Salas DCG3 preamp (line & headphone)

[Salas]

Originally Posted by AndrewT

if you don't approve of source resistors then use emitter resistors in the mirror.
200r on both sides and then a 10r on Q2 side and the 22vr on the Q1 side.
This lets you dial out the offset and lets you see the balance of the LTP.

Big mirror resistors don't harm performance.

Emitter resistors in the mirror I had tried few values. 10-33-47-68-100R. Leaned to third harmonic and the DC turn on bump jumped high. I did not like it.

* They can be seen on the little blue breadboard in crop detail from the first picture of post#2

[AndrewT]

switch on thump would be a problem with headphones.

[Bonsai]

Andrew,

We are still waiting for you to publish one of your projects/designs.

No need to try to redesign Salas' pre/headphone amp. The distortion plots speak for themselves.

[Salas]

He, I just found a forgotten one in the camera from the development phase. The mirror BJTs and the the second 9610 on daughter boards. There emitter resistors no more. That pic has much stuff. Slim uPA on main board, LSK170s and various Toshibas scattered around, LSK389 TO-71 lying at front left, 1uF C5 rejected at upper left. A Cosmos Tocos test pot at upper right that Tham will like, DN2540s aside left after cascoded and made not much sense, various C1s and C2s from hands on compensation trials, hey even a current regulator diode under the uPA that was evaluated for stability vs the BF looking at us over the "101" ceramic. I mean by blowing on them with the hot air station "scientifically" evaluated.

[Salas]

What pot to use? 10K-20K-50K-100K? OK not the extremes. Had it with 20K but how is it with 50K that's smoother for turning up and easier on sources? Well its not bad at all. Nice tone, just a bit rounder. Safer.

[AndrewT]

Originally Posted by Bonsai

Andrew,

We are still waiting for you to publish one of your projects/designs.

No need to try to redesign Salas' pre/headphone amp. The distortion plots speak for themselves.

I am following this Thread with interest and was deliberately not making any comment on topology nor performance. It was looking good.
Until Salas said this:

The LTP's tail CCS forces the units to bias at about 2.5mA Id per LTP side.

That is wrong and I felt forced to speak out and correct his error.

[Salas]

When you want the LTP best balanced you set the trimmer to zero. The servo still copes fine. So simple.

[Tea-Bag]

Originally Posted by Salas

What pot to use? 10K-20K-50K-100K? OK not the extremes. Had it with 20K but how is it with 50K that's smoother for turning up and easier on sources? Well its not bad at all. Nice tone, just a bit rounder. Safer.

Rounder in tone?
Can you describe that. I have only tried 50K. I am pleased.

[Salas]

Originally Posted by Tea-Bag

Rounder in tone?
Can you describe that. I have only tried 50K. I am pleased.

Like when I was listening to Carey from Joni Mitchell's Blue in 192K FLAC with the 20K pot it was upfront and pristine but with the 50K one it came across bit less tiresome. A small difference but it could make the song passable without reaching for testing another filter from the DAC's menu. In this video the Appalachian dulcimer is sounding much softer in tone than the one in the record which is much tinnier in comparison. It won't make it soft like that, it will just put it in the recording space hanging a bit better. (https://www.youtube.com/watch?v=A1kcTjtdVE8)

Maybe it has to do with the necessarily narrower resulting bandwidth vs 20K. Then again all 20K or 50K pots won't be the same, but at least I had same cheap ones of various values. Its more of a system matching choice thing each builder has to do for himself.

[Salas]

Originally Posted by massimo

Another Salas' interesting design! Thank you for sharing, it definitely deserves a try.

I have a few questions:

1. uPA68H (old, fat) or uPA68HA (new, slim)? You had the chance to test both, any prefrence?

I put the fat ones in today to answer you better. But I could not find any subjective difference. Maybe its the aural memory fainting between listening with the slims then desoldering them, soldering the others, and let warm up, but nothing striking.