Kia ora whanau - I wonder if anyone has any ideas as to what is up with the Rotel RC-870BX preamp I am trying to put into my system. I have had this running in the past in my system thus: source > Rotel RC-870BX > MiniDSP 2x4 > power amps > speakers. 
I have introduced new poweramps (LM3886 and Topping PA3) but I get a loudish hum from the preamp. I know that the poweramps both function in other configurations. I have measured ac ~0.3v on preamp out, 50hz. any thoughts?

We are what we pretend to be, so we must be careful about what we pretend to be.

Curiouser and curiouser. I am working on this with the pre-amp hooked up to a pair of active speakers and have noticed the following behaviours: preamp powered off louder hum. Preamp on, no input, quieter hum (but still too loud). Quieter still if I press down on transformer cover. Input attached (RCA > laptop) - louder hum, about the same as when Preamp powered off.

We are what we pretend to be, so we must be careful about what we pretend to be.

Hi Papahemi, sounds like an earthing issue. Either there are too many mains earths in the system causing a ground loop and hum, or the whole system isn’t earthed.
It’s possibly more likely the latter as you say the hum changes when you press on the transformer case.
Try listening to the preamp by itself on headphones without the poweramp connected. If there’s no hum, it’s defo an earthing issue. If there is hum still, disconnect the source as well, so you can see if the preamp itself is generating hum.
Perhaps also check the house wiring for a correct earth at the socket if all that fails.
Let me know how you go!

AMR-iFi R&D

Thankyou for your help Colin. Listened on headphone without source - dead quiet. With source, clean sound. Looks like there is an earthing issue. Of recent times we have had issues with circuit breakers and wonder if this is somehow connected.
"Perhaps also check the house wiring for a correct earth at the socket if all that fails." What is meant by this?

We are what we pretend to be, so we must be careful about what we pretend to be.

Checked mains outlets cocncerned and they seem ok according to what I could find on Google

We are what we pretend to be, so we must be careful about what we pretend to be.

I am still trying to resolve this issue. I have checked my mains earthing and it all seem ok - also the only component that seems to be affected is the Rotel preamp. I have unplugged everything and run just the preamp with phones - all good. but the moment I plug into a poweramp I get a pronounced hum. If I unplug the inputs from the preamp the hum goes away. I feel that I am missing something but can't work out what it is.

We are what we pretend to be, so we must be careful about what we pretend to be.

Sounds like my worst nightmare.

I get hum issues sometimes when I insert various SUTs into my system. SUTs that have previously been fine in the same basic system. Maybe only difference being what else is plugged into preamp, or maybe different cart.

Once the hum disappeared when I changed cables.

Another time I was recommended to systematically unplug any appliance in the circuit and unplugging the fridge got rid of the hum, which was not a long term option!

My sympathies and best wishes to you eventually solving this hideous problem.

Try using a cheater plug (the now unobtainable back to back 3 pin plug with the earth lug filed off) on the preamp only. The preamp will still be earthed through input (i.e. from CD) and output (to power amp) interconnects. An earth loop can result from having both a proper earth (the cheater plug disconnects this) and the earth return on your interconnects.

Thanks for the input. The Rotel is double insulated, a two pin plug for mains, I was thinking that this may be a contributing factor but how I'm not sure. What I am sure about is the level of frustration I feel about not being able to suss this out. I have disconnected everything on the circuit bar the preamp which is quiet on its own through headphones, add a poweramp and get hum. 
I have been reading a thread of Owen's here but this has only added to my confusion. Weirdly this same preamp played without issue a few months back, the only change since then has been the addition of chip amp (chassis grounded, hums with rotel) TP60 (also chassis ground, also hums with rotel) Topping PA3 (SMPS also hums). Buggered if I know!

We are what we pretend to be, so we must be careful about what we pretend to be.

Good to read you’ve checked the house earthing and all seems ok.
Sounds like a ground loop between the source and the poweramp, via the Rotel preamp.
What source are you using, is it earthed as well as the poweramp?
A ground loop would be as follows: the source is earthed to the mains; the poweramp is also earthed to the mains; the source’s audio ground is connected to the poweramp’s audio ground through the interconnects and preamp. So there might be both an earth signal path through the mains earth wiring, and the other through the preamp and interconnects.
The goal is to have only one signal ground from source to poweramp, without compromising the safety earthing of either earthed component.
(Here I mean earth as in connected to the mains earth system. Ground meaning circuit 0V or signal cold reference point.)
Fixing a ground loop via earthing can be easier said than done. Cheater plugs aren’t recommended as they by definition remove the safety earth of a component in order to preserve the integrity (and hum-free operation) of the signal ground from input to output.
It’s better to use a 100 ohm resistor with a power rectifier bridge connected across it, in series with one of the earths of a component. That way any potential fault current flows through the low impedance of the rectifier bridge above around 1.2V and then trips the RCD in the house power board, yet the earth impedance is now 100 ohms (below 1.2V). That’s enough the break a ground loop.

