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Old 31st August 2012, 08:50 PM   #1351
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Quote:
Originally Posted by Zen Mod View Post
Click the image to open in full size.
ZM and buzzforb: Please clarify.
  • Is the primary winding of transformer driven in a differential manner by the dangling connections off the source pins?
  • Is feedback going directly to the junction VF2 in the above schematic. If yes, won't it be positive feedback which may oscillate the power amp?
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Old 31st August 2012, 08:58 PM   #1352
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by Nelson Pass View Post
.....

Can you say Beast With a Thousand Jfets?





Quote:
Originally Posted by Antoinel View Post
ZM and buzzforb: Please clarify.
  • Is the primary winding of transformer driven in a differential manner by the dangling connections off the source pins?
  • Is feedback going directly to the junction VF2 in the above schematic. If yes, won't it be positive feedback which may oscillate the power amp?
naah

that's buffer as you already know it , with output from VF2 node

dangling sources are one connection for LED cascode reference ; second one is ground

follow link where buzz quote Mighty Moi - you'll find all info you need
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Old 31st August 2012, 08:58 PM   #1353
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Quote:
Originally Posted by Nelson Pass View Post
This will oscillate as presented - try putting some Gate stoppers on the Jfets.

Of course with a small amount of Source resistance on each, say 10 ohms,
and a lower supply voltage you can dump the cascoding altogether.



Can you say Beast With a Thousand Jfets?
Just used the schematic posted and have added the gate stoppers on PCB. As far as the cascode and Rs, i might as well add it to the pcb. Jumpers, jumpers everywhere. I probably should take Nelson's hint and drop the cascode...but what does he know

Edit:
I assumed the cascode would help with considerable Ciss of mulitple input pairs, even operated as source follower. Is this not a concern in terns of bandwidth?

EDIT:EDIT:
If nothing else, it will serve as propoer storage place for future FE parts for other amps.
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Last edited by buzzforb; 31st August 2012 at 09:01 PM.
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Old 31st August 2012, 09:10 PM   #1354
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Quote:
Originally Posted by Zen Mod View Post






naah

that's buffer as you already know it , with output from VF2 node

dangling sources are one connection for LED cascode reference ; second one is ground

follow link where buzz quote Mighty Moi - you'll find all info you need
I panicked; fearing that buzzforb's amp may buzz and burn up!
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Old 1st September 2012, 12:34 AM   #1355
lhquam is offline lhquam  United States
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Default Teaser-6 "tweakable" parameters

The "F6 Concept Schematic" and the LHQUAM Teaser-6 schematic create an amplifier containing some parameters that can be "tweaked". This note discusses those parameters and their implications for the resulting sound of the amplifier.

To limit the parameter set, I will start by assuming a single pair of output FETs and a target output of 25 watts into a nominal 8 ohm speaker. This implies roughly 24 volt rails and 1.3 amp idle current for class A operation. However, the following discussion doesn't really depend on the top level choise of wattage and number of FET pairs.

The remaining parameters that might affect the sonic signature of the amplifier are:

Feedback network resistors R2, R1, where R1 is from output to feedback node and R2 is from feedback node to ground.

Output FET source degeneration resistors Rs.

There are not really any other "tweakable parameters", unless some other components are added to the schematic.

See the image below for the effects cause by changes to these parameters. (Can someone tell me how to get a table into the body of the a post without using attachments? White space formatting gets removed, screwing up the columns).

This amplifier is very different from the F5 because of the transformer. The transformer adds a phase delay of about 2.5us. Thus, global feedback will likely have a different sonic signature vs. local feedback, or no feedback.

EXPERIMENT WITH A VARIETY OF PARAMETER VALUES:

The source degeneration resistors Rs can be different for the upper and lower FETs. Small differences might be used to null the 2nd harmonic at a particular frequency. Large differences can be used to deliberately increase the 2nd harmonics.

As you can see, there are a relatively small number of adjustable parameters that can change the sonic signature of the F6. I suggest setting R1=50R and using a 2 watt, 10R or 20R potentiometer for R2 to "tweak" the closed-loop gain. My Teaser-6 PCB has provisions for a .1" spacing terminal block for the source resistors, enabling easy experimentation with different values of the source resistors.
Attached Images
File Type: jpg F6-parameter-table.jpg (20.2 KB, 363 views)

Last edited by lhquam; 1st September 2012 at 12:37 AM.
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Old 1st September 2012, 01:03 AM   #1356
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and you can modulate them from bottom of Rs ....... or from top of Rs ....... or one from bottom and other from top ....... or the other way around .........

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Old 1st September 2012, 01:30 AM   #1357
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Zen Mod View Post
and you can modulate them from bottom of Rs ....... or from top of Rs ....... or one from bottom and other from top ....... or the other way around .........

Interesting. Do you have any subjective experience with those combinations?
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Old 1st September 2012, 01:32 AM   #1358
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nope ...... just came to my mind

but I intent to ....
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Old 1st September 2012, 10:32 AM   #1359
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Quote:
Originally Posted by lhquam View Post
The "F6 Concept Schematic" and the LHQUAM Teaser-6 schematic create an amplifier containing some parameters that can be "tweaked". This note discusses those parameters and their implications for the resulting sound of the amplifier.

The remaining parameters that might affect the sonic signature of the amplifier are:

This amplifier is very different from the F5 because of the transformer. The transformer adds a phase delay of about 2.5us. Thus, global feedback will likely have a different sonic signature vs. local feedback, or no feedback.

.
lhquam: I am glad you are connecting the subjective "sonic signature" of the amp to its objective and measureable parameters. So, your reference loudspeaker will sing with a different voice which is discernible to you with each substantial [and maybe not] tweak. Clearly one "sweet" voice [to your hearing] will eventually stand out and be adopted.

Please reconfirm in your table of parameters the following. Will increasing Rs increase or decrease damping factor? Thank you.
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Old 1st September 2012, 10:47 AM   #1360
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Increasing Rs usually lowers DF buy reducing gain. Dont know about here.
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