Salas DCG3 preamp (line & headphone)

[vgeorge]

I do like it, and after I recently changed the horn arrangement, I am sure that the DCG3 has the best sound I ever had in my system. I am not going to ditch my tube preamps, but the DCG3 beats them up in dynamics, resolution and neutrality.

 

[Salas]

Don't buy lower pf cables, but verify the simulation to make sure, better buy a 20K good switcher pot at a point.

 

[vgeorge]

You could also try verify that bandwidth with a 10K load resistor across two output cables in parallel on a channel and the scope's probe attached at that resistor. Feeding 1V 1kHz sinewave from the generator when the pot is split equally for worst case response and your 2x gain gives 1V output again. Wind the frequency up until you read 0.71V. For finding your practical -3dB supersonic reach.

When I find time I will do. The cables can easily reach to my scope without moving much stuff. The lab is not far away!

 

[Salas]

Your cables are doing very decently at ~1000pF anyway for 12 meters of parasitic, my HD600 cans 3m factory standard cable measures 450pF to give you an idea. Your K712 Pro's detachable did you ever try it on the LCR?

 

[vgeorge]

Don't buy lower pf cables, but verify the simulation to make sure, better buy a 20K good switcher pot at a point.

That meets my experience too, that 20K is good middle ground for all systems.
But I had it in my stash, so I had to try it.
Also the reason I left some space in my build in the pot area, was to be able to try attenuators that take more space.
I would like to find a ladder stepped pot, but goods ones cost serious money. We 'll see.

 

[vgeorge]

Your cables are doing very decently at ~1000pF anyway for 12 meters of parasitic, my HD600 cans 3m factory standard cable measures 450pF to give you an idea. Your K712 Pro's detachable did you ever try it on the LCR?

Not yet, another test for tomorrow maybe.

 

[Salas]

You may find little different BW even if the practical result will generally follow because the sim is with LSK models and you got UPA but the LSK Models are verified so I preferred them for the sim.

 

[vgeorge]

Your cables are doing very decently at ~1000pF anyway for 12 meters of parasitic, my HD600 cans 3m factory standard cable measures 450pF to give you an idea. Your K712 Pro's detachable did you ever try it on the LCR?

300 pF for the orange 3 meter K712 cable.

 

[Salas]

Between each channel and common ground?

 

[vgeorge]

Yes

 

[RCruz]

Hi Salas

Just finished my new mosfet power amp ( http://www.diyaudio.com/forums/soli...r-amplifier-assemblage-vii-6.html#post4980619 ) and it sounds very well with the dcb1 and with keantoken's superbuffer but it distorts at medium volume level with the DCG3.

The power amp has an input impedance of 33k and a series resistance of 560 ohm. Is the low output impedance of the DCG3 imcopatible with this setup ?

 

[vgeorge]

Hi RC,

as the DCG3 has some gain, maybe you are overloading the input of your amp?

 

[RCruz]

Yes... maybe... Increasing series input resistance might help.

 

[vgeorge]

I think only if you increase it a lot, like 10K.

 

[AndrewT]

measure the voltages with the DCB1 and with the DCG3.
does the DCG3 distort even though the speaker voltages are at a similar level to the DCB1?

 

[AndrewT]

Could it be overload inside the DCG3?
with gain the output voltage is higher than the input voltage.

 

[bkdog]

Gain structure is an important concept...
http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html
Setting Sound System Level Controls

 

[Salas]

Hi Salas

Just finished my new mosfet power amp ( http://www.diyaudio.com/forums/soli...r-amplifier-assemblage-vii-6.html#post4980619 ) and it sounds very well with the dcb1 and with keantoken's superbuffer but it distorts at medium volume level with the DCG3.

The power amp has an input impedance of 33k and a series resistance of 560 ohm. Is the low output impedance of the DCG3 imcopatible with this setup ?

If its not about gain structure it could be the power amp does not like low output impedance sources as it is now input filtered & stability compensated. DCB1 has about 270 Ohm Z, DCG3 has 50 Ohm (lab standard) that can change if necessary because its only set by a resistor in line output. Natively it has near zero.

 

[AndrewT]

If its not about gain structure it could be the power amp does not like low output impedance sources as it is now input filtered & stability compensated. DCB1 has about 270 Ohm Z, DCG3 has 50 Ohm (lab standard) that can change if necessary because its only set by a resistor in line output. Natively it has near zero.

If too low a source impedance is the problem that is causing the distorted medium output signal, then it is easy to try a little experiment.

The input +IN node sees R3(33k)||C2(470pF)||{R1(560r)+Rsource}

Rcruz could change the 560r to 750r for the DCG3 to give a total of ~750r+50r = 800r in parallel with 33k||470pF
and compare to DCB1 ~560r+250r = 810r in parallel to 33k||470pF,
or go the whole way and change R2 to 1k
The output impedance of the DCB1 is ~30/50ohms and the 220r adds to that to give ~250/270ohms.

It is unusual for too low a source impedance to cause input stage misbehaviour and create extra distortion. The opposite can be the case: high source impedance can result in increased distortion. Which is not what has been reported here.

I suspect the high gain of the amplifier combined with the high gain of the pre-amp results in a non compatible system.

 

[AndrewT]

Hi Salas

Just finished my new mosfet power amp ( http://www.diyaudio.com/forums/soli...r-amplifier-assemblage-vii-6.html#post4980619 ) and it sounds very well with the dcb1 and with keantoken's superbuffer.................

Have you solved the output offset?
What did you do?