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Old 29th January 2009, 12:00 AM   #121
jkeny is offline jkeny  Ireland
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I'm coming to this thread late mainly by searching for sliding bias topology. So my assumption is that this reworking of the original Mullard amp (brilliant work Lumanaw, LXG & Nordic) has retained this sliding bias topology!

I see this hasn't had activity since 7/2008 - has nobody built this? It looked to have so much potential!

What is the dissipation of the O/P devices - is it operating at a lower bias than full class A as is possible with sliding bias?

I hope there is a good outcome to this post as it seems a shame to let this one die!
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Old 29th January 2009, 11:36 AM   #122
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Thanks - no, it's not dead by a long shot. There are two prototype PCB layouts on this thread, both started by Nordic. I was hoping somebody would take it from there, make a prototype and see how it sounded.

The dissipation depends on the rail voltage(s) and the quiescent current - at +/- 24V and 1.25 A, it's 60W dissipation per ch., but you can safely lower the rail voltages to as low as +/- 12 V, Icq=1A for approx. 6W per ch. output power into 8 ohms., with a dissipation of 24W/ch, Class-AB at max swings.
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Old 29th January 2009, 12:28 PM   #123
jkeny is offline jkeny  Ireland
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Thanks LXG,
Are these not class-A dissipation ranges? What does the sliding bias do?

With those sort of exemplary THD figures the only question is how does it sound - you built some early prototypes, I think - what were your impressions?

Nordic, how did the prototype from the pics work out? And same question to you.
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Old 29th January 2009, 01:54 PM   #124
jkeny is offline jkeny  Ireland
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Forgive my stupidity - it is a class-A amp so no surprise with the dissipation, doh!
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Old 8th March 2011, 01:06 PM   #125
gannaji is offline gannaji  India
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Default A very old Schematic of Mullard Amp

Dear all,
As I was browing the various threads, I stumbled on this thread and read it completely. I made notes on a paper about 40 years back and also copied the drawing. I copied the drawing into LTspice and my notes, what ever I could retrieve are as below. Hope this will be of interest!
I used to be very careful in making notes, but in this drawing, the circuit around Tr2 & 3 and Feed back is not clear to me. Please I want to understand the operation of this circuit as an exercise. Can any body who followed the scheme explain it a bit and show how the Power dissipation etc are derived?


Mullard 10 watt amplifier.
Normal Loading, Iq = 0.4 amps.
Low Loading, Iq = 0.27 amps.
Ripple < 100 mv.
Iq = Ipk/Pi and Pout = Icpk*Vcpk/2
Provided the load does not at any frequency fall below Rl = Vc/(Pi*Iq), crossover distortion cannot appear. With this particular method of class AB, (Pi mode AB), the collector dissipation is maximum under no drive conditions and Pc = Vc*Icpk/Pi which is nearly equal to 2/3 Pout (Max)
---------------------------------------------
The changeover from Class A to class AB operation occurs at 0.4*Pout max.
Overall Negative Feed Back of 44db is applied.
The Stability of the collector currents is ensured by direct coupling the phase splitter to the output stage and by over all negative DC feed back. Thus Iq through output transisters is defined by one Nfb loop and midpoint voltage by the other loop.
The normal collector dissipation of the output Transister = 9.2 watts/trans
Max Pc = (+30%) = 12.2 watts.
If Iq is reduced to 0.27 amps, 5 watts output for constant Sine wave drive.
Max dissipation = 8.3 watts.
Dissipation of OC 81 Z = 500 mw.
Dissipation of TR3 = 160 mw.
If the emitter Resistance, decoupled, is made large enough so that the DC is well defined, then the dynamic Bias condition of the transistor has to adjust itself with drive so as to maintain constant DC. Upto a certain power level, both the transistors operate in class A PP. Beyound this point, each transistor is cut off for part of the cycle. Also each transistor sees half the class A load during that part of the cycle during which its partner is cut off. As the drive is increased beyond this point, the total current will also tend to increase. The operating point will however shift so as to return the overall DC consumption to its previous value. With the increasing drive, the operation shifts to class B and then to class C conditions.
For audio amp applications, the design shold be such that the class B condition is reached at max power output. For this case, the Iq, Ipk, Vc & Rl are related by
Iq = Ipk/Pi = Vc/piRl, where Ipk is the peak current at max output, and Rl is the load seen by each collector independently. The design of the output stages is the same as for a Class B stage, delivering the same power but the ---
------------------------------
Attached Images
File Type: jpg Mullard orig.jpg (541.8 KB, 417 views)
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Old 25th September 2013, 11:21 PM   #126
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Hello guys,
Did someone build the original or the improved schematics of this amplifier?
Could you post the listening impressions or the results?
Thanks in advance
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Old 9th December 2014, 07:10 PM   #127
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Quote:
Originally Posted by gannaji View Post
I used to be very careful in making notes, but in this drawing, the circuit around Tr2 & 3 and Feed back is not clear to me. Please I want to understand the operation of this circuit as an exercise. Can any body who followed the scheme explain it a bit and show how the Power dissipation etc are derived?
I missed this posting earlier - thanks for your schematic. Yes, it's one of the Mullard sliding-bias circuits, and notable for the use of only PNP transistors (in the germanium era, PNPs were the more common transistors).

