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Art VLA Pro Mods
Old 27th July 2013
  #1
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Art VLA Pro Mods

I'm about to undertake some mods to my VLA and thought it would be fun for anyone else keen to do the same to join in the journey.

the objective is to help make the unit more of a frontline comp and to learn some of the technical reasons why modifying/upgrading things can make a difference. rather than a scatter gun approach to parts replacement I was hoping to get an understanding of the why's.

I already like my vla. if I had one complaint it is that it can make things sound less open and a little smaller. so if I can address those things, drop the noise floor, improve the bottom end and generally upgrade the sound i'd be a happy camper.

I thought before we pick up a soldering iron we could talk about some of the possible outcomes with specifics.

the most common mods done are.

1. audio path cap upgrade and bypass
2. power supply cap and diode upgrade
3. opto circuit mod with vactrol and opamp upgrade
4. transformer front end circuit bypass mod
5. transformer output circuit mod
6. tube upgrade
7. tube power supply upgrade

I've attached the schematics. please take a look if you would like to join in. i'm hoping a few people with technical knowledge can help us not so technical types to understand why certain mods work and for what reasons.

if anyone has any other mods they would like to see performed please add those as well.
Attached Files
File Type: pdf Art Pro VLA circuit.pdf (144.3 KB, 400 views) File Type: pdf Art Pro VLA psu circuit.pdf (70.9 KB, 373 views)
Old 27th July 2013
  #2
Lives for gear
ok, to kick things off,

1. caps in the audio path, I have been looking at the coupling/filter caps. C21 & C23 on the outputs, C42 in the tube feedback loop, C29 & C28 on the inputs.
a common mod is to swap out for low esr caps and to add .1uf bypass caps in parallel to improve hf filtering. some people adjust values but the corner freq is already very low from my calcs
I would also like to explore removing C28 and C30 if possible

2. power supply mods include increasing voltage ratings of all caps, replacing with low esr caps and replacing all 1n4003 diodes with uf4007 to reduce hf noise.

3. the vactrol in the art has a dynamic range of 70db. swapping out for a vtl5c1 takes the dynamic range to 100db and increases the attack timing. another mod is to replace the opamp driving the vactrol circuit with a higher slew rate chip. there is also a resistor value change that is said to open up the top end.

4. adding a transformer to the front end allows the entire frontend circuit up to the ribbon connector to be removed from the circuit for improved audio. the documented transformers are all 600/600 ohm but would a 15k/15k transformer be a better option.

5. adding a 600/600 transformer to the output for some colour.

6. plenty of option for tube upgrades. mil spec nos RCA, telefunken, etc.

7. this is a biggy for me and most likely over my head. but increasing the tube supply voltage to 150v, seems increasing to 60v is also a common approach.

ok, thoughts? ideas?
Old 27th July 2013
  #3
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Jazz Noise's Avatar
I think you've an interesting point with 3 - but not from the slew-rate side. The envelope follower circuit has a fairly high impedance - meaning that the release cap takes a considerable amount of time to charge.

It's also a little "cray cray", as they say, that a TL074 is driving the vactrol stage but they use 5332's after that. The most obvious source of distortion is going to come from loading the output - though with the trimmer set to 4K or more the distortion should be pretty low. Even still, the input buffer stages are clearly of a very high impedance - though it hard to mess with this and guarantee your input impedance won't cause distortion from the output of a previous device. Sigh.

The other noise issue is the voltage divider before the tubes - the 20K and 10K voltage divider is extremely big, which is quite odd since while it's also driving the Threshold pot it could lower that divider further without any real distortion increase, though you may find you need the threshold a little lower to get the same Gain Reduction. You'd happily get away with 10k and 5k instead which should drop the Johnson noise abit. Further drops could be done, but gain reduction would become a problem. They really should have been isolated from each other with another buffer, but that's another story.

I guess they may have felt the Tube Stage noise would dominate. Which may be true - I'm not much of a tube guy.

60V is an improvement for the tube stage, 120V would be better. The reason is if you read a 12AT7 spec sheet you'll see that the linear region for a 12AT7 with a low Va (Anode) or Plate Voltage is pretty small. Bringing this up gives you a bigger swing. It also increases the gain slightly. Tube preferences? Use what you gots.
Old 27th July 2013
  #4
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Haz-Mat-Strat's Avatar
 

The mod that you have is a solid mod and changes the speed of the attack and release considerably. The change in the Vactrol makes the biggest difference.

