| markiemrboo |
Well, I have built myself a GC based on carlosfm's non inverted schematic and "snubbered" regulated PSU along with a soft-start from Rod Elliot.
I've got a single channel hooked up, it all turns on an off just fine. The relay click happens very shortly after I flick the power switch. Probably more than 100ms, but definately less than 1s... lets say 500ms :).
I checked the offset, with no input signal, using a 10R 7W resistor, before hooking up a test speaker, and I got 92mv (a little high but probably nothing to worry too much about at this point?).
So next I plugged in the speaker, still without an input signal. Silent turn on, strange high pitch click on turn off. Speaker didn't seem to be blown, but it made that strange click on turn off every time.
The moment of truth. My current amp, a Cambridge Audio A5, has a "PRE OUT", which appears to mean it's the output of the preamp. Seems obvious enough, so I assume that's exactly what it is and I was hoping to use it purely as a pre-amp for now. I plugged one channel in to the pre-out and then plugged the other end in to the GC input. Sat hesitating for about a minute, and, while playing music, turned on the beast with the volume control WAY down. It was LOUD, very loud, and as far as I could tell the volume control was doing nothing at all to quieten it. It was, however playing music. I turned it off, checked everything... and tried again, it was still loud. I think the fuse blew when I turned it off that time.
What worries me is that it was so damn loud and the volume control was doing sweet FA. I measured the pre-amp output and it only seemed to be ~300mv, BUT this figure didn't seem to be changing when I twiddled the volume control either :( I suppose this is something wrong with my current amp. I'll worry about that later I guess.
The fuse I used is a 3.15A quick blow, and I have a 500VA toroidal transformer (originally was going for 300VA, but the 500VA was cheaper at the time!!) which might not be enough I guess? I have a 5A slow blow, but I am a little worried about turning it on with that though!?
Oh, also, as it's not in a case I haven't connected anything to the safety earth pin yet ( yes yes, call me an idiot but what am I supposed to connect it to at this point? :) ). Might this also have something to do with it, or not?
For what it's worth I didn't seem to see or smell any smoke, it just wouldn't turn on so I checked the fuse... and it seems the waxy stuff inside melted. Nice and green in there now :) Hopefully this is a good sign! |
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| AndrewT |
Hi,
how often do we need to tell everyone.
Use a mains light bulb wired in series with the live mains feed to test the unit from the mains, to avoid catastrophic blow ups.
It is cheap and potentially saves a fortune in damaged components if something has been mis-wired, transformers and caps particularly.
Output offset is measured with the output open circuit and the input short circuit. Do it again correctly and if it is still near 100mV then fix it first.
Come back with the correct offset reading while it is still connected through that light bulb. |
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| markiemrboo |
AH! I misread / misunderstood something somewhere then. I thought the output was just supposed to have a "load" to test the offset.
I guess just a wire from input +ve to input -ve on my board is OK? That would be what shorted means right? :)
By the way, I know about the light bulb thing. I just dont have a spare to do it with. My own risk and all that, yes, I know!
PS thanks for your really quick reply!! :D
EDIT: still ~90mv |
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| AndrewT |
Hi,
can you post your schematic?
We should be able to reduce that output offset substantially. |
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| markiemrboo |
| Ah cool, I used this schematic by carlosfm... |
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| AndrewT |
Hi,
first glance shows that Carlos is using mixed AC & DC coupling. That is not good for output offset and offset stability.
I recommend fully AC coupled or fully DC coupled, not mixed.
DC coupled means DC input at the non-inverting input pin AND DC feedback at the inverting input pin.
AC coupled means DC blocking capacitor in the input line to the non-inverting input pin (as shown in the schematic) AND DC blocking capacitor in the lower leg of the NFB loop.
Mixed AC & DC coupling means one of the DC blocking caps is omitted. |
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| markiemrboo |
By DC coupling you be refering to the 3.3uF cap between the power rails, I assume? Or the 300nF between input pins? Or even both of these?
