I build and measure prototypes on the lab bench all the time. In fact, the measurements you see on my website are done without any chassis. I do pay attention to the wiring and, for low noise measurements, I often wrap the circuit in bubble wrap and then aluminum foil (which is grounded). That can reduce the noise floor quite a bit if you're having any kind of capacitive coupling into the circuit from external interference sources.
I have heard subjective reports from my builders that they perceive less noise when the amp is in a chassis. Without measurements, it's impossible to tell whether this is cognitive biases and human cognition at play or whether the noise floor is actually reduced a bit. It could be that builders are a bit more careful and deliberate with the wiring inside the chassis, whereas a prototype on a piece of plywood is often more of a rat's nest build. That could certainly explain why they perceive better performance once the amp is in the chassis.
Tom
I have heard subjective reports from my builders that they perceive less noise when the amp is in a chassis. Without measurements, it's impossible to tell whether this is cognitive biases and human cognition at play or whether the noise floor is actually reduced a bit. It could be that builders are a bit more careful and deliberate with the wiring inside the chassis, whereas a prototype on a piece of plywood is often more of a rat's nest build. That could certainly explain why they perceive better performance once the amp is in the chassis.
Tom
Looks good so far.
Well... to be fair, I'm not finished. This project is still in the "screwing-around-with" phase.
I did have it built already but knew I was going to be taking it apart and building it again to make it pretty inside.
The noise issue does have me perplexed; I think I will slowly get this thing back together again trying to remember all the advice I've read. Once I have it together and tested I will post updates.
I'm feeling happy right now because I just remembered I have a sound card with optical out which will allow me to remove my PC from the equation when trying to track down the causes for the noise. I've had this same PC hooked up to Parasound and Denon amps and the noise issue was still there, albeit less. We'll see...
I did have it built already but knew I was going to be taking it apart and building it again to make it pretty inside.
The noise issue does have me perplexed; I think I will slowly get this thing back together again trying to remember all the advice I've read. Once I have it together and tested I will post updates.
I'm feeling happy right now because I just remembered I have a sound card with optical out which will allow me to remove my PC from the equation when trying to track down the causes for the noise. I've had this same PC hooked up to Parasound and Denon amps and the noise issue was still there, albeit less. We'll see...
Ok, so I have the amp back together. Twisted up all wires. Routed high-current stuff away from signal cables and amp boards. Power supply output ground now connects to chassis through a 1k resistor combined with two opposite-facing diodes.
I turned on the switch for the first time today after the re-do, and... silence! However, I still had my light bulb on the hot wire going into the plug receptacle. I thought I was done, finished, success.
After I switched to a normal unbroken power cable, the noise was back.
Dubbya. Tee. Eff.
I switched my miniDSP to optical input. Disconnected the USB. Noise is still there. I'm starting to think the problem is the miniDSP or my RCA cables, however when the light bulb is connected through the hot wire going into the amp the noise is gone!
I'm so ^!%#$$#@#%#$frustrated.
I don't know what else to do...
I think I'm going to sell the complete amp to anyone interested for cost of parts. Someone who knows how to do this the right way...
I turned on the switch for the first time today after the re-do, and... silence! However, I still had my light bulb on the hot wire going into the plug receptacle. I thought I was done, finished, success.
After I switched to a normal unbroken power cable, the noise was back.
Dubbya. Tee. Eff.
I switched my miniDSP to optical input. Disconnected the USB. Noise is still there. I'm starting to think the problem is the miniDSP or my RCA cables, however when the light bulb is connected through the hot wire going into the amp the noise is gone!
I'm so ^!%#$$#@#%#$frustrated.
I don't know what else to do...
I think I'm going to sell the complete amp to anyone interested for cost of parts. Someone who knows how to do this the right way...
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If that doesn't work, I would leave ground connected and then I would remove the power supply wiring from every amp except one. I would test that by itself and see. If there was hum I would disconnect it and move on to the next amp until you test them all by themselves. If any of them works without the hum, then you can find the problem by finding the difference between them.
I just noticed Scott's reply. Do you have a portable CD player? If you do I would try connecting that. When doing the initial test, I would only connect one RCA input and one set of speakers to the output of that same amplifier. Repeat the test with each of the amplifiers but only connect one at a time.
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sorry if I failed to read the thread fully - did you try moving the amp boards further away from the power transformer - as far as possible ? I had a hum issue in a pre-amp, no doubt waaaay more sensitive than your amp but as you are looking for any ideas I'm just going to mention that the magnetic field from the power trafo infected the input stage of the pre-amp and produced hum. I could stop the hum if the pre-amp was a foot away.
I powered the amps individually when testing during assembly outside of the chassis. Played audio out of my cell phone into each amp board input, and thought everything was fine. All of them start humming as soon as an RCA cable is plugged in that is connected to anything powered from AC, whether it's my CD player, PC, or miniDSP. Connecting to a phone, it's quiet.
I pushed the amp boards out to the corners of the chassis, twisted all current cables, tried to route everything more or less away from each other but nothing seems to help. I ran a solid 12 gauge wire and soldered it to the shield tabs of the RCA terminals, then bolted it to chassis. Tried it with diode "lift" and without. No change.
Personally, I'm not convinced it's even the amp's fault but rather my PC, but if I can't make the two dance together there will be no honeymoon if you know what I mean.
The noise is quite loud. I would say that it's -50 dB below full volume.
All my equipment is powered from the same AC socket through a 6-outlet power strip, so they should be sharing the same ground. I tried mounting everything in a rackmount cabinet and then grounding it outside to a pipe driven 10 feet into the ground and connected with a 4-gauge ground cable. Still hum and noise.
My PC uses a Seasonic power supply which has active power factor correction. Could this be injecting noise into my AC supply?
The last thing I can think of trying is plugging all my equipment into a power conditioner / filter. If I can find a decent one for less than a grand, I'll give it a shot. Otherwise, I'm going to sell the complete amp to someone who is more of an expert than I am.
I pushed the amp boards out to the corners of the chassis, twisted all current cables, tried to route everything more or less away from each other but nothing seems to help. I ran a solid 12 gauge wire and soldered it to the shield tabs of the RCA terminals, then bolted it to chassis. Tried it with diode "lift" and without. No change.
Personally, I'm not convinced it's even the amp's fault but rather my PC, but if I can't make the two dance together there will be no honeymoon if you know what I mean.
The noise is quite loud. I would say that it's -50 dB below full volume.
All my equipment is powered from the same AC socket through a 6-outlet power strip, so they should be sharing the same ground. I tried mounting everything in a rackmount cabinet and then grounding it outside to a pipe driven 10 feet into the ground and connected with a 4-gauge ground cable. Still hum and noise.
My PC uses a Seasonic power supply which has active power factor correction. Could this be injecting noise into my AC supply?
The last thing I can think of trying is plugging all my equipment into a power conditioner / filter. If I can find a decent one for less than a grand, I'll give it a shot. Otherwise, I'm going to sell the complete amp to someone who is more of an expert than I am.
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Ok the problem is, as has been mentioned, the RCA connectors grounded to the chassis. The mod-86 has a balanced input. Assuming you have some shielded twisted pair cable, make up direct runs from the Mod inputs to the miniDSP bypassing your chassis RCAs so the amplifiers are cabled up as per design intent. Then you have the starting point of the way most people run their Mod-86 and we should be able to debug from there.
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