Dac with tube preamp? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th March 2010, 08:08 AM   #1
diyAudio Member
 
Join Date: Jul 2009
Location: Western Europe
Question Dac with tube preamp?

Hi,

I'm about to build the lampucera DAC with tube output buffer (as cathode follower)

http://www.lampizator.eu/LAMPIZATOR/...X/IMG_5143.jpg

I don't need any other sources and I have no need for EQ, so I thought I would skip the preamp and connect the dac directly to my power amp (Quad 303).

As I still need volume control I'm planning to add a tube amp stage with volume control after the cathode follower. The quad has a low input impedance (22kOhms) so I'm not sure which design would be best? My speakers are Quad ESL 57's.

Is this a good idea in general or am I missing something?

Thanks a lot,
Teo
  Reply With Quote
Old 25th March 2010, 10:32 AM   #2
diyAudio Member
 
kavermei's Avatar
 
Join Date: Jan 2005
Location: Lokeren, Belgium
Send a message via MSN to kavermei
Hi Teo,

looks like the signal level is just right for your power amp. This means that you don't need extra gain.

For volume control, you could suffice with a 5kohms stereo pot at the output of the buffer. Your output coupling cap should then be at least 5uF.

If you will be using *really* long cables to your amplifier, or want to use a smaller coupling cap, you can put a second cathode follower behind the volume pot. The volume pot can then be much larger (100Kohms) and the coupling cap could be 220nF or so. But this requires a second tube.

HTH,
Kenneth
__________________
Never send a human to do a machine's job. --Agent Smith
  Reply With Quote
Old 25th March 2010, 01:04 PM   #3
diyAudio Member
 
Join Date: Jul 2009
Location: Western Europe
Hi Kenneth,

thanks for your quick response!
I've checked the specs for the Quad, and you're right: max input level is 500 mV RMS, so with the 2Vpp from the DAC is more than enough!
I'll use a pot as you described then.

Thanks!!
Teo
  Reply With Quote
Old 25th March 2010, 03:03 PM   #4
diyAudio Member
 
Join Date: May 2007
Hey Teo,

First of all, do you take the signal directly from the chip without any buffers in between?

Donīt use the schematic above as is. You will get high distortion due to excessive loading from the small cathode resistors!

Also why is there caps between DAC and CF???? Deleting them would better performance as the cathode-resistors would get higher values. Can redraw if you want to.

Still, using this circuit will degrade DAC-performance due to the added distortion from this mediocre design.

If you have typical voltageDAC, Cirrus etc. only a cap followed by a pot directly from the DAC is needed. No tube!

Personally I doubt it is the best of ideas to only use one of the balanced outputs but it obviously works.
  Reply With Quote
Old 25th March 2010, 04:06 PM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally Posted by revintage View Post
Hey Teo,

First of all, do you take the signal directly from the chip without any buffers in between?

Donīt use the schematic above as is. You will get high distortion due to excessive loading from the small cathode resistors!

Also why is there caps between DAC and CF???? Deleting them would better performance as the cathode-resistors would get higher values. Can redraw if you want to.

Still, using this circuit will degrade DAC-performance due to the added distortion from this mediocre design.

If you have typical voltageDAC, Cirrus etc. only a cap followed by a pot directly from the DAC is needed. No tube!

Personally I doubt it is the best of ideas to only use one of the balanced outputs but it obviously works.
Lars, I completely agree with everything you have said and would take it further wrt to using only one of the balanced outputs: unacceptable..

If this is a balanced output voltage dac I would use a transformer to convert the balanced drive to an unbalanced output - simple, works well and sounds surprisingly good with the right transformer.

Given the gain requirements and the output voltage from the dac a 2:1 stepdown should work well and drive a 5K pot with no problem at all. (I'd look to Lundahl for suitable transformers.)
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 25th March 2010, 04:31 PM   #6
diyAudio Member
 
Join Date: May 2007
Well spoken Kevin. Was trying to be a little diplomatic about the balance/unbalanced ting.

And yes, 2:1 seems to be a good idea wrt levels. Except for helping us get unbalanced out when using transformers we also get rid of the 2,5V DC from the chip.

