Background:
Well I have inverted paralleled lm3875 chip amp here, built after “Application Note 1192 Overture Series”.
All resistors are matched within 0.01% to really balance between the chips. After the first built I measured the temperature on the chips after running the amp pretty hard for an hour and it differed 5 degrees Celsius between them. I replaced one of the chip’s did the same thing again and the difference was almost the opposite 7C. I had a few chips to test and eventually I found two that that had the same temperature after running them hard. After building the second channel with the same hazel and played it for a while. I love it and it sounds great and has tighter bas than my unparallel GC.
Conclusion:
Their is small differences between different LM3875 (I might be wrong, maybe something else affect the temp difference but I haven’t thought about it)
My question:
Is it any fast and easy way to match or measure the chip’s other than doing it the brute force way?
/Magnus
Well I have inverted paralleled lm3875 chip amp here, built after “Application Note 1192 Overture Series”.
All resistors are matched within 0.01% to really balance between the chips. After the first built I measured the temperature on the chips after running the amp pretty hard for an hour and it differed 5 degrees Celsius between them. I replaced one of the chip’s did the same thing again and the difference was almost the opposite 7C. I had a few chips to test and eventually I found two that that had the same temperature after running them hard. After building the second channel with the same hazel and played it for a while. I love it and it sounds great and has tighter bas than my unparallel GC.
Conclusion:
Their is small differences between different LM3875 (I might be wrong, maybe something else affect the temp difference but I haven’t thought about it)
My question:
Is it any fast and easy way to match or measure the chip’s other than doing it the brute force way?
/Magnus
you could try to test them with a small DC signal (DC couple the inputs before), say 0.5V (small enough to prevent the output to clip), and look at the output voltage
repeat the operation with all chips, and adjust the gain (you'll have to use pots for input and/or feedback resistors) so that all the chips output the same voltage with the small DC input
This is a rather complicated way to do this.
You could also simply put a series resistor between each IC's output and the "common" output, say 1R. Exactly like with paralleled voltage regs
repeat the operation with all chips, and adjust the gain (you'll have to use pots for input and/or feedback resistors) so that all the chips output the same voltage with the small DC input
This is a rather complicated way to do this.
You could also simply put a series resistor between each IC's output and the "common" output, say 1R. Exactly like with paralleled voltage regs
Magnus, lost swede in big US... think about this: What happens if you take two lead batteries with different charge status and connect them in parallel? Right, huge current from one of the battery to the other. You have the same problem with two outputs connected together. You must insert a small resistor if you don't want a current between the amps. Those IC's aren't any precision devices. Check the AN9-1192, sensible values!
You know, you can't trim this because you will have temperature drift. The only way to avoid those output resistors is to use DC-servo or doing some selection but how long term stable will this be?
Answer: Connect only one IC at the time and measure the output offset. Then take two IC's with the same offset.
You know, you can't trim this because you will have temperature drift. The only way to avoid those output resistors is to use DC-servo or doing some selection but how long term stable will this be?
Answer: Connect only one IC at the time and measure the output offset. Then take two IC's with the same offset.
After viewing the "Application Note 1192 Overture Series" from the National semiconductor website, I see that the IC's are operated in a "bridge output" mode.
One explanation for a higher temperature rise on one IC might be that the audio being fed to the amplifiers isn't symmetrical.
Almost all audio is asymmetrical in nature. The difference between positive and negative voltages in the waveform can be greater than 10dB. Under such circumstances, one IC might be delivering more power than the other IC.
As Bricolo advised, if the IC's have a slightly different voltage offset at their outputs, you might try adding a small series resistance (.27 ohms/5 watts) between the output of each IC and the loudspeaker. This will help balance the load between the two IC's.
I hope I have been of some help to you.
One explanation for a higher temperature rise on one IC might be that the audio being fed to the amplifiers isn't symmetrical.
Almost all audio is asymmetrical in nature. The difference between positive and negative voltages in the waveform can be greater than 10dB. Under such circumstances, one IC might be delivering more power than the other IC.
As Bricolo advised, if the IC's have a slightly different voltage offset at their outputs, you might try adding a small series resistance (.27 ohms/5 watts) between the output of each IC and the loudspeaker. This will help balance the load between the two IC's.
I hope I have been of some help to you.
Well I know they are not any precision devices, thats why I had to do it brute force, it really works nice now. I had small series resistor between each IC's output and the "common" output from the begining. I have probed the temp for two weeks now, and they stay within one degree all the time.
Just asked to get to know if it was any easier way to find the one's that worked together. I might test with a little dc signal next time.
