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Balanced B1 with Korg Triode
Balanced B1 with Korg Triode
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Old 6th August 2019, 06:33 PM   #1
elwood625 is offline elwood625  United States
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Default Balanced B1 with Korg Triode

This thread is for "How to build a Balanced B1 Korg Triode" preamplifier.
First, the hardware, I purchased two sets of PCB w/FETS and Korg Triode tubes from the diystore. Boards were stuffed the parts from the BOM with Nichicon UES 1000uF filter caps and Panasonic 10uF film caps (these were the extras I mistakenly bought while building a BZLS).
Once both boards were stuffed and the test voltages confirmed for both boards, I marked one board for Left + and Left -. With the other board marked Right + and Right -.
I measured each side of each board using REW software, Akitika 1K generator and a Behringer UM2 sound card, for 1.5% 2H negative, 1% 2H negative, null point of 2H, 1% 2H positive and 1,5% 2H positive. Here are the results:


R+ R- L+ L-

1.5% 8.57 vdc 8.61 vdc 8.84 vdc 9.51 vdc

positive 2H



1% positive 9.53 vdc 9.52 vdc 9.41 vdc 10.53 vdc


Null 2H 11.62 vdc 11.43 vdc 11.41 vdc 11.57 vdc

0.043%


1% 12.44 vdc 12.12 vdc 12.30 vdc 12.30 vdc
negative 2H


1.5% 13.07 vdc 12.50 vdc 12.84 vdc 12.70 vdc
negative 2H


This is the same procedure I used for my B1 Korg Triode preamp, the hard part was adjusting each channel in balanced mode.
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Old 6th August 2019, 06:46 PM   #2
elwood625 is offline elwood625  United States
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Next, I had read the B1 with Korg Triode thread twice and made notes regarding adjustments:
"Ideally then you want to measure the distortion to get it just right. For
distortion this high, it is not that difficult. If you have any sort of decent
sound card on your computer you can use one of the several analyzer
programs to measure the distortion.

Assuming that you can get a numerical figure, start with a high Plate
voltage (pot counter-clockwise), and decrease the voltage until the
distortion is at a minimum, where the 2nd harmonic is being cancelled
and you have 3rd harmonic.

From there you can increase or decrease the Plate voltage until you
reach the distortion number and phase you want.

Keep in mind that the circuit inverts absolute phase. What is "positive
phase" 2nd harmonic at the output of the circuit will become negative
phase when you again reverse the phase at the output, typically at the
speaker connection, in order to have correct absolute phase."



"If you have a means of measuring distortion but can't tell the phase
(for example, using a soundcard to make the measurement) then simply
find the lowest distortion point, and then as you turn the pot clockwise
you go into positive phase, or negative phase with counterclockwise."


"
To the extent the balanced channels are matched, they will cancel, leaving
5th. You can unbalance the 2nd harmonic if you like - the channels are
adjustable as described in the article. You could set one side at negative
phase and the other at positive, or simply make them unequal in
amplitude but not phase"


After re-reading these quotes from NP, I sent him an email for help:


1st reply:
Depends on what you want. If you want to null the 2nd harmonic out,
then you would set the 2 channels identically.

If you want more 2nd, then you would set them differently, for example:

Keeping mind that the individual channels are inverting, if you have a
+ incoming signal it becomes the - outgoing signal and vice versa.

+ in to L ch = -out

- in to R ch = +out

adjusting L ch for more positive phase 2nd gives more negative phase 2nd in balanced out.
(less negative phase 2nd is same as more positive result)

adjusting R ch for more negative phase 2nd gives more negative phase 2nd in balanced out.
(less positive phase 2nd is same as more negative result)

I hope that's clear enough.....



After several hours of adjusting the Left channel in Balanced mode, I sent him my results and his reply:
"Looks to me like you have a good handle on it."


This is what I needed to devote more hours to adjusting the pc boards.



