Hmm, just done some testing and it seems to be the pseudo-differencial cables. With them unplugged it powers up pop free. I wonder if they are upsetting the dc offset servo at power-up?
Down the road I am considering Tom's THAT driver as a buffer in the pre-amp so that would remove that issue I guess.
Down the road I am considering Tom's THAT driver as a buffer in the pre-amp so that would remove that issue I guess.
I just tried on mine: Connected a pseudo-differential cable to the amp and let the other end float. I get a small, but audible, plop on turn-on. The plop goes away if I unplug the pseudo-differential cable and leave the input floating on turn-on.
There is no plop when I plug the pseudo-differential cable into a single-ended (RCA) source.
I think your source may be putting out a little bit of DC. It wouldn't take more than a few mV at most to generate an audible plop. The DC servo in the MOD86 will correct for this, but it will let through an initial transient. That's how DC servos work.
All four channels in my 4xMOD86 amp are plop-free.
As for the protection board: Enough people have asked that I'll put one together at some point. I can't promise a schedule at this point as I have other projects ahead of it.
Tom
There is no plop when I plug the pseudo-differential cable into a single-ended (RCA) source.
I think your source may be putting out a little bit of DC. It wouldn't take more than a few mV at most to generate an audible plop. The DC servo in the MOD86 will correct for this, but it will let through an initial transient. That's how DC servos work.
All four channels in my 4xMOD86 amp are plop-free.
As for the protection board: Enough people have asked that I'll put one together at some point. I can't promise a schedule at this point as I have other projects ahead of it.
Tom
Depends on the issue. The THAT Driver is DC coupled so if your source is putting out DC, adding the THAT Driver won't help. You could easily add an input cap on the THAT Driver, though. Then any DC presented by the source would be eliminated. I'd use a nice polypropylene one.
Tom
See this is why we are all here, your support!!!
With the pseudo cable connect and the RCA floating as you describe at power up I intermittanly get a audible pop. And the cone can move forward or back slowly by quite an amount as the servo kicks in I guess. Nothing happens with the XLR unplugged.
Could this be caused by some interaction with the chssis and grounding? The chassis is all ali and so saftey earthed. The XLR socket is metal and bolted to the chassis. The XLR socket has a earth tab in the rear that I have connected to the screen of mic cable to the amp.
With the pseudo cable connect and the RCA floating as you describe at power up I intermittanly get a audible pop. And the cone can move forward or back slowly by quite an amount as the servo kicks in I guess. Nothing happens with the XLR unplugged.
Could this be caused by some interaction with the chssis and grounding? The chassis is all ali and so saftey earthed. The XLR socket is metal and bolted to the chassis. The XLR socket has a earth tab in the rear that I have connected to the screen of mic cable to the amp.
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Thanks Andrew, so if I understand you correctly
I disconnect the internal sheild connection going from pin 1 to the MOD86 pcb such that the shield is only connected at the MOD86 end.
The chassis mounted XLR has pin 1 connected to the chassis, in my case I think the extra tab located by pin 1 is provided for this purpose.
The pseudo-differential cable takes the RCA ring to both pin 3 via the - cable and to the chassis via the shield to pin 1.
If so that is not how I understood Tom's documentation on page 20 of the design documentation.
I disconnect the internal sheild connection going from pin 1 to the MOD86 pcb such that the shield is only connected at the MOD86 end.
The chassis mounted XLR has pin 1 connected to the chassis, in my case I think the extra tab located by pin 1 is provided for this purpose.
The pseudo-differential cable takes the RCA ring to both pin 3 via the - cable and to the chassis via the shield to pin 1.
If so that is not how I understood Tom's documentation on page 20 of the design documentation.
Figure 13 of the THAT1200 datasheet is exactly how Tom suggests the Mod86 is wired except that the chassis connection shown in the RFI section is taken back to the chassis via the Mod86 PCB and that the gnd connection of the input is also. So as you say taking the Mod86 input ground to the chassis is not needed?
I have countered this as many times as you've brought it up. I am amazed that you still bring it up, in particular as in case of the MOD86 your "advice" is WRONG. I guess some people learn slower than others, but man... This is getting extreme.
The connection from Pin 1 of the board connector on the MOD86 to Pin 1 of the XLR >>IS NEEDED<<. Without this connection the input RFI filter does not work. Also without this connection the input common-mode voltage is undefined.
The plop on startup is >>NOT<< caused by the Pin 1 connection.
I hope I have expressed myself clearly this time. I'm sick of having to explain this every time you regurgitate AES48-2005 without putting it into the proper context or even contemplating the consequences or validity of your "advice". I know you mean well, but you're creating a lot of extra work for me here. I have enough stressors in my life. Thanks.
Tom
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Please connect it as shown in the design documentation (which you already have done, so no changes needed).
As I mentioned earlier, I firmly believe that a little bit of DC is to blame for your turn-on plops. I am not able to reproduce your issue with a single-ended source connected to my MOD86 using a pseudo-differential cable. My source is AC coupled.
Tom
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So would the advice be that for a single ended source to AC couple it? If so what size polypropylene cap would you suggest?
PS thanks for considering the speaker protection board
PPS: Just to verify with some real world tests...
Disconnected the shield at pin 1 of the chassis of the amp. Made up a pseudo cable as per no 17 of the 1985 Rane doc. Leads picked up hum immediately as I touched the outer of the RCA. Connecting them to my passive preamp and was overwhelmed with hum. So now in the process of reconfiguring back to the masters suggestions
Disconnected the shield at pin 1 of the chassis of the amp. Made up a pseudo cable as per no 17 of the 1985 Rane doc. Leads picked up hum immediately as I touched the outer of the RCA. Connecting them to my passive preamp and was overwhelmed with hum. So now in the process of reconfiguring back to the masters suggestions
2.2 uF or greater polypropylene would work.
Disconnected the shield at pin 1 of the chassis of the amp. Made up a pseudo cable as per no 17 of the 1985 Rane doc. Leads picked up hum immediately as I touched the outer of the RCA. Connecting them to my passive preamp and was overwhelmed with hum. So now in the process of reconfiguring back to the masters suggestions
Yeah. That's what happens when the common-mode voltage is being decided through the mains wiring between the preamp and power amp. Thanks for confirming.
Tom
As it happens I have a pair of these from another project. They are huge mainly cos of their 630V spec but I have hooked them inline as a test. And with everything back to Mod86 manual wiring I still see it happpening when connected to the pre-amp or with the pseudo-differential cable RCAs disconnedcted and floating. See this video.
So it suggests I have wired something wrong in the chassis or made the same mistake with component population on both the Mod86 PCBs. I'll do some checking tomorrow in daylight
So it suggests I have wired something wrong in the chassis or made the same mistake with component population on both the Mod86 PCBs. I'll do some checking tomorrow in daylight