Salas DCG3 preamp (line & headphone)

[Salas]

Thanks for the song - very nice. I also like this one for testing head phone amps.

https://www.youtube.com/watch?v=s6_B1AB9nu8&sns=em

Just listened to it on the 770s. Yes good on the Beyer's sonic style indeed.

 

[Salas]

P.S. Maybe try some Beth Hart tracks too

 

[xrk971]

I have a problem

I just measured the spectrum of my DCG3 and it looks like there are some oscillation issues. I can't really hear it all that well but it shows up like a xmas tree on the FFT. My simple 2 transistor amp measures clean so I know the rig and software are not at fault. Using UCA202 and RightMark software.

What do you all think could be causing this?

As a comparison of a clean amp, the same rig measures this:

 

[Salas]

Signal or rails cabling issues? PSU interaction? Bad grounding? Non grounded vol pot body? It can do 1MHZ even with 20K pot in between source and input so it takes some caution. Did you have a pot when running the RightMark?

 

[xrk971]

No pot. Let me take a look at PSU - try different one. I am using 100nF caps instead of 150nF caps in amp. Would that matter?

Must be no earth grounding.

 

[Salas]

In op-amp's rails decoupling? Not likely to matter.

 

[Salas]

Are your C4s 1uF indeed? Looking kinda small size.

 

[Salas]

Must be no earth grounding.

Meaning?

 

[xrk971]

Amp ground is not connected to earth ground. Only AC mains connected to trafo primary for now.

1uF is 50v MLCC.

 

[Salas]

Could even be radio interference or rails oscillation. It shows lower noise floor even to the clean one in areas its not excited and that's weird. If you will ground the Jfet cans with crock clips to the signal ground, does it change something?

 

[xrk971]

Let me try a better PSU and in different room. There seems to be RFi in that room.

 

[thimios]

Let me try a better PSU and in different room. There seems to be RFi in that room.

Are you sure that you don't ovedrive the unit?
Pay attention on RMAA vu-meter

 

[Hikari1]

Almost ready to fire this thing up.

 

[Salas]

Good luck

 

[Hikari1]

Regarding the signal wiring, if running twinax with a sheild should I connect the shield at both ends on all connections inside the amp or just connect it at the source end? Is it better to "complete" the shield throughout the input signal wiring or leave each individual sections with the shield only connected at the source end? Iam thinking it's better to connect all the shields except the one to the amp board input so it all drains to the source. Any opinions?

 

[Salas]

Don't you have single coax for each channel signals? Its better.. Connect all shields to the PCB common. I would connect both ways the amp's wire shield too. I would try differently only in case of hum loop.

 

[xrk971]

I set it up in another room with a known good PSU (7815/7915 based with CRCLC filtering good for solid 150mA bias). Unfortunately got the exact same HD xmas tree bloom. So I added a 50k pot and below a certain level (-2.7dB on VU meter - it says -1dB should be good), the HD goes nuts - just explodes. But a tad below and it vanishes. There is a minimum input load that is required to keep the amp stable. I should measure what that setting on the pot is and just add a hardwire resistor to ground to keep it from happening.

Here is the new FFT with the pot set so that the distortion doesn't blow up. Much better - this amp is predominantly H3 and H7 - I suppose having a differential input pair does that?

 

[Salas]

Its not an H3 preamp when normally set, its like in post#1

Are your C1 10pF & your C2 33pF verified? Does the H profile change if you tweak VR1? Maybe you could make R1 4.7k

 

[Hikari1]

Don't you have single coax for each channel signals? Its better.. Connect all shields to the PCB common. I would connect both ways the amp's wire shield too. I would try differently only in case of hum loop.

Seperate twinax each channel. 2 wires plus shield each channel.

In this diagram should I connect the shield at the vol pot and amp or leave it disconnected at the amp to "drain" like some interconnects do?

For the psu I will probably just connect the shield at all connections.

 

[Salas]

I would first connect everything both ends and only change something in case of noises