Slap a servo on that bad boy!
:thumb:
:thumb:
class A biasing
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
Hey Butta...
ah i see, so in principle it's a good option to have on most output ckts (that'd otherwise use a cap to cancel DC) ye? Like fabio's API brds that have either transformer, transformerless servo and transformerless with caps options...
Cheers!
ah i see, so in principle it's a good option to have on most output ckts (that'd otherwise use a cap to cancel DC) ye? Like fabio's API brds that have either transformer, transformerless servo and transformerless with caps options...
Cheers!
Swedish Chef
Well-known member
as I understand it the DC servo in this instance is basically a 0hz HP filter. The gripe some people have with them is that, as we know, a filter cannot be a perfect brick-wall and a 0Hz corner frequency filter could have an audible effect up to 10Hz...perhaps higher. Some people prefer the sound of a good cap...
YMMVA!! :grin:
cher
YMMVA!! :grin:
cher
NewYorkDave
Well-known member
Servos predate Hardy by many, many years. Read up on early analog computers, bomb-sighting apparatus in WWII, etc.
Unless designed very carefully, a servo on an audio circuit can cause more problems than it solves. Sometimes it's better just to use a cap.
Unless designed very carefully, a servo on an audio circuit can cause more problems than it solves. Sometimes it's better just to use a cap.
> it sounds very class b,,, not very lively.
I don't see how this comes together. Either your observation/conclusion, OR this circuit. Q2 can't possibly be a 2N2222, can it????
But if you want to try: make Q2 the PNP it wants to be, increase both emitter resistors to 33 ohms, add a third diode.
If that op-amp is TL07x.... it isn't thrilled to see that 2K2 load. FOr many uses, you could use 4K7 instead. If you honestly need a powerful output, don't use TL07x.
> Slap a servo on that bad boy!
Why? No clue that it has any problem from DC error. You've started a drift that is probably completely off the mark.
I don't see how this comes together. Either your observation/conclusion, OR this circuit. Q2 can't possibly be a 2N2222, can it????
But if you want to try: make Q2 the PNP it wants to be, increase both emitter resistors to 33 ohms, add a third diode.
If that op-amp is TL07x.... it isn't thrilled to see that 2K2 load. FOr many uses, you could use 4K7 instead. If you honestly need a powerful output, don't use TL07x.
> Slap a servo on that bad boy!
Why? No clue that it has any problem from DC error. You've started a drift that is probably completely off the mark.
tk@halmi
Well-known member
[quote author="buttachunk"]have a circuit here i want to mod. the output looks-a like-a 'dis;
http://img.photobucket.com/albums/v251/buttachunk/consoleout1copy.jpg
it sounds very class b,,, not very lively.
i'm thinking,, jumper r4 and r5, and add an emitter resistor of 150ohms to the bottom transistor-- bias into class a.
potential problems ?[/quote]
What a funky output stage. Just remove R8 altogether to bias the output pair more towards Class-A. Wait, no, don't do that. How much standing current do you want? Do not bypass R4 as it is local feedback (emitter degeneration) and helps with a number of things.
Hmm, are you sure that Q2 is NPN as well?
http://img.photobucket.com/albums/v251/buttachunk/consoleout1copy.jpg
it sounds very class b,,, not very lively.
i'm thinking,, jumper r4 and r5, and add an emitter resistor of 150ohms to the bottom transistor-- bias into class a.
potential problems ?[/quote]
What a funky output stage. Just remove R8 altogether to bias the output pair more towards Class-A. Wait, no, don't do that. How much standing current do you want? Do not bypass R4 as it is local feedback (emitter degeneration) and helps with a number of things.
Hmm, are you sure that Q2 is NPN as well?
tk@halmi
Well-known member
R8 will determine the current through the diodes and to varying degrees the current through Q1 and Q2.
