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Reverse engineering vintage Haz Labs pre

#241
ColdEye said:
Anyone use this on a Warwick? Lol I have a Warwick that needs pickups and a preamp, probably going to try this (vol/vol/stacked B/T as it only has 3 pot holes).
Click to expand...

We have a few clients running these in Warwicks, so hopefully one of them is on TalkBass to answer this directly.

The challenge is finding a stacked Bass Treble pot with a B100K linear taper and center detent that physically fit.

The bass and treble pots that ship with the LHZ are custom manufactured for us to match the original pots used in European Spectors (LHZ Custom Potentiometers – LHZpreamps.com). The critical spec on these are the thread diameter of M8 X 0.75 and thread length of 10mm. Most pots you will find have a shorter thread length of 6.5mm. If the right length is found, the threads are often 3/8" diameter which is too large.

If you can tell me the dimensions of the existing Warwick pots, I can check to see if any of the concentric pots I have will fit.
 
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#244
If this has been covered in your wiring diagram, and I missed it, I apologize. But I have a master volume question.

Would it be possible to run a blend control straight into the input of the preamp, and then put the master volume at the end between the output of the preamp and the jack? If so, what would be an appropriate resistance value for the output pot? Linear taper or audio?

I just received my circuit in the mail today, I’m super excited to install it and for now I’m going to leave the bass as it is with two volume controls, but I’ve always had a master volume and blend before. It seems to me, and again, please correct me if my line of thinking is crazy, but running the pick ups hot into the preamp would make it sound like the volume is “dimed” all the time, even if you turn it down with what basically becomes a master volume on the guitar.

I realize the only expression would be my fingers, but I kind of play that way anyhow.
 
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#245
Good afternoon, @BKsGarageBand!

This is one I'd not been asked before. Am I correct in assuming that you do NOT normally play with the volume at 100%, but instead use the volume on your bass to control your overall stage volume? If so, I get where you are coming from: you want the preamp driven as hard as it can be regardless where you dial in the ultimate volume.

Yes, you can do this. I would recommend a higher resistance potentiometer, say 250K audio taper, wired with the center wiper going to the output jack. The Hazlabs (and LHZ of course) uses an interesting EQ stack: instead of a buffer at the input, it's a treble boost. That boost is counteracted by an RC treble cut on the output so that the signal is flat at center of travel of the Treble pot.

So the goal is to minimize impact to the final RC filter so sweeping the volume does NOT change the EQ curve.

The taper (linear or audio) depends on how you prefer the volume pot to work. Generally, Audio taper pots are used for volume controls as it yields a smoother rate of change across the entire sweep whereas linear will feel like most of the volume change happens in the last 1/4 turn.

Do you need a custom diagram for this?
 
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#246
Thanks for replying!

Yep, you hit the nail on the head as far as how I play. No, thank you, on the diagram.

All my bases I’ve had before the Spector have always been volume with a blend so I was kicking around different ideas as to how I wanted to implement it and I didn’t want to buy something that was incompatible, or would damage the circuit.

I’m really leaning towards the EMG ABC control especially since I have the stock spector Bartolini’s in this instrument and it’s been my experience that passive pick ups sound better, to me at least, isolated by op amps rather than being allowed to directly interact with each others impedances. I plan to call EMG and see if the buffers in their ABC blend come before the potentiometers or after.

I really appreciate how much detail you put into the research on this device! When I get it wired up, I’ll see if I can figure a way to post some sound clips for others, the ones I’ve heard so far have all been based on EMGs.
 
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#247
I can probably save you time since I disassembled the EMG ABCX and traced its circuit to see how they magically get rid of that annoying volume dip at center and made it capable of end-to-end smooth blending.

I assume the ABC is the same basic design.

Basically, pickups go to the blend pot like any other blend, but then each gang's output from the pot one goes through it's own Cap + Resistor before being mixed back together and going into an OPA277 to be buffered. I'd post the schematic, but don't want to raise the ire of the guys at EMG.
 
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#248
Well I’ve got my guitar all buttoned up with the new LHZ preamp installed, and I have to say, I’m in love with the guitar all over again!

Now this is a Euro Spector with the Spector Bartolini pups, I don’t know if that’s an interest to anyone else on the forum, but the preamp works very very well with these pick ups. In fact, one of the main gripes I had with the tone pump was I was getting these strange wolf tones when I tried to adjust the pick up height up where they ought to be. Not so with this new preamp. I can get the pick ups all the way up to where they nearly touch the strings and sound their best and I don’t get any annoying harmonics or anything untoward.

There are SO many more usable tones in this guitar now, I almost never used the J pick up before, but I can run them both wide-open And it just comes alive!

I hope I’m not derailing your thread, but I just wanted to chime in and say how pleased I am with my purchase!
 
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#249
One small thing I’d like to point out, I’m glad that you mentioned in your instructions about being careful of the conductive paint in the body cavity. This particular guitar had some wood fuzz that had been painted with the conductive paint and I was afraid it would get shorted out, so I hot glued a small piece of plastic to the interior before mounting the preamp in place with double sided tape. Just as an extra precaution against any shorts to the internals.
IMG_7148.jpeg IMG_7147.jpeg IMG_7144.jpeg
 
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#250
AgingEngineer said:
We have a few clients running these in Warwicks, so hopefully one of them is on TalkBass to answer this directly.