AMR-iFi R&D

SO far for testing purposes I have been using laptop>RCA>Rotel or Laptop>airplay>basestation>Rotel. Everything else is unplugged.

We are what we pretend to be, so we must be careful about what we pretend to be.

Correct me if wrong:
Laptop audio RCA > Rotel > headphones = no hum
Laptop > Airplay > Rotel > headphones = no hum
Rotel (no source connected) > either poweramp = no hum
Laptop audio RCA > Rotel > either poweramp = hum
Airplay > basestation > Rotel > either poweramp = hum

Is the laptop power supply or the base station power supply earthed?

AMR-iFi R&D

Dec 14, 2022 23:57:48 GMT 12 colinf said:

Correct me if wrong:
Laptop audio RCA > Rotel > headphones = no hum
Laptop > Airplay > Rotel > headphones = no hum
Rotel (no source connected) > either poweramp = no hum
Laptop audio RCA > Rotel > either poweramp = hum
Airplay > basestation > Rotel > either poweramp = hum

Is the laptop power supply or the base station power supply earthed?

Correct, but additionally there is clear sound output on headphones, no output other than hum otherwise.
Airport base station is two prong double insulated
Laptop power supply has three prong plug, but I have used it on battery supply with no change to hum levels

We are what we pretend to be, so we must be careful about what we pretend to be.

Ah ok, so laptop on battery supply, or Airport base station, both not earthed > Rotel not earthed > poweramp with unconfirmed earth.
Can you pls check with a multimeter on resistance setting, that either poweramp is earthed to the mains? Measure from the earth pin on the power plug to the RCA socket shields (signal ground).
If they are earthed, you might be getting hum pickup from cable shielding problems.

AMR-iFi R&D

The TP60 is earthed to mains, 0.7Ω on each channel, the chip amp is not.
Both work fine when directly fed from source.
I am concerned that there is no sound on output from preamp, only hum. Perhaps I need to closely inspect rca/circuit board for fault.

We are what we pretend to be, so we must be careful about what we pretend to be.

Ok great, the Rotel’s output may be at fault.
As a starting point, if you can undo the bottom, check to see that the RCA sockets are all soldered properly and not cracked, and while you’re at it, check the cables you’re using.
It would be good to earth the signal ground in the chip amp with a 10 to 100 ohm resistor. But run the mains earth straight to the same chassis that the power transformer and any power switch or fuse is mounted to.

AMR-iFi R&D

Yes, check out the RCA sockets is the next move. Tomorrow, bit buggered after lawnmowing etc. When you say "earth the signal ground in the chip amp with a 10 to 100 ohm resistor." do you mean to the chassis ground? I could not see why this is done, works fine without it, care to illuminate me, I know shit.

We are what we pretend to be, so we must be careful about what we pretend to be.

Yes, resistor from signal ground to the chassis ground. I presume the chassis is earthed to the mains. Use a 5W wirewound resistor or similar. The high power rating of the wirewound resistor means it won’t get zapped from a static discharge like a small resistor could.
Connecting the signal ground to the chassis helps to reduce some potential hum problems. The chassis (if it’s an enclosure) now becomes a faraday shield for the chip amp circuit inside. Although a lot of stuff has double-insulated power supplies these days, there should still be one earth in the system, so that the circuit is at the same potential voltage as the earth the house is built on.

AMR-iFi R&D

Dec 18, 2022 1:27:03 GMT 12 colinf said:

Yes, resistor from signal ground to the chassis ground. I presume the chassis is earthed to the mains. Use a 5W wirewound resistor or similar. The high power rating of the wirewound resistor means it won’t get zapped from a static discharge like a small resistor could.
Connecting the signal ground to the chassis helps to reduce some potential hum problems. The chassis (if it’s an enclosure) now becomes a faraday shield for the chip amp circuit inside. Although a lot of stuff has double-insulated power supplies these days, there should still be one earth in the system, so that the circuit is at the same potential voltage as the earth the house is built on.

OK - Thanks - that is one resistor from a common signal ground to chassis ground.

I have dismantled the preamp and checked continuity between both L and R output and ground - it seems there is:






looking closely at the board it is hard to tell where the short(s) may be.







Looks like I'll have to take the rca connector off...

We are what we pretend to be, so we must be careful about what we pretend to be.

The schematic is on Hifi Engine to help you with faultfinding.
The output looks to be shorted by relay RY901 while it’s off - that’s probably why you’re measuring a short circuit at the output.

