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Old 30th June 2001, 11:30 PM   #11
mlloyd1 is offline mlloyd1  United States
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Things I did to the Lang:

Added Servo for no DC offset

Changed cascode to some funky design from a Malcolm Hawksford paper I was curious about (supposed to be much lower distortion than conventional cascode). I'll dig it up for a reference if you're interested.

Probably a few other things, too. That's what I remember. It's been a while.

I like my Borbely amp better. My speakers are 4 ohm (Kef 104/2). I spent A LOT of time and attention to the Borbely to make it sound as good as I could at the time: lots of output devices, good grounding, monstrous power supply, matched devices, etc.

Some day :-) I'll get around to tuning the Lang; there's a lot of potential there. I spent my time on the Borbely because I wanted to play with a JFET front end.
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Old 1st July 2001, 03:20 AM   #12
jam is offline jam  United States
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Michael,

I would love to have a copy of the cascode article if you have it.
It should not be too dificult to convert the Lang to fet input after which you could get rid of the input cap and possibly the servo.

Jam
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Old 1st July 2001, 06:30 AM   #13
ppl is offline ppl  United States
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I was thinking the same thing My DIY power amp uses Dual matched BJT's LS-312 & LS-352 and I had to use a Pot to inject a voltage at the invertng input to adjust the offset. Was thing of replacing the LS types with 2SK389 & 2SJ109 Dual Jfets. My amp is all casscoded similar to the Leach.
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Old 1st July 2001, 01:18 PM   #14
jam is offline jam  United States
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Default Fet input modifications

ppl,

I would first set the voltage on the base of the cascode transistors (of the differential pairs) to about 10volts. Replace the differential transistors with the jfets you mentioned. Fets like to see about 10 volts on their drains for lowest noise and distortion.
Reduce the value of the emitter resistors of the differential (if there are any there )because fets have lower transconductance than bipolars.
Hopefully the differentials are current sourced so you won't have to adjust the value of the drain resistors.
Fire the amp up with a variac and check for stability and offset. If everything is ok short out the large electrolytic in the shunt part of the feedback circuit.
You can then play with the values of the source resistors on the differentials or adjust the current sources for zero offset.
I have modified several Aragon amplifiers (which use a similar topology to the Lang and Leach amplifiers) replacing the diffefentials with jfets or mosfets with great results.

Removing the large feedbeck cap has a great impact on the sonics of the amplifier (for the better).
Another area to experiment would be to reduce the amount of feedback used, by reducing the open loop gain of the amp.

Jam

[Edited by jam on 07-01-2001 at 11:36 AM]
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Old 1st July 2001, 07:02 PM   #15
ppl is offline ppl  United States
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Jam: Thanks for your solid advice. My topology is similar not exackly like the Leach. The amp is already Direct Coupled thank god i hate Coup. caps. Some of the other input details are the voltage ref for the input Diff amp cascodes are set at 15 volts (Base of cascodes to Emmiter of Diff amp) A current source supplies the 15 volt Ref. The input stage is a complimentry version of what was in the Mark levonson #23 (I thought that was a cleen amp and felt comfortable with their aproach. My amp is i gess you could say made up of Ideas from manny Designs. Like Passes Cascoded output stage ect. i am also thinking of replacing the BC-107's and BC177's used as the active device on the Second voltage gain stages with Complimentry jfets also like maby 2N5458 & 2N5460? other recomendations would be helpfull. I think the BUT used on the Non active Half of the second stage cascode breing a High voltage Types can stay. One again thanks for you input.
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Old 1st July 2001, 07:58 PM   #16
jam is offline jam  United States
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ppl,

Nelson Pass does not use the cascoded output stage anymore because he says it reduces dynamics (I think this is mentioned somewhere in the A-75 article).
What are your experiences with a cascoded output stage?

Jam


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Old 10th July 2001, 01:53 AM   #17
ppl is offline ppl  United States
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The High frequency reproduction has more Air and delineation of detail in the Upper midrange and High End vs the same Circuit in a normal configuation
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Old 10th July 2001, 06:14 AM   #18
dorkus is offline dorkus  United States
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hmm interesting i wonder why cascoding would reduce dynamics? i always thought it would improve the linearity of any gain/driver stage, not sure why the dynamics would be affected... hmm time to hit the ol' analog textbooks again.

dorkus
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Old 10th July 2001, 06:26 AM   #19
jam is offline jam  United States
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Marc,

Apparently this only applies to the output stage, worth investigation.

Jam
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