Re: power supply
If you have hum with no signal, it is probably not because of small caps. At no load, the ripple is very low and would not be audible.
Possibly your switch from the lab supplies to the xformer also changed some of the supply wiring, especially the grounding scheme? You may have a groundloop.
Jan Didden
suho said:
If you have hum with no signal, it is probably not because of small caps. At no load, the ripple is very low and would not be audible.
Possibly your switch from the lab supplies to the xformer also changed some of the supply wiring, especially the grounding scheme? You may have a groundloop.
Jan Didden
My symasym was DEAD silent, but now I changed somethings in the PSU, and now it's noisy. Blargh, I hate these issues, I'm going to rebuild it at all.
I can't bias Symasym. I need 12mV at the emiter pin of the transistor right? But I can get -12mV, when I get 12mV, the other transistor is far distant from this value. Am I doing anything wrong?
I can't bias Symasym. I need 12mV at the emiter pin of the transistor right? But I can get -12mV, when I get 12mV, the other transistor is far distant from this value. Am I doing anything wrong?
Nando, are you measuring across emitter resistor, or from emitter to ground? You must measure across emitter resistor, or you will also have dc-offset in your measuring. How big is you dc-offset?
Suho, are you using 2x4700µf per Channel?
Still, i use 2x22.000µf for both channels together, halfing that value shouldn't give audible hum. I suspect the wiring/groundloop. How is your PSU built up? With a single rectifier? For a single rectifier you have to use correct polarity from the transformer-output. You have to measure 2x30=60v AC at the AC-terminals of the rectifier. If you measure ~0, you need to revert one polarity, so that both secondary windings are in series. Otherwise you are "pumping" the ground with 50hz.
Mike
Suho, are you using 2x4700µf per Channel?
Still, i use 2x22.000µf for both channels together, halfing that value shouldn't give audible hum. I suspect the wiring/groundloop. How is your PSU built up? With a single rectifier? For a single rectifier you have to use correct polarity from the transformer-output. You have to measure 2x30=60v AC at the AC-terminals of the rectifier. If you measure ~0, you need to revert one polarity, so that both secondary windings are in series. Otherwise you are "pumping" the ground with 50hz.
Mike
Hi Mike !
I biased Symasym, but I can't get 12mV. It's with -15mV at both transistors, my offset is also 15mV.
The amplifier works fine, but I have the power on thump.
I'm using 42V supply rails and 2N5551.
What you think?
edit: I already tested with + - 25v rails and I have the same behavior. What's annoying me its the power on thump, the woofer moves a lot !
All tests with nothing conected to symasym's input.
Best Regards !
I biased Symasym, but I can't get 12mV. It's with -15mV at both transistors, my offset is also 15mV.
The amplifier works fine, but I have the power on thump.
I'm using 42V supply rails and 2N5551.
What you think?
edit: I already tested with + - 25v rails and I have the same behavior. What's annoying me its the power on thump, the woofer moves a lot !
All tests with nothing conected to symasym's input.
Best Regards !
Hi -_nando-_,
-Chris
Then there is something wrong for sure. Mine are quite and have a very soft thump on power up.
-Chris
I don't hear BOOOM, it just moves the woofer backwards and then it leaves the woofer in the normal position.
It sounds perfect, no heat or distortion.
I have two Symasyms, they were running with MPSA18 and 33v supply rails, now I'm using the same modules, but with 2N5551 because of the new 42V rails.
I didn't biased the amp before doing this mod, and it worked fine. But as the speakers (JBLs) hides the woofers, I didn't see if the woofers were moving or not when I power the system on.
Is this happening because of the 2N5551s 😕
It sounds perfect, no heat or distortion.
I have two Symasyms, they were running with MPSA18 and 33v supply rails, now I'm using the same modules, but with 2N5551 because of the new 42V rails.
I didn't biased the amp before doing this mod, and it worked fine. But as the speakers (JBLs) hides the woofers, I didn't see if the woofers were moving or not when I power the system on.
Is this happening because of the 2N5551s 😕
Hi -_nando-_,
The DC correction loop is charging too slowly. The time constant is a hint here. If the gain is low with your diff pair, the input current will be higher. Are your capacitors that couple the feedback network to ground much larger than spec?
-Chris
The DC correction loop is charging too slowly. The time constant is a hint here. If the gain is low with your diff pair, the input current will be higher. Are your capacitors that couple the feedback network to ground much larger than spec?
-Chris
Hi Anatech !
No, all capacitors are exactly the same value shown on the MikeB's webpage (symasym 5.3).
