Markbass Combo Mods... Piezo/Crossover removal & bad sounding DI.

[spinmusic]

Hey folks.

Well, this is my second post on this lovely amp. (It’s an early 2006 model). And being a bit of a tone junkie like so many, I’d like to ask an expert, or a few!.. about bypassing the tweeter on this model (myself, if it’s simple and safe), in addition to having my amp repair guy potentially deal with the weak signal that outputs from the DI, plus any tips on lowering general amp hiss for lower studio recording levels etc..

I use Thomastik flats on my 64 USA reissue Jazz. It’s a great tone all round, but I feel personally that I’ve no need for a piezo to be in the equation, since what with flats and my tone rolled halfway off (most of the time), I’m struggling to see the benefit of it being connected at all! Especially as I would never need to hear much of the bass at such high frequencies, If the amp could even accurately replicate them?

So, to recap!

1. Can someone please explain how to correctly bypass the tweeter (and crossover) completely. I’d ideally like the full range from the amp, to go only to the main 15” driver. I would leave the piezo mounted inside (just for cosmetic reasons!), but just want to bypass the whole thing.

2. As for the weak sounding DI, I’ve checked it a few times and it’s clearly not right. I use an Allen & Heath SQ5 as a mixer, and all of my other DI’s (both passive and active) sound great with just the bass direct into the desk’s pre’s. The Markbass on the other hand is a different story! It’s like there a high pass filter built in. Is pretty unusable for anything really.
Yes, I realise I could just hook up another DI and run it with the amp, but I’d like to know if there’s a fix for this? I’m no electronics expert, but I know a fault when I hear one! It’s a shame really, since the whole combo is ‘nearly perfect’

3. Lastly, is there a potential solution for the level of amp hiss I’m experiencing? Even when I very temporarily disconnected the tweeter, the hiss was quite loud through the main driver.

Sorry about the long post, but I’m hoping this may also help some others who just want to ‘tweak’ an old Markbass

I’ve attached some photo’s for reference.

Anyway, thanks in advance for any help.

 

[Blues Bass 2]

Do the blue/black wires come from the input jack ? If so you could take them off the circuit board and attach them directly to the woofer , bypassing the tweeter .

 

[spinmusic]

No, they were the wires connected to the tweeter. The red & black ones (the ones that are still shown as connected in the image), are the ones from the amp output to the board.
I’ve just connected the amp output speaker wires, straight to the speaker. The crossover and tweeter have since been removed.
I hope I haven’t done anything wrong by this! But I’ve read a few posts that say you can remove both the tweeter and the crossover, then just connect the amp output straight to the speaker.

Fingers crossed it’s all good!

 

[Blues Bass 2]

Cool , you did the right thing . If you ever sell it you can hook the tweeter back up if you want .

 

[spinmusic]

Great to hear! I was worried for a while, but you’ve just made me feel better about it
Can’t say I’ll keep the amp for too long though. I love it, but I’d really like a Barefaced cab and a suitable amp head (with a decent sounding DI!)
I only live about 26 miles from where the cabs are made, and would be really keen to try one.
This old Markbass will take some beating though. It’s a great sound! Just wish the DI output wasn’t so bad!
Thanks again for your advice

 

[woodyng2]

No, they were the wires connected to the tweeter. The red & black ones (the ones that are still shown as connected in the image), are the ones from the amp output to the board.
I’ve just connected the amp output speaker wires, straight to the speaker. The crossover and tweeter have since been removed.
I hope I haven’t done anything wrong by this! But I’ve read a few posts that say you can remove both the tweeter and the crossover, then just connect the amp output straight to the speaker.

Fingers crossed it’s all good!

I have a cmd121p combo. All i did was remove the wire/clip from the tweeter.
The crossover is still in the bass. I typically run all the tone controls flat,and the vpf vbn controls off. I have been really pleased with the sound results for years.

 

[PhatBasstard]

Longtime all Markbass user.

1. Whether you're using the piezo and crossover or not, your speakers are already getting full range.
That crossover is only a passive high pass filter for the tweeter. Removing was unnecessary but harmless.

2. I'm assuming you have an original "Combo Head" that has no line out control. The original Combo Head is just a rebranded Little Mark II (The newer "Combo Head II" is just a rebranded "Little Mark III").

I'm surprised you are having low output issues as every Markbass head I've owned without the line out control,
the XLR output has always been hot, assuming pre-gain is set properly, and needed a board pad or in-line adapter pad (-10db).

