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RF preventing me from using my home studio
Old 11th April 2021 | Show parent
  #61
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Aaron Rash's Avatar
 
🎧 10 years
Quote:
Originally Posted by Radardoug ➡️
Aaron, if you buy a couple of good quality Jensens, you will not hear any audio difference. Or you could sell your piece of crap amplifier and buy something that will work, with proper input circuits and a ground on the chassis. Dont try and flog a dead horse.

I just got this amp to use with my NS-10's I've seen it recommend by a ton of people including Jim Williams among others, but I doubt any of them were close to a transmitting tower. In my situation It does suck...

I'm pretty limited on budget and the Adcom is what everyone recommended to me for NS-10's. I wonder what else would be a good fit in my price range of about $200 or so.

It's either that or retrofit some transformers into it. In that case, I wonder what the most transparent input transformers would be a good fit for the unit.

Thanks for the input.
Old 11th April 2021
  #62
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David Rick's Avatar
 
🎧 15 years
You can put an RC filter on the center conductor of a normal RCA connector with the C between center and shield. Actually, there already such a filter in your Adcom, but it's on the circuit board, not right at the connector where it belongs. The idea with RF is to keep the fox out of the hen house, not try to kill it once it's inside.

Filtering the center conductor won't help anyway, because the RF entry is actually happening on the shield. The way your Adcom is designed, noise on the shield ends up getting into the amplifier feedback path.

If the chassis were earthed, you could probably get away with an hanging a 1200 pF ceramic cap directly from the floating RCA shield to the nearby chassis. But since the chassis is not earthed, it won't be as effective.

It's a bad situation, and I'm not gonna mince words: Your amplifier is poorly designed from a RF immunity standpoint and you've deployed it in a challenging high field strength environment. If you have more time than money, try some of the suggestions I've made here and see if any of them yield an improvement. Maybe you'll get lucky. If you have more money than time, either ditch the amplifier for a more robust one, or buy one of these:

Jensen Iso-Max PC-2XR

I know it seems expensive, but if I were billing you for time, you'd already have spent more than that.

David
Old 11th April 2021 | Show parent
  #63
Lives for gear
 
Aaron Rash's Avatar
 
🎧 10 years
Quote:
Originally Posted by David Rick ➡️
You can put an RC filter on the center conductor of a normal RCA connector with the C between center and shield. Actually, there already such a filter in your Adcom, but it's on the circuit board, not right at the connector where it belongs. The idea with RF is to keep the fox out of the hen house, not try to kill it once it's inside.

Filtering the center conductor won't help anyway, because the RF entry is actually happening on the shield. The way your Adcom is designed, noise on the shield ends up getting into the amplifier feedback path.

If the chassis were earthed, you could probably get away with an hanging a 1200 pF ceramic cap directly from the floating RCA shield to the nearby chassis. But since the chassis is not earthed, it won't be as effective.

It's a bad situation, and I'm not gonna mince words: Your amplifier is poorly designed from a RF immunity standpoint and you've deployed it in a challenging high field strength environment. If you have more time than money, try some of the suggestions I've made here and see if any of them yield an improvement. Maybe you'll get lucky. If you have more money than time, either ditch the amplifier for a more robust one, or buy one of these:

Jensen Iso-Max PC-2XR

I know it seems expensive, but if I were billing you for time, you'd already have spent more than that.

David
Hi David,

I've got more time than money at this time unfortunately.

I just thought of something... What if I were to put a 1200pF cap on the Adcom end of the RCA cable shield and connect the other end of the cap to its own dedicated ground separate from the Adcom.

This way the chassis would still be floating which is a bad design for RF but that could maybe possibly work.

I wish I could just ground the damn RCA shield and Chassis without it damaging the amp.

I paid $350 for the amp. For Jenson transformers it would cost me another $150 which is too far out of my range to spend on an amp right now.

Unfortunate but thats reality. I can also see if the seller will take the amp back since I just got it from Reverb. But then I'm stuck hunting for another amp.
Old 11th April 2021 | Show parent
  #64
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nosebleedaudio's Avatar
 
🎧 15 years
RF can get in via the Speaker cables...
Old 11th April 2021 | Show parent
  #65
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Aaron Rash's Avatar
 
🎧 10 years
Quote:
Originally Posted by nosebleedaudio ➡️
RF can get in via the Speaker cables...
Hi

In my situation its coming through the input cables. Ive already narrowed it down and cut the noise out by shortening the input cables.

The problems is that I can't practically use my interface connected to an amp with 6 inch input cables.


