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Old 25th September 2001, 07:13 PM   #11
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Grey/Amplitone, Thanks for the info.....

cp

[Edited by cp642 on 09-25-2001 at 02:19 PM]
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Old 28th September 2001, 02:32 AM   #12
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Hey tortello,

How did you adjust the current gain to the right value ? What value pot did you use for R19 ?

Thanks for your help !
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Old 28th September 2001, 12:13 PM   #13
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Hello Amplitone,
I use a 39Kohm resistor, in series with a 50K trimmer.
The power Mosfet source resistor are 0.47 Ohm
In this way I can set the bias current between 1.3A and 3A.
You can reduce the "trimmer" range, if you need only a precision regulation: I prefer a wide range because one of my speaker system, Rogers LS3/5A (ante '86, 16ohm) is a very light load: I prefer this speaker in summer! :-)
However, with a narrowes range, you can compensate the mosfet Vgs dispersion: you need only to match them three by three.

Tortello
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Old 28th September 2001, 09:36 PM   #14
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Thanks for the info, tortello.

I am considering a wide range of bias as well (Dynaudio and ProAc mini monitors). What is the math behind the values you chose ? How would I do the calculations for more bias current ? Is the original schematic value of 221K only for 1ohm bias resistors ?

I know 0.47ohms for the bias resistors increases the current through the output mosfets, but how does R19 change the current gain ? Is it a voltage divider into the input of Q6-8 ?

Thanks !
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Old 29th September 2001, 02:21 PM   #15
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Hi Amplitone,
in this circuit, the bias current is set both by the source resistor, both by the "pull-up" resistor, R19.
The source resistor senses the drain (source) current, R19 give a fixed contribution, to avoid an unreasonable voltage drop caused by a source resistance high value, at least I think.
So they are by far contingent.
In the original Aleph 5, the voltage drop trough the source resistor supplies almost all of the required voltage, so the high value for R19.
The net you should consider, for an analytical procedure, is made up by R17-R18, Q5, R19,R20 and R40. I prefer simulations, but for a precision tuning, you should measure the real amplifier.
In my A5 the current may be set between 1.3 and 3 ampere, but usually I prefer 2.5A, to keep the junction temperature below 95C.
If you dicide to increase the bias current with respect to the original design, remember to scale the "negative clipper" also, changing the R15-R16 ratio.
Tortello
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Old 10th November 2001, 03:55 PM   #16
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Default How to match ?

tortello

How to matching network for current source (R17-R18, Q5, R19,R20 and R40) with "negative clipper" (R15-R16) ?
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Old 12th November 2001, 09:37 AM   #17
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Hello Forte,
In the Aleph 5 design, R15, R16 and Q4 form a sort of clipper to protect the active side's hexfets: R15 and R16 fix the maximum value of the current in each hexfet.
I think that this choice is not critical: the current limiter should not operate in "normal" conditions, and the high current capability of the power Hexfet (15A) assures a wide range of values.
In my Aleph 5 -hence with 3 Hexfets per side- biased with 3A -1A per Hexfet- I found a good mix with 220 Ohm for R15 and 150 Ohm for R16. The source resistor is 0.47Ohm.
The clipper operates at about 3.14A per Hexfet, 9.5A for the whole stack.
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Old 14th November 2001, 04:48 PM   #18
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Default 3A Bias for Aleph 5 ?

Did you mean you use 3A bias for Aleph5, not original 2A?
how much voltage per rail are you using?
I think you are making more then 60W from Aleph 5 circuit.
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Old 14th November 2001, 05:57 PM   #19
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My Aleph(s) runs with 3A to achieve a better performance into low speaker impedance.
I don't need high power output into 8 ohms, so the "B+" and "B-" rails are about 34V: this allows to obtain slightly more than 65Wrms.
But with more "heavy" loads, it works better than the original setup.
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Old 14th November 2001, 09:47 PM   #20
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If you've got the heat sinking and the power
supply, 3 amps is definitely better. The
Aleph 5 was limited by the 300 watt dissipation
of the package.
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