Nuuk said:
An externally hosted image should be here but it was not working when we last tested it.
The 3875 is mute (or not turned 'on') when voltages are below +/- 10V.
Now I am not sure how, they do not reveal the U-V Protection Circuit. But it would seem that as the power supply comes 'on' there is a delay before the output devices conduct and that it does so softly. Even if this 'soft delay' is only 100mS before turn-on and near full 52mV DC-Offset occurs, that equates to 10Hz/2 = 5Hz (the cone moves then air pressure settles, causing a half wave 5 Hz).
Naturally not audible.
Hum, on the other hand, is likely to be a different cause. One is a DC problem and the other, hum, AC.
Joe R.
ron clarke said:
Battery could make a big difference. How do you turn 'on' the power supply? If you got a two-pole switch on DC, then the the two DC rails are not applied at the exact same moment in time. Unless you have a perfect mechanical switch or a high precision electronic one? In AC power supplies, this exact synchronisation is not required.
Joe R.
tweeter
Tweeters and mid horns wold not be very happy with this, I assume?
I saw the JBL active monitors schematic-uses LM3886-and they use NO capacitor in the tweeter amp but the 3886 have a mute function.
The Genelec instead, include a capacitor but 2 and a half octaves down the crosover frecuency.
I'd really like to avoid the capacitor but I fear for the tweeters.
Ric
Joe Rasmussen said:
Tweeters and mid horns wold not be very happy with this, I assume?
I saw the JBL active monitors schematic-uses LM3886-and they use NO capacitor in the tweeter amp but the 3886 have a mute function.
The Genelec instead, include a capacitor but 2 and a half octaves down the crosover frecuency.
I'd really like to avoid the capacitor but I fear for the tweeters.
Ric
Re: tweeter
Personally I'd like to see less than 10mV if connected straight to tweeter voice coil - very tricky indeed. If you have a way of connecting 50mV DC to a tweeter voice coil and look at the dome itself, from side on, and see if the dome moves in or out. If the dome doesn't move, then maybe it isn't so bad? Just calculate the DC resistance of the voice coil, let's say 6 Ohm, the 6x(0.05/6) = 0.000416 or 0.416mW. So heat may not be a problem?
Get it down to 10mV and it becomes 0.16mW.
I got curious, and being a 'hands-on' sort of guy, I decided to feed DC into a Vifa tweeter I had on hand. The DCR was 6 Ohm. When I fed 50-60mV there was no noticeable movement, then I gradually increased the V. It wasn't until 0.5V or more (500mV) I could see any displacement of the dome. This is 'better' than I expected. As tweeters have small Xmax, this is important.
So it must be the dissipation that is even more important. Any excess heat in the voice coil erodes the Qe and thus, when exposed to dynamic conditions, reduces transient response. Stating the bleeding obvious, not good in a tweeter.
Hope this has been helpful.
Joe R.
Ricren said:
Personally I'd like to see less than 10mV if connected straight to tweeter voice coil - very tricky indeed. If you have a way of connecting 50mV DC to a tweeter voice coil and look at the dome itself, from side on, and see if the dome moves in or out. If the dome doesn't move, then maybe it isn't so bad? Just calculate the DC resistance of the voice coil, let's say 6 Ohm, the 6x(0.05/6) = 0.000416 or 0.416mW. So heat may not be a problem?
Get it down to 10mV and it becomes 0.16mW.
I got curious, and being a 'hands-on' sort of guy, I decided to feed DC into a Vifa tweeter I had on hand. The DCR was 6 Ohm. When I fed 50-60mV there was no noticeable movement, then I gradually increased the V. It wasn't until 0.5V or more (500mV) I could see any displacement of the dome. This is 'better' than I expected. As tweeters have small Xmax, this is important.
So it must be the dissipation that is even more important. Any excess heat in the voice coil erodes the Qe and thus, when exposed to dynamic conditions, reduces transient response. Stating the bleeding obvious, not good in a tweeter.