1. Also by-pass C41,

2. Replace C44 with Polypropylene. In the VLA 2 the topology is changed to 2-499k resistors in place of the 10k resistors in the input and output sections and C44 is bumped up to a 10uf capacitor.

3. Replace R64 with a 100k 1% resistor. this will give you more voltage to the tube and reduce distortion.

On the Pro VLA2, I have seen an additional 100uf 63v capacitor added at the junction of R62 and the 48v supply. Some units have them some do not. I think these were added during QC and do not have a designated spot on the circuit board.

On one VLA2 that I was modding, after the mod channel 2 had a severe hiss on the output. I traced it down to the tube and then added a 100uf cap to the junction of R98 & 48v and the noise was gone.

I use several different transformers for my mods:

Input: Jensen. OEP and Cinemag

Output: Jensen, OEP and Carnhill

OEP and Carnhill will have the most "colour."

These do make a difference and I wire them in to retain the by-pass switch.

In the "Heavy weight" Mod that I offer ALL of the electrolytic capacitors are replaced with low ESR capacitors and some of the smaller capacitors are replaced with film (1uf and below). Also input and output transformers are added.







Happy Modding!!!
















.
Old 28th July 2013
  #5
Lives for gear
cheer jazz noise. very glad you're joining in.

Quote:
Originally Posted by Jazz Noise View Post
I think you've an interesting point with 3 - but not from the slew-rate side. The envelope follower circuit has a fairly high impedance - meaning that the release cap takes a considerable amount of time to charge.
this is a little over my head. can you point out the release cap.

Quote:
It's also a little "cray cray", as they say, that a TL074 is driving the vactrol stage but they use 5332's after that. The most obvious source of distortion is going to come from loading the output - though with the trimmer set to 4K or more the distortion should be pretty low. Even still, the input buffer stages are clearly of a very high impedance - though it hard to mess with this and guarantee your input impedance won't cause distortion from the output of a previous device. Sigh.
I was going to replace the tl074 with a tl2074. the 5532. thoughts here. if I replace it with something with less dc offset. then I could possibly remove c28 which is keeping dc off the pot and c30 filtering dc before the tube gain stage. if I can't remove those caps then i'll bump them up the 220uf with a bypass.

the input buffer stage tl072 chips and circuit i'd like to completely bypass with the input transformer.

Quote:
The other noise issue is the voltage divider before the tubes - the 20K and 10K voltage divider is extremely big, which is quite odd since while it's also driving the Threshold pot it could lower that divider further without any real distortion increase, though you may find you need the threshold a little lower to get the same Gain Reduction. You'd happily get away with 10k and 5k instead which should drop the Johnson noise abit. Further drops could be done, but gain reduction would become a problem. They really should have been isolated from each other with another buffer, but that's another story..
ok so adjust r56 and r55

Quote:
I guess they may have felt the Tube Stage noise would dominate. Which may be true - I'm not much of a tube guy.

Quote:
60V is an improvement for the tube stage, 120V would be better. The reason is if you read a 12AT7 spec sheet you'll see that the linear region for a 12AT7 with a low Va (Anode) or Plate Voltage is pretty small. Bringing this up gives you a bigger swing. It also increases the gain slightly. Tube preferences? Use what you gots.
excellent, will work towards that.
Old 28th July 2013
  #6
Lives for gear
hi jim, thanks for jumping in. that's very giving no doubt. you've got a lot of expertise with this mod.

Quote:

1. Also by-pass C41,

2. Replace C44 with Polypropylene. In the VLA 2 the topology is changed to 2-499k resistors in place of the 10k resistors in the input and output sections and C44 is bumped up to a 10uf capacitor.
what I was thinking oof doing is bypassing the input stage completely. take a feed from 3 and 2 on the xlr and send it to the transformer with the unbalanced output going to pin 1 on the jumper. that way the bypass switching remain in use.

Quote:
3. Replace R64 with a 100k 1% resistor. this will give you more voltage to the tube and reduce distortion. .
done. will also upgrade c36 to 100v just be sure.

Quote:
On the Pro VLA2, I have seen an additional 100uf 63v capacitor added at the junction of R62 and the 48v supply. Some units have them some do not. I think these were added during QC and do not have a designated spot on the circuit board.

