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Old 3rd July 2004, 07:48 PM   #11
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Default Thanks for the tip Danny

I did as you suggested and googled around a bit. It looks like the load resistor would need to be on the emitter of the first transistor in the pair (I was trying to put it on the emitter of the last transistor).

So are we talking about something like this? The attached file is my stab at a Sziklai output. If I understand it right hFE of the pair is hFEq1 X hFEq2 so that Q1 could be a small signal device and only Q2 would actually pull large current.

Does that look like a workable design? I am not entirely sure about the stability of the bias currents.
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Old 4th July 2004, 12:19 AM   #12
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Default Re: Thanks for the tip Danny

Quote:
Originally posted by mashaffer
Does that look like a workable design? I am not entirely sure about the stability of the bias currents.
I have built something similar:

http://www.diyaudio.com/forums/showt...249#post359249

(in the thread of "simplest amplifier possible with BJTs")

Mine has considerable amount of feedback, primarily because of the non-linear nature of the 2nd / 3rd stage. I suspect that your will work but will have substantial amount of distortion. You may want to try it out in simulation first.

If you are interested in simple designs, here is another of mine. it was posted somewhere here a while ago but I couldn't find the link. the schematic is for a pre-amp and it can be redesigned to run as a power amp (I have done both). Essentially it is the half of the Nelson's latest Zen amp. Very good linearity, sweet sound, but hugely inefficient.
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Old 4th July 2004, 12:39 AM   #13
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Default Mashaffer, thank you related...

The amplifier using Tip31/32 in the output.
Goooood, simple and nice.

tks

u great!

Carlos
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Old 4th July 2004, 04:02 AM   #14
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Default Thanks millwood and destroyer

Millwood;

Thanks for the links. I will look them over. As for simulations I would love to be able to do that but as I am seriously unemployed I can't buy any software right now. Do you know of any freeware that a rookie like me could use to do this kind of simulation?

Destroyer;

Thanks. I wish I could take credit for the design but all I did was adapt it to 38V power supply. The class A thing I am trying to do from scratch.
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Old 4th July 2004, 04:50 AM   #15
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well, I did it for you. Here is the simulation results from multisim.

I adjusted R1 to obtain the best output. THD pretty high, at 6%.
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Old 4th July 2004, 08:03 AM   #16
Geoff is offline Geoff  United Kingdom
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Default Re: Thanks millwood and destroyer

Quote:
Originally posted by mashaffer
As for simulations I would love to be able to do that but as I am seriously unemployed I can't buy any software right now. Do you know of any freeware that a rookie like me could use to do this kind of simulation?
Try Simetrix Intro (http://www.catena.uk.com/).

It's free, it's relatively easy to use and the device/node limitations are sufficiently generous to permit moderately sized analogue circuits to be simulated. The only downside is that it comes with a fairly small component model library, but models can be downloaded from many manufacturers' websites and they are easy to install (unlike some free simulation programmes).
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Old 4th July 2004, 06:08 PM   #17
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Thanks Geoff. I am going to download it as soon as I get the automatic email they send with the download instructions.

Thanks Millwood!!! That is really going the extra mile. This gives me a great starting point. When I get the Sim that Geoff recommended up and running I can use your results as a guide to whether I am modeling it properly. Once I have the basic model down then I can play with the circuit and see how good I can make the circuit without interstage FB. At that point I can decide whether to use additional feedback, howmuch and where.

Did you happen to notice during your run whether the circuit was getting enough gain to reach the full potential of the 15V PS and/or if there was any excess gain (could bypass less of the 1st stage emitter resistor in an attempt to improve THD)?

It might make sense for me to model the input stage by itself to see how much it contributes to the total distortion.

I sure hope I can get this sim software up. It could be a whole lot of fun and very educational.

Well time to go read up on amplifier distortion mechanisms.
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Old 4th July 2004, 06:26 PM   #18
Geoff is offline Geoff  United Kingdom
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Quote:
Originally posted by mashaffer
Thanks Geoff. I am going to download it as soon as I get the automatic email they send with the download instructions.
If you send me an email via the button below, or my website, I will send you the component model libraries that I have compiled over the past three years. They are mainly audio related components but it will be a start and will save you searching the manufacturer's websites, at least initially.
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Old 4th July 2004, 07:41 PM   #19
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Quote:
Originally posted by mashaffer
Did you happen to notice during your run whether the circuit was getting enough gain to reach the full potential of the 15V PS and/or if there was any excess gain (could bypass less of the 1st stage emitter resistor in an attempt to improve THD)?

I didn't play with it much this time. But if you look at my chart, it is doing about 7-8vpp on the output. And distortion is very sensitive to the bias of the first stage: a couple of kohm out and you will immediately see distortion.

I sure hope I can get this sim software up. It could be a whole lot of fun and very educational.

you can get very good results with LTspice (at www.linear.com). It is also free and can be cutomized. You can also use ideal transistors and the result wouldn't be too far off.
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Old 5th July 2004, 06:38 AM   #20
sajti is offline sajti  Hungary
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Quote:
Originally posted by millwood
well, I did it for you. Here is the simulation results from multisim.

I adjusted R1 to obtain the best output. THD pretty high, at 6%.
Try to add some small resistor between the emitter of the 2N3055, and the ground. It will set some local feedback, and increase the linearity.

sajti
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