bob bias using the lm723
Hey, where did you get that clamp stand from, I like it.
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Bias design, amplifier design, filtering etc.
- Thread starter -=PEAKABOO=-
- Start date
Hey, where did you get that clamp stand from, I like it.
bob bias using the lm723
looks like some caps and resistors and a relay do you have a pic of the bottom and where all of them go together if at all?
You can test it with using expensive pills.
Do you have a source for those cheap pills that are used for testing.... LOL
Or how about the ones that don't heat up or blow if the amp takes off into self oscillation?
I could use 4 of them right now.
on another note.....
Did Bob etch that board for the bias?
He did a pretty good job on it if he did.
Personally I would rather just "dead bug" than etch for the time it takes
i have etched boards with ferric chloride in the past but not the bias boards, i drew the basic layout using the component sizes i was using and had some prototype boards etched and drilled,
if i were to do it again i would makes some changes, probably go partly surface mount and not bother with class c to make the boards smaller and neater.
if i were to do it again i would makes some changes, probably go partly surface mount and not bother with class c to make the boards smaller and neater.
Nice stuff Bob... I hate etching boards.
One thing I noticed right away os how thick that board looks. It doesn't even look like copper clad.
I don't know if its just the picture or what, but that board looks really thick and solid.
I remember seeing another bias circuit you bought many many years ago. I think you got it from PSF Tech..but I could be wrong.
I think you know which one I'm talking about anyways.
How did that one work compared to this one?
Boo... I might just send you a PM.
I'm having problems with my old Circuit test scope and don't have a schematic. It must have got damaged when I moved...it worked fine before the move.
I also need a few more parts for the bias and amp that I am waiting for.
Best I can do right now is put a load on the bias output and check the voltage and current draw under load. Without a scope I can't see the output waveform. Without it I am just looking at numbers and guessing if its operating right.
I'm going to wait .
I'm back at work tomorrow fighting with a Mototurbo system all week.
I won't be able to do anything until next weekend.
I'll definatly get shoot you a pm if I get stuck.... but right now I'm not even set up to test it in the amp.
Thanks again for all the input guys.
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bush, the boards are double sided tinned copper on fiberglass, they are thicker than i had anticipated,
i never bought a bias circuit but i remember somebody posting a pic of the psftech board, they were basic, i have not played with a circuit like that since the late 80's,
the active circuit works MUCH better than any passive system i ever saw,
as peakaboo said it would be wise to test your circuit before you stick it in an amplifier, i would clamp the output with a large diode to ground so you dont accidentally smoke the base emitter junctions.
i never bought a bias circuit but i remember somebody posting a pic of the psftech board, they were basic, i have not played with a circuit like that since the late 80's,
the active circuit works MUCH better than any passive system i ever saw,
as peakaboo said it would be wise to test your circuit before you stick it in an amplifier, i would clamp the output with a large diode to ground so you dont accidentally smoke the base emitter junctions.
OK Bob...I thought that was you that was testing a commercially made active bias years ago. It might have been your prototype.
But as far as testing the bias..... The voltage and current has been tested under load. I have a steady voltage and current is limted. There is really nothing else I can check on the bench. I was actually going to just keep it set at 200ma and just put a fuse in series with the ammeter when I test it in the amp. I have a small value resistor and diode just before my rf choke. The diode will function as a clamping diode.
Thanks for mentioning it Bob.... that could have been easily forgotten/overlooked.
But if I can set the voltage at .7 volts or so....... and my current is limited to around 200ma ( will be variable when I test so I can start lower)...and i have a fuse in series with the bias output. ... why not test it in the amp.
I can monitor the bias current, bias voltage, amplifier current. I can increase the bias current/voltage as needed. I just want to get enough voltage and current to turn on the base. Once I have low voltage/current applied to the base I can generate rf in at low power... monitor the bias current...amp current..and bias voltage while checking out the waveform.
I just need the right amount of current and voltage to turn on the base...and some measures in place to protect it from over current draw. Once I have a steady .7 volts ( or so) going into the base and the current is limited and protected.....I should be ok to generate in at low power.
Correct me if I am wrong...but I should be ok to test it in the amp if all those perameters are met.
Current is limited and protected by a fuse...voltage is regulated and steady and brought up from lowest point. I will be generating low power while monitoring heat, voltage and current draw of the amp and bias.
This bias has been tested on the bench under load.
Unless I am missing something I should be ok to test with the current limited at 200ma and a fuse.
What am I missing?