On one VLA2 that I was modding, after the mod channel 2 had a severe hiss on the output. I traced it down to the tube and then added a 100uf cap to the junction of R98 & 48v and the noise was gone.
done also. 100uf 100v added.

Quote:
I use several different transformers for my mods:
Input: Jensen. OEP and Cinemag
Output: Jensen, OEP and Carnhill
OEP and Carnhill will have the most "colour."
These do make a difference and I wire them in to retain the by-pass switch.
are you using 15k line input transformers. from what you wrote above it seems you keep the opamp on the input in the circuit and upgrade components. is that the case?

Quote:
In the "Heavy weight" Mod that I offer ALL of the electrolytic capacitors are replaced with low ESR capacitors and some of the smaller capacitors are replaced with film (1uf and below). Also input and output transformers are added..
this is the plan also. when all is said and done I am a little concerned about lifting traces and I think this is a fairly large danger at the diy level.
Old 28th July 2013
  #7
Lives for gear
I am looking at the output. are the 300 ohm resistor trying to show a 600 ohm load to the next piece of gear inline. or are they there to keep a fixed load on the outputs of A2?
I would like to swap this to a lme or similar then into the output transformer. would I still need the resistors on the outputs or can they be removed.

also looking at the highpass filter on the outputs c21,23 and C29. common mods are to add bypass caps. another is the increase the size of the cap. I have a bunch of mkp10 4.7uf caps that I picked up for next to nothing. I was thinking an alternative here would be to use those. but i'm struggling to feel confident about the corner freq calcs.

c42 & c43 i'd like to use film also. with all of these caps and the HF networks is they need to remain at larger values i'd consider a solen even.
Old 31st July 2013
  #8
Lives for gear
a quick update,

still researching this one but am currently looking to modify the tube circuit to allow the use of a different type of tube.
Old 31st July 2013
  #9
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Jazz Noise's Avatar
Sorry I haven't replied sooner. I can't access the schematic as I'm at work, but if you consider the attack portion as the sidechains LPF, consider the release a sort of HPF. It acts as a reservoir cap, holding and releasing DC. As I said, more on that later.

How are you modifying the tube circuit and what are you hoping to use instead of the 12--7 series?
Old 31st July 2013
  #10
Lives for gear
no problems jazz noise.

my father is a tech and I've been talking to him about this circuit so I can get a better understanding of whats going on. he's struggling to see the point because of the low voltages involved but there's a couple of options on the table.

option 1 is a complete high voltage retrofit.

option 2 is a voltage tripler.

option 3 ecc808

he's also looking at the transformer mod for the output. I am very keen to try and make this circuit as close to direct coupled as I can.
Old 2nd August 2013
  #11
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Jazz Noise's Avatar
Okay, for the power supply design. It's a slight quandary as the input is a mere +/-8v and goes through repeated multiplication to get to 45. Though it's 65V before the regulator apparently (Why they chop off 20V is beyond me).

Taking that 60V and feeding it into a DC-DC converter to try and get something higher - say 200V and regulating it might be an option. You might get it upto 300V, but you're going to have to go component shopping to find a suitable tool. Once you figure out what's available you'll need redo the resisters on the tube section as the power output for the filaments will go from about half a watt to about 4.5 watts. 350 V caps at the minimum. 600V if we don't like smelly things. Can the transistors following this stage also handle that V+ voltage? Probably not. Something worth reading up on, too.

Do you've any idea what the transformer is? Can it handle all this or are we sentencing this thing to a smelly death?

A simple mod I was going to suggest was something I read on the Silonex site about improving Linearity in Opto's. One suggestion they had was a Shunt/Series configuration where because both units would be on the opposite side of their Ron-Roff curve, the non-linearities would partially cancel. It'd go between R29 and C31 if you did it and could be made switchable or even variable with a pot in parallel to it.. I'm not sure if the idea appeals to you, but I said I'd say it.

Those 300 ohms are just to present a minimum output Z incase of a short on another unit so that the whole thing doesn't eat itself if there's a bad cable.
Old 5th August 2013
  #12
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cheers jazz noise.

