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Old 28th January 2011, 03:57 PM   #111
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Wouldent a properly designed VI limiter make an amp almost indestructible in terms of shorting ?
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Old 28th January 2011, 04:00 PM   #112
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Originally Posted by Buckeye View Post
5 per rail? With a 250 Vce, that design should easily run your stage lights.
This has taken its place:


I've never tried darlingtons in an amp before. Generally, the preferred way is to use separate drivers. I don't know if paralleling darlingtons (to increase SOA) introduces stability issues. I may get some to play with.
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Old 28th January 2011, 04:17 PM   #113
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Wow, 8 per rail! That's quite an amp. Instead of watts, do you measure your amps in feet or (I see you're Canadian) meters, eh? (Sorry, couldn't resist.)

Darlingtons are good and make the amp simple, but I haven't found any PNPs with high voltage plus high power ratings. So there goes the cheap full complementary design. The highest voltage rating for a complementrary pair I've found is 120. So I think that limits you to around 60V rails and the max power out into a 4-Ohm load is around 300W.

Seeing your prior creations, I'm not sure that would be enough for you.
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Old 28th January 2011, 04:35 PM   #114
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The MJH11021/22 are 250V, 15A. Less than $5 each.
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Old 28th January 2011, 05:01 PM   #115
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That's a good find. Pd is a little light, just 25W better than the TIP142/147, but looks like a very workable device. At $5 each, you could certainly string a few meters together to dim the lights in the neighborhood.

I'll consider this for the SOC150 design. This could make the working range of rail voltage from about 12 to almost 250. What's really nice is that there's a range of identical devices rated 150, 200 & 250 volts, so you could pick the parts for your rails.

In the US, you can get 2400W out of the typical wall plug, so 150V rails is about all you'd need.


(Couldn't find them because E14/Newark have their data wrong. hfe shown as 15 & no mention of darlington.)

Last edited by Buckeye; 28th January 2011 at 05:07 PM.
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Old 2nd February 2011, 08:34 PM   #116
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Default Properly designed VI limiter?

Originally Posted by Tekko View Post
Wouldent a properly designed VI limiter make an amp almost indestructible in terms of shorting ?
Just wondering, exactly what is the properly designed VI limiter?
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Old 3rd February 2011, 08:50 AM   #117
AndrewT is offline AndrewT  Scotland
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Read Workhorse recent reply to me why the gate protection Zener does not work as a short protection.
There is so much I still have to learn !
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 3rd February 2011, 09:41 AM   #118
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add to that that the same diodes will also not protect from a too reactive load .....

( many or too many times amplifiers presented in the forum with plenty of power and inovative audio circuits are not designed to be idiot proof or have inovative protection methods for a change )
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Old 15th February 2011, 12:39 PM   #119
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Default Simulations for the Cheap150

I've been off learning how to simulate and thought the Cheap150 would be a good circuit to start with. I used LTspice and the learning curve wasn't too bad thanks to the help I got from the diyaudio Software Tools Forum.

So I used the Cheap150 circuit as originally posted by Mr. Fisher. Here is what it looks like in the simulator:


Here is the simulator output showing the frequency response at 33W and distortion at 6, 33 and 100 Watts. The frequency response didn't change much at different power levels so only one curve is shown:


The distortion in the graphs is shown in %THD, and it does not look so good in the simulator -- especially at the higher frequencies. Much of this could be caused by using only two diodes in the bias circuit. This makes the amp operate close to class B.

So I tried a couple simple mods to get the distortion down and yet keep the design very simple. I added a third bias diode to make the amp operate in class AB, and changed a couple emitter resistors for Q2 & Q3. I also added a proper low-pass filter at the input. I don't like using only a 100pF cap across the input as a filter because the corner frequency is set by the impedance of the source signal you plug in. Usually, that's a low impedance making the corner frequency very high, and the filter nearly ineffective. Here's the modified Cheap150:


The frequency response stayed pretty much the same, which was good to begin with. The distortion was significantly reduced, especially at high output levels. Here are the graphs:


Keep in mind these are only simulations as I don't have the equipment to make actual measurements. I am working on another small mod that promises to reduce the distortion quite a bit and only adds a transistor, a pot and a resistor. I'll post it when I have it.

The result might not be audiophile quality, but it will be cheap -- the operative word here. I'm hoping to make it better and still keep it cheap.
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Old 15th February 2011, 01:26 PM   #120
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FYI, I inadvertently changed the PSU voltage to 49 from 45 in the modified circuit. This reduced the %THD at 20kHz/100W by about 0.1%. This was unintentional and didn't change the final results significantly.
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