Regardless, I do seem to recall carlos saying his DC offset with that circuit was < 50mv. Probably partly to do with my crummy PCB layout? :smash: :rolleyes: |
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| AndrewT |
Hi,
the non-inverting input pin sees a DC resistance of 330R + 15k =15330r to ground. All the input offset current has to sink (or source) through this resistance generating a +ve input offset voltage.
The inverting sees 100r//2k=95r2 ohms to ground. This generates a different (lower) -ve input offset voltage.
The sum of these voltages are multiplied by the chip amp gain (21Times) to give the output offset.
As the two input offset voltages are brought more equal in value the output offset reduces.
The easy solution is to insert a DC blocking cap into the lower leg of the NFB loop. but this will still leave 2k vs 15k3 unmatched input impedances. It will be better but not optimum.
Can anyone confirm that this chipamp is not internally compensated to reduce input offset currents? If it were so then balancing impedances is unlikely to offer a solution.
The secondary advantage of balancing the input offsets is reduced thermal variations in the output offset. With a chip amp thermal variations are enormous compared to discrete and very clever component layout is required inside the chip to minimise this critical parameter. Keeping the chip cooler and with as little temperature variation as possible will help. |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
the non-inverting input pin sees a DC resistance of 330R + 15k =15330r to ground. All the input offset current has to sink (or source) through this resistance generating a +ve input offset voltage.
The inverting sees 100r//2k=95r2 ohms to ground. This generates a different (lower) -ve input offset voltage.
The sum of these voltages are multiplied by the chip amp gain (21Times) to give the output offset.
As the two input offset voltages are brought more equal in value the output offset reduces.
The easy solution is to insert a DC blocking cap into the lower leg of the NFB loop. but this will still leave 2k vs 15k3 unmatched input impedances. It will be better but not optimum.
Can anyone confirm that this chipamp is not internally compensated to reduce input offset currents? If it were so then balancing impedances is unlikely to offer a solution.
The secondary advantage of balancing the input offsets is reduced thermal varaiations in the output offset. With a chip amp thermal variations are enormous compared to discrete and very clever component layout is required inside the chip to minimise this critical parameter. Keeping the chip cooler and with as little temperature variation as possible will help. |
All a bit over my head, but I think what you're trying to tell me is that I should be trying to get the feedback down to 2k, the same as the input? :xeye:
I don't know...
I just tried it again with a quiet song, and it comes out pretty darn loud. The speaker still works though :) I'm thinking maybe the fuse is just stupidly under rated for the toroid. The loud(er) song I played before must have put it past 3.15A? :xeye:
It sounded really "airy". I might try with one of my spare speakers at this point. At the moment it's just a cheapo full range driver sitting on my bed, not in an enclosure or anything. I haven't even listened to that driver before! |
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| Nordic |
you could probably drop the gain a bit...or double check the resistor value you used there... also double check the pot with a multimeter to make sure it is working and not dead shorted inside... I have never needed more than a 10k pot.
In fact if the pot is doing nothing, that is what is wrong, cause when set to 0 it is suposed to be very close to a ded short to ground over the input.
Also full range drivers outside of any "box" tend to be very efficient(read loud), could give you even 100db off a single watt in theory. |
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| AndrewT |
Hi,
I am not trying to solve the "too loud" problem.
I am trying to guide you through that nonsensical mixed AC & DC coupling that Carlos has designed into his excellent sounding chip amp.
The volume control problem is almost certainly outside the power amp "box".
Let's get the amp working properly and safely first.
The we can look at the volume control. |
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| markiemrboo |
Hi Nordic :)
Good idea checking the resistors, they seem to check out OK. I had to use a 2.2k instead of 2k for the feedback though.
What pot? :) Should it ideally be a pot between the 15k between input and ground?
