Electronics > Beginners
Trimpot rating when in place of two resistors??
Ian.M:
@Zero999: Unfortunately 'R10' is already on the original schematic, its the emitter resistor of the current sink Q4.
@All,
Please note the PMD16K100 and PMD17K100 output transistors are complimentary Darlingtons. I didn't realise that originally, so my estimate for the max bias voltage across the Vbe multiplier Q6 was wrong. Due to the complexity introduced by the bias limiter (D3, R13), and the varying Vbe drops with quiesent current, I didn't fancy trying to solve the bias cirucit analytically so I have knocked up a quick rough sim of it in LTspice with only one pair of output transistors and default transistor models with only HFE tweaked to get the output transistors in the right ballpark for power Darlingtons of the specified types. Component designators correspond to the original schematic.
It shows a bias point of approx 4V across Q6 and 6.6mA quiescent current per pair of output transistors. Edit: I got the emitter resistr values wrong - see below. The corrected sim shows excessive quiescent current - either the amp's intended to run rather warm in class AB or there's something wrong with the O.P's schematic
Due to the action of D3, R13, Gyro's suggestion of inserting a 100R trimpot between R11 and R12 will *NOT* provide a useful bias adjustment range.
Zero999:
--- Quote from: Ian.M on February 12, 2019, 10:04:56 pm ---@Zero999: Unfortunately 'R10' is already on the original schematic, its the emitter resistor of the current sink Q4.
--- End quote ---
Of course, the designator should be different.
Robbie010:
--- Quote from: Ian.M on February 12, 2019, 10:04:56 pm ---@Zero999: Unfortunately 'R10' is already on the original schematic, its the emitter resistor of the current sink Q4.
@All,
Please note the PMD16K100 and PMD17K100 output transistors are complimentary Darlingtons. I didn't realise that originally, so my estimate for the max bias voltage across the Vbe multiplier Q6 was wrong. Due to the complexity introduced by the bias limiter (D3, R13), and the varying Vbe drops with quiesent current, I didn't fancy trying to solve the bias cirucit analytically so I have knocked up a quick rough sim of it in LTspice with only one pair of output transistors and default transistor models with only HFE tweaked to get the output transistors in the right ballpark for power Darlingtons of the specified types. Component designators correspond to the original schematic.
It shows a bias point of approx 4V across Q6 and 6.6mA quiescent current per pair of output transistors.
Due to the action of D3, R13, Gyro's suggestion of inserting a 100R trimpot between R11 and R12 will *NOT* provide a useful bias adjustment range.
--- End quote ---
Sorry gents, you have lost me a little here, my knowledge is definitely on the up but not quite there yet......
Am I to assume that @Ian.m's original suggestion of reinstating R12 and install the trimpot in place of R11 is still the correct thing to do??
And if so, was my depiction of the schematic the correct way to install it?
Thanks
Thanks.
Robbie010:
--- Quote from: Ian.M on February 12, 2019, 11:21:34 pm ---Why do you think it *needs* a trimpot and how do you plan to adjust it if you fit one?
The addition of R13, D3 to the standard Vbe multiplier circuit adds a bias stabilisation feedback loop, so the bias should be automatic unless any part of the output stage and its bias circuit is defective or seriously out of tolerance.
--- End quote ---
The circuit currently has a trimpot in place of R11 & R12, so I was just planning to replace it like for like while overhauling the amplifier. However, when getting advice from various sources I have been told that a trimpot for bias adjustment is fine but that I should also reinstate one or the other, or both of the substituted resistors.
I have a multimeter, a function generator and an oscilioscope so I was planning on testing the amplifier thoroughly before connecting it up to my hifi system and as I understand it, the DC offset of the output can be adjust, if needed using the trimpot.
One thing to note is that while the amplifier is roughly true to the original schematic, it is not exactly the same with some components being of higher values, so I would assume that the original builder of the amp put the trimpot there for a reason?
Ian.M:
OOPS! |O I missed some flyspeck decimal points in front of the emitter resistor values. I dislike schematics that don't follow the convention of replacing the decimal point with the S.I. multiplier letter, (with R for unit ohms) and *HATE* ones that use decimal points without a leading zero.
I *SHOULD* have noticed 68R was excessive for power amp emitter balancing resistors. 0R68 is a lot more reasonable.
Please amend the values of R16,R18 before running my sim.
More to follow when I've run some parameter sweeps.
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