MKIIA-LATOR

[mark2boogie]

Hello Mark-Owners,

I know that several affordable products exists now to balance the level of the passive FX loop of the MKII amps (A and B, at least) in order to allow the use of front-end pedals in the FX loop of these amps without overloading the effects... So my MKIIA-LATOR is not so useful today, since commercial all built products are available. When I developped it, I didn't knew the existence of such a cheap units, hence :

You can try a G-lab SA-1 signal adapter or their ALA-1 loop adapter. I got an SA-1 for my MKIIA and it works great. My loop is way hot compared to studio or effects loops. Not expensive.

Will be interested to see mark2boogie comes up with.

So here is it - with resized pictures :

A+!

 

[b00g13mk2B]

Looks great. What is your procedure to adjust the trim pots? Do you use a signal generator to the amp input with the amp master volume set at a specific number?

 

[mark2boogie]

Looks great. What is your procedure to adjust the trim pots? Do you use a signal generator to the amp input with the amp master volume set at a specific number?

Thanks ! My original design procedure was as follow :

1 - I checked the maximum signal level that entered the phase splitter fo my MKIIA at the onset of clipping in the 100W mode : it was circa 3.20VRMS (sinus @400Hz / master at max, all tones at half, volume at the usual value of 7, normal rhythm position).

2 - Necessarily, the headroom of the MKIIA-LATOR needed to equal or better excess enough that 3.20VRMS level : as you can read on the schematic, for 18VDC, the headroom is at least 5.2VRMS and circa 7VRMS for 24VDC power supply. So that's correct.

3 - The usual level for the stompboxes is -10dBv, that is to say circa 0.32VRMS. Knowing that the signal level in my MKIIA where it was inserted is 3.2VRMS, my MKIIA-LATOR then needed to attenuate X10 the signal before the effect and recovery it x10 after the effect.

4 - I connected a signal generator set at 5.2VMS at the FROM PREAMP input, I adjusted the SEND LEVEL ADJ. pot at 0.52VRMS on the FX SEND / FX RETURN jacks (attenuation X10), and then adjusted the RECOVERY GAIN ADJ. to obtain 5.2VRMS at the TO POWER AMP output (recovery x10).

Doing that, I am sure that my FX in the MKIIA-LATOR loop will never be overloaded. But in fact, most of the effects today can accept a higher signal level that -10dbV (0.32VRMS), so I think that I probably can rise that level at 0.64V (att/rec x5) or maybe more by resetting the trimpots, in order to achieve a better signal-to-noise ratio (hiss) - coming from the effects, not from the MKIIA-LATOR which is dead quiet.

A+!

 

[BlackBoxy]

Slt,

tu peux attaquer un fuzz au germanium avec ? est il adaptable à des impédances différentes ?

thx

 

[mark2boogie]

Yes,

My MKIIA-LATOR can drive any effect for sure, including Ge Fuzz, but vintage effects like Fuzz may not give a satisfactory sonic result, as these were often intended and designed to be placed just after the guitar to react with it, and not inserted in an FX loop.

A+!

 

[b00g13mk2B]

Looks great. What is your procedure to adjust the trim pots? Do you use a signal generator to the amp input with the amp master volume set at a specific number?

Thanks ! My original design procedure was as follow :

1 - I checked the maximum signal level that entered the phase splitter fo my MKIIA at the onset of clipping in the 100W mode : it was circa 3.20VRMS (sinus @400Hz / master at max, all tones at half, volume at the usual value of 7, normal rhythm position).

2 - Necessarily, the headroom of the MKIIA-LATOR needed to equal or better excess enough that 3.20VRMS level : as you can read on the schematic, for 18VDC, the headroom is at least 5.2VRMS and circa 7VRMS for 24VDC power supply. So that's correct.

3 - The usual level for the stompboxes is -10dBv, that is to say circa 0.32VRMS. Knowing that the signal level in my MKIIA where it was inserted is 3.2VRMS, my MKIIA-LATOR then needed to attenuate X10 the signal before the effect and recovery it x10 after the effect.

4 - I connected a signal generator set at 5.2VMS at the FROM PREAMP input, I adjusted the SEND LEVEL ADJ. pot at 0.52VRMS on the FX SEND / FX RETURN jacks (attenuation X10), and then adjusted the RECOVERY GAIN ADJ. to obtain 5.2VRMS at the TO POWER AMP output (recovery x10).