Good news is, I can adjust the volume via the Windows mixer. It didn't blow my old speakers up, and they actually sounded alright. I wouldn't really like to compare to my current amp at the moment though :) For starters it's only one channel and the LM3886 is on a stupidly small test heatsink while I drill holes in something a little more suitable.
Bad news, the "PRE OUT" seems to be the output from my sound card! I still have the speakers connected to the amp, this might possibly have something to do with it I suppose :bigeyes:
Oh, more good news... there doesn't seem to be any humming or buzzing, even without the star grounding lala! I'm quite happy now. My "work" is starting to pay off finally :) |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
|
Hi :)
| quote: |
I am not trying to solve the "too loud" problem.
|
I figured :)
| quote: |
I am trying to guide you through that nonsensical mixed AC & DC coupling that Carlos has designed into his excellent sounding chip amp.
|
Alright, so... which is the AC coupling and which is the DC coupling? Is it as I said before? The DC being the 3.3uF capacitor between the +ve and -ve rails? The 330pF capacitor over the power rails? Both? Or am I just lost again? Do bear in mind that I'm not an electrical engineer or nout! I only really understand some of the basics. You might have to dumb it down a bit for me, if you're willing to try and explain!
| quote: |
The volume control problem is almost certainly outside the power amp "box".
|
Yeah. I think it's just my integrated amp's PRE OUT not doing what I think it should be doing. I think it must just be taking the output from my sound card, not the pre-amp. As in the previous post, this may be something to do with the speakers still being connected :xeye:
| quote: |
Let's get the amp working properly and safely first.
The we can look at the volume control. |
Had it running for 5 minutes playing with the volume on half on my sound card (which does adjust the volume). Didn't set fire to itself yet :D
Thanks for your replies so far! Much appreciated! |
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| markiemrboo |
More good news: I am officially an IDIOT! (was thinking about leaving it at that just for humour value).
My current amp has REC OUT and PRE OUT. I accidently plugged it in to REC OUT. The volume control now works, and it's no where near as loud! :D |
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| AndrewT |
Hi,
C1 is the DC block on the input. short it out and you have DC coupled input (but only if the source is also DC coupled).
Leave C1 in place and your chip amp is AC coupled at the input.
The DC block is omitted in the NFB loop. This makes the NFB side DC coupled. Add a DC blocking cap in series with R1 and that converts the amplifier to AC coupled at the NFB loop (it will have a DC gain of 1times = 0db gain). As is, it has a DC gain of 21 times =+26.4db gain) and it is this gain that is generating the big output offset from the much smaller input offset due to input offset currents flowing into and out of r1 & r2.
Carlos is going to disagree with me and since he is the designer his reasoning for omitting the DC block in the NFB loop carries more weight than mine.
But I would fit 1mF in series with r1 to make this amp AC coupled and significantly reduce the output offset as well as remain more stable, particularly in the presence of variable sources as well as high temps around the integrated input stage. |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
C1 is the DC block on the input. short it out and you have DC coupled input (but only if the source is also DC coupled).
Leave C1 in place and your chip amp is AC coupled at the input.
The DC block is omitted in the NFB loop. This makes the NFB side DC coupled. Add a DC blocking cap in series with R1 and that converts the amplifier to AC coupled at the NFB loop (it will have a DC gain of 1times = 0db gain). As is, it has a DC gain of 21 times =+26.4db gain) and it is this gain that is generating the big output offset from the much smaller input offset due to input offset currents flowing into and out of r1 & r2.
Carlos is going to disagree with me and since he is the designer his reasoning for omitting the DC block in the NFB loop carries more weight than mine.
But I would fit 1mF in series with r1 to make this amp AC coupled and significantly reduce the output offset as well as remain more stable, particularly in the presence of variable sources as well as high temps around the integrated input stage. |
I see! I can sort of understand that, thanks.