AMR-iFi R&D

Cool - Thanks Colin. Got the schematic but there are some differences in this particular example. IC601 is not present. The IC in its place looks like an opamp IC651 - labelled 4556D/JRC/0054A .
Anyway I checked the circuit board, cleaned up some goo, checked that  the relays were working ok, checked voltage on power supply - all OK. The only other reference voltage stated are on the pins of IC601 which I couldn't check due to it not being there.
Not being able to do anything else I tested it iPhone>RCA>Rotel>PA3
No Hum.
Pleased but puzzled.
I will try it back in the system apopo.
Fingers crossed.

We are what we pretend to be, so we must be careful about what we pretend to be.

Oh ok, sounds like they replaced the headphone amp IC601 from LA4170 to NJM4556. The 4556 was a popular dual opamp used for headphone amp duty in 90s Japanese hifi. It follows the standard dual opamp pinout. Pin 4 is Vee (negative power supply) and pin 8 is Vcc (positive power supply).

AMR-iFi R&D

Gee Colin - you are so knowledgable!
I did measure those pins on opamp and they were within cooee.
Preamp is back in system working quietly. I will slowly add components over the next few days - I tire easily - first up will be the MiniDSP. Still bemused by the hum vanishing - in the disassembly, cleaning, tutuing and reassembly something must have come right. I should be pleased but I'd rather know what it was.

We are what we pretend to be, so we must be careful about what we pretend to be.

I chased a hum for two days then realised I had inadvertantly switched an input on my preamp from balanced to single ended.

Glad the Rotel came right!
The hum fault might resurface at some point though. By disassembling the preamp you could have disturbed the connection that was causing the issue. I’d hazard a guess that it was the output RCA socket ground connection onto the PCB, a common point that breaks with inserting and pulling out cables over time.
That’s a service thing. If a unit came in to be repaired, I’d resolder these connections, as well as go over the circuit operation.
BTW the Rotel is quite moddable for better sound quality if interested. The electrolytic coupling caps would be first to upgrade.

AMR-iFi R&D

Dec 20, 2022 19:12:54 GMT 12 harvey said:

I chased a hum for two days then realised I had inadvertantly switched an input on my preamp from balanced to single ended.

🥹

AMR-iFi R&D

Dec 20, 2022 20:15:57 GMT 12 colinf said:

Glad the Rotel came right!
The hum fault might resurface at some point though. By disassembling the preamp you could have disturbed the connection that was causing the issue. I’d hazard a guess that it was the output RCA socket ground connection onto the PCB, a common point that breaks with inserting and pulling out cables over time.
That’s a service thing. If a unit came in to be repaired, I’d resolder these connections, as well as go over the circuit operation.
BTW the Rotel is quite moddable for better sound quality if interested. The electrolytic coupling caps would be first to upgrade.

Not only a sage but also a seer. Sure enough the hum returned, so disassembly again, resolder connections, reassembly. Bugger! Still there. I braced myself to remove the output RCAs - not as difficult as I anticipated with an extra pair of hands. Lo! The earth pin had broken off at the bottom of the RCA unit.
Nowadays it is problematic to get any parts shipped in small quantities - shipping is also quite steep, so I decided to try and bodge a fix:






Excuse the poor pics - as poor a photographer as a solderer.
But surprisingly I got the spacing right and resoldered the RCAs into place.
It works! (I know I've said this before, but this time I'm sure). Everything is back in the system, wiring a bit tidier, MiniDSP reconfigured, well done! A big thanks to MrF for the assistance, now can you tell me what and where the coupling caps are and what type of cap would be an upgrade, or is removal an option. Now the tonearm on my Lab80 needs a rewire...

We are what we pretend to be, so we must be careful about what we pretend to be.

Great, you tracked it down! I had a hunch after the earthing stuff didn’t work that it was something like a disconnected ground.
Caps…I think it was the 900 series that had the Black Gate caps in them from memory. The 800 series had normal caps. I’ve been using Audionote Kaisei caps lately. I did a shootout of electrolytic caps a while back. Elna Silmic ii, Black Gate, Nichicon Muse, AN Kaisei, normal tantalum, normal bipolar.
The Kaisei caps were most transparent and wonderfully detailed. Not that expensive in the low voltage ranges either. The Black Gate NX series were pretty good too, but no longer available.
If you follow the schematic in the Rotel, find the signal coupling caps and replace them. Use the bipolar ones as these have lowest distortion at bass frequencies. The bass rolls off in the phono stage as a subsonic filter. If you want more bass at the expense of record warps being amplified, you can increase the size of the caps in the main phono circuit (C403, 404).
If you use an MC cartridge, the MC stage opamps (only) can be replaced with the LT1028 which has much lower noise than the humble NE5534AN.

AMR-iFi R&D