Volumax

Hello all,

I'm new here, been browsing a bit and seems like there's a wealth of knowledge here. After not having any luck turning up any info on the Volumax, I thought I'd ask a few questions.

I have a CBS Volumax model 400. http://www.freeinfosociety.com/electronics/schemview.php?id=313
Are there any mods that exist to make this thing a little more friendly to recording? I'm looking for something to give it a little more adjustablity "on the fly", if you will.

Even if I could simply mount the input and output level pots on the front panel, that would be a start. However, the Volumax utilizes T-pad pots for both of those roles, which happen to be ridiculously expensive to replace. Is there some way around this? Is there any way to retrofit a traditional pot? Or even a stacked pot? Something?

Any help or cool little tricks would be greatly appreciated.

-GR04

> T-pad pots

Why not move the existing T-pads to the front?

1K plain-pots will work.

That does not change the fact that the input is about 600 ohms, a heavy load for some modern gear.

I cut my teeth on a Volumax. It does what it does very well; it isn't real adaptable to major changes. The ratio of attack to release time-constants can't be shifted significantly because of driver and buffer impedances. You could swop C14(?) 2.2uFd to change both constants... 1u or 0.5u might make a good drum-whacker, 10u might be a nice slow limiter which leaves an uncompressed edge on initial transients.

Thanks for your response.

I wouldn't mind moving the existing pots to the front faceplate; I've thought about cleaning them and doing so. I'm just worried that I won't be able to get them properly cleaned. They are horribly dirty and scratchy...

If I were to try a 1k, would I put it between the tip of the input jack and the transformer?

And thanks for the tip on C14, maybe I'll put a rotary switch in with some selectable values...

Also, there are so many trim pots in this thing. Are there any that do something useful?

I have replaced all the electrolytics in the Volumax, simply for peace of mind. There was no audible difference, but 40+ year old caps make me weary.

I also brought the controls to the front panel, making it a little more user friendly.

Per PRR's suggestion above, I've added a 4pos rotary switch to select C14 cap values of .47, 1, 2.2, and 10uF.

This took it from an "effect" compressor to something that's usable, though still a "color" compressor.

I"ve also added 1/4" and XLR in/out jacks for convenience.

I've got a few further questions regarding this device, if any of you wouldn't mind...

There are balance trimpots in a few places(R22, R36 in the schematic link above), and I'm curious as to how frequently these require attention. What's the maintenance schedule on something like this? I don't believe any of the transistors have been replaced, so the factory trimpot settings should still be applicable. However, I know that component drift could cause some discrepancies over time...

I don't have any of the tools for proper adjustment, other than a good ear and a screwdriver... Now I can't imagine that would be the best way to go about things, but is it worthwhile to fiddle with them? Or am I simply going to make matters worse?

My other question is in regards to a Non Linear Capacitor, NLC, per THAT technical paper AN103. Found here: http://www.thatcorp.com/datashts/600034-1.pdf
pages 6-8.

Now it sounds like an interesting concept when used to time the RMS level detector they have it on, and I'm wondering if it might be useful in the volumax, as C14? Firstly, will it even work as intended in this application? The rms detector IC is similar to the discrete circuit on the controller board in the volumax, isn't it?

Any thoughts would be appreciated.

I see 3 trimmers intended for nulling of harmonic distortion, and 1 for meter set. Wouldn't take too much to adjust them and verify results, but I wouldn't mess with them if you don't feel confident and they don't seem mal-adjusted. But then, on second thought, if you think of this as purely a color box, you may wish to play with them and see what happens, so long as you don't care if you end up slightly worse. No more time this AM, sorry.......

[quote author="guitarrock04"]I have replaced all the electrolytics in the Volumax, simply for peace of mind. There was no audible difference, but 40+ year old caps make me weary....


...My other question is in regards to a Non Linear Capacitor, NLC, per THAT technical paper AN103. Found here: http://www.thatcorp.com/datashts/600034-1.pdf
pages 6-8.

Now it sounds like an interesting concept when used to time the RMS level detector they have it on, and I'm wondering if it might be useful in the volumax, as C14? Firstly, will it even work as intended in this application? The rms detector IC is similar to the discrete circuit on the controller board in the volumax, isn't it?

Any thoughts would be appreciated.[/quote]

As PRR said, the volumax is not really adaptable to major changes. The biggest issue with adjusting time constants, is that the release or discharge path of of C14, is through R64 and the three diode string, which not only biases the buffer/follower, but sets up a constant current source for discharging which gives a linear rate release/db of limiting. If you change these values, you change the biasing of the follower. If you check the manual, you will see that the gain reduction ratio for the volumax is about 5:1 for the dynamic limiter, and nearly infinity:1 for the "instantaneous" diode peak limiter.