I was told that the power drops from 65v to 45v so that it delivers a smoother more stable supply.

looked at the shunt resistor location. did you mean R31 and C29? I['d be keen to explore any options with regards the opto.

have dug up an ecc808 to 12ax7 adaptor that I could pull apart and rewire so fingeres crossed it fits in the case to allow the tube swap. still waiting on the power reconfig for the tube swap.
Old 21st August 2013
  #13
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M.Chang's Avatar
 

Does anybody have the VLA II's schematics? I think the one OP posted was for VLA one.
Old 22nd August 2013
  #14
Lives for gear
 

yea another poorly designed art tube buffered device.

it would be better for you to learn from building up a simple circuit.



btw B+ of 235V is a nice sweetspot for the 12at7
Old 10th May 2019
  #15
Here for the gear
 

I have both VLA & VLA II. Stock units are in my humble opining awesome value and are useful on the right material.

I found that just changing the tubes to JJ audio in the vlaii at7, gold lions for the ax7 in the vla and the optocoupler an in expensive way to make the unit a keeper.

I think the tubes do make a subtle difference for sure in the way they harmonically distort. I wouldn't call the tube swap a night and day exercise though i think its enough to warrant the exercises.

My biggest gripe with the stock units is the slower attack / release response and because of that the artifact distortion it imparts, neither good or bad just not my taste. Changing the optocoupler to a VTL5C1 was a night and day mod. I cant measure it but by ear it would seem that the slowest setting is now closer to the original fastest setting. Also you get an extra 30db of dynamic range.

I figured you can do alot of cool mods with this unit as this thread elaborates on but for that money & time I bought something different to add to my compressor fetish.

Just my experience with the VLA mod stuff
Old 1 week ago
  #16
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I recently completely bypassed the original input and output stages on a VLA II and replaced them with 600:600 transformers. I used Hammond input transformers and Jensen output transformers. I used Sparkfun 1646 Breakouts as line drivers for the output transformers.

The input transformers really changed the compression characteristics for the better, in my opinion. To me, the stock unit seemed to clamp down and kind of suck the life out of things like mixbus and snare drums. But with the addition of input transformers, I no longer feel that way. The output transformers were more subtle.

I also added the faster Vactrols on spdt switches so I would have a "fast" and "slow" mode. Basically soldered in parallel with the original vactrol, except the positive leg of each vactrol is switched. To my ears the faster vactrol doesn't actually grab as much as the original, but maybe I'm hearing it that way because of the faster release time? The compression seems more transparent. Compression is happening, but it's not as noticeable. Switching back to slow mode you're much more likely to hear the compression working. So far I'm mostly using the fast mode, but in applications where audible pumping is desirable, the slow mode is good to have.

I was able to perform all the mods without removing the main board at all.

I'm really happy with these mods, if anyone want's any more info I'd be happy to share.

Last edited by dshodge; 1 week ago at 10:47 AM.. Reason: no need for title
Old 1 week ago
  #17
Gear Maniac
 

Quote:
Originally Posted by dshodge View Post
I was able to perform all the mods without removing the main board at all.

I'm really happy with these mods, if anyone want's any more info I'd be happy to share.
Yes please...
Old 1 week ago
  #18
Quote:
Originally Posted by Cpl. Punishment View Post
Quote:
Originally Posted by dshodge View Post
I was able to perform all the mods without removing the main board at all.

I'm really happy with these mods, if anyone want's any more info I'd be happy to share.
Yes please...
Here here, a second vote for yes please! Tally ho!
Old 1 week ago
  #19
Gear Addict
Cool, i would like to know where the input transformers go
Old 5 days ago
  #20
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Quote:
Originally Posted by guze View Post
Cool, i would like to know where the input transformers go
The easiest way to add an input transformer is cut pin 2 and 3 of the XLR and solder the primary directly to those pins. The secondary is wired with one end to the buffered input wire, which is the white wire going to the main board and the other end to audio ground which is the black wire. Repeat for the other channel. This completely bypasses the input stage so now the signal will flow from XLR>transformer>main board. Someone smarter than me might suggest a load resistor from that same white buffered input wire to ground, or they might know why its unnecessary, if any one has any suggestions...

There are 3 wire looms connecting the main board to the I/O board, the middle one is for power, the other two are for audio signals. Those are the black and white wires I'm talking about.
Old 5 days ago
  #21
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Quote:
Originally Posted by sameal View Post
Here here, a second vote for yes please! Tally ho!
I did my best to explain the input mod, but I might have to make a video or some kind of guide on these mods, they aren't that hard, but it would be lot easier to have a visual reference. Most of the mods are based on instructions found here: http://www.beautyofdecay.com/mod/prompa2.html
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