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t1 headphone amp - hiss

t1 headphone amp - hiss

I am a noob so please be kind.
I have the SMPS300R and the T1 headphone amp. I have wired the power from a computer cable.

AC Power In
Green striped - slot 1
blue - slot 2
brown - slot 3

DC Power to Amp
I have two wires going from the Ground Out / Positive Out going to the T1 V+ and Ground. On the PSU side, I have both wires soldered directly to those pins, and the connections seem to be solid. Everything on the amp side is only screwed in. Currently, there is no connection to the Negative Out.

Headphones Out
There are 4 wires going to my 3.5mm female headphone jack. I have twisted the Out's together and they are soldered to the middle of the 3.5mm jack. The other two wires are going to the smaller l/r connections on the 3.5mm

Audio In
I have the red and black wires going to the left/right, and the white wire going to the middle connection.

Voltage Dial
I have not touched the voltage dial on the PSU. I assume it is ok where it is.

So, I'm not sure why I have a lot of hiss. Also, the audio is horrible as though it is very underpowered or getting inconsistent power.
The instructions for the SMPS300R say that if there is a lot of hiss, to: "If noise or hum is experienced due to the incorrect GND connection, the GND loop breaker circuit made of R33 and C33 (can be seen in the right side of the mains fuse) should be connected using the dedicated solder pads on the bottom of the PCB."

I can identify the resistor and the cap to the right of the fuse, but I am not sure what to do with this. Which ends are supposed to be connected to where?

Also, I know that the instructions said to use the connectors for the output, but I kind of goofed them up a bit and just went with putting the wires through the holes and soldering them in.

So, what should I do?

Edit: also, the hiss is there from the get-go, it doesnt matter what volume anything is at.
you say that you're using have connected this to a 3.5mm jack. That might be the problem, as the 4 outputs from the amp can't be connected together, the two negative outs are not ground, they are separate negative outputs. This amp can only be used with headphones that has 4 separate cables, not common GND for both sides. Connecting the negatives will likely damage the amp.
Thanks Nick.

I tried it directly into some speakers and ya, it sounds great right out of the box. Very nice.

All the headphones I see out there use either to a 3.5mm jack or a 1/4 inch jack, and they all have a common ground. I think my friend had some STAX that weren't using a headphone jack, but that's the only exception that I know of. I will re-wire a pair of headphones for 4 wires. I can't see using a single channel on the amp and them splitting it to L/R. That wouldn't give me stereo and kind of defeats the point of having stereo headphones.

I think most people will want to use a 3.5mm jack, so if there is a way to design for this in the future, that'd likely be great for most people.
Hi, yes please try to re-wire for 4 wires. It is a limitation of the TK2050 chipsets. This is a speaker amp, but we made this adjustment to it so it could work with some hard to drive headphones after Mike at headfonia reviewed the amp. Those hifiman headphones have 4 separate wire.
So I was testing the amp with actual speakers, and it was sounding fine. I wired a pair of headphones for 4 wires, and there is hiss. I'm not sure how to get rid of it, or what the source is.

This is probably a differrent question: (not related to the hiss)

I am wondering if I have hooked up the audio input correctly. The wire for audio input in the diagram has a black, white and red wire: . The wire I have had a black, white and red on one end, (the board side), but the loose end, it is red, white and then loose strands. I am assuming that the loose strands are the black, but I have seen some wires where there are loose strands which act as some sort of insulation.

Should I use the loose strands as the black wire, or is there an actual black wire in there that I need to go find? I have cut the wire twice looking for the black wire, and not found it.
Hi, the loose strands are the black wire.
Which headphones did you try it with? There could be some noise if they are very efficient. Is the hiss like a ground bzz kind of sound, or more a sshhh kind of noise sound ?
Hi Nick, Thanks tons for the responses.

The headphones are 120db.

A friend has the same cans that I do (but his has 3 wires vs mine now being rewired for 4), and I just tried his earphones with an amp that another friend had. There was hiss present also. So, I think we have confirmed that my amp here is working as intended, its just that these cans are too sensitive. Is there a resistor I can put in the circuit somewhere to limit the output before it gets to the earphones? I see from the instructions on the SMPS that you can try R33 and C33 but I'm not sure from the instructions exactly what to solder to where. Should I connect R33+ to C33+, and R33- to C33- ?
Hi, yes 120db are quite sensitive. The one we have used with this amp are 85-88dB, really hard to drive headphones.
You could try with a 8ohm resistor at the outputs, connected between Rout- Rout+ and Lout- between Lout+. These restors should have a quite high rating to be able to take the power output from the amp.

You shouldn't adjust the smps.
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