Building the Pete Millett Korg Nutube Balanced Preamp

Hi Nash,

I replicated your setting for on my Focusrite 3rd gen and ran internal loop as Bal2Bal. Results are attached.
I'll run 4 tests and will add images soon.
(60Hz mains spike) --> I use GND lift (Rod Elliot style) with 10R.
 

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Here are the plots for the Left and Right channels. All taken with Focusrite gen3 with input to preamp of 1.0v in and output adjusted by volume attenuator.

Settings are as :

Left channel TP3-8.63v, TP5-5.34v
Right channel- TP3-5.61v, TP5-5.14v

nash
 

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No sure how to use this info yet. Just to make sure we have 1:1 comparison: both of your Gain knobs on Focusrite are in MIN position. Right?
What input connectors are used on Focusrite? XLRs or TRSs? It differences in input impedances for these two…
As far as I understood, you adjusted TP3 and TP5 based on harmonics matching on L & R channels and make H2 dominant vs. H3 to make minimal. Am I correct?

Still not clear why I do not have sufficient volume level on my headphones. 13dB gain is probably OK for my Audeze LCD-4, but still very quite.

Let’s see what Pete say.
 
Yes no gain and yes XLR.

Harmonics wont explain why you have insufficient headphone volume. Are you using opamp output and have you installed 10r resistors for R30/31 as per schematics?

Nash


I use TRSs on Focusrite inputs plugs since it is higher input impedance. I can change my plugs to XLRs if needed to compare.
No opamps. I have 170s there on outputs, same as on my inputs. What opsmps do you use? I have many OPA2134 and I hope I can try these.
 
I use it only as a preamp not a headphones amp so I am using a Jfet buffer. If you are also using a Jfet buffer it is not likely to drive headphones. See below from PM description:

"The opamp buffer can drive much more current, and is needed if you want to drive headphones or a 600 ohm load. The opamp needs to work on +/-15V, and be unity gain stable. It is a standard 8-pin DIP single opamp. I used the OPA551, which makes a good headphone driver. Note that you want to make R30 and R31 smaller for headphone use - 10 ohms is generally good, but you can use a value that you like. It will change the damping on your headphones. For 32-ohm headphones you may want to go to near zero."


nash
 
I use it only as a preamp not a headphones amp so I am using a Jfet buffer. If you are also using a Jfet buffer it is not likely to drive headphones. See below from PM description:

"The opamp buffer can drive much more current, and is needed if you want to drive headphones or a 600 ohm load. The opamp needs to work on +/-15V, and be unity gain stable. It is a standard 8-pin DIP single opamp. I used the OPA551, which makes a good headphone driver. Note that you want to make R30 and R31 smaller for headphone use - 10 ohms is generally good, but you can use a value that you like. It will change the damping on your headphones. For 32-ohm headphones you may want to go to near zero."


nash

Yes, I did see that report. Ordered OPA551PA and will see if that resolves my volume level issue. Will report soon.
 
Success!!! Dropped these high current BB opamps and it is running totally fine. It is enough margin for loudness now. Just started to worm it up, but it’s sounding already very good. Will run it overnight and set it with final calibration. Next move will be better than WIMA signal path caps. The challenge will be to find high quality but low profile once.
 
Hi Nash, let me refresh my memory and build my confidence...
I'll ask same question again: have you adjusted your TP3/5 to match for H2/H3 (when H2 is higher and H3 is smaller as possible) on L/R channels without too much care about voltage matching between T3/5? Please let me know.
One more question about your Caps. Please remind me your caps brands/models?
Have you ran you amp with BOM WIMA in the beginning for comparison?

Thank you.
Alex
 
Hi Alex,


H2 higher as opposed to H3 depends on the output level of the preamp. So TP3 and TP5 have been adjusted to obtain the relative proportions as shown in the plots at the different voltage out levels, Also I have tried to get both channels to display similar patterns. What I have found is that you can unwittingly adjust this preamp to get say the same THD at 1v or 0.5v output but much higher THD at lower output which does not sound that good. That's why I check the plots at 4 different output levels.
Look at the Nutube published THD vs output plot.
I used Clarity Cap CSA 12uf for the caps before the Nutube and CDE 940C for the ones after, all mounted under the board as shown in the pics in post #1.
nash
 
Hi Nash, thank you for that info. Have you ran you amp with WIMA caps at the beginning or these big caps were installed pretty much from the fist assy? It is quite $ investment and I'm interested to hear if such upgrade is essential for SQ.
 
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