I've got this AC DM608 on my bench. With nothing else hooked to it, I get MAJOR hiss out of all output RCAs with only my Honeytone attached straight to RCA outputs. The problem can be 'controlled' through the AC software, where input and output gains can individually be changed. With '0' IE NO gain, the problem is quite apparent. If I move the 5v/10v jumper on the AC board over to 10v, the hiss is even more incredible.
So, on the bench this device is doing the same hissing as in my car. I'm going to start combing through it... but hope I don't bump up against one of the computer chips with dozens of pins being an issue.
So, on the bench this device is doing the same hissing as in my car. I'm going to start combing through it... but hope I don't bump up against one of the computer chips with dozens of pins being an issue.
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With ground jumper set to isolated, I was not able to observe stable +- regulated voltage on opamps from several external references. Only with the ground jumper set to GND or 200 ohms can I reliably see +-15.6vDC on opamps. No noise difference any way at all.
With the ground set to isolated, you'll likely get a reliable supply voltage reading with the black probe on the output shields.
I think I found one issue though maybe not the total issue. On several of the output side opAmps, I'm seeing +1vDC on input pins 5 6. This is coming from the ~80 pin Analog Devices IC (AD1933 device ) just above them, through a 4.7k resistor. Actually the Analog Device is feeding 1.5v before the resistor.
Its possible that IC only operates at positive voltage, maybe 3vDC. It could be using the resistor and opamp to drop down to the audio side's ground/shield.
The noise is present on all sides of these connections.
Do these opamps look legitimate? They don't say JCR/NJM on them.
Its possible that IC only operates at positive voltage, maybe 3vDC. It could be using the resistor and opamp to drop down to the audio side's ground/shield.
The noise is present on all sides of these connections.
Do these opamps look legitimate? They don't say JCR/NJM on them.
Attachments
The op-amps appear to have original solder so they're likely original.
The A/D converter operates from a 3.3v supply. The output is likely going to be 1/2 of the supply voltage.
Doesn't the A/D converter output pass through a DC blocking capacitor?
The op-amp's sole duty is to drive its output so that the inverting input matches the voltage on the non-inverting input. If pin 5 has 1v, pin 6 will have the same 1v.
The A/D converter operates from a 3.3v supply. The output is likely going to be 1/2 of the supply voltage.
Doesn't the A/D converter output pass through a DC blocking capacitor?
The op-amp's sole duty is to drive its output so that the inverting input matches the voltage on the non-inverting input. If pin 5 has 1v, pin 6 will have the same 1v.
No cap on the outputs of the A/D converter, though I can add one somehow at that 4.7k resistor if you think that may help.
Also I read page #13 of the following data sheet:
https://datasheet.octopart.com/AD1933WBSTZ-Analog-Devices-datasheet-10622274.pdf
"POWER SUPPLY AND VOLTAGE REFERENCE
The AD1933 is designed for 3.3 V supplies. Separate power supply pins are provided for the analog and digital sections. These pins should be bypassed with 100 nF ceramic chip capacitors, as close to the pins as possible, to minimize noise pickup. A bulk aluminum electrolytic capacitor of at least 22 μF should also be provided on the same PCB as the DAC. For critical applications, improved performance is obtained with separate supplies for the analog and digital sections. If this is not possible, it is recommended that the analog and digital supplies be isolated by means of a ferrite bead in series with each supply. It is important that the analog supply be as clean as possible.
The AD1933 includes a 3.3 V regulator driver that only requires an external pass transistor and bypass capacitors to make a 5 V to 3.3 V regulator. If the regulator driver is not used, connect VSUPPLY, VDRIVE, and VSENSE to DGND."
What I am seeing is, all Grounds Digital and Analog are tied together, INCLUDING VSUPPLY, VDRIVE, and VSENSE to DGND. Also all positive supply pins are tied together - thats Analog AND digital.
Absolute 0 ohms across all POSITIVE power pins.
Absolute 0 ohms across all NEGATIVE power pins.
Seems to me this may have been short-cutting a suggestion from the data sheet, though I'm no designer. I can read.
Also I read page #13 of the following data sheet:
https://datasheet.octopart.com/AD1933WBSTZ-Analog-Devices-datasheet-10622274.pdf
"POWER SUPPLY AND VOLTAGE REFERENCE
The AD1933 is designed for 3.3 V supplies. Separate power supply pins are provided for the analog and digital sections. These pins should be bypassed with 100 nF ceramic chip capacitors, as close to the pins as possible, to minimize noise pickup. A bulk aluminum electrolytic capacitor of at least 22 μF should also be provided on the same PCB as the DAC. For critical applications, improved performance is obtained with separate supplies for the analog and digital sections. If this is not possible, it is recommended that the analog and digital supplies be isolated by means of a ferrite bead in series with each supply. It is important that the analog supply be as clean as possible.
