John Curl's Blowtorch preamplifier - Page 101

uli · 27th April 2006, 07:12 AM

Of course I am, as I am always interested in other people´s input.

In fact you can change my circuit a little bit to avoid loop feedback.
There are different ways to design a single gainstage to a certain
amount of gain. One is to degenerate gain by using source
resistors in a diff stage. Another possibility would be to feed back
the drain node to the gate as I did. Both methods use negative
feedback. The "purist" approach would be not to pick up the
feedback from the joint of the source followers output nodes but
put in two equal resistors in series between the gates of the
followers and use the connection between them as feedback
node. This would be a non overall feedback circuit too AND
you are able to take advantage of X "feedback".

Uli

Justcallmedad · 27th April 2006, 07:47 AM

Hi Uli, it will be clearer with a schematic, your design has some similarities with the Blowtorch and it could be interesting to discuss them, as well as the differences.

EUVL · 27th April 2006, 07:48 AM

> Patrick, maybe could you post it here and we could discuss it, eventually with Uli, of course if he is interested.

I have been reluctant to do so, as a respect to John, as the design discussed here is two stage folded cascode, which is different. And I find the latter more interesting as a circuit topology. The circuit I built is essentially a Borbely EB116, with feedback removed.

> There are different ways to design a single gainstage to a certain amount of gain. One is to degenerate gain by using source
resistors in a diff stage.

Yes, I used source degeneration for both first and second stage, both with JFET cascodes. This is especially necessary for the first stage to give decent bandwidth as I am using 10k gate resistors. Total gain is 20, split as 5 x 4.

Because of source degeneration, I have to use extra CCS for each half of the first stage, as Grey has described in the other post. So that is why I have been quiet up till now. No new inventions. : )

Output (3rd) stage is the Curl complementary JFET follower using 5 pairs of 2SK170V/2SJ74V with 70mA total bias at Idss. I would love to try a MOSFET version as it would be easy to increase bias / voltage / bandwidth with MOSFETs. This is also why I was asking for a MOSFET version of the follower earlier (Post #779). But no one else came up with something different; so either I was on the spot, or totally silly.

Bandwidth is around 105kHz with the 10k input resistors. But as soon as you shunt (for attentation) across the two, you get 350kHz easy. (Proven on the scope).

I shall try to post a schematics and some pictures, and oscilloscope plots later.

Patrick

uli · 27th April 2006, 08:25 AM

I would define a folded cascode as a single gain stage

The Borbely EB116 is rather a classic 3 stage design with non
diff input stage, Vas stage and output follower.

To avoid the 2nd stage I used a folded cascode (see Pass A75)
to be able to connect a follower directly.
Of course the follower can be left out. The drawback would be
that the gain of this circuit depends heavily on output load.

With BL devices the needed outputload (= drain resistor) would
be around 1k, assuming the degeneration resistors are small
enough to couple the complementary stages together. This leads
to an inapropriate maximum output swing with a given bias of
about 5mA.

Uli

Upupa Epops · 27th April 2006, 08:46 AM

For what is this " exercise quadrat ", guys ? " Deadline judge " is human listening, so what are results ? What's are differences between particular solutions ?

No " hifi newspeak ", please....

uli · 27th April 2006, 08:53 AM

Quote:

Originally posted by Upupa Epops
For what is this " exercise quadrat ", guys ? " Deadline judge " is human listening, so what are results ? What's are differences between particular solutions ? No " hifi newspeak ", please....

Thanks for the "correction"!

Long live free speech!

Uli

Upupa Epops · 27th April 2006, 09:00 AM

You don't have another answer, Uli ?

EUVL · 27th April 2006, 09:06 AM

As promised the schematics.
I have left out all values and types, not because I have things to hide, but I think it will make the discussion more interesting.

I am sure it can also be modified to use folded cascode.

Patrick

uli · 27th April 2006, 09:07 AM

As nobody knows the Blowtorch Design exactly (except of course
John Curl himself

) I don´t know how close anybody in
this thread is to the original. This is not the point IMHO, guessing
solutions of famous designers is kind of sports

Uli

EUVL · 27th April 2006, 09:08 AM

Response to 10kHz square wave, no attenuation.

