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Old 18th March 2013, 11:30 PM   #2701
Bigun is offline Bigun  Canada
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Well, the issue is the topology. It uses a phase splitter to drive the output devices. The phase splitter has to work in Class A - because if you let this device turn off then it no longer controls the current flow. And it must control the current flow in at least one of the output transistors even in Class B. In order for the lower output device to turn off (in Class B) you have to let the phase splitter device turn off. At least this is how I see it. So I don't think you can do it - the distortion will be awful.

If you want to have an amp that can operate in Class A and switch out into Class B when it runs out of bias current you need a different topology. A suitable topology would be the more common complimentary output devices so that it's the output devices that do the 'phase splitting' - rather like the JLH Class B amplifier (described on this site: http://sound.au.com/tcaas/jlhab.htm)

I think both the simple Class A amp and his simple Class B amp would sound very nice. But do you need the higher power - you may find with the right speakers that the Class A amp is loud enough.
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Last edited by Bigun; 18th March 2013 at 11:34 PM.
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Old 19th March 2013, 12:22 AM   #2702
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Quote:
Originally Posted by Bigun View Post
Well, the issue is the topology. It uses a phase splitter to drive the output devices. The phase splitter has to work in Class A - because if you let this device turn off then it no longer controls the current flow. And it must control the current flow in at least one of the output transistors even in Class B. In order for the lower output device to turn off (in Class B) you have to let the phase splitter device turn off. At least this is how I see it. So I don't think you can do it - the distortion will be awful.

If you want to have an amp that can operate in Class A and switch out into Class B when it runs out of bias current you need a different topology. A suitable topology would be the more common complimentary output devices so that it's the output devices that do the 'phase splitting' - rather like the JLH Class B amplifier (described on this site: The Class-A Amplifier Site - JLH Class-AB Amplifier)

I think both the simple Class A amp and his simple Class B amp would sound very nice. But do you need the higher power - you may find with the right speakers that the Class A amp is loud enough.
That makes sense.

I was going to go with Class A but thought of trying it running as old tube amps were, biased about 60-70% and then switch to Class A to see if I could hear any difference. Just for curiosity sake. This may take a bit more thought.
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Old 19th March 2013, 12:26 AM   #2703
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Originally Posted by Printer2 View Post
I am going to ask something sacrilegious here. I am interested in the earlier 15W capacitive coupled amplifier and was wondering if you could run in Class AB with about 1A running through the outputs at idle. So up to about -3dB of full output it runs Class A. Or am I missing something here?
The original JLH current steering phase splitter was incapable by itself of
shaping drive currents in a non-linear way that would produce AB output
currents. Non-linearity of the default 2n3055 output transistor bends the
wrong way for natural AB with reduced idle current. Indeed idle current
of the JLH output totem is far MORE than the peak output current and in
wasteful excess of linear class A!

If you wish to reduce idle currents in AB fashion, you'll have to sabotage
the ideal constant drive current source that can be steered only two ways.
You'll either need to restrict excess drive current during the AB crossing,
or shunt it somewhere else than output bases during the crossing.

I've found an easy way to shape the drive currents for square law AB
that can be grafted onto an existing JLH circuit. But my AB solution with
Schottkys for current sensing sometimes oscillates. Not recommending
till I figure why it does that. Fine if you just want to use current sense
resistors (instead of Schottkys) to shape for linear class A behavior.

Search the forum for SRJLH, but don't built the AB option unless you
have some deep insight how to fix the oscillations.

Last edited by kenpeter; 19th March 2013 at 12:36 AM.
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Old 19th March 2013, 02:30 AM   #2704
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Might just be easier to get a Class AB amp and bias it for Class A?
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Old 19th March 2013, 07:43 AM   #2705
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by Printer2 View Post
I am going to ask something sacrilegious here. I am interested in the earlier 15W capacitive coupled amplifier and was wondering if you could run in Class AB with about 1A running through the outputs at idle. So up to about -3dB of full output it runs Class A. Or am I missing something here?
Quote:
Originally Posted by Printer2 View Post
Is there a reason why? I am hoping that the amp just behaves like any other Class AB amp once it goes into Class B. The power supply will have enough current to supply the demand. Or is the subsequent designs more suited to run this way?

