Pre amp/power amp combo buzzing (not humming!) - Page 1
Forum section: electronics / projects

author: icon
author_rank: Regular Contributor
timestamp: April 16, 2015, 09:17:53 pm
content: HiI'm at the end of my tether, trying to work out why this thing is buzzing. The output at the speaker has a gentle buzz that looks like this (triggered off AC with one of the secondaries for reference).The pre-amp consists of a number of PCBs from Rod Elliott (ESP) - a power supply, RIAA pre-pre-amp and pre-amp. Here are some general shots.All the internal wiring is shielded - the shields are in each case only connected to ground at one end, and all the grounds go to a central point. There's a 10R//0.1uF between that point and the chassis (that's the stuff sitting on the big bridge rectifier - it is isolated from the chassis!), but it doesn't seem to be a ground loop problem. There's also a 10R~0.1uF snubber across the secondaries of the transformer.Here's the odd thing - the amplitude of the buzz barely changes with volume, nor does it change when you switch from 'line' to 'phono'.So, other than "put a match to it", what do you think?ThanksJohn

author: techricky
author_rank: Contributor
timestamp: April 16, 2015, 09:31:29 pm
content: Ok the first thing that jumps out at me is the connection of the volume pot low end back to the common ground point.This is effectively letting you hear any voltage difference between that point and the main amp pcb ground, as well as any magnetic pickup from the loop you've created..What you hear is probably mostly the switching noise of the bridge diodes, or external mains noise by the look of it.Just use the cable shield for the pot, there is no need for anything else as the pot is isolated and there is only one ground point for that circuit.

author: icon
author_rank: Regular Contributor
timestamp: April 16, 2015, 10:13:25 pm
content: Quote from: techricky on April 16, 2015, 09:31:29 pmThanks for taking a look. What you say makes sense. I'm not sure what I was thinking...I'll get the soldering iron out tomorrow and see what difference it makes. You've got me wondering about the headphone socket too...RegardsJohn

author: dom0
author_rank: Super Contributor
timestamp: April 16, 2015, 10:24:54 pm
content: Looks pretty wideband. I'd take a look at the spectrum of it, although some mains-related content is definitely there.

author: icon
author_rank: Regular Contributor
timestamp: April 16, 2015, 10:34:22 pm
content: Quote from: dom0 on April 16, 2015, 10:24:54 pmHiBecause it's the speaker output, the 'hiss' that you see is inaudible. It's the little positive- and negative-going blips that occur either side of the AC peaks that seem to be the buzz.Besides, if you think that looks noisy, wait till you see the output when it's switched to the RIAA pre-pre-amp - that ROARS! Fixing that will have to wait it's turn, though...John

author: moffy
author_rank: Super Contributor
timestamp: April 17, 2015, 12:53:16 am
content: The mains waveform looks heavily clipped. What is doing that? The tops are so flat it looks precision, I shudder to think of the currents.

author: Andy Watson
author_rank: Super Contributor
timestamp: April 17, 2015, 01:10:13 am
content: Your speakers (and everything else) appear to be referenced to that star point, as is the centre tap of the power transformer. This is a problem because the main capacitors will charge with very high current pulses. These pulse will be travelling from the centre-tap to then star point and (importantly) via a resistive piece of wire to the common point of the main caps. So all that ripple current is being imposed on your star point. The centre tap of the transformer should go directly to the common point of the capacitors.It is very important to get the grounding right in power amps, both to avoid injecting signals and for stability.

author: icon
author_rank: Regular Contributor
timestamp: April 17, 2015, 08:27:31 pm
content: For the avoidance of stress, I'll start by saying; problem solved. I'll say what fixed it in a mo, but first:Andy/moffy - this is just the pre-amp. The speakers are connected to a power amp downstream. I don't know why the secondary (not mains) is shaped like it is, but the total current draw of this box, when I had it connected to a bench power supply is down in the tens of mA. Not anything you could weld with.Techricky suggested rewiring the grounds of the volume pot(s). This made a big difference, but there was still a distinct buzz on the phono input, but only when the turntable was actually connected. I had a look at the internal wiring at the turntable - the shields of the RCA connectors are not connected to anything, and the earth binding post just seems to shield the box that hey're mounted on. So following the same logic as the volume pots, I've removed the earth wires connecting the phono inputs to the star ground and just connected the board's respective grounds to the RCA shields. Hurray; it's now silent. Strangely (perhaps someone can explain) the general level of 'hiss' is significantly reduced, too - I have to crank the volume nearly to max to hear anything, which I think is pretty good considering how much gain is involved.There is a power amp coming up next. At the moment it's a bench-top lash up:Thanks for the help.John

author: moffy
author_rank: Super Contributor
timestamp: April 17, 2015, 11:18:11 pm
content: Quote from: icon on April 17, 2015, 08:27:31 pmNice to hear (pun intended) that you fixed it. When I mentioned mains I intended the secondary mains voltage. The fact that the clicks were synchronised to the flat peaks of your "mains" looks like heavy currents are being drawn somewhere. If not this device then it is something else connected to your "mains". I have never seen such flat tops!

author: kalinga
author_rank: Contributor
timestamp: April 18, 2015, 09:17:25 am
content: Turntables need a separate grounding and most amps comes with a binding to ground the turntable, mostly connected directly with the mains earth, that's why the phone jacks are not connected to the ground of the inputs to avoid ground loop hums