.7v will turn some transistors on too hard bush,
i would make a test rig up using a cheap audio transistor with similar dc current gain to the 2879's.
when you get to testing in the amp ( using basic tools ) start at under .6v, dont worry about current draw from the bias supply, it needs to NOT current limit at regular drive levels to keep it in class ab, you can look at the bias voltage, if it dont sag under higher drive levels you have enough current limit, you can always set your current limit to start folding back above a certain drive level when you are done,
( if you have poor rf isolation rf can mess with the regulation )
put the meter between the power choke and the power strip on the back of the output transformer so you can measure the idle current of the transistors,
put a dummyload on the amp for safe measure,
turn your bias supply down to its lowest setting,
activate the bias supply and look at the current drawn on the collectors, increase bias voltage while watching idle current, a pair of 2879's should draw about 2-300ma at idle in class ab ( other values can be used and still sound acceptable to the ears ), its handy to be able to measure both sides of the amp at the same time but not critical,
when you have each side of the amp set to the same idle current with no drive you can hook the radio up and watch the bias voltage as you increase drive level,
if the voltage stays stable you are in the ballpark, start warming the amp up and periodically chech your no signal idle current on both sections of the amp to observe how far they have drifted,
a well tracked bias circuit will show little change or a small decrease in idle current,
if you see an increase with temperature you are undertracking, if the idle current has significantly dropped you are over tracking,
keep your eyes open for sudden increases in current draw or silly watt readings some amps with low feedback can be provoked into self oscillation which can easilly end in smoke, i like to test using a current limited power supply whenever i can,
( touch wood i never made any smoke so far )
hope this helps.
i would make a test rig up using a cheap audio transistor with similar dc current gain to the 2879's.
when you get to testing in the amp ( using basic tools ) start at under .6v, dont worry about current draw from the bias supply, it needs to NOT current limit at regular drive levels to keep it in class ab, you can look at the bias voltage, if it dont sag under higher drive levels you have enough current limit, you can always set your current limit to start folding back above a certain drive level when you are done,
( if you have poor rf isolation rf can mess with the regulation )
put the meter between the power choke and the power strip on the back of the output transformer so you can measure the idle current of the transistors,
put a dummyload on the amp for safe measure,
turn your bias supply down to its lowest setting,
activate the bias supply and look at the current drawn on the collectors, increase bias voltage while watching idle current, a pair of 2879's should draw about 2-300ma at idle in class ab ( other values can be used and still sound acceptable to the ears ), its handy to be able to measure both sides of the amp at the same time but not critical,
when you have each side of the amp set to the same idle current with no drive you can hook the radio up and watch the bias voltage as you increase drive level,
if the voltage stays stable you are in the ballpark, start warming the amp up and periodically chech your no signal idle current on both sections of the amp to observe how far they have drifted,
a well tracked bias circuit will show little change or a small decrease in idle current,
if you see an increase with temperature you are undertracking, if the idle current has significantly dropped you are over tracking,
keep your eyes open for sudden increases in current draw or silly watt readings some amps with low feedback can be provoked into self oscillation which can easilly end in smoke, i like to test using a current limited power supply whenever i can,
( touch wood i never made any smoke so far )
hope this helps.
.7v will turn some transistors on too hard bush,
i would make a test rig up using a cheap audio transistor with similar dc current gain to the 2879's.
when you get to testing in the amp ( using basic tools ) start at under .6v, dont worry about current draw from the bias supply, it needs to NOT current limit at regular drive levels to keep it in class ab, you can look at the bias voltage, if it dont sag under higher drive levels you have enough current limit, you can always set your current limit to start folding back above a certain drive level when you are done,
( if you have poor rf isolation rf can mess with the regulation )
put the meter between the power choke and the power strip on the back of the output transformer so you can measure the idle current of the transistors,
put a dummyload on the amp for safe measure,
turn your bias supply down to its lowest setting,
activate the bias supply and look at the current drawn on the collectors, increase bias voltage while watching idle current, a pair of 2879's should draw about 2-300ma at idle in class ab ( other values can be used and still sound acceptable to the ears ), its handy to be able to measure both sides of the amp at the same time but not critical,
when you have each side of the amp set to the same idle current with no drive you can hook the radio up and watch the bias voltage as you increase drive level,
if the voltage stays stable you are in the ballpark, start warming the amp up and periodically chech your no signal idle current on both sections of the amp to observe how far they have drifted,
a well tracked bias circuit will show little change or a small decrease in idle current,
if you see an increase with temperature you are undertracking, if the idle current has significantly dropped you are over tracking,
keep your eyes open for sudden increases in current draw or silly watt readings some amps with low feedback can be provoked into self oscillation which can easilly end in smoke, i like to test using a current limited power supply whenever i can,
( touch wood i never made any smoke so far )
hope this helps.
So I bring up the voltage to .6 ?
Is it even worth while to limit the top end of current draw of the bias circuit?
What should I expect for current draw from the bias circuit idle and with driven power ( or don't worry about it)?
If collector current is 200ma idle... how much current will the bias draw at idle?
Once the base is turned on I should get between 100 to 200ma per?
Current control of the bias ( for overdrive) will have to wait until the end...
Good idea about limiting the amplifer current under testing... I have a small 20 amp supply I can limit current...I have another small class c amp that I can test on.
I wish I didn't have to work....
Thanks for the input my friend.
Its always helpfull
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