LarMar vs Matchless MV (Vox AC Circuits)

[inkomodo.dragon]

I just wanted to let you all know about my experience using both in my Vox AC15C1.

As you may know from the SCHEMATICS, this amp comes equipped with the Matchless type PPIMV also known as the Cross-Line MV. This is basically a 500k pot between the output of the PI's coupling caps. Its simplicity may be appealing but it has a number of disadvantages and most importantly sounds weird at high levels of attenuation. I don't know about it's performance within other type of circuits but on my AC15 it muddied up the overall sound. The low end gets fartier as you turn it down and if that isn't bad enough you get some weird high end distortion that's not pleasing at all. I just don't get why they keep using it other than the low cost.

I struggled with it for a couple of weeks and then decided to test the LarMar and the difference was night and day. I've used this neat MV on a couple of Marshallesque amps and although it sounds great you get the minor inconvenience of "loosing" your presence control as you attenuate. But since there's no negative feedback on AC type circuits this problem is gone. In my opinion there's no better choice in MV for this kind of amps.

Do yourself a favor and install a LarMar on your AC, your ears will thank you.

[CDDESIGN]

I'd also say check out the VVR (variable voltage regulator)from Hall Amplification .

I'm about to start a couple vox style builds and have been researching different master volumes myself. I recently added the larmar PPIMV in a jcm800 build with decent results. I will report back when i get a chance to intall the VVR's in a fixed bias (marshall) and cathode bias(vox) amp.

You can find them here:
http://www.hallamplification.com" onclick="window.open(this.href);return false;

[robert]

Hi,

I would prefer the Hall "Fixed Bias- VVR" ( but only on the screengrids! ) in combination with the LarMar.
BTW: the LarMar PPIMV IMHO sounds a lot better than this strange "Matchless- PPIMV"

Regards

Robert

[inkomodo.dragon]

I've tried Dana's VVR implementation in some amps and it worked well but I must admit I didn't spend a lot of time with it.

The LarMar PPIMV was super easy to implement non intrusively and delivered great results so I'm all for it. Eventually I'm going to try the VVR but I don't feel it's needed at the moment.

Having said that, the main reason for this post was to let you guys know that the cross-line MV is a definitive NO in my book.

[robert]

[darkbluemurder]

I fully agree. I replaced the cross-line master volume in a top boost circuit amp with EL34s, in my Chieftain build and in a Spitfire build with a LarMar PPIMV. In all amps the LarMar was a vast improvement.

Cheers Stephan

[brunomariano]

inkomodo.dragon, can you post the schematic of that larmar PPIMV on your AC15?

[brunomariano]

I've tried Dana's VVR implementation in some amps and it worked well but I must admit I didn't spend a lot of time with it.

The LarMar PPIMV was super easy to implement non intrusively and delivered great results so I'm all for it. Eventually I'm going to try the VVR but I don't feel it's needed at the moment.

Having said that, the main reason for this post was to let you guys know that the cross-line MV is a definitive NO in my book.

Can you post the schematic of what you Did?

Thanks!

[brunomariano]

Hi!

For installing a LAr Mar on VOX AC15C1 I must just remove those 220k resistors or put a jumper on their place?

[inkomodo.dragon]

I don't have a schematic for that yet but the steps would be:

• Remove R81, R82, R86, R89 & VR9
  Solder a wire from C41 & C43 to the CW pins of the Dual 250k Pot (Linear taper works fine)
  Solder a wire from the center pins of the Pot to R65 & R77
  Join both CCW pins and solder a wire from them to the point where R85, R86 & R89 joined
  Remember to solder two 2M2 resistors between each mid point and it's respective CCW pin

Essentially you are wiring the pot as two variable voltage dividers with the coupling caps feeding the input, the screen resistors at the output and grounded through R85.

For more info check the LarMar wiki here at Metroamps

Hope that helps

[rupe]

So would it be of benefit to change MV types in a '97 Chieftain? It's an incredible sounding amp but the master seems to serve little purpose...there are no good dirty tones to be had with the master less than half way up. I run it wide open with the pre-volume at 3:00 to get to the goods but its extremely loud with those settings (Clearsonic panels keep the peace with the soundman). This amp would have been better off no master IMO.

[inkomodo.dragon]

I'm not familiar with that particular circuit but if it doesn't have negative feedback you'll be more than happy and the tones you got now will still be available.

It's one of those win win situations...