This capacitor should be an electrolytic, yes? I'm not sure if I have any 1uF's laying about, and ordering them the postage would probably cost more than the caps themselves! :(
Would the cap probably be the best way to get rid of the DC offset in my case? |
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| AndrewT |
Hi,
1uF in series with r1 will kill all the lower end response.
You need at least 470uF and preferably 1200uF to maintain decent bass response. The nearest value is 1mF.
100r and 1mF = 100mS with -3db @ 1.6Hz and good phase all the way down to 16Hz. Setting the input filter half an octave above this will give Bass performance above 20Hz that should be exemplary if the PSU is also scaled to give complementary performance. |
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| markiemrboo |
Hm. I googled mF before I posted, and it seems like mF = uF.
"MicroFarad (mF,uF or mfd),"
Would changing the feedback resistors work well enough? Changing the 2.2k to 220k and the 100R to 10k? I think this would give ~9.5k in parallel? Much closer to the 15k to ground.
220000R * 10000R = 2200000000R
220000R + 10000R = 230000R
2200000000R / 230000R = 9565.217R
I think this is what you were trying to explain to me earlier isn't it? What would the side effects of this be? I can change the resistors quite easily, I have a good supply of those in all different values (only 5% tolerances but still..)!
Thanks for the reply again :) I'm sure it must be quite stressful dealing with a clueless n00bite such as myself :D |
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| AndrewT |
Hi Marki,
G=*10^9
M=*10^6
k=*1000
m=/1000
u=/10^6
n=/10^9
p=/10^12
note from k and below all are in lower case.
Yes, your calculation for two parallel resistors is correct.
The same method is used for two capacitors in SERIES
Could you tolerate Zin=10k as the load for your source?
If so, then you have matching.
However two problems with this solution:-
The mixed coupling disadvantage still exists (hi DC gain and more offset variation with temperature).
High resistance around the NFB loop will increase noise (will it be noticeable). Carlos specifically warns against both medium and high NFB resistances.
Once you have them matched you can fine tune Zin to reduce the output offset, but only if the input offset current is NOT internally compensated. No-one has come back to answer my query. |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi Marki,
|
Good morning Andrew!
| quote: |
G=*10^9
M=*10^6
k=*1000
m=/1000
u=/10^6
n=/10^9
p=/10^12
note from k and below all are in lower case.
|
Ah, I see :)
| quote: |
Yes, your calculation for two parallel resistors is correct.
|
A good start for me then :D
| quote: |
The same method is used for two capacitors in SERIES
|
That's good to know, thanks!
| quote: |
Could you tolerate Zin=10k as the load for your source?
|
To be honest, I have absolutely no idea. Would this mean the GC has an input impedance of 10k, or 10k and 15k in parallel? I have had a look inside my current amp, which will be used as a preamp, and all I can really tell you about it is that it uses NE5532's, and has a unity gain output buffer (NE5532 again).
| quote: |
If so, then you have matching.
However two problems with this solution:-
The mixed coupling disadvantage still exists (hi DC gain and more offset variation with temperature).
|
If it "sounds good", then would this matter too much? The offset is unlikely to vary to dangerous levels? I'm not sure it bothers me too much at the moment, but I might not know the true dangers of running it like this etc :)
| quote: |
High resistance around the NFB loop will increase noise (will it be noticeable). Carlos specifically warns against both medium and high NFB resistances.
|
This is noise from the resistors, yes? That might be something I will have to experiment with then.. I did read that Carlos said he preferred the "sound" of lower value resistors, and I think alot of other people out there do too, but... (and no offence to anyone here).. I am generally quite skeptical of anything that many "audiophile's" have to say :)
| quote: |
Once you have them matched you can fine tune Zin to reduce the output offset, but only if the input offset current is NOT internally compensated. No-one has come back to answer my query. |
Ah :) Well I read about people tuning pots to compensate for the offset, so might one be able to deduce that it's probably not internally compensated from this? If it is, would the offset just not change much no matter how much you twiddle a pot?
