With a pair of lm3886 designs finished, I find myself in need of a matching preamp, something decent but not overkill. So I went back to Eagle and here is the result of it. It heavily draws on an earlier project.
The overall design will easily fit in either a 1u case (the galaxy gx347 is what I've got in mind) or directly in an amp case to create an integrated amp. It is powered from a 5VDC supply (such as a 1A/5V phone charger).
Features:
- Either active or passive design;
- Remote volume control;
- Remote source selection (4);
- Remote muting;
- Balance control;
- Headphones amplifier;
- One unbalanced output and one balanced output.
The preamp is divided in between two boards (6*10cm each):
- the first pcb holds 4 relays for source selection, a relay for muting, a balance pot, a motorized volume pot (alps rk168) and an arduino nano to control the relays.
- the second pcb holds a dc-dc converter (2W 5v to +/-9V), a input buffer (opa1688), a volume pot for the headphones amp, an output section for line (opa1688/drv134) and one for headphones (opa1688).
Notes:
- The first pcb can be used by itself as a passive remote control.
- If used as passive, there is a -3.6db insertion loss (due to the balance control). If used active, there is a 2.4db gain as the input buffer has a gain of 2.
- The source switching can either be controlled remotely (using (on)-off-(on) switch on the preamp to double up on the remote) or by a simple rotary switch at the inputs of the relays driver.
- If the remote control feature isn't needed, the arduino can be left aside. In that case, a alps rk27 can be substituted to the rk168.
- There is a strict separation in between the gnd of the arduino/motor/relays section and the audio gnd.
- There is nothing smaller than 1206/soic8 or anything requiring special soldering skills.
- Yes, the preamp pcb is missing, still working on it
Comments, criticisms, suggestions ? Anything helpful is welcome.
The overall design will easily fit in either a 1u case (the galaxy gx347 is what I've got in mind) or directly in an amp case to create an integrated amp. It is powered from a 5VDC supply (such as a 1A/5V phone charger).
Features:
- Either active or passive design;
- Remote volume control;
- Remote source selection (4);
- Remote muting;
- Balance control;
- Headphones amplifier;
- One unbalanced output and one balanced output.
The preamp is divided in between two boards (6*10cm each):
- the first pcb holds 4 relays for source selection, a relay for muting, a balance pot, a motorized volume pot (alps rk168) and an arduino nano to control the relays.
- the second pcb holds a dc-dc converter (2W 5v to +/-9V), a input buffer (opa1688), a volume pot for the headphones amp, an output section for line (opa1688/drv134) and one for headphones (opa1688).
Notes:
- The first pcb can be used by itself as a passive remote control.
- If used as passive, there is a -3.6db insertion loss (due to the balance control). If used active, there is a 2.4db gain as the input buffer has a gain of 2.
- The source switching can either be controlled remotely (using (on)-off-(on) switch on the preamp to double up on the remote) or by a simple rotary switch at the inputs of the relays driver.
- If the remote control feature isn't needed, the arduino can be left aside. In that case, a alps rk27 can be substituted to the rk168.
- There is a strict separation in between the gnd of the arduino/motor/relays section and the audio gnd.
- There is nothing smaller than 1206/soic8 or anything requiring special soldering skills.
- Yes, the preamp pcb is missing, still working on it
Comments, criticisms, suggestions ? Anything helpful is welcome.
Attachments
I'm a bit slow to finalize the preamp pcb... it's on its way though.
@MAAC0: I'll look into it. I'm not very motivated however, as it would only display the source switching and that can be done with leds more simply (especially in terms of mechanical mounting).
@Big Cheese: that would be quite easy to add by using a dpdt switch and an extra pair for rca. The only thing that wouldn't be great is that the volume out would be the same for each amp while their gain might not match.
@MAAC0: I'll look into it. I'm not very motivated however, as it would only display the source switching and that can be done with leds more simply (especially in terms of mechanical mounting).
@Big Cheese: that would be quite easy to add by using a dpdt switch and an extra pair for rca. The only thing that wouldn't be great is that the volume out would be the same for each amp while their gain might not match.
There was a lot of empty space in there... so I added a phono preamp and a rcrc PS section to power it.
This small board (60*80mm finally) could easily serve as a preamp by itself. Add balance and volume pots, a rotary switch for sources selection and a small 5V supply and that's it.
This small board (60*80mm finally) could easily serve as a preamp by itself. Add balance and volume pots, a rotary switch for sources selection and a small 5V supply and that's it.
Attachments
The project is not dead. I've converted the mainboard (volume, balance, input, arduino) to through hole parts. It's a tad bigger at 70*100mm (from 60*100).
The idea is to create a suite of small pcbs to complete that mainboard. Each add-on would have its own onboard dc-dc converter, so that the project can be powered from a single 5V.
The possible add-on boards could be:
- a simple SE gain/buffer stage.
- a phono preamp.
- a small headphones amp.
- tone controls.
- balanced receivers-drivers.
- summing/lpf+hpf filter for a subwoofer+satellites output.
- a vu-meter driver.
The idea is to create a suite of small pcbs to complete that mainboard. Each add-on would have its own onboard dc-dc converter, so that the project can be powered from a single 5V.
The possible add-on boards could be:
- a simple SE gain/buffer stage.
- a phono preamp.
- a small headphones amp.
- tone controls.
- balanced receivers-drivers.
- summing/lpf+hpf filter for a subwoofer+satellites output.
- a vu-meter driver.
Attachments
I thought about it indeed and decided it was too much bother/extra cost. The problem is that some cards would best go in front (tone controls, headphones amp) and some in the back. If I use modushop cases, blank front plates are always available if I need to rethink the layout.
The "modular" concept I settled on is more basic:
- all cards must have mounting holes on the 1cm grid.
- all cards must be powered from a 5V supply (with dc-dc converters for audio circuitry, no possible gnd loops).
- all cards should accept a 2Vrms input by default.
- all connections by molex kk or similar.
- and that's about it...
My preference goes to an all analog design. I like the feel of a good old pot so no encoder or touchscreen is planned. Hence the choice of a motorized pot too. Still, I moved 7 output pins of the arduino to a pins header, so extra automatization is possible, beyond volume and source selection.
The "modular" concept I settled on is more basic:
- all cards must have mounting holes on the 1cm grid.
- all cards must be powered from a 5V supply (with dc-dc converters for audio circuitry, no possible gnd loops).
- all cards should accept a 2Vrms input by default.
- all connections by molex kk or similar.
- and that's about it...
My preference goes to an all analog design. I like the feel of a good old pot so no encoder or touchscreen is planned. Hence the choice of a motorized pot too. Still, I moved 7 output pins of the arduino to a pins header, so extra automatization is possible, beyond volume and source selection.
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