Hi
So I'm sold that I do need input caps to avoid the risk of blowing my speakers.
Short story, my 4780 amp is showing about 30mV on one channel and 8 mV on the other channel. What am I missing?
I have a list of possible things to do, which one/s of these are wasted time?
1. Shift speaker ground returns to power supply instead of amp board.
2. The chip is bad, it's not an issue normally so when I fit in the rest of the chips it'll all be OK.
3. Assemble in 3D and use a feedback decoupling cap. The only problem is finding a good cap - the ones I used killed the music completely.
4. Push the input impedance of the amp down to 4.7 K with a resistor. That should bring the offset down to about 20 mV on the bad channel, I think.
Long Story:
I put together a 4780 stereo amp (using Brian/Peter's PCB) using a 220K/10K combo, with a 33K resistor from the + input of the chip to ground (call this Rdc). A 2.2 uF film cap on the input. This thing plays music fine, but I'm getting 80mV on the output of one amp and 30 mV on the other.
I think OK, let's try getting closer to the feedback resistor value so I put a 220K at Rdc, and I get 500mV on the worse channel. The better one is at 200 mV.
Then it's time to change all the resistors. Man, what a pain to resolder stuff into a PTH board. By now the board looks like a butcher's block and the resistor legs do not fit the holes anymore. So I'm holding the resistors in with tweezers and using blobs to hold them... Thank God this is a spare test board that will not be in the main system...
So I try 47K/1.5K feedback and 47K Rdc, and I get about 120 mV/60 mV on the outputs. In desperation, I fit a 100uF feedback decoupling cap the way it is in the National PDF, and the offset is now 1mV on one channel and 2 mV on the other channel.
But now the music has disappeared, it becomes flat, lifeless and dull. This is not acceptable.
In a mad rage I remove all the resistors, and refit the 220/10K feedback and now use a 10K resistor in the position Rdc. my offset is now 30mV on the bad channel and 8 mV on the better one. No cap is used except the input cap.
Edit: Typos
So I'm sold that I do need input caps to avoid the risk of blowing my speakers.
Short story, my 4780 amp is showing about 30mV on one channel and 8 mV on the other channel. What am I missing?
I have a list of possible things to do, which one/s of these are wasted time?
1. Shift speaker ground returns to power supply instead of amp board.
2. The chip is bad, it's not an issue normally so when I fit in the rest of the chips it'll all be OK.
3. Assemble in 3D and use a feedback decoupling cap. The only problem is finding a good cap - the ones I used killed the music completely.
4. Push the input impedance of the amp down to 4.7 K with a resistor. That should bring the offset down to about 20 mV on the bad channel, I think.
Long Story:
I put together a 4780 stereo amp (using Brian/Peter's PCB) using a 220K/10K combo, with a 33K resistor from the + input of the chip to ground (call this Rdc). A 2.2 uF film cap on the input. This thing plays music fine, but I'm getting 80mV on the output of one amp and 30 mV on the other.
I think OK, let's try getting closer to the feedback resistor value so I put a 220K at Rdc, and I get 500mV on the worse channel. The better one is at 200 mV.
Then it's time to change all the resistors. Man, what a pain to resolder stuff into a PTH board. By now the board looks like a butcher's block and the resistor legs do not fit the holes anymore. So I'm holding the resistors in with tweezers and using blobs to hold them... Thank God this is a spare test board that will not be in the main system...
So I try 47K/1.5K feedback and 47K Rdc, and I get about 120 mV/60 mV on the outputs. In desperation, I fit a 100uF feedback decoupling cap the way it is in the National PDF, and the offset is now 1mV on one channel and 2 mV on the other channel.
But now the music has disappeared, it becomes flat, lifeless and dull. This is not acceptable.
In a mad rage I remove all the resistors, and refit the 220/10K feedback and now use a 10K resistor in the position Rdc. my offset is now 30mV on the bad channel and 8 mV on the better one. No cap is used except the input cap.
Edit: Typos
30 mV is nothing to worry about.sangram said:
The input offset voltage comes from two components, the voltage offset (1-10 mV) plus the input bias offset currents. If you have AC coupled input, you should match the (DC) resistance from both inputs. This is not so easy if you have DC gain of 30 and let's say 22 kohms at the input.
If you have 10k/220k as feedback and 10k as pull down at the input you MUST also have your amp AC coupled at the input. If not you will get lower DC resistance than 10k and this will create offset. The input resistor should be 220//10 = 9.56 k.
One way to get rid of all problems is to use a DC servo like I have done in my Gainclone designs. Then you can have the amp totally DC connected with less than 1 mV offset in all times.
Thanks Peranders.
Do you mean by 'matching the resistance' connecting an equal resistance to Rf//Ri at the + input to ground? For the datasheet circuits that would mean about 953 ohms (22//1K), which seems a little low as that also determines the input impedance of the amp... Unless I misunderstand you.
What I find is that the value of the offset is directly proportional, almost mathematically, to the value of the input resistor. It has finally stabilised at about 26 mV on the bad channel and just over 6 mV on the better one, with the same 10K input resistor.
