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Old 27th July 2002, 04:24 PM   #16
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Default To gromanswe

Hello

Well I would like to see your 10 W 1000mV DC output amp drive my speakers.

First of all I would newer connect any amp with 1000mV DC on the output.....would you ?

Secondly 10 Watt is in my humble opinion newer enough since the woltage clipping alone will make it sound like crap !!
(The voltage sving on my tweater amp is about +- 50 V )

Third stop the crap !!

\Jens
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Old 27th July 2002, 05:28 PM   #17
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Default Lots of reasons to bridge

The heat loss will not necessarily be less, but if you build 2 smaller 50 W amps, then the heat load will be divided between them. Could make heat sink selection easier.

For someone who does not have a great deal of experience, building a 50 W amp might also be easier.

When it comes to parts, you have a better selection of output devices, as most suitable ones will have plenty of SOA. Don't forget to take into account when you do look at SOA curves that they must be derated for temperature. There is a possible advantage then by breaking up the thermal load with bridging.

When it comes to selecting your filter caps, you will need at least a 63 V rating, but most likely 75 V to be safe. At that voltage, filter caps get harder to find when compared to say 40 V ones. Also, if you look at the performance curves for a given series of caps by one manufacturer, you will find when you get up to 63V or 75 V, that they don't look as good on paper as the lower voltage units. There are some slight manufacturing changes in cap construction as the voltage goes higher.

As for sound, there are many of us that feel that bridging sounds better. There are active threads on this that you should check out. One reason is that there is less signal modulation of the power supply rails.

MY recommendation is to build a smaller 50 W amp for starters, with an eye on bridging it. You may find that you don't really need 200 W. If you do, then bridge it. I believe that you will be happier in the end this way.

But back to the earlier question: I would use multiple transistors in parallel for the 50 W amp.

Jocko
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Old 28th July 2002, 01:30 PM   #18
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Question To Jocko Homo

Hi There

I thought this up...

Any thoughts

\Jens
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Old 28th July 2002, 09:19 PM   #19
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Without having done any calculations, looks reasonable.

Just a couple questions/comments:

1) What is the purpose of the 100 Ohm resistors in series with the bases of the input diffpairs?

2) Same question about the 10 Ohm resistors in series with the bases of Q22/Q23?

3) Are the Miller caps really big enough at only 10 pF?

4) What is the purpose of C6/R42?

5) I would be inclined to go for a longer time constant than 1k / 330 uF for the DC feedback.

6) I can't really read it, but is the cap across the Vbe multipler only 100 pF? I'd be inclined to make it bigger (100 nF at least) if so...

7) The currents in the input diffpairs are biased with a strong negative tempco, so the gain in the input stage will be rather temperature dependent, it will drop at high temp. Probably OK if the enclosure keeps things cool ...

8) Are you planning to include some protection circuitry, to sense overcurrent and / or power supply faults, and reduce output drive, [ or just live dangerously ]
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Old 29th July 2002, 05:51 AM   #20
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Default To mirlo

Thanks for your time

1) Saw it on an other amp (Pure and simpel 1 :-/ )
2) Current feedback to the predrivers for stability
3) I'm still working on that, I have simulated it, and the open loop gain and fase of the output is ok. The fase never gets above 90 degrees (Is it here ok to let it go to just under 180 degrees ???)
4) To reduce gain at high freq (still working on the feedback system).
5) Why ? the timeconstant sets the lowerfreq cutoff to about 10 Hz
6) the signal does not go through this cap since the amp isrunning a "positive and negative side"
7)I have the current generator it's in the upper and lower right corner
8) Planning protection...don't know what yet

\Jens
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Old 29th July 2002, 06:29 AM   #21
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Thumbs up phase response etc

Phase response starts changing well before the amplitude response. That's why I would be inclined to use a really long time constant in the DC rejection part of the feedback, so that at audible frequencies, the phase is rock solid.

When you are trying to set/simulate the loop compensation, are you breaking the feedback loop for the simulation?

The closer you get to 180 degrees, the more ringing you will get when you hook up the feedback... I'd aim for a phase margin of 70 degrees or so -- that gives decent step responses and plently of margin for error.

Base current for the outputs does flow in the Vbe multiplier. Think of what happens when the output is being driven hard one way. The opposite side essentially turns off. The current required to discharge the bases of the opposite side's output transistors has to flow through the impedance of the Vbe multiplier.