Here are the REW printouts of the final adjustments for the Left and Right channels:
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Old 6th August 2019, 07:08 PM   #3
elwood625 is offline elwood625  United States
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How I was able to measure the combined Balanced 2H measurement:
With the help of xrk971 and Mark Johnson, I wired a TRS and female XLR connector as follows:
XLR: Pin 1 = ground: pin 2 = positive; pin 3 = negative. Tip to pin2 of the XLR, ring to pin3 of the XLR with the sleeve and pin 1 of the XLR with no connection.


This gave me the connection needed from the pc board to the sound card.
With the pc board powered by my lab supply at 24 vdc, the Akitika was adjusted so that 1v rms was measured at the output of each side of the pc board. The XLR adapter cable was plugged into the sound card and the male XLR connector which was wired to the pc board outputs.
With my laptop running on batteries, the REW software was launched, first adjustments were made to achieve 1.5% 2H. Since both pots on each pc board are adjusted, I concentrated on reducing 3H while still measuring 1.5% 2H. Adjusting each board for 1.5% 2H is very easy, reducing 3H to an almost null point requires constant back and forth between the two pots, adding/subtracting 2H all the time staying with 1.5% 2H.
Every time I thought I had good results, I would record the REW screen with the L+, L-, R+, R- voltage adjustments. I would name the screen shot file with the voltage adjustments.
After I was happy with the results of each adjustment, I would power every thing down and retest the next day to verify I could duplicate the results.
With the final adjustments for each channel verifed, I removed the XLR connectors from each pc board, and measured each L+, L-, R+, R- individually to see where each side of the pc board adjustment gave for distortion results. The following graphs file names are the results:
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Old 6th August 2019, 07:19 PM   #4
elwood625 is offline elwood625  United States
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In conclusion: I would like to thank xrk971 for teaching me how to measure distortion levels using an affordable method, Mark Johnson for not giving me easy answers and making me think.
Of course, NP for the design of the B1 Korg Triode, and the incentive and road map for the "how to" part.

For vibration control, I have 3mm rubber mounts for each pc board, with 3M automotive vibration mat for the top and bottom chassis panels, and 2mm thick memory foam for under the Korg tube.
I think I'll take a break and tomorrow start on the chassis using an ALPS shorting type 3 position 4 pole selector switch (thanks Zen Mod), and a 24 step quad 50K log volume control. I found a nice chassis to build it in and knobs which look just like the knobs on Threshold FET 9 & 10 preamps.
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Old 6th August 2019, 09:37 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Balanced B1 with Korg Triode
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Old 6th August 2019, 09:47 PM   #6
Zen Mod is offline Zen Mod  Serbia
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Balanced B1 with Korg Triode
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Old 6th August 2019, 11:25 PM   #7
xrk971 is offline xrk971  United States
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Balanced B1 with Korg Triode
Nice work Elwood! Very thorough work. Thanks for providing such a detailed account.
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Old 8th August 2019, 01:34 PM   #8
elwood625 is offline elwood625  United States
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My final adjustments for 1.5% negative 2H from both channels. Left channel is adjusted for 1.5% positive 2H, and the right for 1.5% negative 2H.
I have a terminal strip for 6 with 2 grounds for the T7-T8 connections, this way I can easily hook up a DVM and make changes.
I'm very curious as to how it will sound with 1.5% negative 2H on one channel and 1.5% positive 2H on the other channel. With all the measurements I've recorded for each side of both channels, all I will need to do in the future is adjust the plate voltage to the desired feedback amount, including null of 2H.

I plan on using this preamp with the F5T v3 balanced mono blocks I'm building.
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Old 8th August 2019, 01:37 PM   #9
elwood625 is offline elwood625  United States
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Just to clarify, in order to get 1.5% positive and 1.5% negative out of the preamp, both channels will require adjustments for 1.5% negative as the Left channel will invert in balanced mode.
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Old 8th August 2019, 01:39 PM   #10
elwood625 is offline elwood625  United States
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Excuce me, 1.5% POSITIVE adjustment for both channels. In the future I should refer to the adjustments as "above 11 vdc, and below 11 vdc" to keep me from getting confused.
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