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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 30th January 2017, 10:46 PM   #1271
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
Quote:
Originally Posted by bambadoo View Post
Prelimanary volume pot (25k - ladder attenuator ebay style ) and prelimanary cabling for testing purpose. In the office setup for testing... line out.. this preamp has potential.


Too early to make serious listening.. will try in a week or so in my main setup..
Now in the home office. Dac-dcg3-audio note soro integrated (have for repair)-dali 350 speakers..

I was settling for approx 150ma bias (30 ohms headphones) using the 7.5r (from tea-bag GB) for r10. I got about 160ma.. Good heatsinking.. Is 160ma ok?

Regarding dc offset. I really didnt notice any improvent with or without the opamp. Opamp is in now when playing. Approx +-1mv offset. Will calibrate again in some days..
Any ideas for optimum grounding scheme? I had non connection between the lineboard and chassis ground so I mounted a small cable strip from line in to chassis gnd - pot is grounded. Works ok.
For that kind of big sink in your pictures 160mA is easy work, no need to hold its bias back.

As I wrote before this preamp may give steady low offset result on its own before servo -especially when on a big sink- but the opamp is your safeguard because drifts can happen at a later point when put in a system and not looking for them anymore. Unless the power amp has an input coupling capacitor, to leave the opamp out is still a future DC risk.

As for grounding, when its silent in your own way, you are done. Line-in RCA ring and a wire to chassis isn't much different than a wire from pot's common to chassis that we usually do anyway.
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Old 31st January 2017, 01:27 PM   #1272
Rothor is offline Rothor  Sweden
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I found two of these toroids lying around (220v/18v 15VA +18v 15VA) and was thinking that maybe I could parallel each one of them and use them for the DCG3. Would that be OK, or am I making things too convenient for me...?

https://www.dropbox.com/s/urd2p04c76...%2044.jpg?dl=0

I hooked them up and got around 22,35V. I guess that could be a bit too much?

//Ronny
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Old 31st January 2017, 01:46 PM   #1273
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
They would normally show less ACV when well loaded. Else they are not in good tolerance for nominal voltage standards.

Even if you parallel two you will have just one 30VA+30VA=60VA total.

In my opinion do it double mono with those if you must use them but you will have to stay at 100mA bias for those toroids to be working at about 35-40C each prolonged.
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Old 31st January 2017, 01:50 PM   #1274
AndrewT is offline AndrewT  Scotland
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18Vac has a peak voltage of 25.46Vpk
your measured 22.35Vdc seems far too low.

Output voltage = mains voltage / rated primary voltage * rated secondary voltage * (1+transformer regulation)
if your 36VA transformer has a regulation value of 12% and you have a mains voltage of 225Vac
then your 220:18Vac transformer will give 225/220*18*1.12 = 20.6Vac
convert thatto peak and you get 29.16Vpk
Subtract 0.5V for the diode drop and you should have ~28.6Vdc across the smoothing capacitor. Attach a load and the PSU voltage will droop.
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Old 31st January 2017, 01:54 PM   #1275
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Salas View Post
They would normally show less ACV when well loaded. Else they are not in good tolerance for nominal voltage standards.

Even if you parallel two you will have just one 30VA+30VA=60VA total.

In my opinion do it double mono with those if you must use them but you will have to stay at 100mA bias for those toroids to be working at about 35-40C each prolonged.
Quote:
220v/18v 15VA +18v 15VA
each secondary winding will have a maximum continuous output of 15VA/18Vac = 0.833Aac
After de-rating for a capacitor input filter you end up with a maximum continuous DC current of ~450mAdc
To run the transformer cooler I suggest you use <=50% of that maximum continuous rating i.e. continuous current <=225mAdc from each secondary winding.
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Old 31st January 2017, 01:57 PM   #1276
Salas is online now Salas  Greece
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Salas DCG3 preamp (line &amp; headphone)
Quote:
Originally Posted by AndrewT View Post
18Vac has a peak voltage of 25.46Vpk
your measured 22.35Vdc seems far too low.
He measured the secondaries with an ACV RMS meter most probably. That's what I understood.
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Old 31st January 2017, 02:11 PM   #1277
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
18Vac has a peak voltage of 25.46Vpk
your measured 22.35Vdc seems far too low.

Output voltage = mains voltage / rated primary voltage * rated secondary voltage * (1+transformer regulation)
if your 36VA transformer has a regulation value of 12% and you have a mains voltage of 225Vac
then your 220:18Vac transformer will give 225/220*18*1.12 = 20.6Vac
convert thatto peak and you get 29.16Vpk
Subtract 0.5V for the diode drop and you should have ~28.6Vdc across the smoothing capacitor. Attach a load and the PSU voltage will droop.
Quote:
Originally Posted by Salas View Post
He measured the secondaries with an ACV RMS meter most probably. That's what I understood.
As I said his measured voltage seems to be far too low.
Maybe they are mislabelled and should be 15Vac secondaries. Or the battery in his DMM needs changing, or he forgot to attach the smoothing capacitor?
But even a 15Vac 30VA transformer will give more than 22.35Vdc after rectifying and smoothing.
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Last edited by AndrewT; 31st January 2017 at 02:14 PM.
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Old 31st January 2017, 02:29 PM   #1278
Rothor is offline Rothor  Sweden
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Thanks for your help!

Yes, I just connected them to mains and meassured directly with a volt meter, and got 22,35Vac.

I also had another one: 220v/18v+18v+8v/77VA. But that's a bit on the small side too then i guess.

Considering other costs, and time spent, I might as well order a proper one.

Thanks again!
//Ronny
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Old 31st January 2017, 02:33 PM   #1279
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Rothor View Post
Thanks for your help!

Yes, I just connected them to mains and meassured directly with a volt meter, and got 22,35Vac.

I also had another one: 220v/18v+18v+8v/77VA. But that's a bit on the small side too then i guess.

Considering other costs, and time spent, I might as well order a proper one.

Thanks again!
//Ronny
Quote:
I hooked them up and got around 22,35V.
ask a friend to check your DMM against theirs.
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Old 31st January 2017, 02:38 PM   #1280
Rothor is offline Rothor  Sweden
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I'll start with getting a new battery for my DMM...
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