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Old 4th April 2004, 10:48 AM   #1
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Default Tube output buffer for DAC....needs advice!

HI!

I have built the "Elektor DAC 2000" some time ago. Efter modding it with AD797 instead of OPA627 the sound has improved a great deal.

I would like to have switchable tube output though. Like the Shanling CD-players have.

I need advice on:

1. What kind of tube design to add (SSRP, etc.)?
2. What kind of tubes to go fore? I have a few 12B4A, 6N1P, E80CC to try out.
3. Where and how to break in to mu exsisting circuit to add the new tube output?

Any ideas?

Best regards,

Peter Lund
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Old 11th June 2013, 09:34 AM   #2
billbo is offline billbo  Australia
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If you have ic sockets on the op amp outputs you can simply remove them and tap off your audio and couple it into a simple cct. like this one NOS DAC with tube buffer output
If you think it is sounds promising you can leave at is or if you don't like it them recover and place op amps back into sockets. Use pcb pins to tap into the sockets.
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Old 15th June 2013, 09:48 PM   #3
wa2ise is offline wa2ise  United States
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Take a look at my web page Modifying CD player DAC circuits where I have a number of circuits for DAC chips with current outputs.
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Old 16th June 2013, 12:39 PM   #4
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this particular lower end dac isn't too bad, but I would waste the time to build one up from scratch.

the design considerations will be how good the PSRR of the tube stage you select.

Personally, a DAC with balance audio out would be better to build up a tube line stage with.

the I to V resistor network would be something to look at. OEMs have a tendency to go on the cheap here on those parts.

the clock is very very important. this is the other place they go cheap at. A PLL type clock is far superior to the basic crystal.
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Old 16th June 2013, 01:16 PM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by DavesNotHere
A PLL type clock is far superior to the basic crystal.
PLLs are always inferior to the crystal clock to which they are synchronised. PLLs are therefore used when some degree of adjustment is needed, as in an off-board DAC box. The PLL in the DAC needs to lock onto the crystal clock in the CD player (or other digital source), and is thus dependent on the low frequency jitter behaviour of that clock.
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Old 16th June 2013, 03:43 PM   #6
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crystal clocks are not accurate. I guess with consumer products it doesn't matter that much.

but the the clocks used to record it is much more critical:

mytek 192cx, apogee big ben, BLA Microclock --- all PLL.
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Old 16th June 2013, 03:48 PM   #7
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Less than 10ps is ok, but I shoot for 1ps on setting up a studio.

Last edited by DavesNotHere; 16th June 2013 at 04:00 PM.
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Old 16th June 2013, 04:02 PM   #8
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I got to go and fire up my $1200 inaccurate clock and record. Have a good day.
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Old 16th June 2013, 04:41 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by DavesNotHere
crystal clocks are not accurate. I guess with consumer products it doesn't matter that much.

but the the clocks used to record it is much more critical:
A good crystal clock is plenty good enough for professional and domestic audio. Low jitter is what matters: obtained by good design and high crystal quality. Long-term frequency accuracy is irrelevant as even a poor crystal clock is better than our ears or any musical instrument.

A PLL, by definition, has to lock onto something else (that is what the first L is: 'lock'). It will have poorer jitter than the thing it locks to.

Quote:
mytek 192cx, apogee big ben, BLA Microclock --- all PLL.
Just out of interest I googled the first clock you mentioned (mytek 192cx). It is a high quality clock plus dividers and buffers. It can take an external clock input too. The blurb does not mention PLL. If in fact it does include a PLL (for frequency multiplying?) then this will degrade the quality of the external clock input, as I have said.
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Old 16th June 2013, 11:32 PM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by DavesNotHere View Post
crystal clocks are not accurate. I guess with consumer products it doesn't matter that much.

but the the clocks used to record it is much more critical:

mytek 192cx, apogee big ben, BLA Microclock --- all PLL.
Crystal clocks can be as accurate as you want. I have one that has long term error of about one part in 10^13. That is totally "nuts" level of accuracy. Cost about $200 to get to that level.

Whenever you talk about more than consumer level, you have to have a "standard" to compare to. We can't talk about accuracy unless you can measure it. Lucky for us that there is now GPS.

As for a cycstal vs. a PPL. The PPL can never be as good as the source it is locked to. Even the best has to add some noise. We can argue if this matters and past a point clear it does not (after all your head moves because of the blood pumping through it) You a part in a billion.

Generally a high-end crystal oscillator will be "tuned" with a varactor diode and can have it's frequency pushed or pulled over some range. THe assembly is housed in a temperature controlled box and the output monitored and compared to a standard, GPS is the most common standard. A system like this is about as good as it gets. but not really needed for audio work
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