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Old 3rd March 2004, 02:08 AM   #121
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Scott,
Yikes, a modification to my sacred circuit! Whatever shall I do? Let me run in circles for a minute or two until my karma gets rebalanced.
Okay, I'm over it.
Actually, I think I remember seeing the version you're referring to, but I don't recall how they had it set up. Therefore, in my best time-honored manner, I'll wing it.
If, for whatever reason, Zeners bother you, here are two more ideas:
--Series two (or more) voltage references. If you can get 9V, great. Otherwise, stack two 5V for instance (i.e. 10V), and use a voltage divider or pot to set the voltage exactly where you want it.
--Don't worry about finding a 9.1V reference...use your 5mA current source across an 1820 ohm resistor. Presto! Instant 9.1 volts.
And a third idea, just for fun:
--If you've got, say, a 6V reference, then simply recalculate the resistors under the tail end of the current source. You'll need something more than the Vgs of the MOSFET, but once you get over that limit, it's a simple E/R=I calculation to set the current.
Think creative thoughts, man. You'll get there.

Grey
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Old 3rd March 2004, 02:42 AM   #122
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Question Grey's current source design?

Scott,
An LM 329 running at 1 ma is fine. The larger current the jfet CCS diode the lower its impedance. You will get much more power supply rejection for the LM 329 voltage reference with the 1 mA current jfet. Don't forget a 1K gate resistor for the ZVP3310. Setting 10 volts across the device is a 150 mW which is no problem for dissipation and helps to lower the drain to source capacitance for the current source. Don't hesitate to put a series resistor between the drain of the ZVP3310 and the sources of the mosfet differential pair to limit the voltage across the drain to source of the current source. It will help to isolate capacitance a little more and detracts nothing from the performance of the current source. I really believe this current source approach is worth implementing since it has several orders of magnitude better rejection and noise. A resistor biased zener with low voltage rails has pretty poor supply rejection and is pretty noisy. Sorry if I clipped anyone's wings............
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Old 3rd March 2004, 04:02 AM   #123
Thunau is offline Thunau  United States
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Quote:
Originally posted by GRollins
Eric,
A single secondary will do in a pinch. You can always create a virtual ground using resistors, since very little current flows to ground in this circuit. A center tapped secondary will be better, and dual secondaries would arguably be even better still.
Start by determining how much wattage you want, then work backwards from there, as that will give you the rail voltage and an indication of where you want your bias set. If you're buying a new transformer, then you'll very likely end up with a center tap or split secondary. If you're buying surplus/used, don't be afraid to go with a single secondary if it'll save you money.


Grey
I'm getting confused now. Might be because of the shear size of the original thread, but most likely because I don't know much about power supplies in general. Sorry, but I just can't seem to put all this info together.
I was using the AXE-1 spreadsheet. If I have a dual secondary 16V transformer and use both secondaries for one channel, do I plug enough rail voltage in the "B12" cell to arrive at ~16V or ~30V in the "B23" cell?
What about if I use one 16V secondary per channel? Probably not enough voltage to make the Mosfets work.
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Old 3rd March 2004, 08:10 AM   #124
Netlist is offline Netlist  Belgium
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Quote:
Originally posted by Coulomb
In the file Axe.zip what resistor is the spread sheet calculating the value for?
If you mean CEL B32 that would be R5/R6 and R40/41.
The calculation is based on the 0.5V that should be over these resistors. Keep in mind this is only a theoretical value.
Quote from Grey: "There's nothing magic about .5V--you're free to set it where you like."

/Hugo
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Old 3rd March 2004, 10:59 PM   #125
Coulomb is offline Coulomb  England
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Quote:
Originally posted by Netlist

If you mean CEL B32 that would be R5/R6 and R40/41.
The calculation is based on the 0.5V that should be over these resistors. Keep in mind this is only a theoretical value.
Quote from Grey: "There's nothing magic about .5V--you're free to set it where you like."

/Hugo
So Cell B33 is the Resistor value I choose to put in for R5/R6 & R40/R41? So B32 is the theoretical and B33 is the practical?

So if I jumper these resistors, can't I take the output for my FET boards directly from the FET Pads? Then I would only need one other wire for the current sense line to each board.

Anthony
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Old 4th March 2004, 01:44 AM   #126
moe29 is offline moe29  United States
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Default Good Enough?

Here are some measurements from my Aleph X.

14V rails .5V across R5

Q1 3.58 Vgs - Q10 3.51 Vgs
Q2 3.80 Vgs - Q11 3.84 Vgs

Diff Pair 3.693 and 3.679 Vgs Q6a 3.87 Vgs

Abs. Offset <50 mV

DC Offset 180mV - 200mV


I guess i'll have to keep popping in FETs to the Q6a slot until i get
a <100mV DC Offset reading. It's just hard to keep desoldering
that piece. I didn't realize that the diff pair had to match to Q6.

or can one live with the DC offset i'm getting.

thanks!
m.
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Old 4th March 2004, 02:15 AM   #127
hifiZen is offline hifiZen  Canada
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The diff pair does NOT have to match Q6. Q6 lives in it's own little world, and doesn't care at all what any of the other transistors are up to. It is the Q5-Q7 pair which must be matched to give you a low DC offset between the speaker terminals.

This mismatch is the source of your DC offset: "Diff Pair 3.693 and 3.679 Vgs"

Hope that helps, and saves you some trouble.
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Old 4th March 2004, 02:51 AM   #128
moe29 is offline moe29  United States
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Chad,

that's only a .014V difference!

doesn't seem to = 200uV of offset...

i could see if it were 0.2V.....

i didn't realize it had to be such an exact match.
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Old 4th March 2004, 03:14 AM   #129
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Default CCS Continued.

Trying to be creative, without getting too far off of the purpose of this thread.

I greatly appreciate the input from both Grey and Fred. Your inputs are greatly helping me understand how the circuit works.

For grins and giggles, lets take this one step further, lets double the bias of the diff pair. ( More Bias is better right?) If I understand this correctly R23, R25 would go from 392ohm to 195ohm so the absolute DC offset track closer to zero and the gain remains the same. The resistor network (R24,R26,VR2) would have to be adjusted to provide control of Q6 at just above its threshold. Depending on the device used the, network would need to control somewhere around 40 or 70 ohms using a 6.9V reference voltage.

If the irf9610 was used for q6, power dissipation wouldn't be an issue, however with the ZVP3310A option, R48 may have to be adjusted slightly to keep power dissipation in a safer operating range possibly with the sacrafice of some linearity. Based on Freds Comments if I was willing to live with 300mW on q6 R48 could actually stay the same.

The only other issue that I can see from my very simplistic point of view, is that if R48 was used at a sufficient value to keep the Vds of q6 around 10V then a 1/2 watt resistor would be needed. Additionally the power dissipated by the diff pair Q5/Q7 would be somewhere above 0.75 watt meaning that a small sink wouldn't be a bad idea providing they are thermally coupled.

Having said that, are there other issues that pop up when upping the bias of the differential pair?

Thanks
Scott
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Old 4th March 2004, 03:19 AM   #130
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Default Heat Sink Construction

For those that can read dwg files, I have attached how the heat sinks were constructed and where the FETS mount.

Have Fun
Scott
Attached Files
File Type: zip general view.zip (59.3 KB, 564 views)
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