Mark Series graphic equalizers

[ytse_jam]

i took a look at some mark series amp schematics and there are some transistors in the graphic EQ section... how do they work? is the tone somehow affected by them? How would you compare the built on mesa graphic eq with turning it off and using a quality graphic eq (i.e. mxr pedals) in the loop?

 

[Boogiebabies]

The transistors are low noise, high gain audio amplifiers. The difference is they are discreet components opposed to op-amp based designs. The Mesa EQ offers and inductance based EQ driven by discreet components for cut and boost functions. It is super quiet, has good drive and the silicon devices are kept to a minimum by using the MPSA63,20 and 70. The modern EQ pedal use multiple op-amps, capacitors and resistance to cut and boost frequencies then amplify them through noisy TL022 op=amps. They are not the audiophile grade of an inductor based EQ and are way noisier, but can provide more drive which is not bad for some people. A good example is using the GE-7 through the loop of a Marshall 2555 Silver Jubilee. It adds gain when you boost the signal and allows you to tweak some of the Marshall mids out and get your Slash tone going. Through a Boogie, it's can sound good, but there is the constant hiss of the circuit. It has a greater ability to cut and boost though as it has more bands.

If you look at older high quality audio equipment like the Neve 1073 or API 550 even to the Yamaha PM-1000, all of there EQ functions driven by transistors. Either Silicon or mostly Germanium. Germanium's tend to sound a bit warmer due to their composistion. So if you look at the Mesa EQ, it is based on very high quality components and design philosophy to provide an audiophile grade Graphic as possible. It's dicreet, has good gain and is extremely quiet. The truest test is to shelve your Boogie's EQ all at center, then play. After a minute, turn the EQ off and your tone should be identical showing it's dead neutral at the center. I cannot say this for out board EQ's because there is always some coloration from my experience. Even with a Graphic EQ on your Boogie, you can benefit from using another Graphic that has cut and boost to other frequencies not on your Mesa's. As long as you do not drive it too loud the hiss should be bearable and you can get some amazing new tones. You can also use them together and drive a cabinet to despair with scooped mids, bottom end and shredding highs. The speakers will see some serious movement.

 

[Russ]

Boogiebabies... That was awesome....

I actually use a GE-7 when necessary for my Marshalls but with my Mark IV I have found no need for it. I tried to use it in the loop even as a boost but found the hiss that was described above and so now when I play my Mark IV it just sits on the shelf. The Mark IV doesn't really even need it (to my ears anyway).

 

[Boogiebabies]

Yes, the MK series EQ's are very well designed. If you run an NS-2 after the GE-7 it helps a bit, but it is a noisy unit, albeit another cool design that is a different approach. You will also see that the Mesa EQ when engaged is almost perfectly level with the amps original signal so it is not over boosting and causing unwanted distortion. 8)

 

[Dutchman]

Wow!!! Boogiebabies your are the well to draw knowledge from. That was really some great info. I often wondered about the quality of the on board EQ. I have mine switchable along with the reverb and channel switching from an old Rockman Octopus That way I turn it on or off clean or dirty. I do notice loss of gain with it scooped though. I make up the gain thru the effects loop with a Intellifex LTD.

Great tutorial......Thanks

 

[Antoine]

Boogiebabies, thanks a lot for this great info about the Mark series EQ.
your knowledge about amps is much appreciated and very helpful here... you're THE BOOGIE MAN ! :wink:

 

[screengrid]

One thing to add regarding the graphic EQ. If you look at the schematics of the earlier mark series amps, say from the MK II A, B C+ versions to the MK III, one can see, that the cirquit design of the EQ always is basically the same, but in schematics of the IIA and B you can see relays used to switch the EQ complete out off circuit in the off position, which would be a true bypass solution. Later again ( IIC+ !!!) the non relays versions were used again, like it is with the MK IV until today. It seems, that Mesa Boogie could not see any advantage with the former true bypass relay versions. the opposite seems to be the case! For me that makes totally sence, because although the EQ circuit is always in the signal way, also in the off position, the amplification factor of the EQ is exactly 1, if all sliders are in the neutral positions or the EQ is switched off ( Q2 and Q3 are working as a differential amplifier! ) In some boogie amps, not sure at the moment but i guess the maverick for example, basically the same EQ circuit, but without the sliders, inductivites and caps are used. Instead of them a simple 10 K Ohm resistor is used. Finally, the EQ doesn't have any negative impact on the all over tone of the Mark series amps, although 4 transistors are used ! And as BB already mentioned, high quality components and a very well done cirquit design is the warranty for good tone, IMO

regards
Arno

P.S. I try to express myself as good as I can and I hope that I'm fairly understandable, although my english surely is not the best
arno

 

[Vigo1999]

Yes, the MK series EQ's are very well designed. If 8)

yeah, i agree, but they're designed to still maintain a certain amount of it even when its off dont?? (i think around 5%, if im not wrong, according to Mesa). and my tech says that all the Mesas that he got to fix, almost all of them was issues with the EQ . . .

i've also heard (or read on petrucciforum or something) that Petrucci mods all their Marks amps to eliminate those 5%, so the EQ will be really out of the path when switched off . . . is that correct???

 

[screengrid]

Hi, since January 1991 I'm using my MK IV. I never had a problem with the amp. I don't know problems caused by the EQ. I guess, that the EQ has a amount, even if it is switched off, but in a very positive way, tonewise. Nothing wrong with this, IMO!

Arno

 

[ytse_jam]

thanks for the great explanation, both boogiebabies and screengrid!

 

[Boogiebabies]

True bypass with LDR's is really a misnomer. When turned off they have an off/dark resistance of 400M. Not much is going to get through here. When you switch out the EQ, LDR 5 ( IIC+) it is a dead circuit. If there is no ground reference, the signal will not follow the path through the EQ, but directly to the phase inverter. In reference to the IIA and B, relays were the fashion, but had the same problem they do today. They pop when engaged with higer voltage, but I do not see them as being a problem for the EQ. As I stated earlier, if the EQ is out, the signal may knock on the EQ's door and quickly see that no one is home and head for the PI. For those of you who want to know, LDR's have an on resistance between 75ohms to 1.2K. They are not truly transparent like relays, but they do the job quietly. If you look at the MK III Relay and LDR circuit for the R2, Mesa used an LDR with a slow rise time to delay the high voltage surge and prevent the pop. The delay however manifests itself in a slow rise to volume. Clever, but annoying.


Weeeeee !!!!

 

[Boogiebabies]

Oh !!!!


As the plate voltages for the later MK series amps went down, so did the need for so much gain to the EQ. They are directly related so the EQ would not add too much gain and remain as transparent as possible. This is my theory on the dropping EQ voltages through the MK III and IV.
As far as JP and his EQ knob, if you put a variable resistor to the EQ power supply, you can basically adjust the amount of gain the EQ will have. Whether I beleive it or not is another matter. Just read the last Guitar World interview with EVH. Can you say SNOWED !!!

 

[Elpelotero]

i think the JP EQ knob acts the same way as a boost slider would in a pedal...it only makes sense to have that feature...

 

[Boogiebabies]

The Mesa EQ slider is a 50K linear taper resistor. To reduce the voltage, a normal taper would be needed because linear tapers only reach full resistance at the top of the knob/slider sweep.

 

[Daevid]

Would the same graphic EQ be that that is used on the Studio 22 etc?

 

[Boogiebabies]

It should be the same in the .22 and is also the circuitry behind mesa's Contour function.