For an entire evening of fun make it a 10k trimer pot and adjust it until:
1. You like the sound.
2. The transistors burn up.
As always PRR's suggestions are better. :thumb:
For an entire evening of fun make it a 10k trimer pot and adjust it until:
1. You like the sound.
2. The transistors burn up.
As always PRR's suggestions are better. :thumb:
NewYorkDave
Well-known member
[quote author="PRR"]Why? No clue that it has any problem from DC error. You've started a drift that is probably completely off the mark.[/quote]
Am I the only one who notices the brilliant pun contained in that last sentence?
Am I the only one who notices the brilliant pun contained in that last sentence?
SSLtech
Well-known member
Arrrrgh!
The lower tranny is still an NPN... not right at all!
And yes, i actually read PRR's post to initially imply a DC drift... had to do a slight mental "recalibration" to figure out what was really meant!
Keith
The lower tranny is still an NPN... not right at all!
And yes, i actually read PRR's post to initially imply a DC drift... had to do a slight mental "recalibration" to figure out what was really meant!
Keith
M P uses sexy opamp input with servo.
If you catch my drift.
Remember that schemo I sent you?
:guinness:
If you catch my drift.
Remember that schemo I sent you?
:guinness:
> these alternate component changes would bias into class A, right?
Yeah, but you have an AWFUL lot of current (88mA) and very little voltage swing (only about 1.5V on the negative peaks). As a 32Ω headphone driver, it works, but very wastefully. As a line driver, it won't even reach the standard 2VRMS 2.8V peak that any hi-fi CD player will do.
Yeah, but you have an AWFUL lot of current (88mA) and very little voltage swing (only about 1.5V on the negative peaks). As a 32Ω headphone driver, it works, but very wastefully. As a line driver, it won't even reach the standard 2VRMS 2.8V peak that any hi-fi CD player will do.
> so raise r4 to 1k, add a 15k resistor from opamp output to v+ rail, remove c3 and tap d1 from v+ rail instead?
????
That forces the output to over +13V. (Remember the collector-base junction.) And since the op-amp plus Q1 probably won't pull much above
+13V, that gives you a maximum output voltage swing of roughly zero. Maybe a volt or so peak-peak?
Is this maybe what you are looking for?
????
That forces the output to over +13V. (Remember the collector-base junction.) And since the op-amp plus Q1 probably won't pull much above
+13V, that gives you a maximum output voltage swing of roughly zero. Maybe a volt or so peak-peak?
Is this maybe what you are looking for?
clintrubber
Well-known member
What kind of load do you want to drive ?
The PRR-additions of the added D3 & the two 33R-resistors do already give class A to a decent output level.
Also have a look here, PRR helped me with redimensioning a comparable circuit:
http://www.groupdiy.com/index.php?topic=186&highlight=540
Bye,
Peter
The PRR-additions of the added D3 & the two 33R-resistors do already give class A to a decent output level.
Also have a look here, PRR helped me with redimensioning a comparable circuit:
http://www.groupdiy.com/index.php?topic=186&highlight=540
Bye,
Peter
clintrubber
Well-known member
Buttachunk, would love to have a look as well if that's OK.
I'm here: chickennerdpig (at) hetnet (dot) nl
Thanks,
Peter
clintrubber
Well-known member
Thanks. OK, we'll await CJ Ritchie.
Carl_Huff
Well-known member
Okay ....
So what does the final, final circuit look like and what is the op amp driving it??
Best Regards,
Carl Huff
So what does the final, final circuit look like and what is the op amp driving it??
Best Regards,
Carl Huff
Carl_Huff
Well-known member
So Buttachunk,
If I am properly following the flow of your posts ....
TR1 is a 2SC2240 npn
TR2 is a 2SA970 pnp
and D1, D2 & D3 are IN4148 diodes
This is a great forum. I learn so much here!!
Thank you PRR!
Best Regards,
Carl Huff
If I am properly following the flow of your posts ....
TR1 is a 2SC2240 npn
TR2 is a 2SA970 pnp
and D1, D2 & D3 are IN4148 diodes
This is a great forum. I learn so much here!!
Thank you PRR!
Best Regards,
Carl Huff
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