Simplified Custom GK Preamp Board

Started by steadystate, November 25, 2023, 11:07:23 AM

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steadystate

I'm about to submit my GK preamp board for production, but I'd like to see if anyone has comments/suggestions/criticisms before I do.

I eliminated the GK/Mix/Guitar switch, S1, S2, the FET power switch, and the LED.  I have never used these features, and it saved enough space to make the board smaller while adding a couple of extras.

The GK Volume pot now connects with a 3-pin header.  I also added a jumper that either enables the pot or ties the wiper circuit to 5V (full volume) if I want to eliminate the pot.  I'm assuming the VG-99 MUST see a voltage on pin 8, otherwise I could eliminate the 5 volt regulator.  Can anyone confirm?

I added a parallel output for the guitar's normal pickups, so I wouldn't have to split wires in the control cavity to accommodate a normal 1/4" output jack.  If it audibly degrades the signal, I won't use it.

I also included parallel input connectors for both the GK-2a and GK-3 pickups, allowing me to use whichever type divided pickup is available.

My main concern was the Pin 9 question, but now I know that I can leave it disconnected for my use case.  I may add S1 and S2 back, each on its own 2-pin connector.  I'm on the fence.

If anyone can see a mistake, I'd appreciate being told that I screwed up.  Brain farts are my specialty.

Elantric

#1
If you omit Pin#9, its best to set the GK Type on the connected GK processor to GK-2A. Not GK-3

GK-3 Pin#9 Explained
https://www.vguitarforums.com/smf/index.php?topic=5259.0

steadystate

Quote from: Elantric on November 25, 2023, 11:40:21 AMIf you omit Pin#9, its best to set the GK Type on the connected GK processor to GK-2A. Not GK-3

GK-3 Pin#9 Explained
https://www.vguitarforums.com/smf/index.php?topic=5259.0
Good point.  Thanks.

steadystate

#3
I added S1 and S2 back, just in case I ever decide to use them.  I'll check it one more time tomorrow morning before I send it.  If I screwed something up, it won't be the first batch of boards that went into the trash.  Even so, at $5 per board (with SMD mounting and shipping), it's still worth it.

I'd still like to know what happens if you leave pin 8 (GK Volume) floating.  I wonder if there is a 5V pull-up on the VG-99 end for such a scenario.

Elantric

Quote from: steadystate on November 25, 2023, 05:10:25 PMI added S1 and S2 back, just in case I ever decide to use them.  I'll check it one more time tomorrow morning before I send it.  If I screwed something up, it won't be the first batch of boards that went into the trash.  Even so, at $5 per board (with SMD mounting and shipping), it's still worth it.

I'd still like to know what happens if you leave pin 8 (GK Volume) floating.  I wonder if there is a 5V pull-up on the VG-99 end for such a scenario.
You must have a voltage on Pin#8 = if left not connected = GK Processor is muted ,   +5VDC on Pin#8  =  Max Volume

steadystate

Quote from: Elantric on November 25, 2023, 05:48:54 PMYou must have a voltage on Pin#8 = if left not connected = GK Processor is muted ,   +5VDC on Pin#8  =  Max Volume
More great info.  I've been searching the site for various topics, but it is SO big.  I don't know how you guys maintain it, but I'm glad you do.

steadystate

I just read about using a SPDT momentary toggle for S1 and S2 on another tread here.  Great idea!  Incorporated.

billbax

Hi steadystate,

At first glance your GK3 schematic looks fine, but do yourself a favour and increase those 10k feedback resistors to 33k (+10dB). GK preamp output signals are meant to be line-level, and a -15dB string single (moderately quiet playing strength) will max out a v-guitar unit string sensitivities.

A GK preamp should be achieving a minimum -95dB noise-floor string signal. Investigating v-guitar headroom sens, and changing GK preamp component values has its rewards.

Black and white.

steadystate

#8
Quote from: billbax on November 25, 2023, 09:52:52 PMHi steadystate,

At first glance your GK3 schematic looks fine, but do yourself a favour and increase those 10k feedback resistors to 33k (+10dB). GK preamp output signals are meant to be line-level, and a -15dB string single (moderately quiet playing strength) will max out a v-guitar unit string sensitivities.

A GK preamp should be achieving a minimum -95dB noise-floor string signal. Investigating v-guitar headroom sens, and changing GK preamp component values has its rewards.

Black and white.

I watched your video.  Awesome.  No doubt the improved noise specs are noticeable, especially when playing with high gain tones.  I love seeing mods taken to the extreme.

