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Old 10th December 2006, 07:37 PM   #1
slepro is offline slepro  Canada
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Default Here is my last creation - DOCAFET class A amp!

Hi everybody,

I would like to introduce you my last creation, the DOCAFET amp (for doubly cascoded JFET). Please don't laugh at his name, I'm not having a huge artistic mind!

The sound seems to be very good, I still did not build it into a casing, but I'm using it as a prototype for now.

I would like to have your opinion on the design because for both channels I built, they are having thermal noise that can be heard through the speakers. By looking at the schematics, does anyone of you have an idea on what may cause this noise?

The amp never get into oscillation, so I can say it is stable. The quiescent current is also very stable. Please note that the 50V dc supply comes from a regulated PSU, and the 35V dc supply is only filtered by huge capacitors.

I thought that the source of the noise may be the zener diodes of the cascoded differential amp, but even with capacitors across them, the noise is still there. Maybe the source is the JFET's? I don't know...

I'm open to all your comments, so don't hesitate to post your opinions!

Thanks all

Sébastien
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File Type: pdf docafet schema.pdf (71.0 KB, 1018 views)
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Old 10th December 2006, 08:10 PM   #2
lineup is offline lineup  Sweden
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Default noise HF filter RC-filter RC input cap

hi
nice work! Potential very interesting schematic!

NOISE.
What could produce noise so big we can hear it
is around input or the input stage itself.



Correctly setup, most input JFET would not create such noise.
Not so we could hear it.


My guess is HF noise. Very high above the audio.
This noise is present everywhere
and any UN-SHIELDED wire will act as an antenna for it.


To possibly cure this, add one 220pF - 470pF - 1nF, parallel across input resistor R13(47k)
and to make this RC filter for HF
you add this Resistor, 470 - 1k Ohm
in series with input Capacitor C5(10uF).
This 470 Ohm resistor could be After or Before C5.

Most usually resistor is placed between C5 and R13,
from end of C5 to 'top of' R13 = in series with input cap.

I would try 470 Ohm + 470pF first.
If needed, I would use 1 kOhm + 1nF, for high frequency roll-off.


If you have some potentiometer or other resistance
before the input of power amp you may skip the resistor.
Then just adding one 220pF or 470pF film cap in parallel Across R13
could be best solution.



This RC filter is a good idea, even if in your amp
the noise have another cause.
But try this first.
Then we may find something else, that possibly adds noise in your amp.


Good Luck!
lineup
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Old 10th December 2006, 11:25 PM   #3
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Seems like a grounding issue for me.
C4 value is drwing mistake, isn't it?
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Old 11th December 2006, 12:44 AM   #4
slepro is offline slepro  Canada
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lineup : Thanks for your help, I'll try that as soon as I can. I am right to say that it would be better to put a capacitor of the same value across R14 as the one across R13?


darkfenriz: I don't believe that is could be a ground issue, since the pcb is having ground plane all around on both sides. For C4, you are absolutely right, the value should be 100pF. The prototypes are mounted with this value, the 4.7nF is an error on the schematic only.

Does anyone think that it may be the zener D2 and D3 or the transistors type for Q7, Q8, Q17 and Q18?
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Old 11th December 2006, 12:44 AM   #5
Leolabs is offline Leolabs  Malaysia
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Quote:
Seems like a grounding issue for me.
It's ok for a schematic session.Just need to pay attention during layout session.

Quote:
C4 value is drwing mistake, isn't it?
Maybe not.But it's kind unusual value.
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Old 11th December 2006, 01:05 AM   #6
lineup is offline lineup  Sweden
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Quote:
Originally posted by slepro
lineup :
I am right to say that it would be better
to put a capacitor of the same value across R14 as the one across R13?
No, slepro
you are VERY wrong, if say so

Because would make a lowpass RxC, RC-filter
with R11 x Cap

you remember I told you could put a resistor, 470 Ohm in series with input cap.
R11, 560 kohm is a little bit higher, in fact >1000 times higher
so the lowpass roll off would be like 1000 times lower!!!!!!!!!!


