Adding an analog input to an DAC-AMP which does'nt have one

[Reactcore]

Hello readers,
I created this thread to share with others my insight on how to built in an analog input into an digital only dac/amp.

I had my eye (or should i say ears) on a Questyle CMA400i and CMA Twelve DAC-AMP unit which i auditioned in a store.
i found this thing sounding exceptional with its patented 'Current mode' amplifier. only i had to find out it didn't had an analog input so i could'nt connect my separate dac. it meant my only option was to go with the older CMA600 if i wanted to also have an inbuilt dac to choose. or go with a amp only which was way more expensive.

So i pulled my trigger on a 2nd hand CMA400 for €400 knowing i was going to modify it.

So lets start.

Needed items:

• Soldering iron with solder and litze for desoldering.
• Pair of side cutting pliers, screwdrivers
• Good quality relay (i used a NEC MR62-12SR with goldplated internal contacts)
• Twin wire for power to the relay
• Good quality shielded wire for audio signal transport to the RCA connectors on the back
• One pole miniature toggle switch for one hole mounting
• 2 good quality isolated RCA female chassis connectors hole mounting
• Drill machine and 6mm and 10mm drills
• Good light on your workplace
• A multimeter to check.

Unscrew the whole chassis of the dac/amp until you only have the main PCB.

Unsolder the volume potmeter using desoldering litze.
Tip: you can also use thick massive wire between the pins and heat up all six contacts at once using lots of solder.

Now cut off the right 2 pins really short as in the picture.




On the PCB, solder 2 wires into the right holes (where the cutted pins were)



Solder 2 short wires on the side of the cutted contacts of the pot and place it back on the PCB, be sure it is straight mounted.



I want to place the relay as close as possible to the potmeter to prevent signal degradation
So in order to create room for the relay i moved one trasistor to the bottom side of the PCB and the second bowed.



Now its time to place the relay and connect the wires, first i soldered the twin wire for the coil



From top to bottom youll see on the relay:

Normally open pins for the analogue input,
Preamp section of the PCB connected to the normally closed pins,
Common contact connected to the potmeter
Twinwire connected to the coil pins

front side view, pay attention that the cutted part and the PCB aren't toughing.
under you'll see there's space for the moved transistor.



Now assemble the unit back together without the top.

Drill the holes and place the switch as you see at the right side.above the Spdif out RCA.
Pay good attention that the holes are centered with enough space around to mount the RCA's and switch.
I laid the unit upside down so metal drill residue won't fall into the electronics.



Now solder one of twin wire from the relay coil on the switch, the other wire just let it go passing by.



Then solder the twin wire to the DC power input of the digital section.
It is the right pair of diodes, the polarity makes no difference.



Place the RCA's into the holes on the back.
be sure they are isolated from the chassis to prevent ground loops and noise.
They come with special plastic rings for that.



Then connect the signal wires between the relay and the RCA's
This is the shortest route possible for the analog audio signal.



Just behind the potmeter there is a test pin marked with GND (signal ground)


Final inside

Now close the top and youre in for a nice listening with your favorite DAC of choice and a Current mode amplifier

Rear view of finished product:

This can be done with most amp/dac brands with a analogue potmeter or resistor ladder with line level pass through, they work pretty much the same.

For the ones interested i include a schematic.
The blue part is original, and red marked is the addon part.

 

[tomb]

Good points on ingenuity!

I've had some experience in using relays with switching between different inputs. In fact, it was input selection between an analog input and a DAC. I often wondered why not a simple double-throw switch? It's a lot simpler and doesn't need power. Plus, many relays develop a capacitance between the contacts. It's enough to allow bleed-through from one input to the other. You may need to ensure the DAC is cut off if you switch to the analog input or vice-versa. Otherwise, you may hear the crosstalk.

The Alpha pot is not a very high-quality volume pot. It would've been a good idea to replace with something else while you were at it. Granted, that would've involved a lot more wiring and difficulty.

 

[Reactcore]

Hi,
I choose a relay just to have the shortest signal path possible (for the inbuilt dac anyway), a switch means extra wires losing quality and the relay is sealed against dust. I have been thinking on placing 2 relays with a ground plate in between though, one for each channel but one just fitted perfectly.

Regarding the pot, the Twelve has a ALPS pot but in the 400i it wont fit, the Twelve has a corner cut out of the PCB to make room for it.

 

[Amber Rain]

Hello readers,
I created this thread to share with others my insight on how to built in an analog input into an digital only dac/amp.

I had my eye (or should i say ears) on a Questyle CMA400i and CMA Twelve DAC-AMP unit which i auditioned in a store.
i found this thing sounding exceptional with its patented 'Current mode' amplifier. only i had to find out it didn't had an analog input so i could'nt connect my separate dac. it meant my only option was to go with the older CMA600 if i wanted to also have an inbuilt dac to choose. or go with a amp only which was way more expensive.

So i pulled my trigger on a 2nd hand CMA400 for €400 knowing i was going to modify it.



So lets start.

Needed items:

• Soldering iron with solder and litze for desoldering.
• Pair of side cutting pliers, screwdrivers
• Good quality relay (i used a NEC MR62-12SR with goldplated intenal contacts)
• Twin wire for power to the relay
• Good quality shielded wire for signal transport to the RCA connectors on the back
• One pole miniature toggle switch for one hole mounting
• 2 good quality isolated RCA female chassis connectors hole mounting
• Drill machine and 6mm and 10mm drills
• Good light on your workplace
• A multimeter to check.

Unscrew the whole chassis of the dac/amp until you only have the main PCB.

Unsolder the volume potmeter using litze.
Tip: you can also use thick massive wire between the pins and heat up all six contacts at once using lots of solder.