Getting the RF out of input cables that are a few feet long is my challenge with a poorly designed amp when it comes to RF unfortunately
Old 12th April 2021 | Show parent
  #66
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Radardoug's Avatar
 
🎧 10 years
Well, now we know you got the amp off Reverb, try and return it. Then you have money to buy another amp. Buy something with XLR inputs and a grounded chassis.
Heres a Yamaha amp on Reverb.
https://reverb.com/item/39831645-435...mplifier-p2250
Its in your price range, grunty as, and made by a good manufacturer.
Old 12th April 2021
  #67
Gear Guru
 
Thomas W. Bethe's Avatar
 
1 Review written
🎧 15 years
New product just out see it here https://www.amazon.com/Protection-El...dDbGljaz10cnVl

Maybe exactly what you are looking for...
Old 12th April 2021 | Show parent
  #68
Gear Maniac
 
borjam's Avatar
 
🎧 10 years
Quote:
Originally Posted by Aaron Rash ➡️
Hi

In my situation its coming through the input cables. Ive already narrowed it down and cut the noise out by shortening the input cables.

The problems is that I can't practically use my interface connected to an amp with 6 inch input cables.


Getting the RF out of input cables that are a few feet long is my challenge with a poorly designed amp when it comes to RF unfortunately
But did you actually try the ferrite beads? It's a really inexpensive solution.
Old 12th April 2021
  #69
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David Rick's Avatar
 
🎧 15 years
The "fix" is on the drawing board, not on Amazon

Hey, Thomas,

Aaron already has a Faraday cage: His amp is built in a steel box. But Faraday cages only work if all the cable penetrations are managed properly. Basically, you need to terminate the cable shields at the box so that the shields become an extension of the Faraday cage. If you route them into the box without proper termination, then the interior cable extensions become antennas, radiating noise inside the box. The same idea applies to conducted interference: it needs to be filtered on each wire at the box entry, not brought inside to contaminate the circuit board. I reiterate: Keep the fox out of the hen house! These fundamental principles were clearly explained by Neil Muncy 25 years ago. He and Jim Brown have both taught AES classes on it. Obviously, the Adcom folks cut class.

Balanced inputs are useful when there is a long distance between interconnected pieces of equipment. Properly implemented balanced interconnects will reject common-mode noise such is often seen when the interconnected equipment is run from different power circuits. Two pieces plugged into the same outlet strip rarely exhibit such problems and will usually work just fine with unbalanced connections. But, since Muncy's seminal paper on the "pin 1 problem", designers of equipment with balanced inputs have tended to get their cable entries correct as well. Even "pro-sumer" recording gear sometimes gets it right these days, because it's possible to spec XLR connectors that tie pin 1 directly to the mounting panel.

David L. Rick
Old 12th April 2021 | Show parent
  #70
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Radardoug's Avatar
 
🎧 10 years
Quote:
Originally Posted by Thomas W. Bethe ➡️
New product just out see it here https://www.amazon.com/Protection-El...dDbGljaz10cnVl

Maybe exactly what you are looking for...
They cant even spell fabric properly!
Old 13th April 2021
  #71
Gear Guru
 
Thomas W. Bethe's Avatar
 
1 Review written
🎧 15 years
Early in my career I worked in all kinds of radio and TV stations. Some had the transmitter, literally in the back yard, and in one case on the roof of the building. We had to do a lot of RF proofing for the audio gear. The suggestions here are all good. It just takes time and lots and lots of trouble shooting. Best of luck to the OP.
Old 13th April 2021
  #72
Gear Guru
Buy some snap on ferrite bead cores and place one on each end of the cable, about 2" away from the RCA connector. Build a good RCA cable, I like Mogami 3080 AES cable. The amp has a 300 pf cap to ground following a 1k resistor, only a -6 db/octave roll-off. The amps are also floated from AC chassis ground due to a two conductor power cable. The chassis ground is connected to the audio ground internally. The RCA cable provides all the screening so it's important to use a good one.
Old 4 weeks ago | Show parent
  #73
Gear Guru
 
UnderTow's Avatar
 
🎧 15 years
Quote:
Originally Posted by David Rick ➡️
I reiterate: Keep the fox out of the hen house! These fundamental principles were clearly explained by Neil Muncy 25 years ago. He and Jim Brown have both taught AES classes on it. Obviously, the Adcom folks cut class.
In their defense, the Adcom amp in question was sold from '91 - '94. In other words, it was designed before the "pin 1 problem" Muncy paper was published.

Alistair
Old 4 weeks ago | Show parent
  #74
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David Rick's Avatar
 
🎧 15 years
A step back and maybe a step forward?

Quote:
Originally Posted by UnderTow ➡️
In their defense, the Adcom amp in question was sold from '91 - '94. In other words, it was designed before the "pin 1 problem" Muncy paper was published.
Thanks for that, Alistair. Since Aaron bought his amp on Amazon, I assumed it was current product, which it isn't. It does appear that its bigger brother, the 555, is still available in a later model revision. I can't find a service manual online, but the photos show that the RCA inputs are still insulated from chassis, which is worrisome. The internal view shows an input board right up against the rear panel, so perhaps they're doing more effective filtering now.

