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Old 1st December 2002, 09:54 PM   #1
LennyK is offline LennyK  Poland
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Question my new amp (comments?)

hi

I'm new here . It's my first post .
I want to build amp for my bass unit (bi amp with push-pull ultralinear EL34 for treble/mid). It will have 100-150W. I post schematic (sorry for quality). Any comments, new ideas? (don't laugh to loud ).
It look like amp form Elektor (autor Giesberts). I made only few changes (especially in the input stage).
"Pre" section will have regulated voltage.
Bias maybe about 200mA

I think about 4 things:
-2 amps/channel (XLR inputs, balanced) would be nice ?
-2, not 4 transistors/channel (especially when balanced)
-different type of output stage (next post)
-different type of driver stage (cascode with constant power, like input stage)

Lenny
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Old 1st December 2002, 09:56 PM   #2
LennyK is offline LennyK  Poland
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Default different output stage

I know that output voltage swing will lover with this wersion, but with BTL and separate PSU for "pre" that is no problem.
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Old 1st December 2002, 10:05 PM   #3
halojoy is offline halojoy  Sweden
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Thumbs up Nice schematics!

I wish I could get such quality
in my schematic pictures.
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Old 1st December 2002, 10:45 PM   #4
Jeff R is offline Jeff R  United States
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I just took a quick look, so I may be in error, but a couple of things I have noted:

1. Your use of AC coupling on the input but not the output. Why not use a fully DC coupled design?

2. I see only a pure resistive feedback network. You should use a capacitor to roll off the high frequency response.

3. You might also wish to use a capacitor in the main feedback loop to reduce the DC gain to 1 (highly recommended in DC coupled amplifiers).

4. Your driving the FETs from a common emitter configuration seems like a bad way to go. I would recommend driving them from emitter followers - with 150 collector resistance like you have, I would think the transient performance would be poor.

Hope this helps.

Jeff R.
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Old 1st December 2002, 10:57 PM   #5
halojoy is offline halojoy  Sweden
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In a current feedback amplifier
you do not use the usually frequency filter capacitor
in the global feedback
It is done somewhere else,
but where?

This one uses the emittors of transistors as the feedback
input (the inverted input)

DC input I use when I want DC outputs.
Like when playing AC/DC.

I use caps,
mainly because my speakers have so bad
response for frequencies BELOW 20 Hz
I know I miss that part of the music
But what can you do what such bad speakers?
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Old 1st December 2002, 11:02 PM   #6
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Just a minor additional comment. All three JFETs could be
replaced with resistors.
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Old 2nd December 2002, 12:56 AM   #7
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Default Re: Nice schematics!

Quote:
Originally posted by halojoy
I wish I could get such quality
in my schematic pictures.
Looks like it was done with Protel for Windows that came out in the early 90's. I have one that they outgrew at work.
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Old 2nd December 2002, 09:53 AM   #8
LennyK is offline LennyK  Poland
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Thank you for comments.

Yes, that's Protel Protel SE form their www page (trial version ).

I forgot to mention that DC offset wil be done with DC Servo, so maybe this capacitor in input is realy bad idea.

Hi frequency behaviour is depended on C7, C8. I hope this is enough. Correct me if I'm wrong. I'm litle worry about oscilations, because I don't have osciloscope.
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Old 2nd December 2002, 11:06 AM   #9
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Lenny if you want to study the concept of current feedback, check ON and National.

http://www.national.com/search/searc...ack&full=1#App

Have some trouble to find at ON but I know they have good documents.
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Old 2nd December 2002, 12:47 PM   #10
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Default Ohmmm....

Meybe you could change first&second stages to one j-fet stage.
You could take a voltage gain and very good input bufer ( it have really high input imedance ). It is a possibility to connetc NFB with the current mode.
There is a great amount of negative feedback in last two stages, I,m affraid it could oscilate.
You have to design a system to prevent volage peaks, because Hexfets could burn to death. Maybe a capacitor across bias stabiliser, meybe zener diodes or system that slowly raises PSU voltages.

Dobra Lenny pogadamy na Aikido.
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