Here is the original JLH running as low as 500ma bias. Lets be clear on the question. Running it at 1 amp as you wish doesn't really fit the accepted definition of Class AB. To me a class AB amp is one running at the correct current for the output stage topology chosen.
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Old 19th March 2013, 11:01 AM   #2706
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Originally Posted by Mooly View Post
Here is the original JLH running as low as 500ma bias. Lets be clear on the question. Running it at 1 amp as you wish doesn't really fit the accepted definition of Class AB. To me a class AB amp is one running at the correct current for the output stage topology chosen.
Your making me want to figure out how to use LTspice. Good thing I have a few projects to finish first, looks like I will have to brush up on my transistor theory. I was just trying to get a head start on this one since I have to get the transistors mailed in (Slow boat from China you know).
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Old 19th March 2013, 11:27 AM   #2707
Mooly is online now Mooly  United Kingdom
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Originally Posted by Printer2 View Post
Your making me want to figure out how to use LTspice. Good thing I have a few projects to finish first, looks like I will have to brush up on my transistor theory. I was just trying to get a head start on this one since I have to get the transistors mailed in (Slow boat from China you know).
If you install LTspice here is the file that will run straight off. Just unzip the folder and keep the two files together. If you want to try it and get stuck just ask... I knew zilch about using spice 12 months ago.
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Old 19th March 2013, 10:52 PM   #2708
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If you install LTspice here is the file that will run straight off. Just unzip the folder and keep the two files together. If you want to try it and get stuck just ask... I knew zilch about using spice 12 months ago.

I had other plans for tonight. Oh well.
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Old 20th March 2013, 01:25 AM   #2709
iperv is offline iperv  Italy
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Originally Posted by AndrewT View Post
No.
You have this all wrong.
If the output bias of a single ended SS amplifier is the maximum output current then the maximum power is given by:
Pmax = Ibias^2 * Rload / 2

If the output bias of a push pull SS amplifier allows twice as much maximum output current then the maximum power is given by:
Pmax = [2times Ibias]^2 * Rload / 2

The JLH operates as a sort of sliding bias where the maximum output current of this amp is intermediate between the SE and PP types.
I've seen estimates of 1.5times bias and 1.6times bias. I have not tested so can't confirm actual peak output current.
The maximum power of a JLH is given by:
Pmax = [Xfactor times Ibias]^2 * Rload / 2

If you know that the maximum output current of your JLH is 1.75Apk then the maximum power into 4r0 is 6.125W, not 12.25W

Had you used an 8ohms speaker for your higher voltage JLH and I assume the Xfactor is 1.5 (as before), then the maximum power would be:
Pmax = [1.5*1.75]^2 * 8 / 2 = 27.56W if the output voltage required for this Pmax is within the capabilities of a 46Vdc supply.
Vpp for that prediction is 2 * 1.5*1/75 * 8 = 42Vpp.
This is possibly within the capabilities of your 46Vdc PSU.

There is no advantage in increasing Vdc of the PSU AND reducing the speaker impedance from 8ohms to 4ohms.

Power is power, nothing else.
We are speaking of PowerRMS:
[W RMS] = [A RMS] * [V RMS]
When i misure Ibias with my digital multimeter I measure it by [A RMS] (Ampere RMS), not Apk, nor App of course.
This is because multimeter makes only measurements of Voltage RMS, even if it is measuring Ampere; in fact it then divide Voltage measurement by a shunt resistor, to obtain correct value of I, expressed in [A RMS].

So if I measure Ibias = 1.75A RMS and I connect a 4ohm loudspeaker, then the output power is:
(1.75^2)*4 = 12.25W RMS.
In this calculation I can ignore the voltage value beacause I know it is sure enough. In fact:
1.75A*4ohm = 6.125V
Vcc(min) = 2*(6.125V+0.7V) = 13.65V (<<48V).
So if I want an output power of 12.25W on a load of 4ohm I have to bias my JLH@ 1.75A rms and 13.65V rms.
But it is not so simple.

If I want to drive 2ohm too @12.25W?
sqrt(12.25Wrms/2ohm) = 2.5A rms circa.
{It's different from sqrt(12.25Wrms/4ohm) = 1.75A rms.}
So: 2*(2.5Arms*2ohm+0.7V) = 11.4V rms (but I've got 13.65V, so it's ok). I have to increase Ibias from 1.75A to 2.5A.

8ohm?
1.75Arms*8ohm = 14Vrms.
14Vrms*1.75Arms = 24.5W rms.
So I need a Vcc = 2*(0.7Vrms+14Vrms) = 29.4V rms. (13.65V is no more ok, and I have to increase a lot the PSU).

At the end of the story a correct couple of values to drive whatever @12.25W rms (min) is:
2.5Arms @30Vrms [12.25W rms A Class @2/4 ohm, 24.5W rms A Class @8 ohm].

I agree with you of course when you say 'There is no advantage in increasing Vdc of the PSU AND reducing the speaker impedance from 8ohms to 4ohms'.
I only said I polarized my JLH @48V when someone asked if anybody out there (cit.) did, not said it's a good choice.

Last edited by iperv; 20th March 2013 at 01:30 AM.
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Old 20th March 2013, 09:59 AM   #2710
AndrewT is offline AndrewT  Scotland
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No.
Ibias is a DC current.
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