PPA v2 construction discussion
May 31, 2008 at 3:25 AM Post #361 of 1,083

amb

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Jelle Schrijver, the Szekeres is a simple MOSFET source follower (aka common-drain amplifier). It has no voltage gain and therefore there isn't the classic "feedback" that we speak of. Feedback is usually applied to reduce the voltage gain (at the same time achieve a lowered output impedance, cancel distortion and extend the bandwidth).

If the amp had been a common-source amp, where the output is taken from the drain pin, then there has to be a resistor from the positive rail to the drain pin, and the ratio of resistance between the drain resistor and the source resistor would determine (roughtly) the voltage gain of the amp. This is called "local feedback", and even though it doesn't look like a loop, it has the same effect.

The same thing applies to a BJT-based common-emitter amp. In fact, it is a basic building block of more complex topologies, where there is local feedback at each stage, and often also a global feedback loop spanning multiple stages, from the output back to the input.
 
Jun 7, 2008 at 7:26 AM Post #363 of 1,083

d-cee

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Jun 7, 2008 at 2:34 PM Post #364 of 1,083

Steve The Egg

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No, I don't already have one. I'm looking to get one and I don't know whether I should get it with these. I'll be using it with a Keces 131 DAC, SR-80, and soon some D2000's. I'm basically looking for an amp that I'll never have to upgrade again. I want it to be as transparent as possible.
 
Jun 7, 2008 at 2:43 PM Post #365 of 1,083

Beefy

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Quote:

Originally Posted by Steve The Egg /img/forum/go_quote.gif
I'm basically looking for an amp that I'll never have to upgrade again.


You don't understand this hobby very well, do you?
wink.gif


In all seriousness, I don't know much about the PPAv2..... but I do know that a STEPS is a very good power supply option. The σ11 over at AMB is a newer design that is arguably 'better', but more expensive. The OPA627 is definitely a premium OPAMP, but there are counterfeits going around - so make sure you get them from a reliable source such as Digikey.
 
Jun 13, 2008 at 1:25 PM Post #366 of 1,083

luvdunhill

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hey guys, I'm a problem with my battery pack. It has been left unused for easily more that the 30 days mentioned in the docs and the cells are depressed. I am using a 24v supply, LM317, R1=1k, R2=4.7R, R4=100k, R5=10k, R6=closed, R7=closed, R8=open. I'm getting 2.05v from the R4/R5 node to V-. I'm assuming something is wrong with my battery pack? This exact configuration has worked in the past, nothing has changed. I'm using a 24VAC @ .45A AC wall wart into a TREAD and am getting exactly 24v DC output into the TREAD.

Thanks!
 
Jun 13, 2008 at 1:43 PM Post #367 of 1,083

tangent

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Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif
I'm getting 2.05v from the R4/R5 node to V


That symptom suggests the opposite of a depressed battery pack: the charge controller would be in trickle-only mode because Vsen was under 1 V.

Check the pack's health by measuring across it with the charging supply disconnected. If you get approximately 1.2-1.6 * cells, go back and measure across each one to see if any are exceptionally low, like under 0.1 V. If the pack is fine, I'd recheck your R4 and R5 values. I haven't run the numbers, but in my experience, you rarely get such common values out of the calculator without going way down the list of possibles.
 
Jun 13, 2008 at 5:46 PM Post #368 of 1,083

luvdunhill

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Quote:

Originally Posted by tangent /img/forum/go_quote.gif
That symptom suggests the opposite of a depressed battery pack: the charge controller would be in trickle-only mode because Vsen was under 1 V.

Check the pack's health by measuring across it with the charging supply disconnected. If you get approximately 1.2-1.6 * cells, go back and measure across each one to see if any are exceptionally low, like under 0.1 V. If the pack is fine, I'd recheck your R4 and R5 values. I haven't run the numbers, but in my experience, you rarely get such common values out of the calculator without going way down the list of possibles.



ok, will do. As for the values I choose, I picked them directly off the bottom of the part selection guide, here, under "12× AAA, 2.8 hour fast charge":

Part Selection Guide

Now, putting 12 cells @ 650mAH, charge time 2.8 hours into the calc, I get:

Supply >= 23.75V

U1 = LM317
Add U2 and U4
Add U3

R1 = 390 Ohms or higher
R2 = 5.6 Ohms (174.72 min. fast charging, 0.28W, 0.22A)