If you haven't defeated the diode peak limiting circuit in your volumax (shown as the "polarity sensor" connected to the primary of the output transformer), you may be getting a lot of clipping on the output, as more peak energy will pass through the active limiting stage into the diode limiter/clipper. With a smaller cap, you'll get faster dynamic limiting and less clipping; the larger caps will give you the opposite.

The non-linear capacitor circuit would be very difficult, if not nearly impossible to implement in the volumax circuitry. It's a feedback detector design, not feed forward, and it relies on quasi-antilog scaling in the side chain; the variable gain device is a biased diode limiter, which will not function properly with the log output of the rms detector.

When I used these as final limiters for broadcasting, back in the day, I added a second time constant circuit to allow greater depth of limiting without "pumping" artifacts. With the 2.2uf cap in place, and depending on the type of audio being fed to it, I used a series 100k resistor/4.7uf cap to ground. You could add to the time constants on your switch by adding combinations of the main cap, and series cap and resistors to adjust rate of limiting.

Hope this helps some.

Thanks for your replies. I noted the resistance values that my balance trimpots were set at, and fiddled with them to see what happened. I never got any improvement, so I set them back where they were.

I'm familiar with the feedforward/feedback topologies; I don't know why I didn't put that together that an NLC wouldn't work here...

After reading through the original manual that someone posted a link to elsewhere, I caught a few interesting things, regarding the polarity sensor. (Up to this point, I've had the manual for a 410 Volumax, while mine is a 400. I now have both.)

There is a jumper required for symmetrical limiting, and open is asymmetrical. I'm pretty sure mine is open. I'm not sure what the audible difference would be between the two? I'll have to experiment a bit with it, and maybe add yet another switch to bring it in/out and sym/asym.

Hi everybody


I just bought a Volumax 440 on ebay, now let's go to modify it  ;D

Guitarrock > Have you done all the modifications you wanted to carry out ?

I'm planning to change all caps, add TRS input and output, change power cord and add a rotary switch for changing the time constant.

What value of caps gives usable results ? (usually I like attack time around 10 and 25 ms, and around 50 and 100ms for release)
Do you have tried the mod with caps and resistor in  parallel, such as jdiamantis adviced ?

thanks a lot for your help !

Phazinhead,

Congrats on the purchase. I'm not familiar with the 440... Was that a typo? If I'm not mistaken, the stereo version was a 411 or something like that, and the newer ones were 4000's.

If you do have something similar to my 400, then by all means, the stuff I've listed above have all been done for a while now, and I'm very happy. I repainted mine and put all of the controls the front panel, added a 4pos rotary switch to select C14 cap values of .47, 1, 2.2, and 10uF, and added 1/4" and XLR in/out jacks. I replaced all of my electrolytics as well, but that was just for peace of mind.

Another suggestion I would make, if it hasn't been done already, is to bridge the contacts on the "polarity sensor" (see schematic) for symmetric control of the signal.

I find myself using it mostly on the faster settings, but it's nice to have options. I tend to use it on acoustic guitars and intense vocals.

If I recall correctly, the factory value for attack time is 10 msec and release is 200 msec.

Feel free to experiment; there's been an awful lot of great advice given here. I never went so far as to try the cap and res to ground that jdiamantis suggested, because I am happy with it as is. That doesn't necessarily mean it won't be what you're looking for.

Hi Guitarrock !


thank you very much for your reply ! Yes I made a mistake, it's not a 440 but the 410 ! this is the mono version of the volumax.

I've just bought it on ebay  (http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=150315459474) I haven't received it by post yet. I'll do like you, changing all caps for peace of minde too, add the rotative switch for changing the time constant and input and output XLR or 1/4". Great idea to add a switch for asymetrical or symtrical limiting detection ! I there any other modification available such as distortion/harmonics level that we could do ? (I read something about trim pots to do that ..)

I forgot something !

What type and value of potentiometer did you use to replace the original "T" pad ? thanks you very much !

Ok, the Volumax 400 was intended for AM broadcast, and the 410 for FM. Not sure what all the differences are, but keep in mind that our units are not exactly the same as you embark upon these modifications. Check out http://www.hobbybroadcaster.net/resources/docs/volumax_410_411_manual.pdf for more information on your particular model. At a glance, it appears to have a shorter attack time and separate release times for high and low frequencies. Also, the C14 in my unit would be C202 in yours.

