Hi
First, I am a beginner, my skill level is about a 1.5 out of 10 and sorry for the long post, I wanted to include as much as possible.
I have been trying to find a 12 volt DC preamp to run some headphones and have run into a problem or two...
The preamp is to be mounted in a 60 watt Mitx case and be powered by the case´s DC 12V supply. The case already has a functioning phono preamp, FM transmitter and a soft latch switch circuit so the power supply can be switched from the front panel. The supply for the case comes from a 12V 60w DC brick. The build isn’t intended to be an audiophile quality system and the preamp will be used to supply some wireless headphones.
I have tried two different (cheap) Ebay headphone preamps (links 1 & 2).
1) Pure Class A Headphone Amplifier AMP 1969 Circuit DIY Kits für HD600 K701 | eBay
2) Portable Headphone Amplifier Board Kit AMP Module Kit For Classic 47 DIY +case | eBay
The preamp in link 1 works and doesn’t cause any problems when it is integrated into the Mitx case with the other components. Unfortunately the sound isn’t great, it is too tiring to listen to for any length of time. Based on this “test” I figured the inclusion of a preamp will work so I thought I would try a different option with opamps.
The link 2 opamp preamp also works well on the bench (powered by a SFX supply) and its sound is surprisingly good. Unfortunately it doesn’t work when it is connected to the other Mitx case components, it barely runs and produces lots of distortion. I discovered the problem is related to the input-output earths. I.e. the preamp doesn’t work when the earths are connected to anything in the case (e.g. phono amp output, case mounted 3.5mm headphone socket). But oddly it works well when the 3.5mm socket is removed from the case and only the RCA centre pins are connected.
I compared the two preamps and found what I think are a couple of issues with the link 2 preamp (I have tested two samples of the link 2 amp). First, it has unbalanced and (what I think is) high voltage outputs. I tested this by connecting the neg output to the multimeter and then moving the mm pos lead between the output pos L and R channels. The output volts are around 0.8V and 1.1V.
I then tried connecting the multimeter neg lead to the DC 12.2Vsupply neg terminal and found the signal input and output neg leads were both showing 6.1V.
I know the amps I have tried are not high quality but am unsure if the issues I am facing are the result of bad design, cheap components or something I haven’t thought of. Do opamp based preamps normally have a high voltage on the I/O neg leads...
I would like to have a preamp in the case but have run out of ideas-talent and I can’t find another 12V DC option that will fit.
So if anyone has a suggestion/an idea that might help or knows of a small(ish) form factor, good(ish) quality DC 12V preamp I could try I would greatly appreciate the input.
Regards.
First, I am a beginner, my skill level is about a 1.5 out of 10 and sorry for the long post, I wanted to include as much as possible.
I have been trying to find a 12 volt DC preamp to run some headphones and have run into a problem or two...
The preamp is to be mounted in a 60 watt Mitx case and be powered by the case´s DC 12V supply. The case already has a functioning phono preamp, FM transmitter and a soft latch switch circuit so the power supply can be switched from the front panel. The supply for the case comes from a 12V 60w DC brick. The build isn’t intended to be an audiophile quality system and the preamp will be used to supply some wireless headphones.
I have tried two different (cheap) Ebay headphone preamps (links 1 & 2).
1) Pure Class A Headphone Amplifier AMP 1969 Circuit DIY Kits für HD600 K701 | eBay
2) Portable Headphone Amplifier Board Kit AMP Module Kit For Classic 47 DIY +case | eBay
The preamp in link 1 works and doesn’t cause any problems when it is integrated into the Mitx case with the other components. Unfortunately the sound isn’t great, it is too tiring to listen to for any length of time. Based on this “test” I figured the inclusion of a preamp will work so I thought I would try a different option with opamps.
The link 2 opamp preamp also works well on the bench (powered by a SFX supply) and its sound is surprisingly good. Unfortunately it doesn’t work when it is connected to the other Mitx case components, it barely runs and produces lots of distortion. I discovered the problem is related to the input-output earths. I.e. the preamp doesn’t work when the earths are connected to anything in the case (e.g. phono amp output, case mounted 3.5mm headphone socket). But oddly it works well when the 3.5mm socket is removed from the case and only the RCA centre pins are connected.
I compared the two preamps and found what I think are a couple of issues with the link 2 preamp (I have tested two samples of the link 2 amp). First, it has unbalanced and (what I think is) high voltage outputs. I tested this by connecting the neg output to the multimeter and then moving the mm pos lead between the output pos L and R channels. The output volts are around 0.8V and 1.1V.
I then tried connecting the multimeter neg lead to the DC 12.2Vsupply neg terminal and found the signal input and output neg leads were both showing 6.1V.
I know the amps I have tried are not high quality but am unsure if the issues I am facing are the result of bad design, cheap components or something I haven’t thought of. Do opamp based preamps normally have a high voltage on the I/O neg leads...
I would like to have a preamp in the case but have run out of ideas-talent and I can’t find another 12V DC option that will fit.
So if anyone has a suggestion/an idea that might help or knows of a small(ish) form factor, good(ish) quality DC 12V preamp I could try I would greatly appreciate the input.
Regards.