Should the 10k be a pot itself, or should a pot be in parallel with it?
As far as progress goes, I've drilled a hole in a much bigger heatsink this morning and mounted it on my other channel. I'm using my other PSU board (one per channel), with all the main filter caps on it (still waiting for my panny fc's to arrive, so the other board only has half the caps on it). DC offset is ~ -30mv on this board. The other one was actually ~ -90mv, not 90mv, if that matters any. This should be acceptable as far as I have read!
I also noticed, at least on the -90mv board, it slowly drops. It got down to -80mv before I had to turn it off because the heatsink on that one got too hot though. The heatsink on that one is literally just a small sheet of aluminium about 3 times bigger than the chip in height, and the same width!
The heatsink now is much bigger, about the size of a Pentium 2 heatsink. I would say it gets quite hot, but I can keep my hand on it for as long as I want. I'd say it's probably around 40-45C. I know that oscillation can cause idle heat, so... is this a "normal"-ish temperature? Room temperature is usually ~30C.
I tried an oscillation test I read about yesterday, just putting a small resistor as the load and seeing if its gets warm. I used a tiny little 0.25W ( I think, could be 0.5W... I dont remember :( ) 10R resistor for 5 minutes or so. It didn't seem to get in the slighest bit warm. I don't have an oscilliscope to test with and can't really afford one, though I really would like to get my hands on one :(
Are there any other cheap ways to test for oscillation? What would oscillation sound like? Strange high frequency noises? Would a sine sweep of 20khz to 20hz maybe be bring out the strange noises a bit better than trying to differentiate them from actual music, or would it be very noticable? |
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| markiemrboo |
Ok, I listened with my ear right up close and there is a very quiet hissing coming from the tweeter (I have 'star grounded' it the best I know how, seperate ground return for speaker, input and power on a disconnecting network consisting of a bridge and 10R resistor).
The speakers on my current amp also have this hiss, so I am not sure it's anything to be worried about? While messing with the grounding I did manage to get a buzzing sort of clicking noise, just from the tweeter and a hum, from both the woofer and tweeter.
Is this OK? Or should it not even really be hissing? (Possibly something to do with the integrated amp and not alot I can do with it on the GC side!?)
Thanks all :D
EDIT: The sound quality from the one old speaker (Eltax Monitor III... hm!) seems good so far! |
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| AndrewT |
Hi,
if you have to get that close to hear the hiss, then it's not a problem.
I hope you normally listen a bit further away.
If all the hum and buzz are completely away, then great.
Now connect your source and check for hum and buzz and hiss again.
Are any of these noises more prominent?
Can you tolerate them? |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
if you have to get that close to hear the hiss, then it's not a problem.
I hope you normally listen a bit further away.
If all the hum and buzz are completely away, then great.
Now connect your source and check for hum and buzz and hiss again.
Are any of these noises more prominent?
Can you tolerate them? |
Well, I can't hear any humming or buzzing :) Just the hiss. It's only noticable if I put my ear right up close to the speaker though, so yeah.. I can tolerate them definately!
I did all the tests with the source connected! :bigeyes:
With no source, and the input NOT shorted, the hiss has an incredibly soft clicking sort of noise to it, and it's ever so slightly quieter. With the input shorted, it is dead silent. Absolutely nothing coming out of the speaker at all :) I did hear that the chips dont like to run without an input, so is this anything to worry about?
I've got the second heatsink drilled and attached now :) All I need is a case, and possibly to sort out the offset on the first board. I measured resistances and the only one that differs is the 330R, I think it was about 5R lower! :xeye: I will try replacing this resistor with one that matches the other board better..
One last question (sorry!). The power supply isn't star grounded at the minute. It's carlosfm's regulated 'snubbered' schematic, and requires independant secondaries. I did have ground wires on both -ve supplies next to the caps (which you say is bad, I think). When I connected them to the star ground the audio stopped, and the regulator heatsink seemed warmer than usual! Probably a stupid thing to do. Where should the star ground be? After the regulators? Does it even need one?
