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Old 8th May 2010, 06:12 AM   #21
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Why waste an op amp or driver IC on a 10 W class A amplifier? Nelson Pass has great projects of such amps that use as little as two transistors and a few passive components.

Pity that the OP does not participate in the thread anymore. Would be interesting to find out whether his idea of a PC speaker's mid range and tweeter amplifier is to have four class A power stages heating away on or next to his working desk. Especially if he lives in a hot country, which is not unlikely, if you look at his name.
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Old 8th May 2010, 09:16 AM   #22
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ThorstenL View Post
Well, 10W can be handled using +/-15V rails. The dissipation to provide this as Class A (single ended) into 6 Ohm is also not that great, as we only need 2A peak current.
are you driving a 6r0 resistor or a 6ohm reactive load speaker?
Allow for peak currents ~ 3* what the resistor draws.
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Old 8th May 2010, 09:33 AM   #23
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by AndrewT View Post
are you driving a 6r0 resistor or a 6ohm reactive load speaker?
We are driving the midrange and tweeter in a fully active speaker. First, either is usually operated well above the fundamental resonance. Second, without multiple drivers and crossovers a driver shows an impedance profile where minimum impedance conincides with minimal deviation from 0degree phase for the impedance.

So if I can drive the voice coil DCR, I can drive the driver fine.

Quote:
Originally Posted by AndrewT View Post
Allow for peak currents ~ 3* what the resistor draws.
Why should I? Single ended Tube Amplifiers have never done such a thing, for the nearly nine decades of their existence and they still drive speakers just fine for that last nine decades.

Of course, if I wanted a full range box with a passive crossover that dips below 1/2 rated impedance in the bass and has a significant phase angle at that impedance I may find that even 3*Current of that to drive nominal impedance resistor is not enough. Of course, I may also find that such a speaker is a very badly designed one indeed.

Ciao T

Last edited by ThorstenL; 8th May 2010 at 09:36 AM.
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Old 9th May 2010, 11:02 AM   #24
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ThorstenL View Post
Why should I? Single ended Tube Amplifiers have never done such a thing, for the nearly nine decades of their existence and they still drive speakers just fine for that last nine decades.
a poor analogy due to ignoring the effectiveness of the output transformer.
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Old 9th May 2010, 11:29 AM   #25
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by AndrewT View Post
a poor analogy due to ignoring the effectiveness of the output transformer.
The effectiveness of the output transformer? How is it effective?

How does it add three times current over quiescent operation? Actually, it does not. All it does is to transform the speaker impedance which is low voltage, high current and hence low impedance into a load the tube can drive well (read high voltage, low current, and hence high impedance).

A 300B SE Amplifier at 350V/80mA operation dissipates 28W and can swing around +/-80mA Peak. With a 3K/8R output transformer the voltage swing is reduced by a factor 20 and the current swing is increased by the same factor, so around +/-1.6A peak current are applied to the speaker (8R in this case) and 10W are developed. There is no magic in the output transformer that increases the available current swing past this.

If we made a single ended (follower) SS Amp aimed at producing the same power (1.6A Peak with 13V peak output voltage we use a 18V rail with 1.6A current and dissipate 28.8W. Our peak output current is 1.6A and our peak output voltage up to 17V (if we design the frontend right).

If we use a choke as load for the SE Solid State Amp (like Pathos does with what they call Inpol) we would need a big chunky choke with around 5Ohm DCR (300B SE OPT's generally have 100R primary DCR), if we use a CCS instead of choke we need to dissipate another 29W or so, but no other relationships have changend.

So, a DHT SE Amplifier is an EXCELLENT example for our case and in fact it illustrates perfectly why in my original example 2A quiescent current suffices for 10W into a speaker with 8 ohm nominal impedance and 6 Ohm voice coil DCR.

Ciao T


This is basically the same as the active halve
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