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Old 28th February 2006, 12:40 PM   #11
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my original circuit DID use resistors instead of current sources, that's how i was told to set the current sources up, by someone on these forums, not only to help improve stability but also to increase the input impedance slightly because the tube stage was haveing problems running it...
So Should i Go back to useing the resistors instead of the CCS and do as sajti says and use bootstrapping?
atached is my ORIGINAL design... that is the one that i posted on here a month or so back..
Would i be better off just changeing some values on this one and adding the bootstrapping? I take it for the bootstrapping i use 2 x 5K resistors in series instead of the current source and sink and connect the midpoint of each of these two resistors to the output?
many thanks,
Owen
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Old 28th February 2006, 12:46 PM   #12
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which would become this ( i think)
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Old 28th February 2006, 12:50 PM   #13
sajti is offline sajti  Hungary
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Quote:
Originally posted by DoomPixie
which would become this ( i think)

Almost good. Just insert 2 pcs. of 47-100uF capacitors between the output, and the common points of the 5kohm resistors.

sajti
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Old 28th February 2006, 01:06 PM   #14
anatech is offline anatech  Canada
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Hi Guys,
I've build similar circuits. I told Owen to go to current sources for a couple reasons. Supply rejection and input impedance are the two main ones. Practically, this solved some problems for me.

DC offset.
No servo required. If you get you input impedance up some more (another stage) it's very easy to reference each drive point to ground through a pair of resistors. Mine are max 40mV from ground, although my circuit is very slightly different. If you really need to add a servo, just drive this common point for DC correction.

You have to remember that Owen is planning to drive this from a tube stage. A bipolar output stage is vastly more linear than a mosfet output stage.

Owen,
You need to use those signal transistors you ordered for the current sources. The BD parts will work, but the others will do a much better job.

-Chris
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Old 28th February 2006, 01:12 PM   #15
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hey chris,
I thought you might reply here, I think i am going to test the circuit with and without the current sources and see which works best for me, The power supply i have made for it has very little ripple on it (surpriseing really as it is very basic)

The real Question is.. Which will work best in this situation... I like the idea of useing resistors instead of the current sources because it cuts down on the active devices in the circuit..
Owen
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Old 28th February 2006, 01:18 PM   #16
anatech is offline anatech  Canada
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Hi Owen,
I don't have a problem with you testing both ways .. I encourage you to do this. You may find your 2 nd harmonics go way up at higher levels with the resistors.

Using resistors also makes the referencing to ground more difficult as the impedance is much lower. This alone may force you to use more active circuit elements in the form of a servo.

Remember that your tubes want to "see" a very high impedance load or the total distortion will be dominated by the tube stage.

-Chris
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Old 28th February 2006, 01:43 PM   #17
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Hi chris,
So useing the resistors and bootstrapping to the output will show a lower input impedance than useign the current sources?
This is all not really important at the moment anyway, i'm pretty sure i am going to have to add an extra stage liek we discussed before to in order to sucessfully drive it from the tube abyway.
I have started to put it together on a breadboard without the current sources, it will be easy to add them later anyway.
Many thanks,
Owwn
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Old 28th February 2006, 01:47 PM   #18
anatech is offline anatech  Canada
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Hi Owen,
Let's see where your trials go. Experimentation is more valuable than talking sometimes.

-Chris
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Old 28th February 2006, 02:32 PM   #19
ilimzn is offline ilimzn  Croatia
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Bootstrap 'current sources' are only as good as their capacitors and the amplifiers performing the bootstrap, in your case, your SS output stage. They 'multiply' the resistor between the cap joining node and the input by 1/(1-Av), Av being the voltage amplification of your SS output stage, which is close to 1. Therefore, the bootstrap resistor is multiplied many times. Even so, effective Av may reduce due to various reasons, or, which is the usual problem with complementary followers, become nonlinear (especially for low bias currents). Because of this it may be prudent to increase the bootstrap resistors as much as possible in your circuit, as your tube front end wants to see a high impedance, the higher the better.
To keep things the same as in the CCS version, you want the same current going through your bootstrap chain as would be through the CCS. The voltage span from the rail to the input is about 20V - (2 x Vbe) so let's say about 28.6V. Your CCS will provide about 1mA (depending on the LED type), so for 1mA across 28.6V (keeping things equal here), you need a resistance of 28.6k. Closest would be 27k. However, for a bootstrap arangement, you want to split that into two resistors. My advice would be 12k each. So, liek this:
+ Supply rail - 12k - 12k - upper input, connect output to joining point of the two 12k with bootstrap capacitor (say 100uF).
- Supply rail, 10k - 4.7k trimmer - 12k - lower input. Connect bootstrap cap between joining point of 12k and trimmer, and output (same 100uF).
This gives you two bootstrap 'current sources' at ~1mA, and a means to adjust offset both into + and -.
With proper bias of the output stage, bootstrapping action will easily increase the input impedance (without bootstrapping it would be approx. 12k) 50-fold at worst, which should be just fine for your tube stage.
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Old 28th February 2006, 05:18 PM   #20
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ok, before i power up.. just want to check.. does the pot have to be on min resistance or max resistance for the lowast bias setting?
Cheers,
Owen
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