Modulus-86 build thread

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The RPS-series is Class II devices so you technically do not need to ground the chassis. I ground it anyway just to be safe. Just take the mains ground (protective earth) to the chassis with a short piece of wire. Ground XLR pin 1. Take XLR pins 1,2,3 to pins 1,2,3 of the input of the Modulus-86. That's all that's needed for grounding.

Tom
 
Thanks Tom, I also had a RPS-300-27 on hand so swapped that in instead. Amp is up and running. Voltages on both channel look good, DC offset on both look good. However, for some reason the left channel is quieter than the right. Note: using shielded RCAs not XLRs.
 

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I'm guessing that by "left" you mean the one that's to the left in the pictures. That channel has its input much closer to the SMPS than the right channel so I would expect it to be the noisier of the two.

Would you mind quantifying "quieter"? Did you measure or is it a subjective experience kinda thing?

Also: Why was C27 omitted?

Tom
 
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So if quieter doesn't mean lower noise/hum, what does it mean? Does it seem like it has lower gain? You could measure that... I go through how to do that here:

If you're using RCA inputs you need to make sure that both pins 1 and 3 on the input of the Modulus-86 are connected to the RCA shell. The centre pin of the RCA should connect to pin 2 of the MOD86 input.

Tom
 
Yes, it's as if the left channel has much lower gain. The RCA shield and ground are intertwined and soldered at the RCA inputs as instructed (I removed and replaced wiring twice to be sure that was not the cause). All inputs isolated from chassis but I'll triple check. I'll take a look at the video. Thanks.
 
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Well, that's intriguing. Thanks for taking the measurements. Now we have data to work with.

If you're confident that the input and output wiring is good, you're dealing with a case of wrong component installed. Most likely you accidentally swapped two resistors. 2 kΩ and 20 kΩ are easy to swap, for example. I'd look around R4, R7, R17, R18 in particular. Compare the coloured bands on the resistors between the left and right channels. I bet you'll find some of them out of place.

If you post a high-resolution picture of the left channel board taken from the top with good lighting I can help you find the offending part.

Tom
 
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The electrolytic capacitors in Rev. 2.4 are not critical. I usually search for: Same pin spacing. Same or smaller diameter. Same capacitance. Same or higher voltage rating as the ones specified. Buy a name brand. I usually go for Nichicon, Panasonic, KEMET, or CDE. I also tend to choose long-life types.

For the LM317 and LM337: It needs to be the 'L' version. So LM317L and LM337L in TO-92 package. The manufacturer doesn't matter.

Tom
 
Well, that's intriguing. Thanks for taking the measurements. Now we have data to work with.

If you're confident that the input and output wiring is good, you're dealing with a case of wrong component installed. Most likely you accidentally swapped two resistors. 2 kΩ and 20 kΩ are easy to swap, for example. I'd look around R4, R7, R17, R18 in particular. Compare the coloured bands on the resistors between the left and right channels. I bet you'll find some of them out of place.

If you post a high-resolution picture of the left channel board taken from the top with good lighting I can help you find the offending part.

Tom

Well the troubles continue. The resistor R7 was indeed the culprit I think - it was 2k instead of 20k. I say I think because no sooner did I replace it with the correct value and turn on the amp to verify... black smoke rose from the right channel. Using the multimeter somehow the LM3886 on the right coupled with the chassis? No idea how as it's been solidly isolated the whole time (some days now) and I repeatedly check to make sure.

This build just doesn't seem like it's meant to be. Quite frustrating.