Are you setting your pre-gain to start to clip (light) on your hardest notes, then backing it off just a hair?
If not, and your pre-gain has been set too low, this might also address your hissing issue.
Also, the original Combo Head/Little Mark II, by default, had their XLR outs set to post (EQ, VLE, VPF).
Meaning any EQ, VLE, VPF settings/changes are going out your XLR. This could also be a factor.

However, you can take the amp out (there won't be a metal lid on top) and there will be a small,
movable jumper in there on one of the circuit boards. Its been a while, but I believe it was labelled
Pre-Po or similar. If you switch it to Pre, your XLR will act like a normal direct box out with the EQ,
VLE and VPF not effecting it.
If your pre-gain is set properly, you should get PLENTY of gain out of the XLR.

Good Luck

 

[spinmusic]

Longtime all Markbass user.

1. Whether you're using the piezo and crossover or not, your speakers are already getting full range.
That crossover is only a passive high pass filter for the tweeter. Removing was unnecessary but harmless.

2. I'm assuming you have an original "Combo Head" that has no line out control. The original Combo Head is just a rebranded Little Mark II (The newer "Combo Head II" is just a rebranded "Little Mark III").

I'm surprised you are having low output issues as every Markbass head I've owned without the line out control,
the XLR output has always been hot, assuming pre-gain is set properly, and needed a board pad or in-line adapter pad (-10db).

Are you setting your pre-gain to start to clip (light) on your hardest notes, then backing it off just a hair?
If not, and your pre-gain has been set too low, this might also address your hissing issue.
Also, the original Combo Head/Little Mark II, by default, had their XLR outs set to post (EQ, VLE, VPF).
Meaning any EQ, VLE, VPF settings/changes are going out your XLR. This could also be a factor.

However, you can take the amp out (there won't be a metal lid on top) and there will be a small,
movable jumper in there on one of the circuit boards. Its been a while, but I believe it was labelled
Pre-Po or similar. If you switch it to Pre, your XLR will act like a normal direct box out with the EQ,
VLE and VPF not effecting it.
If your pre-gain is set properly, you should get PLENTY of gain out of the XLR.

Good Luck

Thanks for your comments Phatbasstard. (That's a great name by the way!)


1. I can't see how/why both the 15" driver & the tweeter would be getting a full range signal as you mentioned, with the crossover fitted? Surely that would negate the need for having the crossover there in the first place, plus just blow the piezo instantly (if that was getting a full range signal).

Maybe I've misunderstood your reply, but with the main driver having a high pass filter set at a predetermined frequency (I'm guessing about 4Khz ish), I would like to think that by removing the crossover/piezo completely, the driver would then be receiving a full range signal from them amp, without any high end filtering happening.

2. Yes, it's the original combo head (2006). Has no separate line out volume. Which I think is a plus point!

3. I did switch the jumper on the board, it unfortunately made no difference to the sound from the DI (Apart from the obvious pre/post gain change to the DI output)

I'll edit the title of this thread, since me saying 'Weak sounding DI' is not very accurate!.. What I was actually meaning, was 'Bad sounding, like there's very little low end coming from the DI'.
The output from the DI is fine (signal-wise), it just sounds very bad sonically compared to any other DI's that i've used with the same bass. I read somewhere online recently that some of these early amps had the wrong type of capacitors for the DI, and that this was causing an impedance issue with a lot of modern mixing desks (Or something along those lines!)

The quest continues..

Thanks again

 

[PhatBasstard]

1. Read my answer again. The tweeter is attenuated with a high pass filter (the crossover), otherwise it would blow.
The 15" is given full range by the crossover, it will sound the same with or without it.
I've talked with Markbass about this and it's also very common in all bass amp passive crossovers.

2. If you changed the correct jumper (there are two, although I can't remember what the other is for),
it will certainly effect what's coming out of the XLR/DI.

Bass Amp DI's 101:
In Pre mode, only your pre-gain will effect it's output.
In Post mode, pre-gain, EQ, VPF and VLE will all effect it.

As far as the capacitor thing, I've had a lot of their amps, including many of the older Combo Head/LM II units.
I've never had the issue nor heard of it.

