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How to choose VAS current
How to choose VAS current
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Old 4th December 2018, 10:17 PM   #11
Itsmee is online now Itsmee  England
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How to choose VAS current
I misunderstood, just looked at the Whammy
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Old 4th December 2018, 10:20 PM   #12
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
I think the feedback JFET's job is more of an AC servo. A DC servo just corrects for DC offset.
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Old 4th December 2018, 10:30 PM   #13
Itsmee is online now Itsmee  England
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How to choose VAS current
Depends on your point of view, if connect your scope ground lead to -V...
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Old 4th December 2018, 11:36 PM   #14
PRR is offline PRR  United States
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Quote:
Originally Posted by JeffYoung View Post
I'm looking for general guidelines.
Answered.

Quote:
Originally Posted by JeffYoung View Post
But let's use this for an example
First specific: this drives MOSFETs. The gate drive current is about zero at DC and quite high at higher frequencies due to gate capacitance. So you need to quantify that _and_ decide how high freq you need to go.

I don't know who drew this or if I am stepping on toes, but I got my boots on....

"LPT trim" seems unnecessary. A current-mirror will force input current balance. That may cause a voltage offset (MOSFETs are so inconsistent) but this can be trimmed while maintaining the more-important current balance. Also the R110 approach assures asymmetric limiting to M9 while a mirror has hope of symmetry (as D.Self(?) shows, not always that simple).

Since input is small-signal and the "VAS" is "medium signal" it seems odd to find 33mA in the input and 5mA in the VAS. If the output was some teeny thing... no, there's 130pFd Gate-Drain, so 1,000pFd loading at least. Counting on my belly-button, isn't all the drive current used-up for full power at 16KHz? While we may never see that in speech/music, we like a lot more headroom, if we can afford it, and I think this can.

Not even going to comment on having a potentially good gain-amp and then omitting all extra NFB around a bunch of ugly output MOSFETs, because I suppose that is a matter of taste.
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Old 4th December 2018, 11:43 PM   #15
Zen Mod is offline Zen Mod  Serbia
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How to choose VAS current
good book for things like this is Valve Amplifiers by Morgan Jones

glass or sand , things are simillar
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Old 5th December 2018, 02:01 AM   #16
Mark Johnson is offline Mark Johnson  United States
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How to choose VAS current
For conventional designs like Blameless et al., I tend to follow John Curl's recommendation. Run em hard, run em hot.

Which means, for me, to select a transistor in a package that can be bolted to a heatsink. Bob Cordell's favorite VAS transistor pair, the Sanyo 2SC3503/2SA1381, meets this requirement quite nicely. They come in a TO-126 package.

I like to calculate the worst-possible-case maximum supply voltage (AC mains +8% above nominal, rectifier diode Vfwd at minimum, etc) and then arrange the VAS transistor(s) DC bias currents so they operate at 60% of their maximum permitted power dissipation, even with max-possible supply voltage. Run em hard, run em hot.

When I cascode the VAS, as Bob Cordell often does, I make sure to run HEFTY bias current in the bias network of the cascode device's base node. When called upon to perform a full rail-to-rail slew, keep that sucker pegged at the correct voltage and don't let it contribute even +2 nanoseconds of rise time degradation.

Of course you'll want to include circuitry (diodes and voltage sources) that let the amplifier clip gracefully and that don't force giant overcurrents during clipping or slewing. Bob Cordell's 50W MOSFET power amp paper in JAES 1984, has a truly excellent discussion of this and some circuits that are easily copied/adapted to the particular needs of your amplifier. link
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Old 5th December 2018, 02:28 AM   #17
mlloyd1 is offline mlloyd1  United States
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How to choose VAS current
zen Mod,

That's a really good quote.
I like it!


mlloyd1

Quote:
Originally Posted by Zen Mod View Post
... always used rule of thumb in meantime , later resorted to thumb of rule ...... just crank it
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Old 5th December 2018, 11:55 AM   #18
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
Excellent stuff guys. I'm learning lots.

Quote:
Originally Posted by PRR View Post
I don't know who drew this or if I am stepping on toes, but I got my boots on....
I drew that. But not worries, squashed toes is how I learn.

Quote:
"LPT trim" seems unnecessary. A current-mirror will force input current balance. That may cause a voltage offset (MOSFETs are so inconsistent) but this can be trimmed while maintaining the more-important current balance. Also the R110 approach assures asymmetric limiting to M9 while a mirror has hope of symmetry (as D.Self(?) shows, not always that simple).
What I have drawn is just a cascode, isn't it? I did try a current mirror (thinking it would perform the voltage protection of the cascode as well), but it didn't. It appears I would need both if I went with a current mirror.

Quote:
Since input is small-signal and the "VAS" is "medium signal" it seems odd to find 33mA in the input and 5mA in the VAS.
He he... it did to me too. Which was the impetus for this thread.

I currently have the LPT at 12mA and the VAS at 28mA, but I'm still playing around trying to figure out if the LPT current affects the H2/H3 ratio.

The driver will be either a SemiSouth JFET or an IRF610 (both are TO-247s). So I can radically increase the VAS current if needed. Based on your, @ZM's and @Mark's feedback, it sounds like at least some increase is in order.

Quote:
Not even going to comment on having a potentially good gain-amp and then omitting all extra NFB around a bunch of ugly output MOSFETs, because I suppose that is a matter of taste.
The original idea was to have the input stage, VAS and OS bias circuits in one box and those ugly MOSFETs in another. So a lack of global feedback simplified things (and might have been an issue over a long wire?). But now I'm thinking driving the MOSFETs over a long wire might also be an issue, so maybe everything belongs in the same case after all. (In which case I'd have a jumper for feedback as I have no idea yet what my taste is.)

Cheers,
Jeff.

PS: the "example" was a bit old; here's my current thinking:

How to choose VAS current-beefcake12-jpeg
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Old 5th December 2018, 08:18 PM   #19
JeffYoung is offline JeffYoung  Ireland
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How to choose VAS current
The LTP current didn't appear to have any affect on the H2/H3 ratio. So I put the LTP at 18mA.

I ran Mark's suggestion and got 60mA for the VAS, give or take. (The limiting factor in this circuit is the CCS BJT rather than the driver JFET.)

Sound reasonable?
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Old 5th December 2018, 08:27 PM   #20
Zen Mod is offline Zen Mod  Serbia
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How to choose VAS current
didn't tried SJEP with so sissy current , but something is telling me that you're wasting it there
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