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Old 24th June 2012, 10:21 PM   #501
Telstar is offline Telstar  Italy
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Originally Posted by Waly View Post
May I suggest also using 2SK389/2SJ109 for small signal? They are excellent matched pairs and you can still buy them.
Where? The j109 in particular.
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Old 24th June 2012, 10:29 PM   #502
AndrewT is offline AndrewT  Scotland
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The datasheets only specify matching of Idss. They show low limit of 0.9:1
That is not close matching although measurement of actual devices does show that typically three times better than low limit is achieved, i.e. 0.97:1. None of the other parameters are claimed to be matched.
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Old 25th June 2012, 04:49 AM   #503
Bonsai is offline Bonsai  Taiwan
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Originally Posted by wahab View Post

. . . .

Higher transconductance of the bipolar would be an advantage
but then you have to look at their loading effect of the previous
stages , wich destroy their apparent advantage , as what is gained
in and end is lost in the other one.

This can be countered by using a triple EF but at the expense
of stability , or bandwith if the arrangement is to be compensated.
Wahab, some good points here from your side.

With the EF3, like many things in electronics, you just need to learn the techniques and get comfortable with them. I think stabilizing (i.e. preventing localized HF parasictic oscillation) in an EF3 output stage is straightforward. However, fail to address the elements that cause instability, and you will have problems. I've built two high power EF3 amplifers - a 280W per channel one with a 21193/21194 output stage (FT circa 4MHz)and the latest 180W per channel amp with 1302/1381 (Ft circa 30MHz) and had no problems. In the Ovation 250, a simple L pad in the driver base circuit and base stoppers in the output devices fixed the 2MHz output stage parasitic oscillation. In the e-Amp, I added stopper RC networks between the driver stage and output stage power lines and was unable to provoke oscillation despite extensive testing with all sorts of loads.
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Old 25th June 2012, 10:29 AM   #504
wahab is offline wahab  Algeria
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Hi , Bonsai

As you know , i simulated your amp as a whole but also specificaly
the output stage s behaviour and it seems after some tries that
its compensation can be quite improved , removing completely
the spikes that appear when testing with a square wave , as
shown in your PDF , moreover letting the high slew rate unchanged.

Still need to check further the numbers and if you dont mind
i ll upload the picture and mods to the OPS compensation
in the dedicated thread about this amp.
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Old 25th June 2012, 10:44 AM   #505
Bonsai is offline Bonsai  Taiwan
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ok - I am interested to see what you come up with ;-)
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Old 25th June 2012, 11:28 AM   #506
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Old 25th June 2012, 12:17 PM   #507
Bonsai is offline Bonsai  Taiwan
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Probably best to post your findings in te Ovation e-Amp Wahab.
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Old 25th June 2012, 12:18 PM   #508
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Default LS impedance dips

Hi KSTR, Steven & Stuart,

Wow, a lot of response on this topic. Thank you!
You have made perfectly clear that and amplifier may be subjected (under certain conditions) to very large output currents. You also made clear that there is not only one reason, rather there are two reasons why this happens:

1. 'Difficult' signals.
2. 'Difficult' loads.

Ad 1, this is also illustrated -for example- by Bob Cordell, 'Designing Audio Power Amplifiers', p. 374 and also discussed Keith Howard in Stereophile. There is nothing we can do about it, but happily such signals are more or less rare, about 1% of the time, depending on the source material, of course. One might argue that under rare conditions and of short duration higher THD level are acceptable, that is, provided that the protection circuit will not prematurely be activated.

Ad 2, as SY replied: They indeed exist. The Wilson WATTs are notorious for that. As far as I understand, the impedance dips are caused interaction between the drivers as well as (ill designed) cross-over filters. I think that designers of powers amps shouldn't accepts this, as, opposed to ad 1, this is something one can do about it.

And finally, I wonder how much amperes you will need to drive 'difficult' speakers with 'difficult' music as well? Maybe 100 A?

Anyhow, I refuse to design 'Wilson WATT friendly' amps. Speaker designers have to take their own responsibilities and make the impedance curves as flat as possible. If they refuse to do that, we, as amp builders should refuse to buy their stuff.

Cheers,
E.
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Old 25th June 2012, 12:26 PM   #509
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bonsai View Post
Probably best to post your findings in te Ovation e-Amp Wahab.
Indeed , i suggested it in the last line of my previous post...


Quote:
Originally Posted by Edmond Stuart View Post

Anyhow, I refuse to design 'Wilson WATT friendly' amps. Speaker designers have to take their own responsibilities and make the impedance curves as flat as possible. If they refuse to do that, we, as amp builders should refuse to buy their stuff.

.
Sure that s a big constraint , the same one as if speaker s designers
where challanged to make practical designs that would have to deal
only with high output Z amplifiers, SET like...
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Old 25th June 2012, 12:39 PM   #510
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Hi Wahab,

>high output Z amplifiers
That's another story.
I mean, if the LS impedance is flat and resistive, instead of 100A, just 10A will do.
Wouldn't that be far more economical?

Cheers,
E.
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Last edited by Edmond Stuart; 25th June 2012 at 12:50 PM.
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