I see the current generators for the bias; they are Vbe/R. Looks like you are going to get about 3 mA per side at room temp. But Vbe drops fast with temp. Because the input stage is degenerated, its gain won't really change so much. It would perhaps be an issue with less degen; as it is it should be OK; I spoke (wrote) too soon.

I don't know if it makes sense to reduce the output level at HF via global feedback via C6/R42. One thing you might want to try instead is connecting the 10 pF to one or the other side of the Vbe multiplier, to leave the output stage, with its (relatively) large parasitic capacitances and slowness, out of the HF loop. As it is I don't think it really does much for the stability, since it increases the loop gain at HF. Does this make sense?

Anyway, this is fun. I get to think about a circuit without the tedium of simulating it or the responsibility for messing it up
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Old 29th July 2002, 06:30 AM   #22
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Question Current feedback to the predrivers?

... for stability.

Could you explain this a bit more?

Thanks,

Mirlo
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Old 29th July 2002, 06:33 AM   #23
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First of all, I want to say sorry for asking these questions. I am a bit rusty and want to freshen my memory on these:

1. What is Bridge and Unbridge terminology in Amp?
2. What is SOA?

Thanks,
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Old 29th July 2002, 06:46 AM   #24
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Bridged means you connect the load between two amplifiers, in antiphase, (ie one is inverting, one noninverting, to get twice the voltage swing on the speaker.

SOA means Safe Operating Area, the range of voltages and currents that don't induce "second breakdown".
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Old 29th July 2002, 11:19 AM   #25
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Default Current feedback to the predrivers

Hi there..

The idear was to protect the surcuit against oscelation by means of local feedback. Much the same way the Output transistors are forced to work more or less the same by means of the Emitter resistance.

I don't know if this is the way to do it.

\Jens
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Old 29th July 2002, 11:40 AM   #26
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Default Open loop gain

Ups I forgot this

The open loop gain i simulated with a great big cap of 99999F !!
just after the feedback network to ground.

I the apply a very small signal to the input, and get the OL gain by : 20*log(Vout/Vin) (Transfer funktion)

\Jens
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Old 30th July 2002, 02:18 AM   #27
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Default on current feedback ....

I think the base resistances can help with thermal stability, but not with preventing oscillation. How does it change the simulated results? I hope someone else comments on this, too...

I suppose C3 does what I was suggesting the larger cap across the Vbe multiplier should do. How is the value of the cap C15 across the Vbe multiplier chosen, then?

A problem with my suggestion of using a large time constant for the DC reject part of the feedback network, is getting a good cap that is so large. I have always been disappointed that Panasonic discontinued their low leakage "Z-series" capacitors. I don't know of anything else quite as good that would be appropriate for this application; again, I'm sure someone else can comment!
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Old 30th July 2002, 06:42 AM   #28
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Default Thermal Stability

Hi all

Well i must say that I haven't done any thermal simulations yes....but I suspect the amp will need to be modified around the Vbe multiplier before it's thermally stabel.

The Idear of C3 is a "speed up" cap, to help turn off the output part of the amp.

I thinking about trying a double pole compensation consept to set the domenating pole (And to get more OL gain). Don't know if this is a good idear i sure have some problems getting the fase of Vout to stay within - 180 degrees. Does anyone know how to make the calculations with a double pole ?? I'll post a conceptual schmatic later...

Also I'm not sure if it could be a smart move to use the feedback consept of Mr. Leach's amp design to get within the phasemargin ??

Thanks for any input

\Jens
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Old 30th June 2004, 10:31 PM   #29
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Default Re: SOA for BJT

Quote:
Originally posted by JensRasmussen
The amp is only to be used in non briged mode, with rails of about 65 Volts (+-).

Transistors are MJL3281A/MJL1302A from ON (Former Motorola I think)

Thanks

\Jens
If you want to do something somewhat complicated, then use MosFET HEXFET transistors to switch output collector current on and off as referenced to the audio signal.

Basically, build a circuit that when the audio is above say +10-15 volts on the base of the NPN output, then the voltage on the collector of the PNP output switches off until the signal voltage is below +10-15 volts and then switches back on to 63V. Don't forget the pulldown resistor. Do the same thing with the other polarity, and you will solve the problem with breakdown voltage of "off" transistor, it could then only have 70V max. Must heat sink FET's though.
The only drag is you have to use a P-Channel PWR FET for the positive side. and an N-Channel. (maybe ECG2382 &ECG2383) sometimes the P-Channel is hard to find. $$$

I may post a drawing later to explain better.
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