As I said, despite this strange thing, the amp works perfect.
Best Regards !
No, all capacitors are exactly the same value shown on the MikeB's webpage (symasym 5.3).
As I said, despite this strange thing, the amp works perfect.
Best Regards !
Maybe it goes away if you match your input transistors? With matched transistors you will get ~3mv dc-offset insteadof 15mv.
Mike
Mike
The system is sounding so sweet... Heatsink is little warm, but I don't hear any oscilation ! Finally the combination of chipamp + symasym worked well !!
MikeB said:
Yes, I think so ! These transistors seems to be very far from each other. They're On-Semi.
The worst brand in my opinion is ST. Their regulators are crap, and transistors are very difficult to match, Fairchild is far better ! I think that I have the same problem here with On-Semi.
You know what, my opamps also "thump" at power on, so I'll put a relay with a delay time to connect the woofer / tweeter to the amps at the start. 😀
Nando, please, inform where you have posted your Symassym picture
Symassym do not make "thump".... no subsonic movement into the amplifier.
I will be happy to watch, to see, a real image of this misterious amplifier.
Écadauma...putz!
Carlos
Symassym do not make "thump".... no subsonic movement into the amplifier.
I will be happy to watch, to see, a real image of this misterious amplifier.
Écadauma...putz!
Carlos
Re: Nando, please, inform where you have posted your Symassym picture
Indeed. Mine is completely silent on power up, and on power down fades into distortion as the PSU caps discharge.
destroyer X said:
Indeed. Mine is completely silent on power up, and on power down fades into distortion as the PSU caps discharge.
15mV of DC offset is strange, however. Even for unmatched input devices symasym easily keeps 3mV.
Symasym does not have DC gain (just 1x), for the reason of capacitor from feedback divider resistor to ground.
Symasym does not have DC gain (just 1x), for the reason of capacitor from feedback divider resistor to ground.
-_nando-_ said:
So, your preamps are creating the turn-on thump?
Build better preamps with symetrical powersupply...
Mike
Mike,
most of opamps have initial thump after turn-on PSU. With symmetrical PSU. The thump depends on internal circuit structure of the OA.
most of opamps have initial thump after turn-on PSU. With symmetrical PSU. The thump depends on internal circuit structure of the OA.
MikeB said:
If my mom wouldnt have thrown my good old scope into the waste I would have figured out what the prob is earlier ... It is the wire from the PC-souncard to the amplifier which picks up the 50Hz. Is there an easy way to filter this out, bypassing it to the ground might change the music-signal itself right? I mean it is not that annoying cause you can only hear it when the pot is in turned on fully.
Hi,
it is more likely that the PC is adding 50Hz to the output signal and sending that composite down the wire.
If you notch it out you lose some of the wanted signal.
After you have notched it out, you'll probably find that some 100Hz remnants become the source of annoyance.
Guess what, after you notch that one out, you'll find that the annoying harmonics go way up into the mid range. Too many notch filters to be useful.
Forget filtering the wanted signal to remove the offending hum.
If you are really lucky, it is as you said, pick up between the PC and receiver. Good screened cable should be able to prevent this hum contamination. A double lapped copper screen + metallised film wrap gives excellent rejection of interference. Some are available with an aluminium foil wrap and even more expensive is the copper foil wrap.
An alternative for even better rejection is balanced signalling for both send and receive, but you PC is probably not worth all this effort.
it is more likely that the PC is adding 50Hz to the output signal and sending that composite down the wire.
If you notch it out you lose some of the wanted signal.
After you have notched it out, you'll probably find that some 100Hz remnants become the source of annoyance.
Guess what, after you notch that one out, you'll find that the annoying harmonics go way up into the mid range. Too many notch filters to be useful.
Forget filtering the wanted signal to remove the offending hum.
If you are really lucky, it is as you said, pick up between the PC and receiver. Good screened cable should be able to prevent this hum contamination. A double lapped copper screen + metallised film wrap gives excellent rejection of interference. Some are available with an aluminium foil wrap and even more expensive is the copper foil wrap.
An alternative for even better rejection is balanced signalling for both send and receive, but you PC is probably not worth all this effort.
suho, I found a really neat isolation transformer from dak industries on the web. It is a 47k isolation transformer with two rca cables attached. Only 29 dollars u.s. I completely eliminated all of the problems i had running material from my computer sound card to the amp/preamp. It is a quality item and worked well for me. You might want to check it out.
Regards, Tad
Regards, Tad