"The Wire AMP" Class A/AB Power Amplifier based on the LME49830 with Lateral Mosfets - Page 200 - diyAudio
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Old 18th November 2012, 08:33 PM   #1991
IanAS is offline IanAS  England
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Thanks, I guess that would make it about 5C/W. I was thinking of more like 3C/W might be needed.
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Old 19th November 2012, 04:47 PM   #1992
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oops I put the following in the wrong thread.

I am still doing testing on the boards and found a couple of things. With some old PC motherboard heatsinks cut up and glued to the LME heatsinks. The LME chips now are just warm instead of too hot to touch. The heatsinks in the old Kenwood poweramp I am using appear to be a bit small so I was trying to find a compromise between bias current and too hot to touch standby opperation. 200 ma appears to be about the limit in my situation. With larger heatsinks I could get away with a higher bias, but that's for the next build. I noticed this morning that the bias was a lot less than 200 ma reading I was getting yesterday. I was trying to see what was going on and found that the bias was very slowly creaping up a ma or so every minute or two. I then tried to monitor the gate to gate voltage to see if it was also going up in relation with the bias current. Once the smoke cleared out of the basement I realized putting a meter across the gates caused the amplifier to oscilate, a lot. Thank goodness for 1/4 watt resistors. I am using 1/4 watt 10 ohm resistors in series with the fets to protect them. I can measure one gate to ground without oscilation. I am waiting for the amplifier to cool so I can start measuring again. The other thing, with +/-68 volt rails the amplifier can easily put out 35 volts rms into an 8ohm load. The feedback resistor is 5.49k 1/8 watt and with about 35 volts across it, it is disapating about 1/4 watt, hmmmm, I am using AC coupled single ended version. I guess for the next build I will have to change the resistors.
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Old 20th November 2012, 06:28 PM   #1993
rsavas is offline rsavas  Canada
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Default Good feedback!

Good info "multisync".
Good catch on the feeback resistor power dissipation. I was surprised to see that the National app note used a "MFR-25FBF-6K81' resistor (1/4W/1% Metal Film)
My calcs indicate 70Vpeakx0.7071=~50Vrms squared/6810=0.36W. I was going to use a 1206,1/4W resistor, in my layout, but will change to a 1210,1/2W. We have to realize that we are never going to run the amp at full output 100% of the time so, avg. PD will be significantly less. People talk about thermal modulation distortion, so it is better to over size this particular resistor to minimize this effect.

If you have a scope, suggest to use that to make your measurements, at least if it oscillates, you will see it happening.

Wondering about your statement about 'Thank goodness for 1/4 watt resistors. I am using 1/4 watt 10 ohm resistors in series with the fets to protect them". I thought that you are to be using 220/330ohm resistors in the FET gates?

I have done a layout as well, yet to fab, so I will incorporate all the feedback that everyone is feeding back. Still contemplating changing the LME49830 HS from what was spec'd by Owen. Lower temp Silicon = higher reliability.

Cheers
Rick
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Old 20th November 2012, 09:09 PM   #1994
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the 10ohm resistors are in series with the fets to the fet power supply. I am measuring the voltage across these resistors to set the fet bias. It was not the first time the colour code on the resistors has changed to black black black black. Yes I could set the DVM's to read current but then I would have to change the DVM fuses and probably the fets.

I did have a scope on it but at the output, that's how I knew it was oscilating.

The measured output offset is 0.0000V on one channel and 0.0006V on the other.

90% of the time the finished amplifier will be operating in the milli watt range. 9% in the less than 5 watt range and 1% in the "it's time to dust of the speakers and the neighbours are not home" range.

Rick when you layout your boards leave room for much larger LME heatsinks.

I had the amplifier on for several hours today to see how much the bias current changed over a period of time from cold to hot. Last night before I turned it off I set the bias on each channel to 150ma. This morning the bias was 99ma on one channel and 101 ma on the other. I just checked the bias again after this afternoon's champions league games have finished and the bias has crept up to 144 ma on one chanel and 142 ma on the other.

