DIY Pre-Amp for UcD400 or UcD180

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#21
chrisb03 said:
The DACT preamp has a selectable 0,6 and 12db gain. How is this interpreted when connecting with the Ucd400 which has a 26db gain?

ghemink tried explainging the 26db for the UcD400's for me, but I didn't get it. :xeye:
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chrisb03 said:
Here's my source, P3A dac, it quotes a 2.5V output, 100ohm output impendance. What sort of specs do I require in a preamp (in and out) to have the best performance.

What do you recomend, I've got no idea and need serious help😕 :smash: .

http://www.av123.com/manuals/2_P3A_oman_h.pdf
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Hi,

26dB gain means a factor of 20 gain. If your source has 2.5V output, you can get 20*2.5=50V output. This is enough to drive the UcD400 to fullpower, in fact slightly beyond. 2V input voltage would drive the UcD to full power output when a 60V powersupply is used for the UcD. The UcD400 has an input impedance of 100k, your sourse has 100Ohm output, so using a potmeter in the range of 1k-10k should be OK as a passive preamp.

If you add a preamp, you would have more gain than needed, 0dB gain for the preamp would be enough (this means no gain), 6dB or 12 dB (a factor 2 and 4 respectively) would give you to much gain and would increase the noise level of your setup. So I think you could use a passive preamp or only a potmeter if you use only one source. If you have other sources that have less output voltage you may have to use an active preamp.

Hope this helps

Best regards

Gertjan
 
#22
matjans said:
a slightly modified version of my pre-to-be.
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How about adding a variable gain (-6 dB - +6 dB) in the feedback loop for the opamp (like in 10 a. here but the Input to ground when in noninverting configuration: http://www.linkwitzlab.com/filters.htm#10). That would be useful for active speakers, and could be after measurements replaced with static resistors. Also could be used as balance control in stereo use.

Edit: The highpass could also be in the feedback patch of the opamp (no need for caps at output).
 
#24
What is the common output voltage/impedance on regular mass market RCA-out? A common CD player?

I'd like to add a switch to my UcD amp, choosing from 2 inputs.

One fixed, goes to the pre-outs of my surround receiver - of course volume control on the receiver this way

One variable, goes to the output of my CD/DVD player, with passive attenuator. This will be used for critical 2 channel listening (maybe i'll modify source & the UcD's)

If my listening levels aren't extremely high, can I get away with a passive between a regular CD player and unmodified UcD400's?
 
#25
Yves Smolders said:
What is the common output voltage/impedance on regular mass market RCA-out? A common CD player?

I'd like to add a switch to my UcD amp, choosing from 2 inputs.

One fixed, goes to the pre-outs of my surround receiver - of course volume control on the receiver this way

One variable, goes to the output of my CD/DVD player, with passive attenuator. This will be used for critical 2 channel listening (maybe i'll modify source & the UcD's)

If my listening levels aren't extremely high, can I get away with a passive between a regular CD player and unmodified UcD400's?
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CD players have enough output for the UCD's. Just change the gain on the UCD's and than you have full control with a passive.
 
#30
Yves Smolders said:
Thanks for the reply Bgt!

If I don't change the gain, will I still have a reasonable output level?

Thanks,
Yves
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Yes, no problem. Only thing is you have to turn your pot/step attenuator a bit high up.


Pasi P said:
Im gonna make passive pre using DACT CT2 stepped attenuator 10k and Bent Audios Dact Attenuator Remote Control.

http://www.dact.com/html/attenuators.html
http://www.bentaudio.com/index2.html
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The way to go Pasi, I use also a DACT CT2 with a passive preamp with relay input switch overs. I also switch every ground at the same time. This reduces humm/crosstalk. There is no electrical connection between the relay PSU(outside the preamp housing) and the analog ground line. Only a small cap. 100nf/100V between relay ground and analog ground.
 
#32
Bgt said:


The way to go Pasi, I use also a DACT CT2 with a passive preamp with relay input switch overs. I also switch every ground at the same time. This reduces humm/crosstalk.
Click to expand...

Thank you for the tip!
I havent think about that at all.

I have ordered these relays http://www.farnell.com/datasheets/922.pdf

These have twin contacts and i am gonna use separate relays in L+R channels, so switch also grounds is possible.


I use 6-ch DACT (volume control after Linkwitz Orions XO to reduce hum from active filter)
 
#33
Pasi P said:


Thank you for the tip!
I havent think about that at all.

I have ordered these relays http://www.farnell.com/datasheets/922.pdf

These have twin contacts and i am gonna use separate relays in L+R channels, so switch also grounds is possible.


I use 6-ch DACT (volume control after Linkwitz Orions XO to reduce hum from active filter)
Click to expand...

Relay looks ok, gold contacts. Whats the Linkwitz Orion XO?
 
#36
Looks nice but I want audiophile quality, the less components in the audio path the better!
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Dear Yves:
If you don't mind spending a little more, consider Transformer based Volume Control (TVC). They pair nicely with UCD with any modern source (I even tried it with my DDDAC1543 wich has 1.6Vout max). I think there can't be a more purist solution. Appart you can configure the in/outs as balanced/unbalanced (with a risk of buzz with certain sources, I'm affraid).
The best would be balanced source->balanced TVC->UCD
I even consider puting one TVC into each monoblock to save one IC, in the future. 😀
Ready made or DIY. Mine costed me US$600 (before Tx increased in price) 🙁

http://www.bentaudio.com/index2.html

http://www.6moons.com/audioreviews/audiozone4/pretone_2.html
The last is a review about a TVC by our forum member Peter Daniel.

Good luck 🙂
M
 
#37
does anybodty know why I cannot get the complete input voltage on the output? Did I connect something wrong?

An externally hosted image should be here but it was not working when we last tested it.


its from a new preamp I've just build.

An externally hosted image should be here but it was not working when we last tested it.

The pot is from
http://www.diyfidelity.com.au/product_info.php?cPath=0_33&products_id=32

inside:
An externally hosted image should be here but it was not working when we last tested it.


The mains unit is next to the amp. You just plug it in the mains and plug your poweramp in the socket.

rear
An externally hosted image should be here but it was not working when we last tested it.
 
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