On occasion, I've used phono preamplifiers that are powered by wall warts of the AC-DC variety, and I've noticed that preamplifiers without any rectification beyond what is in the wall wart are very sensitive to any sort of AC being picked-up by the wall wart's cord.
I've also used the inexpensive but not bad Rolls 2-way crossover. Although this unit does have on-board rectification, it is similarly plagued by power-cord induced noise.
On the TC-750 phono preamp I've been modifying, the designer included some 1000uf 25v caps on the DC input that should block any AC picked up by the cord (I think). But I've removed these because the rating was insufficient for the higher voltage I'm using (I'm running a 24v adapter instead of the stock 12v).
I do intend to determine a way to shoe-horn some 50v capacitors in there (perhaps one above the board, one beneath), but in the meantime, the device is sensitive to routing of the wall wart's cord.
And I've even figured out a way to do that, to keep it nearly as quiet as running on 24v battery (qty. 16 AA).
But I was wondering, if the problem is coupling with AC magnetic fields, would chucking the DC plug in my cordless drill and spinning the two-conductor flat cord be a possible aid in reducing the coupling? While it is a very small cord and the conductors very close to each other (which should work nearly as well as a twisted pair), I wonder if twisted, would be even better?
A more expensive option may be to use the Inter-8 weave cable, or a DIY version of it. But this would require a little more effort/expense, as I'd have to cut the existing cord off the adapter I'm using:
INTER-8® Weave Cable
Anyways, I thought it was sort of general interest as so many here have fought wall wart issues, so figured I'd ask.
I did post this over at the Steve Hoffman forum but wasn't sure how much response I'd get there, so I'm sort of "cross-posting" this here due to the more technical nature of this site.
I've also used the inexpensive but not bad Rolls 2-way crossover. Although this unit does have on-board rectification, it is similarly plagued by power-cord induced noise.
On the TC-750 phono preamp I've been modifying, the designer included some 1000uf 25v caps on the DC input that should block any AC picked up by the cord (I think). But I've removed these because the rating was insufficient for the higher voltage I'm using (I'm running a 24v adapter instead of the stock 12v).
I do intend to determine a way to shoe-horn some 50v capacitors in there (perhaps one above the board, one beneath), but in the meantime, the device is sensitive to routing of the wall wart's cord.
And I've even figured out a way to do that, to keep it nearly as quiet as running on 24v battery (qty. 16 AA).
But I was wondering, if the problem is coupling with AC magnetic fields, would chucking the DC plug in my cordless drill and spinning the two-conductor flat cord be a possible aid in reducing the coupling? While it is a very small cord and the conductors very close to each other (which should work nearly as well as a twisted pair), I wonder if twisted, would be even better?
A more expensive option may be to use the Inter-8 weave cable, or a DIY version of it. But this would require a little more effort/expense, as I'd have to cut the existing cord off the adapter I'm using:
INTER-8® Weave Cable
Anyways, I thought it was sort of general interest as so many here have fought wall wart issues, so figured I'd ask.
I did post this over at the Steve Hoffman forum but wasn't sure how much response I'd get there, so I'm sort of "cross-posting" this here due to the more technical nature of this site.
I've done that, actually. With the arm on the rest, I record a sample and then use REW to take a look. I've attached a screen shot.
FWIW, I've improved cord routing since this point, and noise has dropped. But I'd like less sensitivity to the cord routing.
Thanks for any/all help.
FWIW, I've improved cord routing since this point, and noise has dropped. But I'd like less sensitivity to the cord routing.
Thanks for any/all help.
Attachments
If I understand correctly, you have modified your phono preamplifier by replacing the 12V DC power supply with a 24V supply, and as a result had to remove the smoothing caps, and now you have ripple on the output, and you want to fix that by using different wire. Have you considered putting things back the way they were?
Yeah that essentially sums things up BUT, I did follow a guide here:
TC-750 Phono Pre-Amp (Continued) | Super Best Audio Friends
They essentially said those caps should be unnecessary given a better power supply. The stock power supply is a 12v unregulated transformer. The wall wart I'm using now is a 24v linear regulated unit.
Those original caps were 1000uf/25v, the original supply was 12v, the new one is 24v, so I don't know that I can simply reinstall those caps, or if I have to find something rated higher.
Interestingly, the board's screening shows 220uf/16v for those two locations. And things are tight in there. 220uf/35v caps would be easier to fit than 1000uf/35v caps.
And the question on the twisting of the cord does sort of remain, whether twisting will better help it avoid magnetic fields or not.
I got it sorted for now with a couple of 470uf 50v caps I found in an assortment I once purchased to help fix something else. I also reinstalled the 470-Ohm resistor that had been removed.
I then swung the untwisted cord around and couldn't detect much (any?) sensitivity to placement. So I guess the question of whether twisting the cord makes sense is moot.
I think the original guide I was following, expected one would use a linear power supply that wasn't connected via a 6' cord, to the preamp. Something right next to the preamp, perhaps.
Just figured I'd update in case anyone ever reads this.
I then swung the untwisted cord around and couldn't detect much (any?) sensitivity to placement. So I guess the question of whether twisting the cord makes sense is moot.
I think the original guide I was following, expected one would use a linear power supply that wasn't connected via a 6' cord, to the preamp. Something right next to the preamp, perhaps.
Just figured I'd update in case anyone ever reads this.