Now cut off the right 2 pins really short as in the picture.




On the PCB, solder 2 wires into the right holes (where the cutted pins were)



Solder 2 short wires on the side of the cutted contacts of the pot and place it back on the PCB, be sure it is straight mounted.



I want to place the relay as close as possible to the potmeter to prevent signal degradation
So in order to create room for the relay i moved one trasistor to the bottom side of the PCB and the second bowed.



Now its time to place the relay and connect the wires, first i soldered the twin wire for the coil



From top to bottom youll see:

Normally open pins for the analogue input,
Preamp section connected to the normally closed pins,
Common contact connected to the potmeter
Twinwire connected to the coil pins

front side view, pay attention that the cutted part and the PCB aren't toughing.
under youll see there's space for the moved transistor.



Now assemble the unit back together without the top.

Its time to drill the holes and place the switch as you see at the right side.above the Spdif out RCA.
Pay good attention that the holes are centered with enough space around to mount the RCA's and switch.
I laid the unit upside down so metal residue wont fall into the electronics.



Now solder one of twin wire from the relay coil on the switch, the other wire just let is go trough.



Now solder the twin wire to the DC power input of the digital section.
It is the left pair of diodes, the polarity makes no difference.



Place the RCA's into the holes on the back.
be sure they are isolated from the chassis to prevent ground loops and noise.
They come with special plastic rings for that.



Then connect the signal wires between the relay and the RCA's
This is the shortest route possible for the analogue signal.



Just behind the potmeter there is a test pin marked with GND (signal ground)


Final inside



Now close the top and youre in for a nice listening with your favorite DAC and a Current mode amplifier

Rear view of finished product:




This can be done with most amp/dac brands with a analogue potmeter, they work pretty much the same.

For the ones interested i include a schematic.
The blue part is original, and red marked is the addon part.

That is silly seriously impressive, well done. I would never attempt anything like that. However I have a CMA600i

 

[Reactcore]

That is silly seriously impressive, well done. I would never attempt anything like that. However I have a CMA600i

I haven't got the chance to hear the 600.
But i like the new design with the amber led indicators. And the 400i is smaller.

I heard the Twelve and 400i side by side and i couldnt tell a sound difference other than the Twelve having more spare power.
The 400i really gets loud enough with my 300 ohm senns.

 

[Reactcore]

Oh for anyone interested in this particular amp..
Here a link to the thread:

https://www.head-fi.org/threads/new-questyle-cma400i.851737/

 

[tomb]

Hi,
I choose a relay just to have the shortest signal path possible (for the inbuilt dac anyway), a switch means extra wires losing quality and the relay is sealed against dust. I have been thinking on placing 2 relays with a ground plate in between though, one for each channel but one just fitted perfectly.

Regarding the pot, the Twelve has a ALPS pot but in the 400i it wont fit, the Twelve has a corner cut out of the PCB to make room for it.

It's the fact that the relays are so small that they develop capacitance between the contacts. It's like putting a capacitor in front of each competing channel. It will stop DC, but you may end up hearing the audio from both sources. Just be certain that if you use the relay and switch sources, you may need to turn the other source off, or you may hear it in the background.

 

[Reactcore]

It's the fact that the relays are so small that they develop capacitance between the contacts. It's like putting a capacitor in front of each competing channel. It will stop DC, but you may end up hearing the audio from both sources. Just be certain that if you use the relay and switch sources, you may need to turn the other source off, or you may hear it in the background.

I agree with this fact.
I do lightly hear the internal dac if i feed it while switched to analog without signal. But its easy to have a non used digital input selected.
I have a Denon S10 pre amp which switches inputs with the same relays but they're designed using NO contacs only, so one signal source for each relay. this can't bleed. But i feel no need to make this in a headphone amp..

I must say this amp sounds stunningly good with external dac. If they would have analog input af fabric, they wouldn't sell their bigger brother 600i anymore for 2.5x the price

 

[Voxata]

Regarding the pot, the Twelve has a ALPS pot but in the 400i it wont fit, the Twelve has a corner cut out of the PCB to make room for it.

How have I missed this thread? Nice modding work! Though, a nice ALP RK27 pot can fit the 400i if you are willing to do some light modding. Bottom mount two vregs, drill out the back of the front plate to accomodate the larger screw from the RK27 shaft (same volume knob fits) & spacer. Lay the rear pins forward with a small tip section downward and solder seat them on the rear pins. Solder front normal.

 

[Reactcore]

How have I missed this thread? Nice modding work! Though, a nice ALP RK27 pot can fit the 400i if you are willing to do some light modding. Bottom mount two vregs, drill out the back of the front plate to accomodate the larger screw from the RK27 shaft (same volume knob fits) & spacer. Lay the rear pins forward with a small tip section downward and solder seat them on the rear pins. Solder front normal.

Thats cool to see that alps sitting there
Are you creating a analog input too?

 

[Voxata]

I am not, though if the DAC out could be converted to balanced in wiring into the pot that'd be amazing.

 

[Reactcore]

for true balanded u need 2 dac chips to completeley seperate the signal grounds.. not possible with this unit

 

[Voxata]

I thought the unit was balanced end to end? or so it is touted to be.

 

[Pings]

Sorry to bump this but I have some questions. I have the 400i and my USB input went out and I just seen this. Currently I have a Topping D10 using S/PDIF out then going into 400i just to give my PC connectivity. As you probably know Questyle's support is horrible, like send it in to get repaired and never see your unit again horrible. So this has led me into using it way, and I think I could be the fix I'm looking for plus give me a better DAC. What features are lost? Can you switch to the pre-amp with Function button?

 

[Reactcore]

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