I was touching the various parts to see how warm they got after being on for about 7 hours. I got a pretty good tingle when I put my finger across the tops of the filter caps. Sure enough there was -75 volts on the top of one the lme caps and -65 on the top of the fet caps. I ohmed out the other set of parts I have and the caps measure several megs from either lead to the metal cap hmmmm.

I will probably take the board apart and replace the gain resistors to be on the safe side.
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Old 20th November 2012, 11:06 PM   #1995
rsavas is offline rsavas  Canada
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Your input solidifies my action to put on a much larger HS. The HS on the National app note is not that much greater Tsa, I will check again, these #'s. Puzzled by the varying bias V. Do you reason that the bias changing has to do with the die temp of the LME? Run a fan on the LME and see how much the bias I changes? Seems to be a rather long Temp slew to stabilize?
"I got a pretty good tingle when I put my finger across the tops of the filter caps."
Silly man, touching live HV ckts with your bare fingers. I work with 347Vac, so stand back when hitting the sw.
I put Ohmite FCSL110R050FERCT on my PCB as an option for source r's
"Sure enough there was -75 volts on the top of one the lme caps and -65 on the top of the fet caps" This sounds correct.
"I ohmed out the other set of parts I have and the caps measure several megs from either lead to the metal cap hmmmm." I don't follow you?
Good to here from people ahead of me thx Rick
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Old 21st November 2012, 12:29 AM   #1996
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multisync, is the voltage across the bias setting trimmer (VR5) changing between amp turn on and and warm up?

I would not have expected the bias to change that drastically, even though you are down in an area where thermal changes for the FET will have an impact on the current I'd expect something like a change from 100mA to 105mA ... I haven't built mine yet so I'm going off datasheets only.
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Old 21st November 2012, 04:47 AM   #1997
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"I ohmed out the other set of parts I have and the caps measure several megs from either lead to the metal cap hmmmm." I don't follow you?

I was measuring the tops of the caps and had -75volts on the metal top. On the other cap I was expecting 0.0 volts but read around 0.34 volts. WTF I thought I had a bad solder joint or something but all the large bypass caps with metal tops C72, C74 C83 C84 were reading about the same ie not 0.000 volts but a few tenths. So I took out the other caps from the remaining two kits and ohmed out the leads to the metal cap and all the caps read several meg ohms from either lead to the metal cap. So is there a cap expert here who can tell us why the caps ohm out high but have a voltage on them. Never mind I think it is because they are capacitors.

I was monitering the gate voltage but one of my meters died. I went out today and got a fresh batch of 9volt batteries. Before the meter died, the gate voltage did appear to increase over time but not as much. I might have a little bit of coolant spray left so I will try and cool down the lme chip and see what happens.

I am waiting for some fuse holders and banana plugs so I can put this whole thing together and make some noise.

If I decide to change the resistors to a higher value, R43 and R44 are going to be tough for my aging eyesight and shakey fingers. The offset is so low I might leave the input resistors R44, R46 alone and remove the shorting jumper and put a resistor in its place and then change R39 on the bottomn.
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Old 21st November 2012, 04:57 AM   #1998
qusp is offline qusp  Australia
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I asked opc about that and he said that increasing the value isnt such a good idea, I asked him about it the other day because I was working out what caps I need on my NTD1 IV stage, basically if your source doesnt have low enough source impedance to deal with the 1.2k, youve built the wrong amp.

he was saying its not a mild effect either
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Old 21st November 2012, 05:08 AM   #1999
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the balanced version uses values of 1.2 and 30k. So I am pretty sure I can find something between 5.4k and 30k that will work. I have a LMe49810 working for a couple of years with 33k as feedback restors and input resistors. I know I will get a bit more of Johnson noise from the slightly larger values. But I don't thing they make 1/2 watt-1 watt 0805 size resistors.
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Old 21st November 2012, 05:09 AM   #2000
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i am using Owen's headphone amp as a preamp so there is lots of low Z drive available.
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