Akai VX600 Linear FM

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Brownian Lotion
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Akai VX600 Linear FM

Post by Brownian Lotion » Fri Sep 11, 2020 10:26 am

I’m not sure if this counts as DIY but I can’t see a forum specifically for synth mods. Apologies if it’s in the wrong place.

So I did it. I converted my VX600 from exponential to linear FM. For each voice card I desoldered R3 and soldered a 0.1uF cap from the upper pin to a 150k 1% resistor which is then soldered to the legs of where R5 and R6 join. It was mildly delicate work but not too bad.

So how does it sound? Better thanI expected. Check back tomorrow for sound examples. It’s bed time now.

Some details before I forget. I took tips from the Oberheim Xpander schematic. They used a 422k resistor where I used 150k, which means that the upper half of the fm range is atonal in my VX. I like that.

Will it replace the Hydrasynth I have in the mail? Heck no. But it does sound pretty effin great. I never used the exp fm because it sounded wonky. It probably had DC bias because Akai didn’t use a capacitor before R3. It doesn’t have that anymore.
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dp4
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Re: Akai VX600 Linear FM

Post by dp4 » Fri Sep 11, 2020 11:20 am

wow, unique mod.. interested to hear demos and compare

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mush
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Re: Akai VX600 Linear FM

Post by mush » Fri Sep 11, 2020 11:33 am

Isn't it just AC-coupled Exponential FM? How does the capacitor convert the signal from exponential to linear?
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Brownian Lotion
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Re: Akai VX600 Linear FM

Post by Brownian Lotion » Fri Sep 11, 2020 5:07 pm

mush wrote:
Fri Sep 11, 2020 11:33 am
Isn't it just AC-coupled Exponential FM? How does the capacitor convert the signal from exponential to linear?
No, the cap just cuts the DC because good circuit design. The CEM3374 has an IREF input which can be used for linear FM. It’s in the Xpander service manual.
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Re: Akai VX600 Linear FM

Post by roglok » Sun Sep 13, 2020 12:40 am

Watching this with interest as I rarely use the stock FM myself...

Brownian Lotion
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Re: Akai VX600 Linear FM

Post by Brownian Lotion » Mon Sep 14, 2020 8:29 pm

Here's the mod.
linearfm.png
I'm waiting on the local hardware shop to restock their shelving units so I have enough space to set up my computer to record some examples.
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Re: Akai VX600 Linear FM

Post by roglok » Tue Sep 15, 2020 4:43 am

That's sweet. I'm really curious about the sound examples.

I just compared the Oberheim and VX600 schematics. Your implementation is basically the same as the Xpander but I'm wondering about RX65. Does it make sense to add a pull-down resistor at the output of the analog switch?

VX600
vx600_linearfm_brownian-lotion.png
Xpander
xpander_linearfm.png
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Brownian Lotion
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Re: Akai VX600 Linear FM

Post by Brownian Lotion » Tue Sep 15, 2020 6:42 am

Touché, perhaps it does need a pull down. That might quell the brief higher pitch transient when switching to VCO fm.

Edit: Also, thanks for taking an interest!

What do you think the pull down is for?
Last edited by Brownian Lotion on Tue Sep 15, 2020 7:30 am, edited 1 time in total.
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Re: Akai VX600 Linear FM

Post by roglok » Tue Sep 15, 2020 7:00 am

Are you going to give it a shot? I believe it says 10K but the value is probably not too critical...

It would be nice to have the modulation switchable between linear and exponential with a switch on the back. But then again that would require flying wires which would mess with the serviceability of the voice cards...

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Re: Akai VX600 Linear FM

Post by roglok » Tue Sep 15, 2020 7:22 am

Pin 5 of the 4053 is actually not connected so it should be possible to hook the other pole up to the exponential FM input like it's done on the Xpander. However, I'm not sure if such an "always on" mode would create any issues...

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Re: Akai VX600 Linear FM

Post by Brownian Lotion » Tue Sep 15, 2020 7:34 am

roglok wrote:
Tue Sep 15, 2020 7:22 am
Pin 5 of the 4053 is actually not connected so it should be possible to hook the other pole up to the exponential FM input like it's done on the Xpander. However, I'm not sure if such an "always on" mode would create any issues...
I did think about this. I think the behavior is the switches is to one by one enable or disable the vco and vcf fm. So you’re right, the vco fm would be “always on” and either linear or exponential. I figured that wouldn’t be desirable.

It would be great to have a switch between exp and linear, but to be honest I never used exp fm. For patches that did use it, I could get similar sounds by using the upper range of the linear fm where it starts to pitch bend upwards.
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Re: Akai VX600 Linear FM

Post by roglok » Tue Sep 15, 2020 7:47 am

Brownian Lotion wrote:
Tue Sep 15, 2020 7:34 am
So you’re right, the vco fm would be “always on” and either linear or exponential. I figured that wouldn’t be desirable.
Ah, of course. It would not be possible to use filter FM without any modulation of VCO 1...
Brownian Lotion wrote:
Tue Sep 15, 2020 7:34 am
It would be great to have a switch between exp and linear, but to be honest I never used exp fm. For patches that did use it, I could get similar sounds by using the upper range of the linear fm where it starts to pitch bend upwards.
That's cool. When modding vintage instruments I always try to keep the stock functionality intact somehow. But in this case I agree that linear FM sounds much more useful...
Last edited by roglok on Tue Sep 15, 2020 7:50 am, edited 1 time in total.

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Re: Akai VX600 Linear FM

Post by roglok » Tue Sep 15, 2020 7:48 am

Brownian Lotion wrote:
Tue Sep 15, 2020 6:42 am
What do you think the pull down is for?
I'm not sure, maybe to suppress switching artifacts/clicks?

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Re: Akai VX600 Linear FM

Post by Brownian Lotion » Tue Sep 15, 2020 8:04 am

roglok wrote:
Tue Sep 15, 2020 7:48 am
Brownian Lotion wrote:
Tue Sep 15, 2020 6:42 am
What do you think the pull down is for?
I'm not sure, maybe to suppress switching artifacts/clicks?
That might suppress the pitch warble as the cap charges up. However, since even changing menu pages on the VX600 causes pitch glitches due to the cpu drain, it is possibly just in line with the charm of the instrument ;-)

It would be great to compare the sound with an Xpander.
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Re: Akai VX600 Linear FM

Post by Brownian Lotion » Tue Sep 15, 2020 8:15 am

roglok wrote:
Tue Sep 15, 2020 7:00 am
Are you going to give it a shot? I believe it says 10K but the value is probably not too critical...

It would be nice to have the modulation switchable between linear and exponential with a switch on the back. But then again that would require flying wires which would mess with the serviceability of the voice cards...
Yes, I think I’ll add the pull-downs.

I agree it would be ideal to preserve the original behaviour somehow. But worst case scenario it isn’t a difficult mod to reverse. Much better than adding a bunch of wires everywhere.
Will module for food.

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