Amp Buzz

[DreadPirate]

Conventional BJT amp in mono configuration. I'm converting them to stereo units and the first one, where I reused the internal wiring from the other, works fine. I had to rewire this one using a foil shielded two conductor (15awg) wire, shown here: AUVIO 25-Ft. Roll of 15AWG Round Speaker Wire : Speaker Wires | RadioShack.com. The original wiring in these units uses a two conductor cable with braided shield with the shield tied to ground on the source (rca) end only.

Amp is buzzing from both channels and goes away completely when I insert shorted plugs in both rcas. If one at a time, the shorted channel goes dead quiet, other continues to buzz.

Is it the unshielded wiring that is causing this buzzing? Any suggestions for further evaluation of the source of buzzing?

 

[Speedskater]

Unbalanced RCA inputs?
Are the RCA jack's connected directly to the chassis? (they should be)
RCA jacks need co-ax interconnect cables.
Speaker cables shouldn't be shielded.

 

[gootee]

Need photos and schematic.

@Speedskater: Why would you want the input jacks grounded to the chassis? That would create a ground loop, unless the amp input section's signal ground had no other ground connection. And he is talking about the internal input signal and input signal ground wiring, not the interconnects. The internal wiring is best done with shielded twisted pair, as he described.

Quiet when inputs shorted means the buzz is not being caused by enclosed loop area in the input signal/ground pairs.

So maybe the signal ground conductor that connects to the power supply ground is sharing some length of conductor with the rectifier-cap current loop. That would induce time-varying voltages across the conductor, which would appear back at the non-PSU ends of the ground conductors, and would be arithmetically summed with the input signal. Does the PSU transformer have a center tap?

It's best to ensure that the input signal ground conductor connects to nothing else except the ground end of the resistor at the amp input, and then run a ground reference wire from there to the star ground, or the smoothing caps' ground just AFTER the last smoothing cap. (NO grounds for the amp board should connect to anything else except that same point, just downstream from the last smoothing cap.)

If that cable has a drain wire, connect it to the chassis at the input end, and to NOTHING at the other end. Make sure that the input jack's ground does NOT connect to the chassis.

 

[DreadPirate]

Schematic is not available. The input grounds are tied together where they attach to the board. They are isolated at the chassis. Jumpering grounds at the chassis has no effect. To recap, the wire I am currently using is a two conductor (15awg), not twisted with a foil shield not connected at either end. There is a buzz on both channels, one louder than the other, can hear at 5 feet from speaker. Buzzes with nothing connected. Shorting the inputs completely eliminates the noise. Touching and moving the wires in place affects the buzz somewhat, but does not go away. Upon desoldering the cable at the board end, discovered traces are all buggered up, the board is cheaply constructed. I am going to have to put a screw terminal here, will allow experimenting with different cables. I plan to use some Mogami 2534 microphone cable I have once I get the terminal blocks in. The original cable that worked fine initially is a single twisted pair with the braid grounded at the input end.

Mogami 2534
MOGAMI® - Neglex Quad Cables

 

[Speedskater]

@Speedskater: Why would you want the input jacks grounded to the chassis? That would create a ground loop, unless the amp input section's signal ground had no other ground connection. And he is talking about the internal input signal and input signal ground wiring, not the interconnects. The internal wiring is best done with shielded twisted pair, as he described.

*********************

If that cable has a drain wire, connect it to the chassis at the input end, and to NOTHING at the other end. Make sure that the input jack's ground does NOT connect to the chassis.

It took me awhile to find a description of the situation. It's the classic "Pin 1 Problem" from the 1995 Neil Muncy AES paper. Yes the Pin 1 is in an XLR connector, but the problem refers to all shielded cables balanced, unbalanced, control or power. All shields must be joined to the metal chassis at the connector.

This paper by Jim Brown, who is both the AES committee chair on EMI/RFI and a Ham radio operator was originally written about audio interference and noise. Then it was expanded with a lot of Ham information.

See page 5 for the "Pin 1 Problem" section.

A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing
Revision 5a 5 Jun 2010
by Jim Brown K9YC
Audio Systems Group, Inc.
Audio Systems Group, Inc. Home Page

http://www.audiosystemsgroup.com/RFI-Ham.pdf

 

[gootee]

I quite agree.

At the very least, that is the very next thing that the OP should have tried.

No buzz with inputs shorted indicates that it is NOT pickup of hum via enclosed loop area between signal and signal ground conductors.

 

[AndrewT]

the page 5 diagrams show a "shield" and a hot signal feed wire.
They all omit the cold or signal return wire.

If the screen does a dual duty of "shielding" and the return wire then we have a pin 1 problem.

The avoidance of the pin 1 problem by chassis connecting all shields can ONLY be achieved using 3 conductor interconnects.
1: signal hot
2: signal cold or signal return
3: shield

 

[DreadPirate]

Buzz issue was resolved by routing the cable away from the transformer. Using Mogami Quadflex, with two twisted pairs, one pair with the signal, the other connected to the shield only on the input end. I did get the buzz even with this cable, only went away after re-routing.