Maybe it's worth reviewing how we got here: Aaron needs an external power amp because he's mixing on Yamaha NS10's. Those were a very big deal among mix engineers back in the day, but the idea wasn't to check your mix on them because they were good, but rather because they were so bad. That idea is kind of passé today: many of us think that speakers with the flattest possible response are the best tools to produce a "universal" mix. Even back then, the NS10's were often just a secondary check; studios had "big" monitoring systems that were as accurate as possible.

So perhaps it's time for Aaron to rethink his whole monitoring strategy. I get that he has a tight budget, but when my daughter moved out house, she remarked that she missed hearing music though my high-end playback system. This year I bought her a pair of Kali LP6 powered monitors as a holiday gift. They cost me $300. They're not perfect, but I'm amazed at how neutral they are and how good they sound for the money. Would I mix on them? If the alternative was a pair of NS10's, you're darn right I would!

David
Old 4 weeks ago
  #75
Gear Guru
All the powered monitors I've heard hiss, some a lot. The Adcoms are silent. It would be tough to put a 555 into a small speaker cab.

The reason the RCA grounds are isolated from the chassis is because Adcom used a star ground design. There is one ground cable connecting to the amp chassis to avoid internal ground loops. The GFA amps are classis Nelson Pass designs, a textbook example on each stage on how to design a power opamp. Since the screening is provided by the input cables it's important to use good ones and upstream must be grounded.
Old 4 weeks ago | Show parent
  #76
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Aaron Rash's Avatar
 
🎧 10 years
I've just been mixing on NS-10's forever, I know their sound really well and setting up at home I really wanted to go with what I know.

The studio I've always worked out of had a Bryson 4 but I don't have Bryston 4 money... and after weeks of research and reading Adcom was the best bet.

Also, I got it from reverb, not amazon.

Oddly enough, I haven't seen one single case of anyone else having RF problems with the Adcom 545. I guess I'm the only person to use one within 2 miles of a radio station?

I've ordered ferrite beads and I ordered the Mogami 3080 cable Jim suggested to build a solid RCA cable.

Fingers crossed that the new cable with the ferrite beads will get me back in business.

For now I'm mixing with tiny little 6 inch cables and its short enough to keep the RF out.

Even more funny, if I plug a mic into my Apollo interface the ground/shield from the mic cable leaks into the shield on the 6 inch cables into the Adcom and I hear radio again. It crazy how determined RF is to find away into things!

I'll update soon after the ferrite beads and cables arrive. Thanks everyone for all the suggestions and help.
Old 4 weeks ago | Show parent
  #77
Lives for gear
 
Aaron Rash's Avatar
 
🎧 10 years
Quote:
Originally Posted by Jim Williams ➡️
All the powered monitors I've heard hiss, some a lot. The Adcoms are silent. It would be tough to put a 555 into a small speaker cab.

The reason the RCA grounds are isolated from the chassis is because Adcom used a star ground design. There is one ground cable connecting to the amp chassis to avoid internal ground loops. The GFA amps are classis Nelson Pass designs, a textbook example on each stage on how to design a power opamp. Since the screening is provided by the input cables it's important to use good ones and upstream must be grounded.
I see a Adcom GFA 545 MK1 on eBay. Are the MK1 versions better than the MK2 in regard to RF? I thought I had read somewhere that the early version doesn't have the RCA shield isolated from ground like the MK2 does... But I could be wrong.

If you think the MK1 would be a better option in my situation I can buy that and sell the MK2.
Old 4 weeks ago
  #78
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David Rick's Avatar
 
🎧 15 years
According to the service manual I looked at, the shield is tied to ground on the GA 545 Mk 1. Pictures of the amp's internals show that it would be quite easy to add additional RF filtering on the back side of those RCA connectors. I saw one picture that had a little circuit board right there, so maybe they did that as a running production change

David
Old 4 weeks ago
  #79
Gear Guru
Those small pcb's are mounts for the RCA connectors, nothing else is there. It's a series 1k resistor into a 300 pf cap to ground across a 22k resistor. When modified here I use 27 pf to obtain a 200k hz bandwidth. No one has rf issues with those amps.

If this amp in question is fed from another device it's likely the rf would change, could be worse, could go away. Too many variables that are unknown.
Old 4 weeks ago
  #80
Lives for gear
 
Aaron Rash's Avatar
 
🎧 10 years
Just wanted to update on the RF problem with my Adcom. I was able to completely knock out the RF with the ferrite beads David suggested. I actually ordered the ones from the link David posted at Digi Key. The ferrite type was 28.

Initially, I put the ferrite beads on both ends of the cable and I was still getting RF so what I actually had to do was switch to a smaller shielded cable. I used Mogami W2368 and wrapped it through the core 2 times on both ends of the cable and now the amp is dead silent. Without switching to a smaller cable, I wouldn't have bean able to do more than one pass through the beads and that's what ended up being the magic trick.

So I just wanted to thank everyone for all the help. Hopefully if anyone else has this problem in the future they can find this thread.

I actually can't believe how quiet the amp is now. I'm super happy.
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