R4 = 75K, R5 = 8.2K(Vsen: 1.06 - 1.83V)
R4 = 82K, R5 = 9.1K(Vsen: 1.08 - 1.86V)
R4 = 91K, R5 = 10K(Vsen: 1.07 - 1.84V)
R4 = 82K, R5 = 8.2K(Vsen: 0.98 - 1.69V)
R4 = 91K, R5 = 9.1K(Vsen: 0.98 - 1.69V)
R4 = 100K, R5 = 10K(Vsen: 0.98 - 1.69V)

R6 short, R7 short, R8 open (177 mins)

The discrepancy is in R2 and 100K/10K is the last option in the table. What would you recommend doing? The LED is indeed always on (it doesn't blink).

I'll measure the cells when I get home tonite.
 
Jun 13, 2008 at 11:33 PM Post #369 of 1,083

tangent

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So yeah, 100K and 10K aren't great because they're not giving you as wide a Vsen range as you could get with the first choice on that list. But that calls into question why such a divider is letting 2.05 V through to Vsen. 2.05 V * 11 (the reduction factor) is 22.55 V, almost your full supply voltage. Maybe the regulator's shorted?
 
Jun 13, 2008 at 11:42 PM Post #370 of 1,083

luvdunhill

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Quote:

Originally Posted by tangent /img/forum/go_quote.gif
So yeah, 100K and 10K aren't great because they're not giving you as wide a Vsen range as you could get with the first choice on that list. But that calls into question why such a divider is letting 2.05 V through to Vsen. 2.05 V * 11 (the reduction factor) is 22.55 V, almost your full supply voltage. Maybe the regulator's shorted?


yes. So, total voltage across the pack is 0 volts. There are 2 cells that measure zero volts on the 200mV range of my meter. The rest measure:

.025
.059
.918
.032
.050
.031
.092
.047
.031
.043

Also, looks like my regulator is a LDO. I'm using LM1086CT-ADJ. All the resistor values measure fine and the diodes check out using the diode check. Perhaps U2/U4 is bad, or is definitely due to the 0v cells...

Thanks so much!
 
Jun 14, 2008 at 1:28 AM Post #372 of 1,083

Beefy

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Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif
There are 2 cells that measure zero volts on the 200mV range of my meter. The rest measure:


Wow, they are really dead. Regardless of how the rest of it works out, I would definitely beg/borrow/steal a proper battery conditioner in order to bring those cells back to life.

Or perhaps invest in some newer, higher capacity, low self discharge NiMH batteries like Sanyo Eneloop or Powerex Imedion. These batteries really are the best you can buy IMHO.
 
Jun 14, 2008 at 7:24 PM Post #373 of 1,083

luvdunhill

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problem was a faulty cell carrier. I have worked a fix for it and have raised R2 to 5.6 ohms, as I had one on hand. Seems that you are recommending to change R4 = 75K, R5 = 8.2K, which are the first items in the calculator output and will give Vsen: 1.06 - 1.83V. Off to the store to pick up some resistors then.

Do you recommend using another charger to bring up the cells, or just fix the voltage divider let the battery board do its thing? I'm getting less than 1v across the entire pack.
 
Jun 15, 2008 at 2:02 AM Post #374 of 1,083

Beefy

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Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif
Do you recommend using another charger to bring up the cells, or just fix the voltage divider let the battery board do its thing? I'm getting less than 1v across the entire pack.


As I said in my previous post, beg/borrow/steal a battery conditioner. These cells will need to be very carefully charged.

At an absolute minimum you should charge them at a very slow rate in a trickel charger, do a partial discharge, then pop them back in the PPA charger.
 
Jun 15, 2008 at 2:53 AM Post #375 of 1,083

luvdunhill

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Quote:

Originally Posted by Beefy /img/forum/go_quote.gif
As I said in my previous post, beg/borrow/steal a battery conditioner. These cells will need to be very carefully charged.

At an absolute minimum you should charge them at a very slow rate in a trickel charger, do a partial discharge, then pop them back in the PPA charger.



I had some credit at Amazon, so I just "bought" / "got for free" a LA CROSSE BC-900 Battery Charger as I need one around the house anyways
smily_headphones1.gif
Looks like this will recondition the cells for me...
 

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