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Old 11th December 2004, 02:12 PM   #1
djdamix is offline djdamix  France
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Default improving my VAS

Hello,

I'm devopping an AB class amp, but I the slew rate seems too slow my sinwave at 20Khz is a bit distorded (in reality, in fact not so much in sumulation) any advice is welcome.

David
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Old 11th December 2004, 02:21 PM   #2
jaycee is offline jaycee  United Kingdom
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MJE340's for the input LTP ? I'd say that's the problem. Use some small signal transistors like BC546/MPSA42/etc
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Old 11th December 2004, 02:26 PM   #3
MikeB is offline MikeB  Germany
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Hi !

You should add a driver stage between vas and mosfets, they
have very big inputcapacity, resulting in high load to vas for higher
freqs. I recommend a pair of mje340/350, connected their emitters
with ~500ohm. This gives ~14ma current, so they will need small
heatsink. (Dissipipation ~1watt)

And you could make your vas bootstrapped, means making 2x 1.5k
out of r9, connecting the middle via cap (100uf) to output.

C10 might be too small, increase resistors in feedback/input, or
increase c10.

Mike
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Old 11th December 2004, 02:28 PM   #4
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The load for your VAS is just a resistor. You will achieve far better results by bootstrapping or using an active CCS.

Just wondering why you have chosen such low impedance for your feedback network. It will mean that you need >>0.25W resistors for these components.

Ditch R29 and work out R2 so that you balance the currents flowing in the LTP.

I have seen practical success with driving these MOSFETs directly from the VAS with only <10mA VAS standing current. However, It's not a good design. You should either buffer the VAS (making kind of darlingtons with the MOSFETs), or run the VAS at much higher current and use a MOSFET or buffer/darlington for the VAS.
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Old 11th December 2004, 02:32 PM   #5
quasi is offline quasi  Australia
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Default Hey djdamix;

Is C13 10 micro-farad? If so then this is way to big. In all of the amps that have this capacitor across the voltage multiplier (Q20 in your cct) the value is 0.1 micro farad.

The 10uf as it stands would place a huge load on your driver stage and cause the distortion you described.

Why are the gate resistors (100 ohms) shorted out and going nowhere?

Cheers
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Old 11th December 2004, 03:46 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi, why will changing c13 change the VAS loading? and why will this 10uF cause distortion?
I think the VAS load is r9//gate capacitance of output FETs.
r33 feeding the LED seems a bit small, 69mA flowing gives 5Watts!!! try something =>4k7. or maybe a 33v zener & cap as intermediate volts dropper?
regards Andrew T.
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Old 11th December 2004, 08:42 PM   #7
djdamix is offline djdamix  France
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jaycee=> I use MJE340 because they are rated for high votlage in fact that's the only reason.

MikeB =>I've increased C10 from 10pF to 100pF but it slows the amp too much

quasi =>I've remove the 10u and put a 0.1uF however it is still hard to have 20Khz clean sine wave. The 2nd output pair is not connected on the sim but is connected in reality ...

richie00boy =>I'll put an extra stage : IRF610 & IRF9610 and a 330 Ohm as on www.aussieaudio.com (AV800), I'll try that and tell you the results

Thank you all
David
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Old 11th December 2004, 11:54 PM   #8
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Quote:
Originally posted by quasi
Is C13 10 micro-farad? If so then this is way to big. In all of the amps that have this capacitor across the voltage multiplier (Q20 in your cct) the value is 0.1 micro farad.

The 10uf as it stands would place a huge load on your driver stage and cause the distortion you described.
????????dont think so????????

hallo djdamix

you may also try 47 ohm gate resistors too
and reconsider changing q21
by the way, how do you cope with thermal issues?
cheers
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Old 12th December 2004, 09:04 AM   #9
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Default Re: improving my VAS

Quote:
Originally posted by djdamix
Hello,

I'm devopping an AB class amp, but I the slew rate seems too slow my sinwave at 20Khz is a bit distorded (in reality, in fact not so much in sumulation) any advice is welcome.

David
My friendly advice here is that you look at other amp designs. It seems that you have drawn your schematic straight from your head.

Some inspiration can be found here.

If you check my 2 x 300 W mosfet amp, the QRV-01 (link above) is more or less an exact copy of it. Only the transistors are changed to smaller types.
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/Per-Anders (my first name) or P-A as my friends call me
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Old 12th December 2004, 12:23 PM   #10
Did it Himself
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C13 is not a load on the VAS at all. It merely reduces the impedance of the Vbe multiplier at higher frequencies. 100nF is too small. Depending on the resistor values forming the multiplier and the VAS standing current, 470nF to 10uF is fine.
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