Output stage for the Soekris R2R

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robertrowett

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#1
Hi all, this is an area to discuss output designs for the DAC. Obviously with easy access to the raw 1.4v signal the possibilities are endless!

For now I have settled on raw output to 6111 tube Lampizator board for main drivers and SE output from on board opamps to stereo subs. Great tone, open sound, and NO GLARE. 😀
 
#2
Boberly Buffer-Spencer's version

Maybe something along these lines...to kick of what promises to be an interesting thread.

🙂
 

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#8
This is the kit I am talking about.

Lampizator - STEP 1

I chose it because I used it previously to buffer spdif. I repopulated the board for stereo and fed from raw output stage.

I compared the sound to the unbuffered output and the opamp buffered. In my system it is the most relaxed sound with the NOS filter loaded. I like the raw output second best. To be fair I tapped the balanced outputs for se use (+ and ground), so I have not listened to the balanced outputs as intended.
 
#16
robertrowett said:
Hi all, this is an area to discuss output designs for the DAC. Obviously with easy access to the raw 1.4v signal the possibilities are endless!

For now I have settled on raw output to 6111 tube Lampizator board for main drivers and SE output from on board opamps to stereo subs. Great tone, open sound, and NO GLARE. 😀
Click to expand...

This is interesting but I thought Soren explained that his DAC outputs voltage and has effectively a pure output resistance (~600 ohm?) so any the external Zin of the next stage merely acts as voltage divider only and will not affect the overall sound quality - so a simple direct drive to an amplifier

I would think the improvement you are getting comes from the band limited response of your buffer that is effectively acting as a filter - so no "Glare" as you put it.
So going along these lines a simpler arrangement with good quality transformers would also work as the transformer filters out the high frequency noise.
Maybe even a simple RC low-pass filter.

Anyway, just my thoughts .... :2c:
 
#17
acko said:
This is interesting but I thought Soren explained that his DAC outputs voltage and has effectively a pure output resistance (~600 ohm?) so any the external Zin of the next stage merely acts as voltage divider only and will not affect the overall sound quality - so a simple direct drive to an amplifier

I would think the improvement you are getting comes from the band limited response of your buffer that is effectively acting as a filter - so no "Glare" as you put it.
So going along these lines a simpler arrangement with good quality transformers would also work as the transformer filters out the high frequency noise.
Maybe even a simple RC low-pass filter.

Anyway, just my thoughts .... :2c:
Click to expand...

Hi Acko! I appreciate the insight. I am heavy on anecdotal experience and light on science. Good to have both to find solutions!
The 6111 provides 20x voltage gain, so those that want more than 1.4 volts can step up based on resistor value, tube selection and attenuate response with coupling cap choice.
My amp is 500mv sensitivity so tube is unnecessary, even with reduced gain of NOS setting on filter.
Tonight I ran direct out to overbuilt resistor ladder passive pre and results were the similar (better?) to with tube. So I think you are onto something with the analog attenuation of the headache inducing glare.
So can't we do better than the onboard smd output filter? If we calculate for resistor and cap we should be able to match voltage to our amp and adjust frequency rolloff. Is it possible to bypass or remove smd lowpass filter so we can do this?
It looks to me like the opamp outputs are also redundant with the smd lowpass. I see the use for headphones, but what is the purpose of this design for driving power amp? Same as designing a discreet output gain stage from the "raw" taps?
Also: I see you use v-caps on the DAC on your website. In your opinion are they worth it? What would be a 95% second best?
 
#18
robertrowett said:
The 6111 provides 20x voltage gain, so those that want more than 1.4 volts can step up based on resistor value, tube selection and attenuate response with coupling cap choice.
My amp is 500mv sensitivity so it serves no purpose, even with reduced gain of NOS setting on filter.
I see you use v-caps on the DAC on your website. In your opinion are they worth it?
Click to expand...

Thanks, I see your point here. Also, had a closer look at Soren's DAC and there is already a simple RC low pass filter@ 270k corner on board. This may interact with the next stage if impedance is not high enough. So buffer can be beneficial. Tweaking the on board RC filter to match the next stage looks like another simple option if direct drive is preferred.
Totally agree to limit the op-amp function for basic headphone drive or monitoring. Better to tap and match the raw outputs for final amps to speakers

V-Caps are on the Ref voltage of the DAC and appears to further improve the clarity on my system. Others with same DAC have not tried it for cost reasons so no further comparisons are available at the stage. Looks like V-Caps are highly recommended for tube designs so you may want to consider them. Again, cost is a factor for most DIYers. 2nd best will be the much cheaper commercial grade PP film caps
 
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#20
robertrowett said:
Does anyone know the value of the onboard cap? I am willing to risk surgery if I have enough info. As it stands any stage that is added will be redundant as acko observed.
Click to expand...

Well the roll of is 270KHz, and the output impedance is around 560 ohms.

I'm curious what the dielectric is of this cap (ceramic?) - my experience is that the quality of this part has a significant impact on sound, even if the roll-off is well above the audio band.
 
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