You should have a silent amplifier at idle, so any notable noise/hum is not expected. An amp with no inputs connected, but outputs connected, should have next to no noticeable noise on the speakers or something is not setup correctly. If you experience noise/hum *after* you connect something to the input, its likely that device is the reason for the loop.
Since you mentioned it only happens when you connect to the input on the RCA, have you tried a different input source? I've had ground loops in the past because of misbehaving input devices and their interaction with the amp.
The way I've tracked these down in the past is to get the old DVM Meter out and start measuring for continuity points and looking for unexpected voltages on the grounds indicating currents flowing and loops. As a general rule, the circuit's return should have continuity to earth/chassis ground at a single "star ground" point point and otherwise isolated from that point. That start point is only connected to chassis ground the 3-type isolator (diode bridge, resistor and cap). Take a look at this really well put together article on this topic for some other ideas that also explains why this is important:
Audio Component Grounding and Interconnection - diyAudio
For your ACA, check to ensure you haven't accidentally connected the negatives somewhere to the chassis. I did this accidentally on my very first build where I bent the negative on the RCAs too far and it touched the chassis in a tiny place. The DVM will catch these cases %100 the time - just listen for the beep as you probe around!
--Tom
the noise is not like white noise, its more like lightning noise (uncommon noise). therefore i wonder is it something wrong with the ground input or the negative terminal touched the chassis.
apart from touching chassis and causes noise, is there any possible reason ?
when all the volt measured is 100 % accurate.
Ideally you would set the midpoint voltage using a scope and representative load impedance and adjust for maximum undistorted output swing.
My simulations of the amp with an 8 ohm resistive load show somewhere around 12.6v as around optimum but I would doubt anyone in a blind listening test would be able to reliably pick this out.
Sound quality would unaffected by any slight imbalance away from this optimum.
I use a linear bench supply for all of my initial board build testing and bias setting these days primarily because I rely on its current/voltage limiting function; whether or not its SMPS or linear is not really important for setting the bias. What is important is to measure what the actual input supply is, split it roughly in half and tune the bias towards that. That gets me within some small % of what the final supply is going to be so that after I assemble the entire amp/preamp, I go back and just fine tune it. For me anyways, there is no drama of exploding transistors or caps that way. Weather or not you are exactly %50 of the supply is not critical, just that you are limited by that supply voltage and cannot exceed half of it or you will get asymmetric behavior.
So if your supply is only 23.4vdc, don't worry if you set your bias point at 11.5vdc as you'll be just fine. And as another post said, +/- a couple of volts here isn't going to make much if any, of a difference.
--Tom
Steve Guttenberg the Audiophiliac posts first coverage of ACA construction
Robert built the Nelson Pass designed $327 Class A Amp Camp Amp - YouTube
to be followed by full review.
Robert built the Nelson Pass designed $327 Class A Amp Camp Amp - YouTube
to be followed by full review.
Very cool. It seems all too rare that DIY work gets a review outside of our forums. Looking forward to this!
Steve Guttenberg the Audiophiliac posts first coverage of ACA construction
Robert built the Nelson Pass designed $327 Class A Amp Camp Amp - YouTube
to be followed by full review.
Cool video!
I am particularly glad there is a discussion of sound impressions that gets into a discussion about "best" in the video. I keep trying to explain to folks that half the fun of DIY is building stuff you WANT to listen to because its tailored to suit your particular tastes; there is no "best" per se. The trick is figuring out what you like, and of course then how to build it. Thanks again Papa Pass for all of your efforts in helping me and the community be able to achieve this.
--Tom
I use ACA's on my 8 ohm 4" compression drivers and the amps are very low noise. I can just barely hear a slight hiss if I put my ear right next to the compression driver in the waveguide. Don't forget to add a protection cap. More here. The ACA's sound excellent on compression drivers. Have fun!
There isn't room in the standard ACA chassis. Also the intent of the ACA is not to have Mains voltages inside. And the Meanwell brick is generally considered very good.
Then there are lots of people who have incorporated a SMPS or linear PS (transformer / diode / capacitor) in their own builds in a larger chassis.
There is also another thread ACA with premium parts to get stuck into: ACA amp with premium parts
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Okay, thanks for that. I was thinking that the Schiit Audio Vidar and Aegir are in a very similar sized chassis, and they have a quite beefy transformer and power supply inside the chassis.
I am a newbie, and know only enough to be dangerous.
I have heard that an amp is, at its core - a modulated power supply, so naturally the quality of the power supply has a big affect on the amp.
I am a newbie, and know only enough to be dangerous.
I have heard that an amp is, at its core - a modulated power supply, so naturally the quality of the power supply has a big affect on the amp.
The diyaudio store better stock up.
I have a feeling they are going to be inundated with orders.
COMPARISON TEST: Nelson Pass Amp Camp Amp (kit), Schiit Aegir, First Watt J2 and... - YouTube
I have a feeling they are going to be inundated with orders.
COMPARISON TEST: Nelson Pass Amp Camp Amp (kit), Schiit Aegir, First Watt J2 and... - YouTube