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Reverse engineering vintage Haz Labs pre

#101
AgingEngineer said:
Great question, and I'll apologize in advance for what'll probably be a long answer!

The LHZ is a clone of the 80's HazLabs preamp. The 80's HazLabs is basically a copy of a reference Bandaxall EQ design from the 1950's fed by a treble-boost buffer stage. The Bandaxall stage is tuned so that the response was flat at center sweep of the 100k linear bass and treble pots, and would provide +/-15db of bass and treble cut and boost.

Of course "flat" is subjective. In simulation the frequency response is not mathematically flat: there is approximately +/- < 0.25dB variance from 20Hz to 20kHz in this design. But for practical purposes, that's flat enough for what it was originally designed to be: a bass preamp EQ.

The tone control in a passive EQ circuit is cut only: it does not boost any frequencies. So if a tone pot is "full on" that means it is passing the signal through with little or no cut. Basically, flat response. So a full-on passive tone control will equate to the bass and treble controls of an active cut/boost preamp being at center position, mid sweep.

To use the LHZ without bass and treble pots to just provide flat response, you'd have to replace each pot with two 50k resistors wired to the pot connectors on the preamp. This is necessary as the Bandaxall circuit (as designed) needs to see those two 100k potentiometer in the circuit.

Coming from a world of passive EQ to active EQ on my first Spector Rebop was confusing. I understand bass and treble controls, but the Spector had no center detent so I was never sure where the "flat" point was. It didn't help that the official documentation was vague and internet folklore said that the TonePump was a "boost only" EQ (it isn't). This is why I currently use bass and treble pots with a center detent so I know exactly where the flat response point is.

Sorry about the lengthy post... The short answer is: passive EQ at "full-on" = active EQ at center detent. But, I thought it might help to have some context as to why that is!
Click to expand...
Sorry I was driving when I made my reply.
I ment having the tone controls on full boost. To achieve a full boost shall I just connect the two post on the pre with a jumper wire where the tone pot would go?
 
#103
r_bnks said:
Sorry I was driving when I made my reply.
I ment having the tone controls on full boost. To achieve a full boost shall I just connect the two post on the pre with a jumper wire where the tone pot would go?
Click to expand...

My apologies, I misunderstood... Assuming you are asking about the LHZ preamp and have the installation diagram handy: you would connect a jumper on the PCB where pins 3 & 2 of the pot connect AND a 100k resister between where pins 2 & 1 of the pot connect. You'll need to do this for both the Bass and Treble pots.

If you have an LHZ you are doing the installation on, drop me a PM and I'd be happy to draw a custom diagram for you.
 
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#104
Awesome, thanks a bunch! I don't have it right now, but I'll order it soon.
Jumping the tone controls was what I needed to know before ordering, because one of my basses only have a single vol pot with a very tight cavity.
Always wanted to put a Haz clone in it but you'd need to enlarge the cavity in order to do it.
Since your Haz clone is very small, that's a game changer for me.
 
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#105
r_bnks said:
Awesome, thanks a bunch! I don't have it right now, but I'll order it soon.
Jumping the tone controls was what I needed to know before ordering, because one of my basses only have a single vol pot with a very tight cavity.
Always wanted to put a Haz clone in it but you'd need to enlarge the cavity in order to do it.
Since your Haz clone is very small, that's a game changer for me.
Click to expand...

Happy to help! Almost all the installation diagrams in the manual come directly from conversations just like this with folks who have unique installation requirements. Each time we solve a challenge, it is added to the manual so it's available for future users.

You can grab a PDF copy of the installation guide here: [Invalid or Expired Link Removed]

Another option to consider is putting miniature trimpots on the PCB. I've attached a picture of a Bourns trimmer for reference. The connection pads are a standard 0.1" (2.54mm) pitch, so this is super easy to do. This would give you the ability to dial in exactly the EQ you want, then close the cavity.

I've done this on older projects where I knew I wanted to permanently lock in a setting, but did not know exactly what the values should be. So I put the trim pots in and experimented until I got it just right... Then I measured the trimpot values (resistance between pins 1&2 and between pins 2&3) to determine what resistors I would replace the trimpot with.

So, you have options!
2023-07-20_142045.png
 
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#106
AgingEngineer said:
Happy to help! Almost all the installation diagrams in the manual come directly from conversations just like this with folks who have unique installation requirements. Each time we solve a challenge, it is added to the manual so it's available for future users.

You can grab a PDF copy of the installation guide here: [Invalid or Expired Link Removed]

Another option to consider is putting miniature trimpots on the PCB. I've attached a picture of a Bourns trimmer for reference. The connection pads are a standard 0.1" (2.54mm) pitch, so this is super easy to do. This would give you the ability to dial in exactly the EQ you want, then close the cavity.

