Dear people,
I'm looking for (build in) pre amplifier for a LM3886 chip amp, see pictures below. The amp is powersupplied by a Toroidal Transformer 230v - 2 x 25v 6A. The amp sounds nice but I miss bass and power.
Anyway, I ordered this pre amp board by Dollatek. It fits the 47K Ohm values the amp needs. But now I need a dual 12v up tp 20v AC psu/transformer/voltage regulator. Can anybody advice me? Can I branche power for the toroidal? Do I need a second smaller transformer? I worry about possible hum.
I'm looking for (build in) pre amplifier for a LM3886 chip amp, see pictures below. The amp is powersupplied by a Toroidal Transformer 230v - 2 x 25v 6A. The amp sounds nice but I miss bass and power.
Anyway, I ordered this pre amp board by Dollatek. It fits the 47K Ohm values the amp needs. But now I need a dual 12v up tp 20v AC psu/transformer/voltage regulator. Can anybody advice me? Can I branche power for the toroidal? Do I need a second smaller transformer? I worry about possible hum.
Why not use a pair of voltage regulators, to give you +-15volts from the amplifier boards main smoothing capacitors?
If the amplifier voltage is less than +-35volts an L7815ACV and L7915ACV will work from that voltage giving you +-15 volts required to run the preamp ICs.
The +- voltage can be connected directly to the AC input connector with the ground in the centre. It doesn't matter which way around they connect, the bridge rectifier will take care of the polarity.
I would place the regulators on small heatsinks if only for aesthetics as they won't run hot.
Don't forget decoupling capacitors mount a pair of 10uF capacitors on the ground pin up to the input and output of each regulator.
If the amplifier voltage is less than +-35volts an L7815ACV and L7915ACV will work from that voltage giving you +-15 volts required to run the preamp ICs.
The +- voltage can be connected directly to the AC input connector with the ground in the centre. It doesn't matter which way around they connect, the bridge rectifier will take care of the polarity.
I would place the regulators on small heatsinks if only for aesthetics as they won't run hot.
Don't forget decoupling capacitors mount a pair of 10uF capacitors on the ground pin up to the input and output of each regulator.
Hello there,
I really enjoy reading in this forum about all your DIY projects, so I started some of my own.🙂
To drive my beloved Grundig active speakers, I needed a line level preamplifier with remote control capability.
So I put one together using this DollaTek board.
The good news: It sounds really good at higher volume with my active speakers.
The bad news: I‘ve tried for hours but I could not get this board noise free. There is a slight hum present…sounds very much like bad grounding, it does not get louder with turning up the volume and starts as soon as the board gets power. Also when I short the inputs on the board, it does not go away.
What I have done to the board:
It is powered by a torodial transformer 2x18V.
Does anyone have the same issue with this preamp?
Greetings
I really enjoy reading in this forum about all your DIY projects, so I started some of my own.🙂
To drive my beloved Grundig active speakers, I needed a line level preamplifier with remote control capability.
So I put one together using this DollaTek board.
The good news: It sounds really good at higher volume with my active speakers.
The bad news: I‘ve tried for hours but I could not get this board noise free. There is a slight hum present…sounds very much like bad grounding, it does not get louder with turning up the volume and starts as soon as the board gets power. Also when I short the inputs on the board, it does not go away.
What I have done to the board:
- Exchange the two NE5532 for two OPA2604
- Exchange the volume control for a motorized one.
It is powered by a torodial transformer 2x18V.
Does anyone have the same issue with this preamp?
Greetings
It looks like you already have 2 voltage stabilizers cooled by those 2 heatsinks on the "Dollartech" preamp board.
Mine was neighter completely silent but not at a level that I could hear it at 2 meters distance from the speakers. Was it silent with original volume potentiometer?
I gave this set to a neighbor.
Hi,
I am happy to report that I finally got the preamp noise-free.
What did I do?
All of that lowered the hum a little bit. But gamechanger was the transformer.
I noticed a much smaller toroidal transformer 2x15V in my parts box. Just out of curiosity I hooked it up and now it is completely silent.😀
I really don‘t know why the preamp did not like the bigger 2x18V toroidal transformer. The small one has 15VA power rating, the big one has no marking but I guess at least 3x that…really strange.
But now it works and sounds really good.
😊
I am happy to report that I finally got the preamp noise-free.
What did I do?
- Delete the power-switch on front
- Reduce the gain of the first opamp by 70%
- Short all of the potentiometer-housings to signal ground
All of that lowered the hum a little bit. But gamechanger was the transformer.
I noticed a much smaller toroidal transformer 2x15V in my parts box. Just out of curiosity I hooked it up and now it is completely silent.😀
I really don‘t know why the preamp did not like the bigger 2x18V toroidal transformer. The small one has 15VA power rating, the big one has no marking but I guess at least 3x that…really strange.
But now it works and sounds really good.
😊
Hi,
well…I kind of have to take the last statement back.
The preamp feeds two 3-way-active speakers which I usually really like.
But even after 3 weeks of daily use I could not get used to the sound of this setup. After trying different tone settings I got rid of it completely by connecting the speakers directly to the DAC, which thankfully has a volume control.
The sound is way better now. What I noticed the most is distortion in high and midrange with sudden changes in volume, for example a piano solo. Thats completely gone now. I can hear more detail now too.
Thats kind of disapointing…but I will not give up on my preamp.
Weeks ago I ordered one of those C3850 boards. Since this also uses also dual 15VAC it should be almost a drop-in-replacement.
As soon as I can find the time I will try that and report again.
well…I kind of have to take the last statement back.
The preamp feeds two 3-way-active speakers which I usually really like.
But even after 3 weeks of daily use I could not get used to the sound of this setup. After trying different tone settings I got rid of it completely by connecting the speakers directly to the DAC, which thankfully has a volume control.
The sound is way better now. What I noticed the most is distortion in high and midrange with sudden changes in volume, for example a piano solo. Thats completely gone now. I can hear more detail now too.
Thats kind of disapointing…but I will not give up on my preamp.
Weeks ago I ordered one of those C3850 boards. Since this also uses also dual 15VAC it should be almost a drop-in-replacement.
As soon as I can find the time I will try that and report again.
Hi there,
yesterday I put the new C3850 into my preamp. I will post how it sounds in the dedicated thread.
Just a comparison to the NE5532 Tone Control Board:
The C3850 board sounds a lot better…no more distortion whatsoever.
So yes…when it comes to sound-quality there is a clear winner.
I also inspected the first board looking for damaged components but all seem to be in good condition…I dont think I will put any more time into this…maybe it was never ment to be hifi…
Happy sunday!
yesterday I put the new C3850 into my preamp. I will post how it sounds in the dedicated thread.
Just a comparison to the NE5532 Tone Control Board:
The C3850 board sounds a lot better…no more distortion whatsoever.
So yes…when it comes to sound-quality there is a clear winner.
I also inspected the first board looking for damaged components but all seem to be in good condition…I dont think I will put any more time into this…maybe it was never ment to be hifi…
Happy sunday!