 

[spinmusic]

I've read your answer. (again) The only part that confused me was you saying 'Whether you're using the piezo and crossover or not, your speakers are already getting full range.'.. This full range signal fed from the amp obviously doesn't apply to the tweeter, but I get what you meant!
As for the 15" getting full range with or without the crossover, that makes sense too. Albeit that's not the way that a crossover normally works by default in a regular PA sub (as an example). I'd just assumed that the crossover wasn't sending a full range signal to the main driver.

I'm well aware how pre/post work on any amp setup, but the issue is certainly wth the DI, so I think I'll need to get an expert to look at it. I wish it was simple to upload a couple of comparison clips on here, as I'd then be able to let you hear the problem.

As for the capacitor concern, I found this thread here. Worth checking i think.

MarkBass LMII DI

"The Markbass DI problem is caused by the output capacitors being too low in value. If the DI is plugged into a desk with a low input impedance e.g. 600 Ohms, this has the effect of behaving like a high-pass filter i.e. results in bass cut.

This could be considered a design flaw. The fix is to increase the values of these capacitors.

It is a straightforward modification for anyone who is competent with a soldering iron.

The LMK has the same issue, and can be fixed in a similar fashion.

This issue was originally solved by figuredbass (see quote below).


OK, here is the DI mod to the LMII:

Basically the DI output capacitors are WAY too small to provide full bass into PA boards with a very low input impedance. They are only 1uf and should be at least 47uf/63V, ideally 100uf/63VDC. This would be C52 (which feeds XLR jack pin 2), and C58 (which feeds XLR jack pin 3). Because of the surface mount type of preamp PC board and very limited real estate on it, it will be impossible to directly replace these two caps right where they currently are. So they'll have to be placed elsewhere, with their leads paralleled on the original caps, OR: the original caps will have to be jumpered across (bypassed) and the new caps will then be inserted into the circuit right at the XLR jack. If done this way the caps must be mounted in such a way that they cannot move at all, even by vibrations. (I find combinations of super glue, electrical tape, and silicone adhesive effective.) The positive pole of the cap should lead towards the XLR jack, the negative pole leads towards the amp. This mod will allow the DI to see any load impedance down to 600 ohms without any low end roll off.

A small amount of the roll off in this amp already happens in the preamp stage before it even reaches the DI (as xgabriele, Shelley, and others noted in their comparisons with the UL502) and I'm working on improving this as well, which would improve the low end extension of the stage sound (and therefore improve the DI sound even further). I'm flying blind without a detailed schematic, so it'll be a while before my analysis on this part is complete! For this second mod I'll post the info in a separate thread when I've figured out exactly what has to be done.

Just another caveat regarding mods to this amp: mount all components in such a way that keeps their leads as short as possible in order to minimize picking up excessive noise from the switching power supply.
On the LMK, the capacitors are C29 and C30 by the way.

I hope this helps.

 

[DaDo625]

I have a cmd121p combo. All i did was remove the wire/clip from the tweeter.
The crossover is still in the bass. I typically run all the tone controls flat,and the vpf vbn controls off. I have been really pleased with the sound results for years.

I have this Amp and would also like to try it w/o the tweeter.
Being a total noob to working with amps but able to follow instructions - how do I do this?
Thanks in advance for the input.

 

[thetragichero]

thankfully, this changing the smd output capacitor is a rabbit hole that you don't have to go down

it mentions IF you're using a mixer with an input impedance ~600ohms, then you may have a problem. which is true, because that puts the corner frequency of the hpf somewhere above the 12th fret on the g string of a bass (wow!... although even at 600ohms 47 or 100uF is quite excessive... no need to send all those subsonic frequencies to the mixer)

technical data sheet of YOUR mixer states that the xlr/mic input impedance is >5kOhm, bringing the corner frequency to about 32hz. if anything the output capacitor is TOO BIG in your situation, because you really don't even need to pass frequencies this low

seems likely this is a gain situation somewhere or other. which is good, because tweaking some gain knobs, or turning off a pad or something digital on the mixer end is a whole lot easier than soldering to some smd electrolytics

source: Invalid Link Removed
helpful so that I don't have to figure out the corner frequency myself: Guitar Pedals: R-C Filter Calculator

 

[woodyng2]

I have this Amp and would also like to try it w/o the tweeter.
Being a total noob to working with amps but able to follow instructions - how do I do this?
Thanks in advance for the input.

1.remove front metal grill
2.unscrew piezo from front baffle board
3. Pull wire/clip off piezo
4. Probably not necessary,but i wrapped clip in black electrical tape to prevent contact.
5.reverse steps 2 and 1.

