How to make boss ge-7 true bypass?

[Mark Hammer]

Pre-emph/de-emph is a commony used strategy for improving noise specs.  Think of it as the poor man's Dolby.

Note that at least part of the reason for using it, though, is because on FET-switched pedals the input and output stages are on all the time, even in bypass mode.  So in part the pre-emph/de-emph strategy is to offset the noise during bypass.  Since you plan to use TB, that noise is much less of an issue, unless your plan is to leave the EQ on most of the time.  The oher thing is that the TL022 op-amps, while known for their low current requirements, are not especially reverred for their noise specs.  Their certainly not "noisy" as far as I know, just not as noise-free as some others.  So, if you were going to be using a wallwart supply anyways, higher-current-but-quieter chips are a perfectly acceptable substitution.  The schem you link to suggests battery consumption as around 6ma.  Even if that was doubled to 12ma, there is still reasonable battery life possible so it's not a great hardship to change chips.

[DVB_master]

Clear as ever Mark!
In conclusion the scheme posted by Seljer (Thank you very much man!  ) should be fine if  I use TL072 chips. Right?
http://g0g0g0.dyndns.org/uploads/050114/ge7pg28ocmod4pe.gif

Is R12, which has been removed by Seljer, unnecessary because there is a 1Meg resistor near the input?

If you confirm that the above scheme is almost ok, I'll begint to build it Can I?

[Seljer]

Hmm, yeah. Add another 1Mohm resistor that goes from the right side of C19 to ground to prevent popping noises when switching it on/off

I think the polarity of C19 might also be a bit wierd?


and yep, any opamp should work in it (placing it all out is another story though)

[DVB_master]

Could you tell me a good substitute for the HA1457 ? (IC5)

[Mark Hammer]

Given the small resistances involved, I'm going to suggest an NE5534.  Those can be found in many places.  If you are williing to live with only 6 bands of EQ, rather than 7, you can easily build this with four NE5532 chips.  Any of the sections with its corresponding op-amp can be dropped without affecting functionality.

BTW, based on the redraw you just posted, you need to flip the C19 cap around, and terminate it with a 100k resistor to ground to avoid switch-popping.

[DVB_master]

Ok.  Let me understand.
First of all the 100k resistor must be added before output connected to ground and C19. Right? (it's ok a 1M resistor as Seljer has suggested?)
Next, the op amp: I can substitute the IC5 (single op amp) directly with a NE5534 that is a single op amp too. Right?
Do the four NE5532 replace the TL022 or TL072? But why 4? 6 bands means 3 dual op amp not 4, am I wrong?

Can I add a band and use only four NE5532? So 7 band + 1 IC5 that means 4 dual op amp? (Instad of 3 NE5532 and a NE5534)

Finally the IC4: Is the RC4558 ok?

One last question: If I change those op amp,must I remove/add resistors or capacitors?

Thank you for your support man!

[DVB_master]

I understood badly. My apologise 
If I change the op amp (IC5) with NE5534, must I change also the capacitors and resitors values?
I've found that a LM833 is also quite good for band filters, what are the main differences between NE5532 and LM833 (current, voltages) What do you suggest? Do they work good at a 9V supply voltage? (I ask this because this website: http://sound.westhost.com/project64.htm suggests a 15V supply) A question as a newbie: Does A higher supply voltage give a higher gain?
Can IC4 be replaced by NE5534 or LM833 without further changes at the resistors and caps values?

Can I add more eq bands simply adding another of those repetitive groups? 

Thanks as ever

[Mark Hammer]

I would feel comfortable sticking an LM833 where an NE5532 would go and vice versa.  They should be fine for 9v, since there is not a lot of gain in the circuit.  A higher supply is good if you feed in line-levl signals and want to get a lot of clean gain for complex wide-bandwidth signals (such as a recording of multiple instruments), but for a single medium-bandwidth instrument-level signal, 9v is fine.  After all, you don't hear people complaining about their GE-7, do you?

Yes, you can add more bands.  The thing to keep in mind is the coverage of the bands, however.  Normally, the capacitor values are selected based on how wide each band needs to be if the entire spectrum is only going to be covered by 5 bands, 7 bands, 15 bands, etc.  Take another look at the article on parametric EQ at GEOFEX.  I am pretty sure he goes into the math behind the band narrowness.

[harpplayer]

Hi!

I know it has been almost 8 years since the last post, but that thread has helped me a lot.

Still it left me with another question and I feel it would be silly to start another topic. So, I converted my Ge-7 to true bypass because I was getting an anoying crackling distortion when using it with my harmonica microphone (when tight cupping it and thus overloading the signal)... I am pretty sure it had something to do with the buffer, and indeed it made the problem less noticeable. But I am also thinking it also may have to do with the preamp IC... If I skip it and connect the input directly to the first IC (the TL072), would I lose too much signal?

As for now, I connected the 1uF cap (C19) directly to R12 (gound) and the output, that means I skipped the last transistor, which deals with the de-emphasis. So I thought the next logical step was to remove C20 and R28, since I didn´t need any pre-emphasis. DIdn´t make much of a difference, since there is certainly more hiss when compared to the bypassed signal. I am thinking this must have something to do with the HA12017.

[zombiwoof]

If one already has a Boss GE-7, it may have a pretty good buffer in it already (some buffers are better than others).  If it is used as an "always-on" pedal, true bypass would not be necessary in any case.  Also, an EQ pedal near the beginning of your chain, when you have mostly TB pedals, could be used in place of a dedicated buffer pedal to drive the chain and avoid tonal loss in the chain, and be a dual use pedal in that type of usage, a tone shaper and buffer for the pedal chain.  So, it depends on how you want to utilize the pedal in deciding whether you want it to be TB or buffered bypass.  If it is used as a tone shaper that you turn on and off for a certain effect, you might want to have it be TB, as long as you have at least one other buffered pedal in the chain.  Just some thoughts.  I know that Boss generally uses good buffers in their pedals, so that's a consideration too.  You might want to think about these things even if you are building one from scratch, although I can appreciate the idea of a simpler design with a lower parts count.

Al

[harpplayer]

Good considerations!

In my case, it is not on all the time, and to be honest, it is not necessary sometimes (I got it second hand, and it has been used quite a bit apparently). I basically need it in some tunes/parts when the band is playing louder and the place reverberates too much, thus favoring feedback through my mic+amp. A slight cut around 1khz, and some increase on low/higher frequencies will help a lot to be heard. That´s nice, except for the hiss which could be a little lower.

Well, a good sound tech and a more elaborate work on the band dynamics would be the best scenario, but that´s another challenge...

Anyways, I too favor the low part count as much as I can on my builds and thought the less is more is also applicable to the ge-7, so I removed all the buffer parts, emphasis and de-emphasis stuff and it got better to my perception. Tl072 are pretty quiet on my other builds, so my suspect remains the HA12017... I may try changing the old electrolytics and tantalums first to see if it helps first, though. Well, if anyone can confirm that the pre-amp IC won´t interfere much, then I guess I can skip the trouble and just remove it.

Tks very much for the insights,
André

[MartyMart]

You could try my "HiFi" mod from many years ago !
You replace the tantalum caps used in each band with equal Poly caps, on the original
it helps reduce noise considerably - look for the post for a full explaination.
OP amp changes also helped a bit, IIRC I used 3x TL072's

Regards,
Marty.