Salas DCG3 preamp (line & headphone)

[r_jik45]

The DCG3 board is from a GB done in 2017. No 1.03 sign on it.

Here are two pics to illustrate the shielded vs simple connection wires:

 

[Salas]

Both are going to work without drama. In case of strong interference the first one is more able to withstand it.

 

[bambadoo]

Last one is mine. It did not make any difference with the i-select board or without so I guess we can conclude that it worked out fine

 

[Salas]

 

[r_jik45]

Thanks bambadoo for your input.
The pics were saved while going through the whole thread, good to hear that your configuration worked out fine.

 

[Theodosis O.]

Hi Salas,

If we parallel resistors to pot, like in the photo, will be any improvement ?
I guess we can change also the value of the pot with this way.
.

 

[Salas]

To linear pots fake law resistors can change the situation enough because they are the low value element. But in Log pots you can't put a low enough value before the Log curve is going to be altered too strongly. In any case they still make a little difference.

 

[r_jik45]

Bambadoo,
Any idea regarding the size of these wires, bigger than 24AWG?

 

[alexkosha]

Whatever I did with the wiring layout proper arrangement, transformers rotations and Earth/GND setup, I still can't get rid of the main 60Hz pickup. All is started when I dropped in Khozmo (relay based, 20k leather, LED volume level indicator, remote).
It is absolutely clean with simple Alps.
Consulted with Kozmo numerous times and no luck fixing it yet.
This attenuator is running 5VDC and I have separate simple LDO PSU also supplied Khozmo for it. Initially, I got huge noise when I connected 5VDC to this unit and they advised to connect PSU "0" to attenuator GND pads. That step removed noise, but left 60Hz and 180Hz spikes (about 14dB high) and this is only if some device is connected to DCG3 input. Same huge noise when I switch i-Select to open Input (unshorted).
It might be some design mistakes with that attenuator type or in general faulty unit. Do not know what next step should I take....
Really liked remote control idea, but my 47 steps 25k Goldpoint on other unit is working as doc proscribed with absolutely beautiful sound out and no any issue.

 

[Salas]

Should be simply more susceptible due to the wide loop area of a relays attenuator board vs the short enclosed path of a pot. An extra PSU is involved too.
Also try GND it through the input RCAs common instead. But better show us a picture first so we understand the configuration better.

 

[RCruz]

I have been listening to my dopa build for a while now and although it sounds very well it is not comparable with the DCG3..... it seems that Salas minimalist design is difficult to surpass.

Thank you for your work Salas

 

[Salas]

You are welcome

 

[alexkosha]

Salas DCG3 preamp (line & headphone)

Should be simply more susceptible due to the wide loop area of a relays attenuator board vs the short enclosed path of a pot. An extra PSU is involved too.

Also try GND it through the input RCAs common instead. But better show us a picture first so we understand the configuration better.

Here it is (forgive me for my Flemish school painting skills). I hope that image can be magnified in order to see all details clearly.

directly Connection to safety Earth or through GND lift for all tested by me points with jumper cable did not improve any situation.



I forgot to mention one more detail last time.

I do see that they are coming (these 60Hz and 180Hz spikes) when unit has no main power cable connected, but cable getting close to enclosure. It is just Fucusrite connected/running and I see floor nose grass.

 

[Salas]

I thought it was about their relays attenuator. When about a mechanical pot its metal body must be at chassis potential, not insulated.

 

[alexkosha]

Salas DCG3 preamp (line & headphone)

I thought it was about their relays attenuator. When about a mechanical pot its metal body must be at chassis potential, not insulated.

It is relay based leather type 20K constant input impedance new design unit. It is dimension-wise same and looks like one with the mechanical concept attenuator.

 

[alexkosha]

Forgot to mention that body of that attenuator is heavily anodized with not conducive coating and in order to GND it, I need to scratch it what will damage it’s “new” condition. BTW, I think I tried jumper to GND it and I did not see any changes in signal behave.

 

[Salas]

Test by wrapping it in grounded Alu foil

 

[alexkosha]

Salas DCG3 preamp (line & headphone)

Test by wrapping it in grounded Alu foil

Well, I stared from scratch my GND setup to get appropriate troubleshooting capability.

I separated returns for L and R on first generation I-Select (joined GND on PCB) and run L and R returns independently all the way trough.

Please see updated image, attached.

So, I have L and R signals and returns insulated from each other and none is connected to GND (10R elevated or enclosure/Earth).

I found the way to connect attenuator body to enclosure (Earthed). It is no noise up to 0dB. Above 0dB, noise is coming up and if I Earthing attenuator rod, then I see only 60 and 180 Hz picks up to attenuator’s MAX position (Step 63, +13.7dB, it is probably clipping).

I still do see that high level noise on opened inputs, even with Earthed body and its rod.

I still need to find BKM to connect Earth to attenuator rod, and have several ideas that I’m going to pursue tomorrow.



When that is done, I’ll go to advised GNDed Alu foil around Attenuator. Let’s see then… If that will reduce 60 and 180 Hz spikes, then I’ll use 0.3mm GNDed permalloy (Mu Metal) for better blocking of low frequency airborne pickups.

Will update soon.

 

[Siberia]

Personal revelation.
On cans, I prefer my old cheap SMD DACT type pot to TKD 2CP-2511.
And that's with with my "japanese made" bias and undecided character.

 

[Salas]

If closed back cans, they usually have some midbass hump. You would prefer the brighter pot between the two. Listen over various cans, open backs included, and when possible on speakers too.