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Old 10th June 2011, 12:13 AM   #901
AKSA is offline AKSA  Australia
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Stanley,

The quality of the lag comp cap is important. Silver mica is recommended; ceramic is often used, but there is a sonic difference for some reason.

Hugh
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Old 10th June 2011, 12:19 AM   #902
wahab is offline wahab  Algeria
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Originally Posted by sng001 View Post
Hi Wahab,

I will try your mod. Can I use 2SK117GR for Q4?

I only have a few K170BL but I have got a bunch of cheaper Toshiba 2SK117GR with Idss ~ 6mA.

Cheers, Stanley
Hi , Stanley

Not sure but it should do it .
Will do a sim, though..

cheers,
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Old 10th June 2011, 01:52 AM   #903
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I like the idea of running a .wav file through the simulator, but I doubt it will be accurate since there is not real-time thermal simulation, unless manually implemented. And even then it can only be crude unless you create a discrete version of the component model with added support.

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Old 10th June 2011, 03:26 AM   #904
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Originally Posted by sng001 View Post
Thanks Hugh,

I will try that with a 15pF ceramic cap as I don't think I have the 15pF silver mica.

Cheers, Stanley
Hello

If you don't have silver mica, use a NPO type ceramic cap, they are much better than the standard ceramic cap.

Bye

Gaetan
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Old 10th June 2011, 05:36 AM   #905
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Originally Posted by lineup View Post
With +/-50 Volt supply.
Two pair HEXFET IRFP240/9240 and cascoded JFETs.
IRF610/9610 for VAS.

100 Watt into 8 Ohm with low distortion.
Hello LineUp!



I want to follow this scheme because I now have all the parts fit this scheme. JFETs 2SK170BL except that I do not have, but I have 2SK30A JFETs can I change K30A K170 with it?
I like this circuit! because it matches the speakers have low sensitivity and 4 ohm impedance only for me.
if I need to change any component or value in this circuit?
and then the circuit has good activity and stability after change it JFETs ?


Thank you much!
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Old 10th June 2011, 06:11 AM   #906
mikelm is offline mikelm  England
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Quote:
Originally Posted by wahab View Post

The only mods i made was to increase the values of the feedback
resistors, as their original values were too low for some obvious reasons,
that allow to get rid of an eventual ugly lead comp. cap and replacing it by
a well more stability enhancing lag cap compensation.
I wonder why you call "lead comp" ugly for me it is the most desirable method becuase it brings stability without sacrificing OLG and is therefore beautiful

By choosing the lag cap you are loosing mid & treble OLG which will diminish mid & treble resolution.

For me it is much more satisfactory ( for the sound ) to leave OLG intact and and add compensation direct from the VAS to the i/p Jfet source.

As Hugh states sometimes a bit of both are required but, if designed carefully they perform different actions that operate at different frequencies.

When you add the CCS as the i/p FET drain load on this design you do indeed a miller cap but I would advice keeping it as small as possible and using the lead compensation ( cap from VAS to i/p Jfet source ) to make the amp stable with awkward loads.

With the design swordfishy has settled on, a miller cap is not required and that for me is the one of the best features of it and infact with simple loads no compensation is required at all which is amazing but the amp will be a bit more stable and probably sound a bit better with some ( lead ) compensation added.

Last edited by mikelm; 10th June 2011 at 06:15 AM.
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Old 10th June 2011, 06:25 AM   #907
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I am assuming that lag compensation is having the miller cap from the VAS output directly to the negative side of the comparator (in this case the source of the Jfet(s)).

In this case lag compensation will actually increase OLG, but decrease amplifier gain at the same frequency. Lag compensation is tempting always because it does not cause any extra load on the preceding stages, but most often it doesn't work because there are positive feedback loops in other areas of the amplifier. Lead compensation (miller cap right at the VAS) carpet-bombs most of the issues that can occur there, unless you have fancy cascodes or something exotic with major parasitic feedback paths. Often an RC snubber in the right place can work like magic.

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Old 10th June 2011, 06:36 AM   #908
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In the schematics here:

JFET input, MOSFET VAS, LATERAL output = Perfect!!

Some lag compensation might be appropriate to make up for the extra current the Jfet must provide for the miller cap.

Also, you could always experiment with TMC, which would have less of a performance drop than lead comp I think.

- keantoken
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Old 10th June 2011, 06:44 AM   #909
mikelm is offline mikelm  England
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I think we're getting our lead & lag terminology muddled up.

wahab seems to be calling a miller cap "lag compensation" and a cap from VAS to Jfet source "lead compensation"

To avoid confusion, can we agree on this terminology or can some enlightened soul tell us a more standard terminology ?

mike
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Old 10th June 2011, 06:52 AM   #910
AKSA is offline AKSA  Australia
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Miller cap = Lag Compensation, local feedback from output of VAS to input of VAS (drain/gate, or collector/base), the conventional way of compensating an amp so that loop gain is just below unity at the upper pole frequency when negative feedback turns positive.

Phase lead = JLH technique (and others) of straddling the VAS output and the fb inverting node with a small cap. This is interstage rather than local compensation and it improves the amplifier's tolerance of difficult loads.


Both techniques, and TMC (OS, you there?) have profound influence on amp stability, and curiously, on the sonic presentation of the amplifier.

Hugh
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