My Alesis RA-100 amplifier has hummed since new even before input and output connections are made. I understand that it uses a toroidal main transformer and I think this is where the hum is coming from. However I have not read any other report of such a problem with this amp.
Neither the manual or schematic shows the part type of T1 but I'm wondering whether to try to buy a 240V/48V centre tapped transformer of x VA rating or whether I could re-impregnate or conformal coat the existing transformer to try to reduce the hum?
Ian
Neither the manual or schematic shows the part type of T1 but I'm wondering whether to try to buy a 240V/48V centre tapped transformer of x VA rating or whether I could re-impregnate or conformal coat the existing transformer to try to reduce the hum?
Ian
Toroid's can hum and this can be dependent on the purity of the mains supply. There is an excellent article on this site that explains the problem in detail, however in accordance with the wishes of Rod Elliott, this has to be the sole link.
Elliott Sound Products - The Audio Pages (Main Index)
Elliott Sound Products - The Audio Pages (Main Index)
According to other forums, this appears to be a common fault of the RA-100, dating back more than 10 years. It is identified as mechanical noise from the transformer, though that doesn't necessarily mean it must be due to DC on the mains, since it can happen in areas where there are no industrial users or significant DC levels present.
I find that amplifiers or equipment with substantial idle current, notably Class A, can be noisy with all types of power transformer - particularly low cost, poorly impregnated ones. In one case, the transformer had become a transducer, buzzing along with the music. Simply buying a different brand transformer was the complete solution. There are electronic solutions too but these may need tweaking for the individual application and this is not often simple or cheap.
I find that amplifiers or equipment with substantial idle current, notably Class A, can be noisy with all types of power transformer - particularly low cost, poorly impregnated ones. In one case, the transformer had become a transducer, buzzing along with the music. Simply buying a different brand transformer was the complete solution. There are electronic solutions too but these may need tweaking for the individual application and this is not often simple or cheap.
Nigel,
I wrote "My Alesis RA-100 amplifier has hummed since new even before input and output connections are made", so I meant the hum is from the transformer.
Ian
Ian,
I might try to buy a replacement toroid but the neither the parts list nor the schematic mentions details of the T1 transformer. The RA-100 rails are +48/-48V but no mention of the transformer output. I have not worked out the FW diode drop but typically the a.c. sec average would be (48V/0.45)/2 = 53.3V
Ian
trying to impregnate an insulated toroid transformer may do nothing of worth.
The core may need impregnating.
The primary may need impregnating.
The secondary may need impregnating.
All three of those regions are heavily insulated from each other.
Getting varnish into the outermost of those three layers will be the best you may be able to achieve.
An EI with all three sections exposed, is a much easier proposition for after the event re-impregnating.
The core may need impregnating.
The primary may need impregnating.
The secondary may need impregnating.
All three of those regions are heavily insulated from each other.
Getting varnish into the outermost of those three layers will be the best you may be able to achieve.
An EI with all three sections exposed, is a much easier proposition for after the event re-impregnating.
It's tempting to assume something as basic as the transformer must be the cause, but we don't yet know whether you do or don't have DC on your mains. 'Best to be certain and find out first since it would be foolish to pay for a new transformer only to find it was also noisy.
Having read Rode's ESP article, I'm not sure that there will be a simple method of testing for DC, other than to make up his final suggested design, fit it temporarily (with full and appropriate precautions for mains voltages) and see if it kills the noise. (not you, please!) I don't like the safety aspects of this for people not familiar with mains safety requirements and adding circuitry without proper guidelines, so please don't take this as any suggestion to go ahead unless you are fully confident of what parts and assembly techniques are required.
I'd agree with Andrew in saying you won't be able to DIY impregnate a toroidal transformer. Many aren't even impregnated at all but they are wrapped with a lot of tape between core, windings and outer insulation and this makes simple DIY attempts at reprocessing a waste of time. A professional job requires a vessel suitable for high vacuum and >250 kPa air pressure to remove the entrapped air and force the resin in. A certified safe, flameproof and ducted exhaust stove, capable of 150C is also needed. Yes, all that means just too expensive unless you are on good terms with a motor rewinder or another transformer manufacturer who would do this type of work routinely.
As for VA rating, I understand the amplifier is rated 100W/4R and 75W/8R. A standard 300VA transformer would be plenty for this amplifier and you can usually verify from the dimensions of toroids. 35VAC windings produce about 52V (no load) and 30VAC windings produce about 43V (no load). It may be difficult finding an off the shelf model with 33VAC windings, which may be necessary for 48V or otherwise lose a little with lower rails. Measure the AC voltage of your secondary windings and know for certain.
Having read Rode's ESP article, I'm not sure that there will be a simple method of testing for DC, other than to make up his final suggested design, fit it temporarily (with full and appropriate precautions for mains voltages) and see if it kills the noise. (not you, please!) I don't like the safety aspects of this for people not familiar with mains safety requirements and adding circuitry without proper guidelines, so please don't take this as any suggestion to go ahead unless you are fully confident of what parts and assembly techniques are required.
I'd agree with Andrew in saying you won't be able to DIY impregnate a toroidal transformer. Many aren't even impregnated at all but they are wrapped with a lot of tape between core, windings and outer insulation and this makes simple DIY attempts at reprocessing a waste of time. A professional job requires a vessel suitable for high vacuum and >250 kPa air pressure to remove the entrapped air and force the resin in. A certified safe, flameproof and ducted exhaust stove, capable of 150C is also needed. Yes, all that means just too expensive unless you are on good terms with a motor rewinder or another transformer manufacturer who would do this type of work routinely.
As for VA rating, I understand the amplifier is rated 100W/4R and 75W/8R. A standard 300VA transformer would be plenty for this amplifier and you can usually verify from the dimensions of toroids. 35VAC windings produce about 52V (no load) and 30VAC windings produce about 43V (no load). It may be difficult finding an off the shelf model with 33VAC windings, which may be necessary for 48V or otherwise lose a little with lower rails. Measure the AC voltage of your secondary windings and know for certain.
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