I considered this mod, but decided to use the 10k resistors for the first run of boards (assuming there is a second).  With my VG-99, I'm running the Sensitivities between 27 and 40 to reach full scale on a moderate strum.  Perhaps the VG-99 has more available gain than other units.  Still, it makes sense to have more gain earlier in the signal chain.  My concern is whether the Sensitivity increments are much larger towards the bottom of the range (I've never tested it).  So far, I haven't had an issue adjusting the Sensitivities, and I only use the VG-99 live in very noisy environments.  I've attached the final circuit and layout.  I'll gladly send the gerber file to anyone who wants it, because, F Roland's policies towards US customers.

If the build proves unsatisfactory, I'll try again with the 33k's, and I might try to find another company that offers smd installation of a better op amp.  Thanks for the suggestion.  This is an amazing forum.

cags12

You also might want to review the topic below in regards to the mono pickip connection:
https://www.vguitarforums.com/smf/index.php?topic=17111.0


steadystate

#10
Quote from: cags12 on November 26, 2023, 04:14:43 AMYou also might want to review the topic below in regards to the mono pickip connection:
https://www.vguitarforums.com/smf/index.php?topic=17111.0


Hmm.  While I get why this could happen, I've never experienced a problem with loading of my passive PAFs or active EMGs from the opamp on the preamp board.  They have plenty of output and highs. I always use the "Mix" position. I have the same pickups in guitars with and without the GK, and they sound the same. Seems to me that, if loading was an issue, that a bypass jack would make the normal pickups sound different at the 1/4" jack than on pin 7 when using only the GK cable, which to me is undesirable.  If there is a difference in my case, it is very subtle.  Now I'll have to do some critical testing.

GuitarBuilder

Quote from: steadystate on November 26, 2023, 10:48:37 AMHmm.  While I get why this could happen, I've never experienced a problem with loading of my passive PAFs or active EMGs from the opamp on the preamp board.  They have plenty of output and highs. I always use the "Mix" position. I have the same pickups in guitars with and without the GK, and they sound the same. Seems to me that, if loading was an issue, that a bypass jack would make the normal pickups sound different at the 1/4" jack than on pin 7 when using only the GK cable.  If there is a difference in my case, it is very subtle.  Now I'll have to do some critical testing.

Agreed. If wired correctly wrt power, the active pickups work just fine.
"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

Elantric

#12
Quote from: cags12 on November 26, 2023, 04:14:43 AMYou also might want to review the topic below in regards to the mono pickip connection:
https://www.vguitarforums.com/smf/index.php?topic=17111.0



It's important to use a 1/4"  switching output jack that removes the normal guitar output signal from getting loaded down by the unpowered  Opamp buffer in the GK-KIT-GT3 internal kit, WHEN THE GUITAR IS USED WITHOUT GK 13 CONNECTION

Review this post.
https://www.vguitarforums.com/smf/index.php?topic=73.msg73532#msg73532


When you insert a 1/4" guitar cable This jack allows the normal guitar  signal removed from the GK-3 circuit - essentially placing the guitar back to "stock" to prevent GK-3 circuit interaction that occurs
Review the method Fender employed on Roland  Ready Strat and GC-1

A Switchcraft 12A 1/4" switching jack which isolates the GK Preamp away from the passive Strat electronics when the 1/4" output is occupied

Details
https://www.vguitarforums.com/smf/index.php?topic=133.0

steadystate

#13
Quote from: Elantric on November 26, 2023, 12:26:50 PMIt's important to use a 1/4"  switching output jack that removes the normal guitar output signal from getting loaded down by the unpowered  Opamp buffer in the GK-KIT-GT3 internal kit, WHEN THE GUITAR IS USED WITHOUT GK 13 CONNECTION

Review this post.
https://www.vguitarforums.com/smf/index.php?topic=73.msg73532#msg73532


When you insert a 1/4" guitar cable This jack allows the normal guitar  signal removed from the GK-3 circuit - essentially placing the guitar back to "stock" to prevent GK-3 circuit interaction that occurs
Review the method Fender employed on Roland  Ready Strat and GC-1

A Switchcraft 12A 1/4" switching jack which isolates the GK Preamp away from the passive Strat electronics when the 1/4" output is occupied

Details
https://www.vguitarforums.com/smf/index.php?topic=133.0


Now I get it.  Unpowered.  I buffer my pickups before the board (20M input Z and under 75 output Z), and bypass the buffer when I'm using the GK via a push-pull pot.  Makes sense now that some people would have issues.  The bypass jack is a great idea.  Probably better than buffering the pickup against the load of the unpowered board.

gumtown

or a tiny little change over relay, so with the absence of GK power, the normal pickup is direct to the jack.
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