Doing that, I am sure that my FX in the MKIIA-LATOR loop will never be overloaded. But in fact, most of the effects today can accept a higher signal level that -10dbV (0.32VRMS), so I think that I probably can rise that level at 0.64V (att/rec x5) or maybe more by resetting the trimpots, in order to achieve a better signal-to-noise ratio (hiss) - coming from the effects, not from the MKIIA-LATOR which is dead quiet.

A+!

I seem to not be able to fully drive my MKIIA with the G Lab SA-1. I don't have a scope handy, so would like to ask mark2boogie to look at the GLab specs.

http://www.glab.com.pl/sa-1_en

This says I should be able to get up to 17V from the output, assuming I can drive boost side enough. I'm coming from a Midiverb2 in the loop. No problem with adjusting the attenuate section to match the Midiverb input. When adjusting the Midiverb output and SA-1 Boost, I can get to the clipping light on the Boost side blinking. But the amp in nowhere near as loud with the loop unused. Seems the SA-1 can't drive the power amp in fully? Maybe I am doing something wrong. I am Considering getting a C-lator. Should borrow a scope and check the signal voltages.

 

[b00g13mk2B]

I did some more investigation, using only the SA-1 in the loop. As long as I don't turn the Boogie master volume much past 2, I can match the signal coming out of the SA-1 to not having it in the loop. Most of the amp volume comes by 2, but there is some left past 2. Beyond that, the SA-1 recovery side will clip. I think I blasted out my ears and lost my loudness reference.

I also have an ADA TFX4 (I have not purchased time effects in a long time!). It can take the Midiverb output (or in the loop by itself) and drive beyond what the MKIIA preamp out gives. For some reason, the tone is lacking, not transparent like the SA-1.

I bet mark2boogie's MKIIA-lator would be nice. Not having the time to build one myself at the moment. The 18V swing is a plus.

 

[mark2boogie]

Hi b00g13mk2B,

Sorry to answer so lately, I am not so often on the BB forum these times - and you may have found the key to your problem in the meantime...

I do not remember if I explained it, but I rewired my MKIIA's PREAMP OUT POWER AMP IN and SLAVE OUT jacks :

- the PREAMP IN POWER AMP OUT is now rewired as the PREAMP OUT only = FX SEND
- the SLAVE OUT is now rewired as the POWER AMP IN = FX RETURN

These jacks are inserted after the master volume pot and before the phase spliter 12AT7. At that point, for full output power before clipping, the AC signal is 3.2V true RMS. On a Dumble ODS, the FX loop jacks are located at the same place.

Nonetheless, I don't know how your MKIIA is wired about these jacks : do you use an Y jack in the PREAMP IN POWER AMP OUT ? Or another solution ? Did you tried to take the RMS volts here at full power @ clipping onset ?

On last Wednesday, fortunately a friend came at home with a G-LAB SA-1, and we could check that the performances were comparable to my MKIIA-LATOR once the G-LAB settled correctly on my MKIIA. Here are the main differences :

- The G-LAB can attenuate and recovery up to 12-14dB, while my MKIIA-LATOR accepts 20dB on 2x9V battery power. This allow for more headroom.

- The attenuator of the G-LAB is passive, while mine is Hi-Z in / Lo-Z out active. This allow for a better treble preservation.

But this doesn't make a huge difference on my MKIIA.

A+!

 

[b00g13mk2B]

Thanks for your reply. My MKIIA is stock, the jacks have not been rewired. I take the preamp signal from the bottom of the chassis to drive my effects and bring it back to the PREAMP IN POWER AMP OUT jack using standard 1/4" plugs.

In the meantime, I purchased a C-lator from eBay. It has much more gain, I can really drive the amp. There is something odd that goes on adjusting the knobs on the C-lator, like there is stray signal somewhere. But I can get everything adjusted amp sounding good. The C-lator output control makes a nice master volume.

I did not notice treble loss with the SA-1. It seems like a great device, the MKIIA just has a hot signal.

 

[McBarry]

If anyone has the tenacity and skill to make this work, you certainly do Bruno ..!!!

Congrats on the circuit and implementation...!!

Of course, I'll see how it really goes when U come Down Under to show it off in person... hehehe..!!!

Cheers, db