Hope this has been helpful.
Joe R.
Hi Nuuk,
Long time no see.
I'm been working like crazy here at work...
Do you remember a thread I openned some time ago with some tests and conclusions about resistors, M-T pots, DC offset, etc?
I was testing the LM3886, and that thread cost me some time to write.
You have some answers to your questions there.
Curiously, I can't find it anymore! 
3 pages of threads in "Chip Amps" and nothing...
Administratooooor!!!...
Oh, BTW, yes, the chips have a huge production drift.
Long time no see.
I'm been working like crazy here at work...
Do you remember a thread I openned some time ago with some tests and conclusions about resistors, M-T pots, DC offset, etc?
I was testing the LM3886, and that thread cost me some time to write.
You have some answers to your questions there.
Curiously, I can't find it anymore!
3 pages of threads in "Chip Amps" and nothing...
Administratooooor!!!...
Oh, BTW, yes, the chips have a huge production drift.
testing DC offset
I was recently hooked on this Gainclone fever. But as a newbie, could anybody tell me how to test the DC offset on the speaker terminal? I read the FAQ on the Dungeon site, but I did not understand how I should fasten the multimeter.
Thanks in advance.
best wishes
Jan Ove Tangen
I was recently hooked on this Gainclone fever. But as a newbie, could anybody tell me how to test the DC offset on the speaker terminal? I read the FAQ on the Dungeon site, but I did not understand how I should fasten the multimeter.
Thanks in advance.
best wishes
Jan Ove Tangen
I know how to use the multimeter But I wanted to know if I should fasten the red alligatorclip (on the mulitmeter) on the red output terminal and the black alligatorclip on the black output terminal, with an 8 ohm resistor between the terminals.
But thanks for answering. Og fra en nordmann til en annen: underlige steder en kan finne landsmenn?
best wishes
jan ove
But I wanted to know if I should fasten the red alligatorclip (on the mulitmeter) on the red output terminal and the black alligatorclip on the black output terminal, with an 8 ohm resistor between the terminals.But thanks for answering. Og fra en nordmann til en annen: underlige steder en kan finne landsmenn?
best wishes
jan ove
I know how to use the multimeter
It doesn't matter which way round you attach the meter leads to the terminals. It is the difference in voltage that is important, not its polarity. But you have the right idea, one meter lead to each speaker terminal!
Jan Ove Tangen said:I know how to use the multimeter
But thanks for answering. Og fra en nordmann til en annen: underlige steder en kan finne landsmenn?
best wishes
jan ove
looks like you got the answers you needed
undelig? det er en del av oss her, flere en en kanskje skulle tro..
-marius
Thank you for your kind help. I have now measured the DC offset on my finished gainclone. On the right channel the value was 30,9 mV and 13,8 mV on the left channel.
If I understand the earlier answers to NUUK in this tread, this is nothing more than to expect, and nothing to worry about. Or?
I do not hear the faintest of hum or any other alarming sounds, which pleases me a lot! Neither do I hear any thump at all. And the amp plays music very well. But the exitement of getting it together may colour my listening abilitities
My amp i based on the Brian GT PCB and the great help and service of Peter Daniel. Thanks Peter!
And again, thank you for your help! Greatly appreciated. This Internet is a great thing: allowing people to share knowledge and experience.
Best
Jan Ove
If I understand the earlier answers to NUUK in this tread, this is nothing more than to expect, and nothing to worry about. Or?
I do not hear the faintest of hum or any other alarming sounds, which pleases me a lot! Neither do I hear any thump at all. And the amp plays music very well. But the exitement of getting it together may colour my listening abilitities
My amp i based on the Brian GT PCB and the great help and service of Peter Daniel. Thanks Peter!
And again, thank you for your help! Greatly appreciated. This Internet is a great thing: allowing people to share knowledge and experience.
Best
Jan Ove
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