I've recently finished building the wooden chassis for my LM3886 monoblocks from the BrianGT kits, which have been almost 5 years in construction! (For convenience I've been using a lighter-weight Arcam integrated amp as I've moved multiple times since first building these.)
With no load connected, there is a very slight ground hum and almost inaudible hiss, however I believe both of these may be addressed by improved grounding strategy. At present I do not own a suitable pre-amp to drive the monoblocks, so I have been using the output of my Walkman A45 (24bit/96kHz DAC with headphone amplifier) - I understand this must present an impedance matching issue, however I've also noticed that using different 3.5mm to RCA cables can significantly worsen the hum issue - thus I believe the grounding strategy to be the main target. I'm also aware that the port proximity leaves a little to be desired, however I'd prefer to avoid altering the chassis if possible.
My questions are as follows:
With no load connected, there is a very slight ground hum and almost inaudible hiss, however I believe both of these may be addressed by improved grounding strategy. At present I do not own a suitable pre-amp to drive the monoblocks, so I have been using the output of my Walkman A45 (24bit/96kHz DAC with headphone amplifier) - I understand this must present an impedance matching issue, however I've also noticed that using different 3.5mm to RCA cables can significantly worsen the hum issue - thus I believe the grounding strategy to be the main target. I'm also aware that the port proximity leaves a little to be desired, however I'd prefer to avoid altering the chassis if possible.
My questions are as follows:
- As the chassis is insulated, might it be best to remove the AC Earth connection, and if so, where should the CHG (chassis ground) connection on the PCBs be connected to?
- Are there differences that could be made in relating the RCA input ground (SG-signal ground) to the amplifier ground rails (PG- or PG+), for example connecting them directly, or any other changes that could be made in the amplifier to counteract the difference in hum between input cables?
- As the amplifiers are monoblocks, I wish for them to reside close to the speakers. Is there a best-practice alternative to long (1m) single RCA cables (i.e. shielded, but not in a pair)? It's my understanding that RCAs are kept close to each other to minimise ground loops from http://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf
- Do you have any recommendation threads for preamps/buffer circuits to allow hum-free connection to such a handheld player, or laptop DAC source?
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Hi,
Have a look there : Gainclone building contest details (from BrianGT group order)
Post #125 from BrianGT talking about Mono...
Some tricks explained in this thread.
I've got mine running with no hum or hiss.
Seems your "Gnd" needs some more investigations.
Regards
Have a look there : Gainclone building contest details (from BrianGT group order)
Post #125 from BrianGT talking about Mono...
Some tricks explained in this thread.
I've got mine running with no hum or hiss.
Seems your "Gnd" needs some more investigations.
Regards
Sorry, maybe a clear word?
It is not only your grounding scheme, but about all your wires. A well made example of how not to do it. Have a look at some amps in the gallery. You should get an idea how to do it. A hint: twist wires. It really helps. Don´t ever use this ugly kind of paralleled loudspeaker cable inside any amp.
PS you can get any amp hum free. The little hiss is tolerable and unavoidable with most constructions.
It is not only your grounding scheme, but about all your wires. A well made example of how not to do it. Have a look at some amps in the gallery. You should get an idea how to do it. A hint: twist wires. It really helps. Don´t ever use this ugly kind of paralleled loudspeaker cable inside any amp.
PS you can get any amp hum free. The little hiss is tolerable and unavoidable with most constructions.
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Thank you for the signposting to that thread! I'm using the configuration from #125:
----------------------------------------
IN/SGND - to rca
OUT/OG - to binding posts
+V/-V - to power supply rails
CHG - chassis gnd, tie this to your central ground
That should be all that you need to use, with the rest of your grounds from your transformer, and AC GND going to a central ground in your case.
----------------------------------------
I've checked and each of SGND, OG, CHG, PG-, PG+ all have paths to ground. The only connection to the Earth pin of the UK plug for the amplifier is directly to CHG (since there is no metal chassis to connect to as a star ground) - should this scheme be improved upon? I am using dual bridges, and as mentioned all rails I would expect to be grounded seem to be.
My transformer is https://static.rapidonline.com/pdf/82719.pdf . I've connected the "blue" and "brown" inputs for the primary to live and neutral from the mains, "grey" and "violet" together (so that the primary is 0-230V rather than 0-115V). On the secondary, the inputs are connected directly to the briangt/carlosfm Power supply PCB. Thus I don't believe there are any more "grounds from the transformer" as discussed in post #125 above.
With no input the amp is effectively silent (apart from minimal hiss), there is some hum audible with my ear directly above the transformers (i.e. originating from them directly) but it is not audible through the speakers. If anyone has good tips to attempt to address this I would appreciate them - I've started noticing it on my other equipment and it's infuriating!
The 3.5mm - RCA cable used for testing has proven unreliable and a likely cause of the hum, one channel is clean but the other is consistently bad, and moving the 3.5mm connector results in drastic differences in hum volume. I'm wiring up my own shielded cable, most likely to include an ALPS 20k Pot for volume control - should I consider connecting this to an earthed chassis? or will the RCA ground connection suffice?
Thanks for the help.
Update: After a few hours of debugging, I've isolated the cause of the hum to the RCA input ports, I believe they were too close to the power & amplifier outputs causing a tiny but fatal feedback loop. Currently listening to the amps via a direct shielded cable and enjoying the famed LM3886 clarity.