Thanks :) |
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| AndrewT |
Hi,
don't test your amp with the input RCA disconnected. Some amplifiers oscillate when the input is opencircuit. If your tweeters are connected then new tweeters.
The input shorting plug should be used whenever you test output noise or offset. If this is completely silent then your amp is probably working well.
If you connect your source and the output becomes noisier then the source is to blame for the extra noise. Do the tests in the wrong order and it becomes difficult to work out what is coming from where.
After you connect the source you should check that output offset again, it should be unchanged.
So it appears that the slight hiss you have is from your source, forget about it, until it starts to annoy you.
OOPS, that grounding.
The dual rectifier to separated smoothing caps works a bit differently to the single rectifier version.
Sounds like you connected the wrong wires and that caused the overheating, I hope you have not damaged something.
The output from each of the regulators is +ve and -ve.
Take one +ve to the amplifier +ve and take it's partner -ve to audio ground.
Take the other -ve to amplifier -ve and it's partner +ve goes to audio ground.
The G at far right is the audio ground and should be used as your central star ground.
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
don't test your amp with the input RCA disconnected. Some amplifiers oscillate when the input is opencircuit. If your tweeters are connected then new tweeters.
The input shorting plug should be used whenever you test output noise or offset. If this is completely silent then your amp is probably working well.
|
I see, thanks! Well, the tweeters still seem to be working OK. I was doing it all very quickly though, no more than a couple of seconds. I listened for any hiss, and turned it off right away. I might have got away lucky this time I guess, but I definately shalln't be doing it again if it can potentially cause damage!
| quote: |
If you connect your source and the output becomes noisier then the source is to blame for the extra noise. Do the tests in the wrong order and it becomes difficult to work out what is coming from where.
|
This makes sense :) It's definately the source making the hiss then. I might build myself a pre-amp at some point too, but for the moment lack of funds say I must use my current amp as a pre-amp :)
| quote: |
After you connect the source you should check that output offset again, it should be unchanged.
|
Yep, no change with the source connected. Do you think it's worth while changing that 330R resistor to see if that changes anything?
| quote: |
So it appears that the slight hiss you have is from your source, forget about it, until it starts to annoy you.
|
Done and done :D
| quote: |
OOPS, that grounding.
The dual rectifier to separated smoothing caps works a bit differently to the single rectifier version.
Sounds like you connected the wrong wires and that caused the overheating, I hope you have not damaged something.
|
Well, it all seems to work just fine still. I've been playing it for half an hour and it's still going! I think it's ok.
| quote: |
The output from each of the regulators is +ve and -ve.
Take one +ve to the amplifier +ve and take it's partner -ve to audio ground.
Take the other -ve to amplifier -ve and it's partner +ve goes to audio ground.
|
Alright, thank you! I put some extra holes around this area so I think I should be able to do this without much of a problem if I want to do it :)
You have been a great help, thanks so much! |
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| markiemrboo |
Just to let you know, changing the 100R and 2.2k to 1k and 22k did nothing for the DC offset. I guess it must be the chip?
I also get a thump, from both treble and bass drivers it seems, if I turn on / off the pre-amp while the GC is on. It sounds quite nasty! I measured the DC voltage coming from the pre-amp at about 0.300mV when this happens, though my multimeter isn't that good and I guess it could be more. The voltage at the speaker is also "only" about 0.300mV. Anything much to worry about? It sorta confused me that this DC voltage got through to the speakers, as I have an input cap. I thought this was supposed to block DC :xeye: |
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| AndrewT |
Hi,
it'd not DC, it's a pulse.
That pulse includes some or a lot of higher frequencies in it.
The higher frequencies are not attenuated sufficiently by the filter.
You can do a variety of fixes.
The easy one is to invoke the chipamp shut down pin.
Alternatively you fit an input mute or output relay.