I decided to revisit this topology after being inspired by the Amp Camp Amp. In particular, the use of +/- 19V supplies derived from laptop AC adapters seems to be viable. I do have an updated schematic with DMOS output devices, which I'll post in due course - there's also a PCB layout in progress.
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Old 10th December 2014, 02:53 AM   #128
gannaji is offline gannaji  India
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Thank you linuxguru, Eagerly waiting for your new take on this.
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Old 10th December 2014, 10:11 AM   #129
AKSA is offline AKSA  Australia
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And from me, too, LXG is a brilliant man........

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Old 10th December 2014, 12:36 PM   #130
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Hi
The input stage operate as current feedback units, not like standard voltage feedback that is generally used now. Q1 derives feedback from the emitter of Q2, and should be fed with a current (high impedance) source ideally. Q2 then has its emitter voltage controlled by feedback, so this point is low impedance, but the current is taken from the collector, so this is a high impedance output (current fed) output.
The feedback resistor R12 in the output stage then converts the input current to voltage output.
The starting bias conditions can be calculated by guesstimating that the output rail will be approximately the same as the collector voltage of Q3 (you could also assume 0.7V lower from two Vbe's and the drop across R23 at 0.3A). So then we have for Q3
Vb~ R/(R+18k)*Vc (or Vc-0.7)
where R=2.2k plus mid-pot 500 ohms (as a starting point)
Ve=Vb-0.2 (approx)
Ic=Ve/560 (approx)
Vc=Vcc-3.3k*Ic
Vc=Vcc-3300/560*Ve
Let A=2.7k/(2.7k+18k)
then
Vb=A(Vcc-3300/560*Ve)
Vb=A(Vcc-3300/560*(Vb-0.2))
which after a shuffle gives
Vb=4.5
so this would suggest that the centre rail does operate about half the supply. It should be a bit higher to allow for the voltage drop in the lower part of the circuit, perhaps around (-)28V.
To conduct the same current the lower half controlled by Q4 will conduct approx. the same current in the collector setting the voltage across R22 to about 4.5V, setting Iq to about 300mA. Detailed calculations can be done as above (there is no DC link between Q3 and Q4 so these can be solved separately).

This circuit may have been novel at the time but Q3 operates with a swinging collector while Q4 will remain relatively low, so there is an imbalance between the halves of the circuit and Early effect distortion will be rather high.

With approx. 300mA bias current in idling, the power dissipation will be approx. 7W per output transistor. One problem with such circuits is that if the DC bias point changes, there will be a DC ripple effect through the circuit which will change again when the signal reduces again, causing spurious low frequency currents arising in the output.

The frequency performance of Ge transistors was not great, either, so not much positive to say about this as a hifi- more of historical interest.


John
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