I've done this on older projects where I knew I wanted to permanently lock in a setting, but did not know exactly what the values should be. So I put the trim pots in and experimented until I got it just right... Then I measured the trimpot values (resistance between pins 1&2 and between pins 2&3) to determine what resistors I would replace the trimpot with.

So, you have options!
View attachment 5131419
Click to expand...
Thanks a bunch, I can't wait to try it.
Absolutely, more challenges and ideas solved, the more flexible your pre will be.

Oh yeah the trimpot, It was my second guess and depending on the cavity size, I might use it if I can. This way the tone will be more flexible.
 
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#111
I am excited to share feedback from Spector-endorsed artist Lynn Sorensen who recently installed the LHZ-01 into his red Spector Euro LX (same bass used on the "Bad Company: Live at Wembley" album and video):

"Just installed your pre into my Euro LX 4, tried it at a gig I do every Wednesday night and it is simply amazing. It is the Haz Lab I've been after but always too late to get! I'm a dynamic player, and with my right hand it gives me 2 1/2 different bass guitars.

With the EQ dimed the warm overdrive that kicks in the low mid to low frequencies and the rattle of the high end are an aggressive, artistic joy to behold. You just don't get that with a TonePump.

Back off with the right hand and it suddenly becomes a very well-behaved former beast. All without touching a knob.

I'll be getting one for my LX 5 soon."​

Name and quotation used with permission.
 
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#112
No matter what kind of on board preamp I'd ever try to install in my NS4CRFM she would still output its unique CZ sound.
No matter what kind of preamp I'd try to put in my NS2-O baby she would properly still put out its magic Spector growl.
A few rare birds of USA made basses came with Aguilar OBP1 preamp, yet Spector growl embedded.
 
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#113
My first Spector is not even in my hands yet, but already thinking the TonePump might need to go, and this preamp seems like a great alternative.

Couple questions @AgingEngineer : How would this work with a sweepable mid control like EMG or John East? Have you tried it, or where would you put it in the circuit?

Also, when using a blend pot, have you tried moving the master volume after the tone section? Would there be any benefit or would it just cause impedance issues?

Sent you a PM as well
 
#114
HitHouse said:
Also, when using a blend pot, have you tried moving the master volume after the tone section? Would there be any benefit or would it just cause impedance issues?
Click to expand...
The output impedance is equal to 5.1 kOhm steady state at the output jack.
Once the vol.-pot was placed after the preamp then the output impedance (at the output jack) won't by consistent anymore.
 
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#115
@HitHouse: I would highly recommend spending a little time with the new Spector and TonePump before replacing it. A lot of people love the TonePump, so it might just be all you need. I'm a bit of a gearhead, so I have to force myself to adhere to a waiting period before I rip out the electronics of a new instrument. (Even so, I still end up doing brain surgery on every instrument I own even after the waiting period!)

I've not had anyone ask about that specific configuration (sweepable mid) before. The LHZ is intended to take either the direct output from pickups OR the output from Volume / Blend / Coil Tap circuitry. So if you are talking a standalone sweepable Mid, I would expect you'd wire the pickups to the volume, blend, and mid control first, with the final output going to the preamp.

There should be no problem having the master volume after the preamp. Or the sweepable mid, for that matter.
 
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#116
There are at least two benefits present that come with lowish (and also consistent) output impedance such as 5.1 kOhm.
Its possible to use very long instrument cables without suffering any sound issues due to capacitance of cable.
Its even possible to plug the instrument without any issues into input impedance 10kOhm such as is at line-in 1/4" input jack at a mixing console.

Of course once the instrument was exclusively plugged into Hi-Z inputs and instrument cable length may not exceed more than ~30 ft, though it was even possible to place the vol.-pot behind the preamp.
 
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#118
Thanks for all the comments! Agreed, I will spend some time with the Tonepump before trying to solve a problem that doesn't exist. The bass will be here tomorrow. As I said, I have zero experience with Spector's. I just know I've always been a fan of the tone. (i.e. Sound in my head comingled with confusion, ADD, OCD, and OLD).

I'm just used to having a lot of mid control onboard, plus vol/blend, thus the anticipation. And with the above brain conditions, it's hard for me not to anticipate (lol).

I was not thinking about replacing the Tonepump anyway, just the possibility of adding a stand-alone mid and swapping out the 2 volumes for a vol and blend. Problem is the Tonepump uses 10k and 100k eq pots, so that's not easily done with 4 holes. With the LHZ, it seems the treble and Bass could easily be stacked with a B100k/B100k concentric pot, freeing up a hole for a Mid/fre stack while keeping 2 holes for the vol and blend.
 
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#120
fibz said:
@HitHouse : which model is it by the way?
Click to expand...
Euro 4 LX 2017. I'm letting her settle in now from traveling. The truss rod needs a turn and the action is a little low for me. First impression through 6" monitors is - decent tone, al least with all the controls dimed. But then it's way too hot. Gonna go look for that trim pot while the neck is settling.
Euro 4 LX.jpg
 
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