As far as the T pads, I simply cleaned them and remounted them to the front panel. Standard 1k pots would work if something was defective with one of yours and you couldn't get a replacement for whatever reason. They do not maintain balanced operation, however, and while it won't hurt anything, isn't recommended.

The trimpots are already in the unit, and are for biasing and setting the meter. No different than the little trimpot in a guitar amp you would use to bias it's output tubes. If you have access to an oscilloscope (which I do not), you can follow the instructions in the manual link above and set everything back to factory specifications. It may or may not be an issue for you, depending upon whether or not any of your components have ever been replaced or how much they've changed over time.

Hi guitarrock,


thanks for the precisions... yes you're right mine is 410, I hope that it will be easy to modify like yours
I don't have any oscilloscope so I will make the setting with my ears too  ;D

Those limiters are for broadcast... so maybe they will add a pre emphasis to the sound.. is it right ? is it necessary to modify the circuits so as to avoid this pre emphasis in high freqencies ?

.. and what about the CBS 4440 Audimax.. do you think it is a better sounding compressor than the 400 or 410 serie ?

thanks for your reply !

As much as I can remember, the differences between the two are minimal, so it shouldn't give you much trouble, and I'm confident you'll be happy with the results.

To clarify: I didn't make any adjustments to any of my trimpots, I am happy with it's operation and don't wish to get carried away with tasks of such depth.

I've never seen or heard one of the 4000 series compressors, so I can't make any comparison between them. I have a feeling these older ones will have quite a bit more character, and better suited to recording.

The reason behind the separate release times is to provide a cleaner compression. It prevents excessive high or low frequencies from triggering the limiter, when it wouldn't be necessary to the rest of the mix.

I encourage you to set it up as received and experiment a bit with it. Once you've identified what you like and don't like about it, set about to make modifications. Don't always assume every possible change you can make will be necessary.

In my shop I've had the opportunity to service and play with just about every model of limiter ever made and I must say, so far the Volumax is my least favorite. I had 2 of them come in a while ago and they seemed to be capable of only two things -- either doing nothing to the signal at all, or smashing it to pieces. Both acted exactly the same and alignment didn't really help. I was left with the impression that what I was seeing was normal, but even for radio, I can't imagine that dynamics like that would be very useful.

Good luck, I think this one is a definite candidate for mods. Sorry for the downer message, maybe it was me. I'd be interested reading more info on this odd bird.

Hi guys !


thanks a lot for you replies. I'll try the volumpax without any modifications, and then try to recap it and add the mods we talked about. I looked at the Volumax 410 schematic, and I have to admit that it is a bit difficult to understand whate are the mods to do !

When you talk about 2 different release times depending of the frequencies, does it mean that there are two different circuitry in the sidechain, and the need of two capacitors for those 2 release time ? thanks !

Hi everyone!

As the owner of a "5-second-release ;D" 410, I follow this with great interest! When my schedule allows, I will try and replace C202 with some smaller value caps. Also the C203 on the treble side (AGC #2 board) might need some tweaking... (Yes, there are two separate sidechains, or AGC boards) Some more headscratching: Can you join both the AGC boards at some point to get a "normal" compressor? (Are we blessed to have a multiband device or cursed with a re-esser?) Is there a way to remove the "pre-emphasis" circuitry? (Are the separate AGCs actually forming the pre-emp.?)
???

Best,

/Dave

Okay, very interesting to have a double band compressor ! this would maybe allow to have 2 pots for the release, and why not to add a switch so as to make the sound go in only one AGC board, or to split it into 2 bands such as it is normaly ?

Hi everybody, Happy New Year to all !


I just finished the mods of the CBS 410 and tried it a bit, I really like the way it compress the sound, even if it is very dirty and hard sounding when the input gain is pushed hard. Does somebody have an idea of what caps and resistor to change on AGC board #1 and #2 so as to have different attack and release time ? I don't have any idea of what components to change and what value to use, because of the  two band with differents settings.
When input level is hight, sound become very dirty in the low frequencies, especially when the music contains lots of very low frequencies (electro music), so I wondered if it would be possible to add a high pass filter in the side-chain or to modify some caps value so as to make a kind of high pass filter. Do you think it could be achieved easily on this limiter ?

thanks a lot for your help !

Some guesswork: Try reducing the (first) coupling caps on the AGC board (C10/11), should give you less bass sensitivity. As for time tweaking, I still bet on altering C202 and C203. Lowering value of R216(?, resistor following conn. F on control board) *might* speed up charging of cap..? ???

/Dave