Maybe a really stupid question, but what happens if I cross the feedback paths, could that balance them, is it even possible to do? Building one more, to play around with, I'm to happy with this one to play around with it anymore.
/Magnus
Just asked to get to know if it was any easier way to find the one's that worked together. I might test with a little dc signal next time.
Maybe a really stupid question, but what happens if I cross the feedback paths, could that balance them, is it even possible to do? Building one more, to play around with, I'm to happy with this one to play around with it anymore.
/Magnus
Oh no! Do not cross the feedback paths.
That will give you positive feedback and an oscillator that can deliver lots of power.
I'll look at the application notes again. You might be able to add a little bias voltage to the inputs of IC's through a balancing potentiometer. That might null out the dc offset.
That will give you positive feedback and an oscillator that can deliver lots of power.
I'll look at the application notes again. You might be able to add a little bias voltage to the inputs of IC's through a balancing potentiometer. That might null out the dc offset.
OK, I will not cross the feedback, but I thought if it is truly parallel, that both half was the same? You are the one with the knowledge, could you try to explain to me why it would be positive feedback?
plz?
Well i guess I will continue doing it the hard way, just testing chip till I foun pairs that works together.
Thanks for taking your time!
/Magnus
plz?
Well i guess I will continue doing it the hard way, just testing chip till I foun pairs that works together.
Thanks for taking your time!
/Magnus
Sorry, I didn't read the entire application note.
I made the incorrect assumption that you were operating the two IC's in the bridge mode rather than in the parallel mode.
I guess it pays to read your posting more carefully.
Crossing the feedback paths will not cause positive feedback but it will produce some undesireable results.
The d.c. offset will probably be much worse because the feedback will attempt to correct the offset on one IC by applying the correction voltage to the other chip.
One sure way to balance the load between the IC's is to insert a large capacitor between the output of each IC and the common load. This will block the offset voltage. You will want to keep a low value resistance in series with each capacitor to provide a.c. load sharing.
I made the incorrect assumption that you were operating the two IC's in the bridge mode rather than in the parallel mode.
I guess it pays to read your posting more carefully.
Crossing the feedback paths will not cause positive feedback but it will produce some undesireable results.
The d.c. offset will probably be much worse because the feedback will attempt to correct the offset on one IC by applying the correction voltage to the other chip.
One sure way to balance the load between the IC's is to insert a large capacitor between the output of each IC and the common load. This will block the offset voltage. You will want to keep a low value resistance in series with each capacitor to provide a.c. load sharing.
Thanks Frank,
"One sure way to balance the load between the IC's is to insert a large capacitor between the output of each IC and the common load. This will block the offset voltage"
But do I really want to put big capacitors on my output (how big are we talking) I guess it would be pretty big ones like 1000uf if I don't want to block wanted LF and how would this affect the sound. And putting quality capacitors of that size would have me living on noddles for 6 months (in my mind I could see a 1000uf AudioCap custom made that weighs in on 40pound and cost $10K)
I better keep my 8mV DC offset..
It is not bad tasting, the best booze in the world comes from Västra Götaland "Götene" and is served in plastdunk!
/Magnus
"One sure way to balance the load between the IC's is to insert a large capacitor between the output of each IC and the common load. This will block the offset voltage"
But do I really want to put big capacitors on my output (how big are we talking) I guess it would be pretty big ones like 1000uf if I don't want to block wanted LF and how would this affect the sound. And putting quality capacitors of that size would have me living on noddles for 6 months (in my mind I could see a 1000uf AudioCap custom made that weighs in on 40pound and cost $10K)
I better keep my 8mV DC offset..It is not bad tasting, the best booze in the world comes from Västra Götaland "Götene" and is served in plastdunk!
/Magnus
I would not like to become Arnold either, right now he just reminds me about Magnus Uggla "IQ kan räkna till hundra"
I having some fun here, I couldn't resist, crossing the feedback. It doesn't sound great more like Dynaco ST70, the tight bass is gone, but it sure swing, playing music, fun to listen to and I have had it ran for ten minutes without fire now. Is it the cheap way to build Dynaco replicas...
I don't dare to check the DC offset now..
Giving my some new things to test like output tranformes on chip amps, life is wonderful to live!
/Magnus
I having some fun here, I couldn't resist, crossing the feedback. It doesn't sound great more like Dynaco ST70, the tight bass is gone, but it sure swing, playing music, fun to listen to and I have had it ran for ten minutes without fire now. Is it the cheap way to build Dynaco replicas...
I don't dare to check the DC offset now..Giving my some new things to test like output tranformes on chip amps, life is wonderful to live!
/Magnus
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.