-3dB roll off frequency would be:
( PI ~3.14 )
f = 1/ ( 2 x 3.14 x R x C)

and
using 560000 Ohm and 0.000000000470 Farad
we get f ~ 604 hertz
.... would mean your amplifier would have certain problems, for example with 1 kHz sounds or higher......

..... instead if using 470 Ohm and 470pF
we get f ~ 720484 hertz = 720 kHz
which can't effect audio at all = 20-20.000 Hz

---------------------------------------------

besides, the input transistor
is having contact with wires, signal cables, that come from outside the amplifier circuit
maybe even from outside amplifier chassis, via input jacks.

This is why you do not need such a cap at the other JFET input.
The input RC filter limits what enters your amplifier.

I guess you are only interested in Audio signals, in this case,
and you do not want to use your amplifier as a radio receiver
catching those antenna produced RF signals from signal input wires.

The simplest radio antenna is an un-shielded wire, of suitable length,
as most people probably know is working fairly good.



lineup
a former DX-ing radio amateur - now limited to the audio frequency band
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Old 11th December 2006, 03:46 AM   #7
fab is offline fab  Canada
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Default noise problem...

Hi Sébastien,

Nice schematics.

my guess on your noise problem if we reject the grounding scheme issue and you have followed lineup recommendations:

1) without having an appreciation of the level of noise you have, the 2N5458/5461 are not the best jfet for low noise application. You may try 2SK170/2SJ74 for comparison purpose. Ensure to have the appropriate IDss current for your circuit.

2) you can bypass all zener with about 100nF cap.

3) R11 560K for feedback resistor is an unsual high value (hiher resistance value gives higher thermal noise. Use 47K like R13 and it will have a better DC offset optimization. Of course ensure to reduce R14 accordingly.

Notes:
I can not see the Zobel at your output.
C1 and C10 are unusual for compensation cap location.
Have you simulated your schematics to have an idea of the stability of your amp (gain and phase margin) in resistive and capacitive loads?

Good luck
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Old 12th December 2006, 01:59 AM   #8
slepro is offline slepro  Canada
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Default Re: noise problem...

Quote:
Originally posted by lineup


No, slepro
you are VERY wrong, if say so

Because would make a lowpass RxC, RC-filter
with R11 x Cap

you remember I told you could put a resistor, 470 Ohm in series with input cap.
R11, 560 kohm is a little bit higher, in fact >1000 times higher
so the lowpass roll off would be like 1000 times lower!!!!!!!!!!


-3dB roll off frequency would be:
( PI ~3.14 )
f = 1/ ( 2 x 3.14 x R x C)

and
using 560000 Ohm and 0.000000000470 Farad
we get f ~ 604 hertz
.... would mean your amplifier would have certain problems, for example with 1 kHz sounds or higher......

..... instead if using 470 Ohm and 470pF
we get f ~ 720484 hertz = 720 kHz
which can't effect audio at all = 20-20.000 Hz

---------------------------------------------

besides, the input transistor
is having contact with wires, signal cables, that come from outside the amplifier circuit
maybe even from outside amplifier chassis, via input jacks.

This is why you do not need such a cap at the other JFET input.
The input RC filter limits what enters your amplifier.

I guess you are only interested in Audio signals, in this case,
and you do not want to use your amplifier as a radio receiver
catching those antenna produced RF signals from signal input wires.

The simplest radio antenna is an un-shielded wire, of suitable length,
as most people probably know is working fairly good.



lineup
a former DX-ing radio amateur - now limited to the audio frequency band
It seems like you misunderstood me I'm not talking about putting a cap in // with R11 (feedback resistor) but in // with R14 which is the gate grounding resistor, as R13 for Q14 and Q9. I though that if some noise is generated across R13, there's should have some across R14, no?