30mV seemed high to me as my earlier implementation with Ci cap resulted in <3 mV offset so I was a little alarmed. The problem is that this chip stays a little toasty (on a section of extruded aluminium heatsink, quite beefy) but gets better/cooler as the volume is turned up. I don't think it is oscillating (yet), can't hear the telltale hiss but don't have access to a scope so I can't be sure.
The amp input is/has to be AC coupled, as I modded my CDP and removed the output caps and muting transistors - so there's a permanent ~2mV on the CDP output. with my Ci implementation I could DC couple the amp but since the Peter/Brian board doesn't have space for it I couldn't put it in and I have to keep a capacitor in one position, either at the + input or the Ci...
I was keenly studying servo and other 'stable' offset correction systems, but IMO injecting essentially a part of the DC supply voltage into the input of the amp is really not something I'd like to be interested in for this particular design.
Would you think that unloading the ground line would make sense, if I moved the speaker returns back to the supply grounds... Though, in final implementation this is going to be a 4-chip bridge/parallel setup so it won't matter then as I won't be using supply ground.
One last question, OT but while we're on it. For a bridge parallel setup will I need both chips of one channel to tie to a common power ground reference or can they float with their individual supplies? I am looking at running them off a fully balanced input off a DRV 134 or from a Delta 66 card, so the inputs will be referenced to the cable shield or chassis graound, will I also need to tie their power grounds together?
Do you mean by 'matching the resistance' connecting an equal resistance to Rf//Ri at the + input to ground? For the datasheet circuits that would mean about 953 ohms (22//1K), which seems a little low as that also determines the input impedance of the amp... Unless I misunderstand you.
What I find is that the value of the offset is directly proportional, almost mathematically, to the value of the input resistor. It has finally stabilised at about 26 mV on the bad channel and just over 6 mV on the better one, with the same 10K input resistor.
30mV seemed high to me as my earlier implementation with Ci cap resulted in <3 mV offset so I was a little alarmed. The problem is that this chip stays a little toasty (on a section of extruded aluminium heatsink, quite beefy) but gets better/cooler as the volume is turned up. I don't think it is oscillating (yet), can't hear the telltale hiss but don't have access to a scope so I can't be sure.
The amp input is/has to be AC coupled, as I modded my CDP and removed the output caps and muting transistors - so there's a permanent ~2mV on the CDP output. with my Ci implementation I could DC couple the amp but since the Peter/Brian board doesn't have space for it I couldn't put it in and I have to keep a capacitor in one position, either at the + input or the Ci...
I was keenly studying servo and other 'stable' offset correction systems, but IMO injecting essentially a part of the DC supply voltage into the input of the amp is really not something I'd like to be interested in for this particular design.
Would you think that unloading the ground line would make sense, if I moved the speaker returns back to the supply grounds... Though, in final implementation this is going to be a 4-chip bridge/parallel setup so it won't matter then as I won't be using supply ground.
One last question, OT but while we're on it. For a bridge parallel setup will I need both chips of one channel to tie to a common power ground reference or can they float with their individual supplies? I am looking at running them off a fully balanced input off a DRV 134 or from a Delta 66 card, so the inputs will be referenced to the cable shield or chassis graound, will I also need to tie their power grounds together?
If you have DC gain set to 1, 22 kohms is the right value. You see those input bias currents are causing you some trouble.
Thanks, I already went through that but it doesn't answer my question. The AN-1192 primarily talks about a high-power amp running off a single-ended input by running one pair as inverting and one pair as non-inverting, and adding each pair in parallel.
My question was on balanced inputs and power grounding, whether it was OK to leave the power floating on both the amps (assuming independent transformers/rectifiers) or would I need to reference them to a common ground at some point?
I know that the signal inputs need to be referenced to something, and my second question was, whether to refernce them to the signal ground (shield wire) or to the other side of the balanced connection. Obviously in the second case we cannot tie the grounds of the amps together at any point or we will short the inputs to common ground...
This is complex...
My question was on balanced inputs and power grounding, whether it was OK to leave the power floating on both the amps (assuming independent transformers/rectifiers) or would I need to reference them to a common ground at some point?
I know that the signal inputs need to be referenced to something, and my second question was, whether to refernce them to the signal ground (shield wire) or to the other side of the balanced connection. Obviously in the second case we cannot tie the grounds of the amps together at any point or we will short the inputs to common ground...
This is complex...
Thanks P-A. No worries, I've already floated questions over to Peter and some other members who've built similar setups, hopefully I'll get a reply.
FYI I have two sources, a computer soundcard which can run in fully floated mode, and a CDP which I hope to be able to convert to fully floated by using a DRV134.
Thanks again. I am assured by Peter that my current offset voltages are nothing to worry about, except that it does get very warm (almost worryingly so) at idle or very low power levels.
FYI I have two sources, a computer soundcard which can run in fully floated mode, and a CDP which I hope to be able to convert to fully floated by using a DRV134.
Thanks again. I am assured by Peter that my current offset voltages are nothing to worry about, except that it does get very warm (almost worryingly so) at idle or very low power levels.