The challenge is finding a stacked Bass Treble pot with a B100K linear taper and center detent that physically fit.

The bass and treble pots that ship with the LHZ are custom manufactured for us to match the original pots used in European Spectors (LHZ Custom Potentiometers – LHZpreamps.com). The critical spec on these are the thread diameter of M8 X 0.75 and thread length of 10mm. Most pots you will find have a shorter thread length of 6.5mm. If the right length is found, the threads are often 3/8" diameter which is too large.

If you can tell me the dimensions of the existing Warwick pots, I can check to see if any of the concentric pots I have will fit.
Click to expand...


Do these help? If you need me to measure anything else let me know.
IMG_6682.jpeg
IMG_6681.jpeg
 
#251
BKsGarageBand said:
Well I’ve got my guitar all buttoned up with the new LHZ preamp installed, and I have to say, I’m in love with the guitar all over again!

Now this is a Euro Spector with the Spector Bartolini pups, I don’t know if that’s an interest to anyone else on the forum, but the preamp works very very well with these pick ups.
Click to expand...

That's awesome, @BKsGarageBand, and I'm happy you are digging it! I really appreciate you taking the time to post; I regularly get email from potential clients asking if the LHZ will work with Barts, Dimarzios, and other pickups. Every time someone takes the time to publicly share their configuration and the outcome, it's extremely helpful.

Trust me: You are not hijacking the thread. This forum is where the LHZ started with me asking all kinds of kindergarten questions and sharing what I've learned as I reverse-engineered the Haz. It's become a living history at this point, moving from a pet project into discussion by users about how it's integrated and works.

Cheers!
 
#252
#254
AgingEngineer said:
Yes, you can do this. I would recommend a higher resistance potentiometer, say 250K audio taper, wired with the center wiper going to the output jack. The Hazlabs (and LHZ of course) uses an interesting EQ stack: instead of a buffer at the input, it's a treble boost. That boost is counteracted by an RC treble cut on the output so that the signal is flat at center of travel of the Treble pot.
Click to expand...

I’m surprised to hear you say 250k, I’m used to lower values on active preamps. For instance, The Michalik (tone pump) shows a 50K master volume and all the EMG stuff I used to have was usually 25K.

I have 10k and 100k leftover from TP and 500k leftover from a volume, also the 50k from discussing the “boost only” mod with you ;). Would you recommend any of these or should I just order some 250s? The circuit is definitely your baby. Im ignorant about how it would affect your tones stack output impedance.

My goal is to maintain all the input tone and drive of the pickups wide-open into the preamp, and also all of the sparkle and brightness on the output of the preamp.

As an aside, once I heard your preamp in its stock configuration, I decided against the boost only mod. It retains so much clarity even with the treble turned all the way down, I’ve never had a preamp remain so clear before!
 
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#255
You are correct that active pickups and preamps usually use 25k pots. And to be clear, you CAN use a 25k master volume AFTER the preamp... It will just knock off a couple dB from the overall level and roll the high end off very (very!) slightly.

The Hazlabs preamp was designed to have the volume being before the preamp. The output circuity assumed that it will be directly connecting to either an amplifier or a stompbox of some sort. Almost all amps and stompboxes have high impedance inputs... The signal will typically go through a cap and resistor before anything else.

(NOTE: Most modern preamps have an output buffer circuit, so you can basically connect anything to the output and it won't change the tonal characteristic of the preamp. The Hazlabs was NOT designed like that and does NOT have a dedicated output buffer. The LHZ is exactly the same as it is a component-level clone of that circuit.)

From the perspective of the preamp output: it will only see a high impedance device whose resistance never varies. The final components on the Hazlabs (and the LHZ) aren't just the typical output isolation cap and resistors: it is actually a tuned filter that provides a treble cut that perfectly matches the treble boost applied by the first stage of the input.

By going straight into a potentiometer, we are introducing new resistance into a tuned filter which will change how that final filter works.

If the resistance is high enough the impact to the output filter is negligible because that is pretty much what it would see when connected to a pedal or amp. BUT, if it's a low value, it'll start cutting the output dB level and change the curve of the treble cut.

That's why I recommended 250k: it's high enough that it's impact on the filter isn't really noticeable. That is also why I recommended that the pot be wired with the wiper (pin 2) going to the output: this keeps the resistance the preamp sees as constant.

During simulations, values above 100k started having very little impact, so of the pots you have on hand, I'd recommend the 100K for your output volume pot.

Sorry for writing a book! lol
 
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#258
bigsnack said:
Does anyone offer a stacked VOL / VOL pot with the correct resistance value for the input of the pre? I’m also in the three hole dilemma . Amazing thread and project by the way!
Click to expand...

Like @fibz noted, I keep coming back to EMG because their potentiometers are goldilocks solutions: not too small, not too big, just right! I am 90% sure they are sourcing pots from Noble, so I reached out to them to see if these are stock parts. They only do custom OEM builds, so no luck there.

Another source is here: EMG - Potentiometer - 25K Stacked Volume Control (Hardwired) - Best Bass Gear

They seem to carry a few Noble pots under different brand names (Seymour Duncan, EMG, Noble, etc)
 
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