MiniDSP + 4 channel amp = HUM - diyAudio
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Old 5th June 2011, 03:56 PM   #1
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Default MiniDSP + 4 channel amp = HUM

I am using a miniDSP as a two-way active crossover feeding a 4-channel 3886 Chipamp.com based amplifier (4 channels in the same chassis). I am getting a 60hz hum through my speakers when I connect all four channels of this amp to the miniDSP. If I disconnect two of the four channels (one full "stereo" amp), the hum disappears and the remaining two channels sound as they should.

As a workaround, I have been using two channels of the four channel amp, plus a separate two channel 3886 based amp (Same components, but half as many) for the remaining two channels. Obviously, the two amps don't like being in the same chassis, even though they are not sharing a chassis ground. I have tried connecting the four channels' chassis grounds together and also tried separating the two amps chassis grounds. The hum is lowest when I connect all four channels chassis grounds together. The hum gets worse if I actually tie them to the chassis.

I've played around with every grounding configuration I can think of and have minimized but not eliminated the hum by leaving the four chassis grounds tied together but not tied to the chassis itself. The chassis is only tied to the ground plugged in to the wall.

The four channel amp is built using two chipamp.com stereo 3886 kits in the same chassis. The hum goes away when I unplug RCA inputs from one side (two of the four channels).

Anyone run across hum issues using the miniDSP with a four channel amp? Any help would be appreciated.
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Old 9th June 2011, 01:27 PM   #2
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Hi, i know you've tried everything, but i wondered if you have securely grounded the -ves of the input RCAs ?
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Old 10th June 2011, 03:41 AM   #3
tnargs is offline tnargs  Australia
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So it isn't just me that this always happens to .....

I assume the miniDSP is fully floating? It should be.

And the 4 RCA inputs on the power amp are all isolated from the chassis? They should be.

If still hum, try cutting 3 of the 4 shield wires where the RCAs enter the power amp. A bit drastic but....
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Old 10th June 2011, 03:00 PM   #4
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Quote:
Originally Posted by tnargs View Post
So it isn't just me that this always happens to .....

I assume the miniDSP is fully floating? It should be.

And the 4 RCA inputs on the power amp are all isolated from the chassis? They should be.

They are

If still hum, try cutting 3 of the 4 shield wires where the RCAs enter the power amp. A bit drastic but....
I have a feeling this might work...although it seems odd that one would have to resort to these drastic measures. I'll try it this weekend with a temporary setup. Thanks for the suggestion!

Brian
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Old 10th June 2011, 03:03 PM   #5
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The input RCA grounds are soldered to their respective input ground thruholes on the Chipamp PCB. The chassis grounds have been both connected and disconnected to the chassis. There is less hum with them disconnected.

Brian

Quote:
Originally Posted by mrwireless View Post
Hi, i know you've tried everything, but i wondered if you have securely grounded the -ves of the input RCAs ?
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Old 10th June 2011, 07:16 PM   #6
mvaldes is offline mvaldes  Italy
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If you are powering the MiniDSP from your power amplifier power supply, try with a separate PS for the MiniDSP only.
I had the same problem and solved in this way
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Old 14th June 2011, 01:06 PM   #7
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In randy slone's power amp book he suggests putting a low value resistor (e.g. 4.7 ohm) between the signal input grounds and the star earth (HQG point.). I quote:


"An optional technique that is often employed in high-gain audio circuitry is to install a low value resistor between the input signal ground and the HQG point"

"suppose a poor electrical connection developed in the HCQ point so a relatively high resistance resulted between the HCQ point and circuit common, e.g. 0.1 ohm"

"The AC injected voltages on the circuit common line (coming from the amplifier circuitry) would see this resistance, but at the same time they would see the parallel resistance of going back through the signal source"

"This second pathway might represent an equal resistance, causing half of the "hum" signal to feed back through the signal ground and create a hum problem in the amplifier"

--It's a cheap enough mod to try out. I didn't have a hum problem but installing the resistor did reduce the already extremely low hum heard when putting my ear right next to the speaker.
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Old 14th June 2011, 01:16 PM   #8
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....and directly connect the input grounds to the star earth point (with the low value resistor in line). Don't rely on the amplifier's input ground which is not necessarily connected to ground.

Given that the miniDSP outputs are floating (i.e. no real ground present) this could be a source of the problem. I just go by experience, nothing else....good luck
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Old 17th June 2011, 04:14 AM   #9
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The miniDSP has its own power supply.

Quote:
Originally Posted by mvaldes View Post
If you are powering the MiniDSP from your power amplifier power supply, try with a separate PS for the MiniDSP only.
I had the same problem and solved in this way
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Old 17th June 2011, 04:17 AM   #10
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I will try the resistors. Thanks for the advice!

Quote:
Originally Posted by mrwireless View Post
....and directly connect the input grounds to the star earth point (with the low value resistor in line). Don't rely on the amplifier's input ground which is not necessarily connected to ground.

Given that the miniDSP outputs are floating (i.e. no real ground present) this could be a source of the problem. I just go by experience, nothing else....good luck
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