Hey Sangram,
well i too am facing problems with regard to DC offset.
i have so far tested LM4780 in bridged, 4780 in parallel(Peter's).
Also, as i type this i have the 3886(Peter's) playing beautifully.
Now the Dc offsets were less than .5mV in the bridged board.
the 3886 showed around 7-8mv(fluctuates).
So i guess, those two are fine.
Will not bother to check the other bridged and 3886 board.
But, the paralel 4780 is showing DC of 230mV with input signal and 135mV with input shorted.
Any ideas as to how i can bring the offset to the levels of the boards?
Cheers,
Sachi
well i too am facing problems with regard to DC offset.
i have so far tested LM4780 in bridged, 4780 in parallel(Peter's).
Also, as i type this i have the 3886(Peter's) playing beautifully.
Now the Dc offsets were less than .5mV in the bridged board.
the 3886 showed around 7-8mv(fluctuates).
So i guess, those two are fine.
Will not bother to check the other bridged and 3886 board.
But, the paralel 4780 is showing DC of 230mV with input signal and 135mV with input shorted.
Any ideas as to how i can bring the offset to the levels of the boards?
Cheers,
Sachi
Yes, tiltedhalo is right, but the true DC-offset is what the speaker sees as DC across its terminals, AFAIK not just the value present at the amp output. For a quality implementation the DC difference shold be zero across channels, so in bridge configuration you only get the input offset current results like P-A was talking about earlier in this thread.
I found one very simple cure to offset - a Ci cap. This brings offset well below 10mV, which is quite acceptable. But then the sound was suffering. Some people hve also used 0.1% resistors to bring the offset of the amp down, and some even more audacious people have connected the two halves of the amp to the same feedback loop and found very low offset values. For a parallel amp maybe some of these can be tried. I assume you have input blocking cap?
I found one very simple cure to offset - a Ci cap. This brings offset well below 10mV, which is quite acceptable. But then the sound was suffering. Some people hve also used 0.1% resistors to bring the offset of the amp down, and some even more audacious people have connected the two halves of the amp to the same feedback loop and found very low offset values. For a parallel amp maybe some of these can be tried. I assume you have input blocking cap?
sangram said:
Hey guys,
went along with the datasheet and added the Ci cap and replaced the 680 ohm( Ri) with 1k.
the DC offset cleared up immediately.
Stayed at 8.4 mV.
Am faced with some new problems.
Now, when i remove the input to the amp, i get a very faint buzzing sound.It disappears when i reconnect the input RCA plug.
Also, i hear loud pops/blare when the amp is on and i connect/disconnect the RCA pin from the input of the amp(with the preamp off).
Why is it so and how do i make it go away?
Lastly,can i use a 10 ohm(1/4 watt) resistor along with the 0.1uF cap for zobel?
Thanks,
Sachi
Any way to solve this problem?
1 You have electrical fields and a rather high impedance input. It's natural that this happens, but a bit irritating maybe.sachi said:
2 You input bias currents and you connect/disconnect you'll get a voltage drop over the input resistor and this is amplified. If you have AC coupled input you would get a lot less of these pops.
3 This will work as long as you don't test your amp at 100 kHz and full throttle (= smoke) or play 20 kHz sinus for a long time at full output power. I would put a metall film 0.6 W which can withstand a bit more. 20 kHz, 24 V out => 0.9 W approx. 24 V out = 72 W at 8 ohms.
A few points:
1. Zobel is a good idea unless you're pretty sure your amp is not oscillating. I prefer 1 watt resistors.
2. I do hope your Ci cap is not changing the sound. I didn't like Ci cap at all and am not using it.
3. The buzz you hear with open input will happen. So will pops when you connect/reconnect. As long as the amp is buzz free when operating in real conditions, you should not be too bothered.
4. P-A is right, AC coupled inputs are less susceptible to this kind of noise as the input currents have to face varying impedances with connected and disconnected inputs when the amp is DC coupled.
5. I finished my 4-chip bridge/parallel LM4780 kits and I'm getting just 20 mV from the tested kit with open input (no Ci). The input signal has a 20mV DC-offset, so the final offset figure is about 120 mV with a connected input, but once it is finally AC-coupled (before the phase inversion and bridging) it should be OK. Bridging does have its upsides, low offset being one of them.
1. Zobel is a good idea unless you're pretty sure your amp is not oscillating. I prefer 1 watt resistors.
2. I do hope your Ci cap is not changing the sound. I didn't like Ci cap at all and am not using it.
3. The buzz you hear with open input will happen. So will pops when you connect/reconnect. As long as the amp is buzz free when operating in real conditions, you should not be too bothered.
4. P-A is right, AC coupled inputs are less susceptible to this kind of noise as the input currents have to face varying impedances with connected and disconnected inputs when the amp is DC coupled.
5. I finished my 4-chip bridge/parallel LM4780 kits and I'm getting just 20 mV from the tested kit with open input (no Ci). The input signal has a 20mV DC-offset, so the final offset figure is about 120 mV with a connected input, but once it is finally AC-coupled (before the phase inversion and bridging) it should be OK. Bridging does have its upsides, low offset being one of them.
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