Same question, different year

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I posted this question about my Yulong Amp One last year.

Can anyone help me?

Last year's post:


I'd like to know if anyone could tell me how to make this pretty quiet Yulong Amp One I have --- even quieter in the background.

I could replace caps, resistors, etc. if someone would make specific suggestions.

Also, is there a DC offset adjustment for this amp? And, if so how do I do that?

Mark W.
Inside photo of Yulong Amp One

Here's a photo of the insides I gathered up from somewhere.

I have never seen a schematic for this unit.



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There looks to be no mains inlet filtering, also the trafo is a relatively noisy toroid. However replacing the toroid with a quieter EI core looks impractical because of the low profile of the case. If you're serious about getting low noise, build an external power supply for this amp would be my suggestion.
And yes, the transformer does create a tiny (tiny) bit of self noise. But what I'm getting noise-wise is hum. And to me that's a filtering problem.

I'm wondering if increasing the values of caps might help. And to what? And which ones.

I am hoping someone will come up with a schematic.

There are two kinds of filtering needed for amp PSUs. Common-mode and normal (differential) mode - your existing caps are doing the latter. Not very well though because there are no series inductors. The noise filtering that's addressed by changing the trafo to EI is the former.

Increasing the capacitor size might help or it might make matters worse.
I understand the addition of an inductor in a filter (As opposed to a resistor, which must be the case here?).

But, if I just simply took out the toroid and put in an EI transformer this condition would still exist, right?

I'm not understanding how the simple substitution of a transformer would get the desired result? But hey, I'm just a kit builder. I have no real understanding of electronics, so you should know that.

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You could post up close up photos of the inside of your own amp, that would be a help. I doubt there is a resistor there, its not currently particularly fashionable, but it would be a start towards having a decent differential mode filter.

If there's something you don't understand then ask - but make sure the question is specific. Rather than just saying you don't understand, ask about a particular point. The point about changing the trafo is to reduce its self-capacitance (primary to secondary) - EI types have lower capacitance than toroids, especially if you can find one with a split bobbin.
Stay tuned. I'll get a photo posted.

I'm not really willing to change a transformer in this amp. If that's the only way to get this quiet, then the design of the amp is seriously flawed.

And I'll sell it to someone and let them know there's a small amount of background noise.

I had a HLLY T-90 amp that made so much background hissssssssssss it was unlistenable. I ended up just scrapping that.

Once you've posted up a pic so that I can see what devices its using I might be able to confirm its seriously flawed (or not). Depending on what kind of noise you're getting then it might be intrinsic (unfixable because its limited by the chip(s)) or it might be layout dependent. Meaning you'd need to modify the PCB. Or it might be fixable with filtering.

So is it just hum that's bothering you? If so then is the hum present with the inputs shorted and no source connected? If it is then its probably layout and as you say you're just a builder, not a designer its going to be tricky for you to fix that.
Ah I see its not a chipamp, rather using discrete transistors. This makes it much more difficult to guess the schematic and hence the correct layout.

You might find the hum is due to magnetic coupling to the input wires - in which case moving them away from the trafo might be all you need. Or even rotating the transformer - toroids have maximum stray flux where the leads come out so point the lead exit point away from the pcb and the inputs and their wiring.
Photo of transformer and other wiring

The wires from the RCA inputs are about as far away as they can be. The leads from the board to speaker binding posts are indeed close to the transformer. Should they be lengthened. far as rotating the transformer itself to thus move its wires. Not sure how I could get them any further away.

See photo.



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The problem that I can see here is there doesn't look to be any connection of the mains earth to the circuit 0V. Or is there and I'm not seeing it? The wiring of the inputs sucks too because any common mode currents have to flow down that thin lap-shielded screened cable and hence that current appears as a voltage in series with the input voltage.

You've so far not responded to my question about the nature of the noise (hum).
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