the polarity is reversed. it's all you need to do. Keep the 11k resistor as it is.

Best regards

Ricardo
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Old 18th March 2003, 12:31 AM   #24
jleaman is offline jleaman  Belgium
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Send a message via AIM to jleaman
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it is weird he never updates his pages or emails yu back any more.

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Old 18th March 2003, 01:12 PM   #25
Diode is offline Diode  United States
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Thanks Ricardo!

OK, I put 5 1N4148 diodes in series, treat them as they were 1 zener, and keep the polarity the same as the zener in the diagram? This may sound a bit tiring but you know the consequences if a mistake is made! BOOM!

Also you say to put a 100Pf cap on each base of the outputs to its corresponding rail, right? Do I do this with the drivers too?

I'm being so detailed as I'm just about finished with a reasonable layout and I'm about to start buying parts. I'd like to get it right the first time!!!!!!!

You said to set the bias pot to set the pot to the maximum position. Can you tell me which side to set it to. I look at the diagram and it is between 2 parts or easier said, in the diagram, do I set ot "up towards the 100 ohm resistor" or "down where the 2 poles of the pot are tied together?"

Again, thanks for your help!

Chris
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Old 18th March 2003, 01:17 PM   #26
Diode is offline Diode  United States
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Hey Jason,

Yup, that's the problem. Now, do you think that I'm going to trust him with my money? I don't think so........ If he won't answer inquiries about his projects, why should I think that he will send me his PC boards? OK, I'm finished being rude.

The other problem is a suitable place to buy FET's. I have chosen this project and will stick with it. I'm pretty excited about it and will post my stuff and changes to the circuit when finished if someone wants it.

Chris
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Old 18th March 2003, 01:33 PM   #27
Philo is offline Philo  United States
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Default 400W enough?

Randy Slone is offering a 400watt version of his amp in a kit on his web site. It is a very durable versatile design. Here is a pic with the website. The great part is his kits are complete, just add the rest of your case and PSU(which you can get from him if you want)
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Old 18th March 2003, 02:05 PM   #28
rickpt is offline rickpt  Portugal
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Hi Chris

A diode is forward biased device and a zener is a reversed biased device, so you will have to reverse the diodes in relation to the zener on the circuit. Sorry I wasnít very explicit on my last post regarding this subject.

I you put the resistors on the power rail's like I said earlier you wont have a boom!!! Just some very hot resistors!

The pot is wired like a variable resistor. So at maximum (maximum position is down where the 2 poles of the pot are tied together) it will have 200ohms, at minimum it will have 0 ohms. Measure the pot on the circuit with a multimeter just to be sure.

About the bandwidth limit cap, Iím going to attach a picture that I hope it will clarify some things look at the RC filters on that output stage.

Glad I can help you, I will try to answer your questions as best as I can

Best regards

Ricardo
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Old 18th March 2003, 02:08 PM   #29
rickpt is offline rickpt  Portugal
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upsss

the pic
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Old 19th March 2003, 12:51 AM   #30
Diode is offline Diode  United States
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Hey Ricardo,

In the previous post, you said that the capacitor went between the base and the rail...... Did you do it only on the drivers or on each output transistors as well? I really appreciate all of the support! I used to make Native American (Indian) flutes and I belong to different forums and help others with my other passion. I really like to hear when I tell someone something and they do it and are successful in making their own flutes. Anyway.........
I understand the picture!

As always, Thank you very much...

Chris
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