I didn't had time today to test all that, but I'm impatient to see the results.


Quote:
Originally posted by fab
Hi Sébastien,

Nice schematics.

my guess on your noise problem if we reject the grounding scheme issue and you have followed lineup recommendations:

1) without having an appreciation of the level of noise you have, the 2N5458/5461 are not the best jfet for low noise application. You may try 2SK170/2SJ74 for comparison purpose. Ensure to have the appropriate IDss current for your circuit.

2) you can bypass all zener with about 100nF cap.

3) R11 560K for feedback resistor is an unsual high value (hiher resistance value gives higher thermal noise. Use 47K like R13 and it will have a better DC offset optimization. Of course ensure to reduce R14 accordingly.

Notes:
I can not see the Zobel at your output.
C1 and C10 are unusual for compensation cap location.
Have you simulated your schematics to have an idea of the stability of your amp (gain and phase margin) in resistive and capacitive loads?

Good luck

Thanks for your suggestion of JEFT's I will have a look at that!

The Zobel circuit may be added outside the pcb if necessary, but I would like to avoid it.

The reason I put C1 and C10 where they are, is to gain some output span of the voltage gain amplifier section. And I don't see any problem with these capacitors.

It is a good idea the scale down R11 a bit. But will it make a real difference since I'm driving JFET gates wich don't need any current to be driven (I know that at higher frequency's it not the same thing because of Cgs, but for audio frequency the effect is negligeable).

So, I will try (as soon as I can) to put a 330pF resistor across R13, I will add a RC network in series with C5, between C5 and R13, and I will scale down R11 to 47k and R14 to 3.9k.
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Old 12th December 2006, 02:23 AM   #9
lineup is offline lineup  Sweden
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no
I did not misunderstand you
"I am right to say that it would be better
to put a capacitor of the same value
across R14 as the one across R13?"


And the resulting RC filter
would be the value of R11 x Cap = 560k x 470pF (or 220pF)
even if Cap is across R14, attached to ground.

So my posted comment is still correct.
It would destroy the bandwidth of your amp.

You should wait with adding little cap in the feedback side
because you might not need it.
----------------


i agree with previous poster here,
Reduce the resistor divider
R11/R14, (560k/47k) setting the gain of amp =11.9+1 ~ Gain 12.9

for example using 100kohm for R11
and 6k8 for R14
would give Gain:
(100/6.8) + 1 = input x 15.7

-------------------------------------------------------------------
for a voltage supply of 35 V DC. ( 50V DC in input/VAS stage)
you may set the gain to 15-20, something

Gain=15.7, would make 1.0 Volt RMS input
have peak voltage swing output: +/- 22.2 Volt

Gain=20, would make 1.0V RMS input
have peak voltage swing output: +/- 28.3,
which still leaves a few volt margin before output transistor start clipping
35.0 - 28.3 = 6.7 Volt, across output transistors including emitter resistors 1R = 1 Ohm = R20,R23,R25

-----------------------------------------------

Start calculating, R11 and R14 and what gain you want.
But Keep R11 - the feedback resistor <= 100 kohm.
Most amplifiers do not have this resistor higher than 47 kohm.
And there are good reasons for this!


Regards
lineup
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Old 12th December 2006, 04:28 AM   #10
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I think, hat You shuld put small (~7 Ohm) resistors in bases of output transistors. The noises You hear may be results of HF oscillations.

I also think that C1 and C10 shuld be throw off. I din't done any calculations, but I think that open loop gain will be high enough without them. well, a bit subjectivism - I simply don't like caps in such way javascript:smilie('')

C4 seems to be too high in value. C4 and C8 shuld be equal (look at Leach amplifier, You can learn a lot from that, and i'ts very similar to Your). Again, I didn't done any calculations, but C4 is just to big for me javascript:smilie('')

I also agree with all above about input lowpass filter and grounding. You shuld carrefully check grounding topology;
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