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HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles
HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles
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Old 21st October 2019, 05:06 PM   #491
stevebogus is offline stevebogus  United States
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Originally Posted by Nakib View Post
Now don't get me wrong here. Why should someone like Randy Slone post something in his book that shouldn't work as intended?
Thing is, I too was trying to build this amp until I found this thread in this page.
Any possible way to make this circuit work with stability?
I cannot speak for the late Mr. Slone.


Are you still attempting to build this amp design? And which one (Slone had several in his book)?


The problem stems from attempting to use current mirrors in the input stage, which do not provide a fixed voltage reference for the VAS. Bob Cordell came up with a method to solve this. Unfortunately, that alone will not produce stable VAS current in Slone's design, the VAS needs a little modification as well. If you want details I can provide them, at least it is stable in simulation. However, I don't know what other problems may lurk in the design. These modifications require additional parts. Not expensive, a couple of transistors and some additional resistors, and changes to a few values. But if you are already have a circuit board you will need to figure out how to fit them in.
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Old 21st October 2019, 11:03 PM   #492
Nakib is offline Nakib
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I was looking forward to this design mainly.
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Old 21st October 2019, 11:11 PM   #493
Nakib is offline Nakib
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I was also thinking about a solution like this. Sorry for my crude drawing but will this method work?
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Old 22nd October 2019, 12:11 AM   #494
Edmond Stuart is offline Edmond Stuart  Netherlands
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Look here: HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles
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Old 22nd October 2019, 01:46 AM   #495
bimo is offline bimo  Indonesia
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Originally Posted by Nakib View Post
I was looking forward to this design mainly.
Bob Cordell used a resistor which connected to colectors of the current mirror to solved the undefined VAS current.
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Old 22nd October 2019, 04:38 AM   #496
stevebogus is offline stevebogus  United States
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Originally Posted by Nakib View Post
I was also thinking about a solution like this. Sorry for my crude drawing but will this method work?
No, it will not help.


See the attachment.


Cordell's method uses an extra transistor and the VAS is referenced to the base of Q3 in the schematic. The value of R4 determines how much control Q3 has over the operating voltage at the VAS input. A lower value gives greater control, but also limits the gain of the input stage, since the IPS is trying to drive a lower impedance. 22k seems to be a decent compromise, midway between the recommended 10k to 47k.


R1 and R2 may not be absolutely needed, but they do provide several benefits. These emitter degeneration resistors, if matched, help alleviate any variations in Q1-Q2 due to mismatched beta or VBE characteristics. The effectiveness of the current mirror is limited by how well matched those transistors are. These resistors help linearize the response. Also, the combination of resistor and transistor in this application is quieter than a transistor alone, though that might be irrelevant compared to the noise of the input transistors. And, since we are setting a reference voltage for the VAS stage, any voltage drop across R1-R2 will be added to the VBE drops of Q1-Q2+Q3. That will turn out to be useful.
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Old 22nd October 2019, 09:20 AM   #497
Edmond Stuart is offline Edmond Stuart  Netherlands
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Originally Posted by Edmond Stuart View Post
Sorry, above link is broken. This is the correct one: MCP A so called common mode control loop (CMCL) does the trick. (see Q5, Q10, Q11 & Q12).

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Old 22nd October 2019, 12:12 PM   #498
vzaichenko is offline vzaichenko  Russian Federation
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HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles
I fully agree with Edmond (Cheers Edmond ) - in this kind of design, where the positive and negative halves of the front-end "compete" with each other, CMCL is the way to go. It's practically a common-mode servo, tracking the VAS current and regulating it via a common mode negative feedback.
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Old 23rd October 2019, 04:09 PM   #499
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See post #190 in the "Bryston 4B SST clone" thread - this method works very well too. Basically you make the VAS transistor one leg of a current mirror and use the other leg to set the VAS standing current. First saw it used by Giovanni Stochino in his "300uS Power" article in Electronics World April 1997 but not sure if he invented the technique. Electronics World sold pcbs for the design and I never saw any complaints or discussion so it must have been a robust amp. Works fine for me too, as you can see in the thread I referred to.
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