The AD1933 includes a 3.3 V regulator driver that only requires an external pass transistor and bypass capacitors to make a 5 V to 3.3 V regulator. If the regulator driver is not used, connect VSUPPLY, VDRIVE, and VSENSE to DGND."
What I am seeing is, all Grounds Digital and Analog are tied together, INCLUDING VSUPPLY, VDRIVE, and VSENSE to DGND. Also all positive supply pins are tied together - thats Analog AND digital.
Absolute 0 ohms across all POSITIVE power pins.
Absolute 0 ohms across all NEGATIVE power pins.
Seems to me this may have been short-cutting a suggestion from the data sheet, though I'm no designer. I can read.
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The analog and digital grounds may read 0 ohms, just like the resistance between the head unit's ground and the amp's ground (on the other end of the vehicle) will essentially read 0 ohms but to audio, they are very different.
With the jumper set to iso, what's the resistance between the IC's ground and the primary ground?
You might want to ask, on the DAC page of the forum, if this IC is known for being noisy.
https://www.diyaudio.com/forums/digital-line-level/
With the jumper set to iso, what's the resistance between the IC's ground and the primary ground?
You might want to ask, on the DAC page of the forum, if this IC is known for being noisy.
https://www.diyaudio.com/forums/digital-line-level/
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The GND jumper has an affect on the IC referencing ground. The IC is either grounded, open (isolated) or 200 ohm. Depending on the jumper. So, this IC -can- be isolated from general ground by the jumper.
Interestingly referencing output RCA shields; No matter the jumper, output RCA shields all directly connected to ALL grounds of the IC; Digital and Analog, and also VSUPPLY, VDRIVE, VSENSE as well.
Interestingly referencing output RCA shields; No matter the jumper, output RCA shields all directly connected to ALL grounds of the IC; Digital and Analog, and also VSUPPLY, VDRIVE, VSENSE as well.
BTW if you are glancing at the photo I posted of the AD1933, PIN 1 is right-most, bottom pin near U38. There a little white circle also next to Pin 1.
This component uses the same techniques as amplifiers to prevent engine noise (that's the jumper's purpose.
In the iso position, it's like the old ZED amps that had no connection between the primary and secondary grounds. That's why you had to use a secondary ground to measure secondary voltages.
In the grounded position, it relies on a balanced input to eliminate engine noise.
In the 200 ohm position, this component is like the amps that have a 1k resistor connecting the primary and secondary grounds.
Dis all of your signal sources have grounded shields?
In the iso position, it's like the old ZED amps that had no connection between the primary and secondary grounds. That's why you had to use a secondary ground to measure secondary voltages.
In the grounded position, it relies on a balanced input to eliminate engine noise.
In the 200 ohm position, this component is like the amps that have a 1k resistor connecting the primary and secondary grounds.
Dis all of your signal sources have grounded shields?
Head unit in my truck:
RCA Shields to head unit rear metal case - 0 ohms
Metal case to chassis - 18.5 ohms
On my bench I can go several ways with devices with shield to case or grounding. My HoneTone mini amp makes no irregular noise from my bench sources. My frequency generator would be isolated from everything however, but even it makes no noise.
Noise originates from this device no matter what is inputted or otherwise connected or disconnected. From within software I can even 'mute' everything and there is still noise, though muting inputs or outputs separately changes the noise amplitude and noise irregularities. It would seem to me that the noise increases more and more as the signal passes through more and more opamps and ICs. It would also appear, marginally so that the PS of this device may be 'bleeding' into most circuits on this device. To isolate further, I connected a small 3A fused 12v battery alone to supply the AC608, and then just the MiniTone amp - and there is still noise even when everything is on isolated pure battery power.
It may become a no-go for me with this device. I went back to an old Kicker crossover in my truck and its a breath of fresh silence.
RCA Shields to head unit rear metal case - 0 ohms
Metal case to chassis - 18.5 ohms
On my bench I can go several ways with devices with shield to case or grounding. My HoneTone mini amp makes no irregular noise from my bench sources. My frequency generator would be isolated from everything however, but even it makes no noise.
Noise originates from this device no matter what is inputted or otherwise connected or disconnected. From within software I can even 'mute' everything and there is still noise, though muting inputs or outputs separately changes the noise amplitude and noise irregularities. It would seem to me that the noise increases more and more as the signal passes through more and more opamps and ICs. It would also appear, marginally so that the PS of this device may be 'bleeding' into most circuits on this device. To isolate further, I connected a small 3A fused 12v battery alone to supply the AC608, and then just the MiniTone amp - and there is still noise even when everything is on isolated pure battery power.
It may become a no-go for me with this device. I went back to an old Kicker crossover in my truck and its a breath of fresh silence.
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