It doesnīt have to be the most expensive LL either. The amorphous 1544A is a great choice among the smaller ones. A good source for Lundahl in Europe is Benny at Aquablue.
  Reply With Quote
Old 25th March 2010, 10:46 PM   #7
diyAudio Member
 
Join Date: Jul 2009
Location: Western Europe
Hi revintage,

thanks for your reply,

Quote:
Originally Posted by revintage View Post
First of all, do you take the signal directly from the chip without any buffers in between?
Yes, I will be taking the signal directly from the chip.

Quote:
Originally Posted by revintage View Post
Also why is there caps between DAC and CF???? Deleting them would better performance as the cathode-resistors would get higher values. Can redraw if you want to.
Isn't there always a cap there in a cathode follower design?

Quote:
Originally Posted by revintage View Post
Still, using this circuit will degrade DAC-performance due to the added distortion from this mediocre design.

If you have typical voltageDAC, Cirrus etc. only a cap followed by a pot directly from the DAC is needed. No tube!
But would I then not be missing a buffer since I want to connect the dac immediately to the amp which is low impedance?

Thanks, I'm still rather new to audio electronics and I appreciate your help!

Regards,
Teo
  Reply With Quote
Old 25th March 2010, 11:54 PM   #8
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
That cathode follower design you posted is 100% garbage - unless you want around 20% distortion.

A cathode follower needs a good amount of voltage (let's say 50V) across it's cathode resistors - this has almost none.

Use a transformer - or just a cap , but TRASH that Cathode follower!

Regards, Allen
  Reply With Quote
Old 26th March 2010, 12:52 AM   #9
diyAudio Member
 
Join Date: Jul 2009
Location: Western Europe
Hi Allen,

thanks for your answer!
Actually the schematic is not mine but from Mr. Fikus at Lukasz Fikus Lampizator. He seems to use it quite successfully in many of his projects.
But I don't want to question you guys, because as I said, I'm a complete newbie.

I'm just worried that with just a cap after the DAC I would not have any output buffer before my amp.
  Reply With Quote
Old 26th March 2010, 07:50 AM   #10
diyAudio Member
 
Join Date: May 2007
Quote:
Actually the schematic is not mine but from Mr. Fikus at Lukasz Fikus Lampizator. He seems to use it quite successfully in many of his projects.
But I don't want to question you guys, because as I said, I'm a complete newbie.

I'm just worried that with just a cap after the DAC I would not have any output buffer before my amp.
Hey Teo,

Mr. Fikus might be a man with a lot of enthusiasm about CD but the schematics from him shows that he havenīt any skills to design acceptable tube-circuits. As Allen says this one is 100% garbage. Forget it!

You might get away with it using a 6H30 with an Ug of 7V/10mA and a single BJT CCS, still not what I would recommend.

The DAC you have has just a few ohms Zout. Adding a 5k pot gives you a worst case Zout of 1,25k so there will be no problems whatsoever to drive your QUAD. No need for a buffer unless you use extremely long and high capacitance cables. The output cap must be choosen for 5k which gives you 6,8u for -3dB at 5Hz.

You still have the problem with taking the signal from only one side which is should be a No-No even if it will give signal. Go for a transformer! the 1544As will only set you back 60EUR/each.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
need diy tube preamp (line-stage) recommendation and general tube advice Musiclover4 Tubes / Valves 16 18th May 2013 03:19 PM
Running a tube preamp into a solid state preamp? Pckid212 Solid State 5 6th January 2013 10:12 PM
FA: Eico HF-85 Tube Preamp & HFT-90 Tube Tuner joe-electro Swap Meet 0 31st May 2009 04:14 PM
Tube Preamp Popping on Source Input Change (?) / 6DJ8 Preamp On Chip Amps MartyM Tubes / Valves 3 16th October 2008 04:52 AM
Scott Nixon's DAC kit and Tube DAC Sid-W Digital Source 0 5th March 2004 12:38 AM


New To Site? Need Help?

All times are GMT. The time now is 08:17 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2