Salas DCG3 preamp (line & headphone)

Happy camping

Hi Salas,

It's about time I posted a pic of the finished work. I recently reset the DC offset; the music (AWB today) is centered and rock solid. I love this thing!

Thanks again Salas.

Keith
 

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i have heard that discrete opamp not are the best in servo,,,is it something with the use of jfet in normally opamp ?
He tried the OPA1642 also I think. Bipolar is not best here due to high source resistor but some still work well.
All industrial JFET input dual chips of 12nVrtHz or less noise spec that can handle +/-18V rails at least and have low THD at 10K load are technically competent to roll here. Given the resulting offset is low. I have measured and listened with some that met or bested the above criteria during the development, the OPA2604 too. They all passed with no additional noise or extra THD on the preamp's FFT. Except the traditional old TL072 that showed some measurable worsening contribution. Its below than modern average spec naturally allowed that. But the DC offset was fine.
 
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Hi Salas,

It's about time I posted a pic of the finished work. I recently reset the DC offset; the music (AWB today) is centered and rock solid. I love this thing!

Thanks again Salas.

Keith

Congratulations! Nice low profile box build, Hammond allowing. Tell us a few words about the system it plays in. I see (EL34?) glass bottles there...
 
Congratulations! Nice low profile box build, Hammond allowing. Tell us a few words about the system it plays in. I see (EL34?) glass bottles there...

Actually, the Hammonds are normally under cover. And yes, you see right...EL34's driven by a 'Dynamutt' noval board. Magnaquest OPT's.
The speakers are from the diyAudio Fast, Fun OB project elsewhere on this site.

The phono pre (and its pricey tubes) is one of Jim Hagerman's original board kits. The sale of its tubes will buy a planned tubed phono stage (diy of course) and a few records. Sumiko arm on the table, Audio Technica 33PGTII, Kelly/Bergeron speed controller (can be searched under Maxon' GB'), Cinemag SUT,.

All good wife-approved fun...what could be better:)

Keith
 

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Guys, I just finished my first listening session after BC327-40 upgrade and I have quite significant and very positive change in sound. Salas, please accept my respect and applause. I do not know what happen to Beardman unit, but my unit is singing. My mids and mid highs are way clearer. Very pleasant extension. It is kind of more round presentation now. I didn't trusted only my ears and asked my wife if I'm not hallucinating. She also has an absolute pitch and she was a musician in the past. Same exactly feeling. Even humbled U2 ripped CD played from my iPhone sounded like hi-end. No doubt, my second unit with LDRs volume control will be upgraded next weekend.
 
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Alex.
it is what i will call a different experience with the BC327..

i will say the same with the more round sound,,but i miss the clearness in the sound..

to me it gave a more polite sound

the best sound right now i have ,is whit the opa2134 this is more the sound i like



I agree. We all are bit different in perception. This is a nice thing about humans. Otherwise, all will be the same and boring....
Can you tell what is your Bias set to and what volume attenuator do you use? I did not test op-amps and still use one advised by Salas. Very happy about latest test results.
 
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I am looking into this design as a future pre-amp.

As I will only use it as a pre-amp (no headphones) I guess with a > 50k load the bias current can be much less maintaining class A operation.

Any suggestions or maybe even some component change in the last amplicifation stage ?

R10 controls the bias setting. The higher its value the less bias. Class A you may maintain with less Ib on easy loading but on the other hand you probably wouldn't want to compromise the transconductance (gfs) of the output Mosfets or pull them out of their zero tempco curves region. The 100mA DCG3 bias option is a good base for those fast Vertical PMOS in my opinion.

Of course nothing stops you to test with double Ohm R10 than in the BOM or more and decide if the sound quality is still good in your application. Goes without saying that the heat will drop much significantly both on the DCG3 and the DCSTB sinks, if that's the goal. The DCSTB's pass transistors will work with progressively higher Zo the lighter it gets loaded is another negative side effect.
 
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Guys, I just finished my first listening session after BC327-40 upgrade and I have quite significant and very positive change in sound. Salas, please accept my respect and applause. I do not know what happen to Beardman unit, but my unit is singing. My mids and mid highs are way clearer. Very pleasant extension. It is kind of more round presentation now. I didn't trusted only my ears and asked my wife if I'm not hallucinating. She also has an absolute pitch and she was a musician in the past. Same exactly feeling. Even humbled U2 ripped CD played from my iPhone sounded like hi-end. No doubt, my second unit with LDRs volume control will be upgraded next weekend.

Alex.
it is what i will call a different experience with the BC327..
i will say the same with the more round sound,,but i miss the clearness in the sound..
to me it gave a more polite sound
the best sound right now i have ,is whit the opa2134 this is more the sound i like

I agree. We all are bit different in perception. This is a nice thing about humans. Otherwise, all will be the same and boring....
Can you tell what is your Bias set to and what volume attenuator do you use? I did not test op-amps and still use one advised by Salas. Very happy about latest test results.

Yes it is good we all hear different
my volume is a TDK 2500 20kohm
i cant remember just now what my bias is,,will measure later to evening
you should try the opamp change..but i am not sure if the opa2134 will be good with the BC327


Beardman, Alex, IMHO your different 327 subjective synergies lie in the warm TKD pot VS the more neutral Goldpoint switcher and in your room sound of the enormous GR Research LS9 line array speakers VS the more agile Acoustic Zen Adagios. Alex, maybe you could also find a servo chip or other parts that best offset the 327's bit ripe effect and maintain its extra resolution effect that you mostly appreciate. In the airy LDR controlled second DCG3 build of yours possibly nothing more will ever be needed though. Anyway I am happy that either my servo chip or mirror BJT to try suggestions worked for both of you guys complimenting your own needs and systems. :)
 
Salas, I have a few questions about the DCSTB.
1. What is the minumun load in order to keep a decent Zo? It may be possible increase Rb to compensate a lighter load?
2.Why split the filter cap into C1 and C2 with a fuse in between? Fuse holder and fuse resistance/inductance seems too low to form a P filter... Layout and/or cap availability considerations? Why not a single 4700uF cap? Higher ESR?
3. Same for C5-C6-C7. Why 3 elcaps instead of a single 330uF?
Thank you.
 
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1. When pulling below 100mA from it better increase the R bleeding rate to compensate. Been designed with 100-150mA range* in mind.
2. Taking the fuse's parasitic resistance to our advantage and put it into creating a gentle Π filter. Left alone it would simply make a small RC.
3. Paralleling to lower ESR & ESL while maintaining a narrower board profile to match the preamp's width when still having dual mono PSUS.

*Its MJE pass transistors keep sufficient HFE for up to 2A-3A given the sinking is allowing.