Since the problem is in the pre that is where you should do the fix.
A tiny signal relay supplied from one diode on the PSU bridge with a delay to the pull up voltage.
Any or all of these should be "instant off" on mains failure and "delayed on" at start up. |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
it'd not DC, it's a pulse.
That pulse includes some or a lot of higher frequencies in it.
The higher frequencies are not attenuated sufficiently by the filter.
|
Ah, that explains it. Thanks.
| quote: |
You can do a variety of fixes.
The easy one is to invoke the chipamp shut down pin.
Alternatively you fit an input mute or output relay.
|
Of course, there's also always the "don't turn the preamp on before the power amp" option too :) I don't feel confident modifying this preamp really. I think I will probably soon build my own preamp though. An opamp one based on an ESP project schematic probably...
| quote: |
Since the problem is in the pre that is where you should do the fix.
A tiny signal relay supplied from one diode on the PSU bridge with a delay to the pull up voltage.
Any or all of these should be "instant off" on mains failure and "delayed on" at start up. [/B] |
Noted and noted. :)
A small update, I think I already mentioned I tried slightly higher resistor values to try and get the DC down, and it did nothing, so I put them back to the schematic values (and "upgraded" to 1% metal film while I was at it).
I did some heavy searching and eventually came across a thread, http://www.diyaudio.com/forums/show...=&pagenumber=2, which explained things quite well. The calculations seemed to show that in order to get it to decrease I would need to go "the next step up" to 220k and 10k in the feedback.
I've just now modified the boards slightly and added a polarized :( electrolytic as Ci at 470uF, and the offsets are now reduced to 3mV and 1mV. I know 470uF isn't really enough apparently, it's -3dB at 3.4Hz? And that could be out of phase all the way up to about 34Hz? I just found them laying about and decided to give it a go.
Another question then: If I go up to 22k and 1k in the feedback (from 2.2k and 100R) and continue to use a 470uF, is this far too much? This would apparently be -3dB at 0.3Hz? Should I be looking at something more like 220uF (0.7Hz) or even 100uF (1.6Hz)?
I'm still waiting on my case to arrive, but I imagine I will be putting pictures of my monstrosity up soon and telling you all how impressed I am (hopefully) at the sound quality and things :) |
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| AndrewT |
Hi,
why go all the way to 22k/1k?
Carlos says stay low, so try 4k3/200r with the 470uF |
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| markiemrboo |
| quote: | Originally posted by AndrewT
Hi,
why go all the way to 22k/1k?
Carlos says stay low, so try 4k3/200r with the 470uF |
Ah yes, my incredible math skills are starting to show their true colours. A fantastic idea. I still get pretty much the same gain with that kind of combo too, it was just easy for me to see that moving up to 22k/1k would still give me a gain of 22, basically the only reason I suggested those :) :smash:
The cap I have in there is also "only" 25v aswell. I have 28.5v rails, but doesn't that cap "see" the output voltage and not the rail voltage, so this should be "adequate" and quite safe, I think?
Thanks once again Andrew! |
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| AndrewT |
Hi,
no, the divider action of 2k2 / 100r reduces the output AC signal back down to the same level as the input.
Remember that a power amplifier is really just an overgrown discrete opamp.
and we all know that the output of an opamp is equal to the difference between the inverting and non-inverting pins times the opamp gain.
This analogy confirms that the two sides of the LTP see almost exactly the same voltage.
HOWEVER.
if the output stage fails and pulls the output to a fixed high DC bias then the DC blocking action of the NFB capacitor ensures it sees the DC bias across it.
Two problems:
1. the inverting input is subjected to this high voltage and probably fails.
2. the cap is subjected to this DC and probably fails IF the bias exceeds it's rating (the bias could be in either direction).
To avoid both problems, some designers fit a pair of Zeners across the cap or a pair of dual diodes to limit the DC to a maximum of between 1.4V and 3.9V |
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