 

[DaDo625]

1.remove front metal grill
2.unscrew piezo from front baffle board
3. Pull wire/clip off piezo
4. Probably not necessary,but i wrapped clip in black electrical tape to prevent contact.
5.reverse steps 2 and 1.

Thank you...
That was both quick and satisfying....
The hardest part (and it is not hard) is easing out the tweeter and that just takes patience.
Of course I could not resist a test ride...and it made me smile...

Thanks to the OP spinmusic for the thread...

 

[agedhorse]

Do the blue/black wires come from the input jack ? If so you could take them off the circuit board and attach them directly to the woofer , bypassing the tweeter .

Bad advice, this is not at all how the "crossover" is wired.

thankfully, this changing the smd output capacitor is a rabbit hole that you don't have to go down

it mentions IF you're using a mixer with an input impedance ~600ohms, then you may have a problem. which is true, because that puts the corner frequency of the hpf somewhere above the 12th fret on the g string of a bass (wow!... although even at 600ohms 47 or 100uF is quite excessive... no need to send all those subsonic frequencies to the mixer)

technical data sheet of YOUR mixer states that the xlr/mic input impedance is >5kOhm, bringing the corner frequency to about 32hz. if anything the output capacitor is TOO BIG in your situation, because you really don't even need to pass frequencies this low

seems likely this is a gain situation somewhere or other. which is good, because tweaking some gain knobs, or turning off a pad or something digital on the mixer end is a whole lot easier than soldering to some smd electrolytics

source: Invalid Link Removed
helpful so that I don't have to figure out the corner frequency myself: Guitar Pedals: R-C Filter Calculator

IMO, this appears to be good and accurate advice.

1.remove front metal grill
2.unscrew piezo from front baffle board
3. Pull wire/clip off piezo
4. Probably not necessary,but i wrapped clip in black electrical tape to prevent contact.
5.reverse steps 2 and 1.

In the case of THIS product, this will work and is safe imo.

 

[spinmusic]

Thanks for the replies folks. I think it’s probably best that I get my amp guru to take a proper look. As I’ve said a few times, the only issue I have with the DI output is the bad tone that it’s sending to the desk (or any mixer for that matter). Have tested the same Jazz bass on multiple DI’s, with a couple of different mixers, and all sound much better than the Markbass one built into th

It’s not a gain issue with the amp (at least not using the amp on its own, as that sounds fine), so there’s clearly a fault somewhere with regards to the DI.

Thanks again

 

[agedhorse]

Have you tried a different XLR cable? An open pin 2 or 3 will cause much the same symptom. Being thorough on troubleshooting helps prevent drawing wrong conclusions.

 

[thetragichero]

do you have somewhere else you can send the di signal to to try to isolate the issue in the amp vs I the console? if it's in the amp, maybe a tech can 'fix' it, but if the problem is in how the di from the amp interacts with the board, you might as well just send the bench fee to my PayPal because (s)he can't troubleshoot a systemic problem with only the amp in the bench anyway
the reason why I've mentioned gain is that low frequencies require (quite a bit) more energy than higher frequencies, so an absence of gain would cause a thin sound
your mic inputs have a -20db pad on them - have you tried using the di with the pad off versus on?
other simple (and free!) things I try to remember while troubleshooting is making sure connector contacts are cleaned with contact cleaner... I doubt that's the problem but I've also solved more problems with contact cleaner than I'd care to admit
push some buttons or tweak some knobs before spending cash is a good mantra - helps me from spending to much on gear too lol

 

[agedhorse]

Indiscriminate use of "contact cleaner" has also created entirely new problems, I see the results of this too.

"Contact cleaner" and pots is generally a bad scenario, only the tiniest spray of DeOxit D-5 is what I have seen as reasonably safe and effective over say 20+ years of track record. Less is more.

 

[spinmusic]

Hey guys, thanks once again for your thoughts on this problem.
To hopefully answer both of your questions.

Yes, I've tried a few different XLR's, in addition to running the DI output to about 4 different mixing desks at various gigs. I've tested with my own SQ5, another Presonus, an LS9, and another A&H console. All of the other engineers have mentioned that my DI signal into their gear sounds bad! Again, when I've used a Radial DI (or even a cheap Samson!) instead of the DI from the amp, they all sounded fine.
And yes, tried the Markbass with both the pad in and out. Still sounds nasty!

Tis but a mystery..