27th April 2006, 07:12 AM	#1001
uli diyAudio Member Join Date: Dec 2002 Location: Vienna, Austria	Of course I am, as I am always interested in other people´s input. In fact you can change my circuit a little bit to avoid loop feedback. There are different ways to design a single gainstage to a certain amount of gain. One is to degenerate gain by using source resistors in a diff stage. Another possibility would be to feed back the drain node to the gate as I did. Both methods use negative feedback. The "purist" approach would be not to pick up the feedback from the joint of the source followers output nodes but put in two equal resistors in series between the gates of the followers and use the connection between them as feedback node. This would be a non overall feedback circuit too AND you are able to take advantage of X "feedback". Uli __________________ 'Jazz is not dead, it just smells funny' F.Zappa

27th April 2006, 07:47 AM	#1002
Justcallmedad diyAudio Member Join Date: Jun 2004 Location: Paris	Hi Uli, it will be clearer with a schematic, your design has some similarities with the Blowtorch and it could be interesting to discuss them, as well as the differences. __________________ Richard Perez

27th April 2006, 08:25 AM	#1004
uli diyAudio Member Join Date: Dec 2002 Location: Vienna, Austria	definition I would define a folded cascode as a single gain stage The Borbely EB116 is rather a classic 3 stage design with non diff input stage, Vas stage and output follower. To avoid the 2nd stage I used a folded cascode (see Pass A75) to be able to connect a follower directly. Of course the follower can be left out. The drawback would be that the gain of this circuit depends heavily on output load. With BL devices the needed outputload (= drain resistor) would be around 1k, assuming the degeneration resistors are small enough to couple the complementary stages together. This leads to an inapropriate maximum output swing with a given bias of about 5mA. Uli __________________ 'Jazz is not dead, it just smells funny' F.Zappa

27th April 2006, 09:07 AM	#1009
uli diyAudio Member Join Date: Dec 2002 Location: Vienna, Austria	As nobody knows the Blowtorch Design exactly (except of course John Curl himself ) I don´t know how close anybody in this thread is to the original. This is not the point IMHO, guessing solutions of famous designers is kind of sports Uli __________________ 'Jazz is not dead, it just smells funny' F.Zappa

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27th April 2006, 07:48 AM	#1003
EUVL diyAudio Member Join Date: Oct 2003	> Patrick, maybe could you post it here and we could discuss it, eventually with Uli, of course if he is interested. I have been reluctant to do so, as a respect to John, as the design discussed here is two stage folded cascode, which is different. And I find the latter more interesting as a circuit topology. The circuit I built is essentially a Borbely EB116, with feedback removed. > There are different ways to design a single gainstage to a certain amount of gain. One is to degenerate gain by using source resistors in a diff stage. Yes, I used source degeneration for both first and second stage, both with JFET cascodes. This is especially necessary for the first stage to give decent bandwidth as I am using 10k gate resistors. Total gain is 20, split as 5 x 4. Because of source degeneration, I have to use extra CCS for each half of the first stage, as Grey has described in the other post. So that is why I have been quiet up till now. No new inventions. : ) Output (3rd) stage is the Curl complementary JFET follower using 5 pairs of 2SK170V/2SJ74V with 70mA total bias at Idss. I would love to try a MOSFET version as it would be easy to increase bias / voltage / bandwidth with MOSFETs. This is also why I was asking for a MOSFET version of the follower earlier (Post #779). But no one else came up with something different; so either I was on the spot, or totally silly. Bandwidth is around 105kHz with the 10k input resistors. But as soon as you shunt (for attentation) across the two, you get 350kHz easy. (Proven on the scope). I shall try to post a schematics and some pictures, and oscilloscope plots later. Patrick

27th April 2006, 08:46 AM	#1005
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	For what is this " exercise quadrat ", guys ? " Deadline judge " is human listening, so what are results ? What's are differences between particular solutions ? No " hifi newspeak ", please....

27th April 2006, 09:00 AM	#1007
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	You don't have another answer, Uli ?

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