Oh No...Another DIY Rack Preamp Thread

Discussion in 'Amps [BG]' started by Passinwind, Nov 17, 2009.


  1. Passinwind

    Passinwind Charlie Escher Supporting Member

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    This time I decided to build something very simple, meant to be somewhat along the lines of an Ampeg and/or G-K vibe. It kind of morphed into something much different, but so goes DIY building.

    I started with some readily available project circuit boards from ESP and Welborne Labs. The ESP one is Project 94, the Universal Preamp/Mixer. The power supply kit from Welborne is this one, spec'ed for +/- 15 volts. I already had a nice toroidal transformer that I salvaged from an old BGW crossover. I went with OPA2132 opamps for the first two stages, and LM4562 for the mix bus.

    I wanted a transformer-balanced output, and settled on the JT-11-DMCF from Jensen. The pots are from Bourns and Noble. I found some really nice stepped Noble 50K audio pots on eBay that were NOS surplus from the Hafler factory. Finally, the case came from Par-Metal, and is fully RFI shielded except for the front panel. Front Panel Express will be engraving graphics for me in the next few days.

    Initially this build was meant as a simple preamp/effects mixer with just bass and treble controls in the Baxandall configuration. Before I did the effects mixer part I tried the basic preamp out on a few gigs and had some other players use it at some open mikes too. It sounded really good -- actually better than my Fender-based design from a few years back, which I've happily gigged with a ton. So I decided to eschew the effects mixer and try to come up with a workable active midrange control scheme with the second half of the ESP board. I lifted ideas from Ampeg and G-K schematics and came up with my own take, using two opamp sections. I started at ~800 Hz for the center point, which has often been a keystone frequency for fretless bass IME. I considered a swept mid or two, but that is really something I want to do as completely my own take, at a later time. I already own a great sounding parametric EQ if I feel the need for that anyway.

    I gigged that configuration for around 6 months pretty contentedly. But at a few GTGs we noticed that the mid control didn't always do all that much with other guy's basses. So a few weeks ago I took another stab at this, and moved the nominal center point to around 620 Hz while I was at it. I should mention that all this time the design was essentially optimized for one cab, my fEarful 12/6, and also mainly for my EUB. Now we're cookin'!

    Not one to leave well enough alone, I dive into the effects mixer idea some more. I get it working pretty well, but start seeing some polarity/phase issues. After a few days of head scratching I realize that I have one unused opamp section sitting there, so I re-do the whole gain structure yet again and insert the extra opamp inline. Much to my surprise, noise and distortion specs both improved, and the effects mixer section works phenomenally well with my Hafler T-2 tube distortion widget and Boss GX-700 modeler. My reference mixer is a Crest XR-20, and I'm easily matching the results I get using that now. Being able to blend in just a tiny bit of modeled signal is pretty cool in my book, much nicer for my purposes than using an inline approach.

    So, here are a few pics of the build:

    [​IMG]

    [​IMG]

    As I said, graphics are currently in process.

    Measured specs and basic frequency response curves of the tone controls:

    [​IMG]

    [​IMG]

    Some of the more unusual tone control response curves:

    [​IMG]

    Any questions? :cool:
    maestrovert likes this.
  2. Implosion

    Implosion

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    I have a question. Why do I have a Master's degree in Mech Eng instead of EE? This is way more interesting.
  3. rpsands

    rpsands

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    Where do I buy one? :)

    Definitely in the market for a high output super light 1u preamp without too many derned features. No one seems to make a 1u version that's super shallow, lightweight, and low noise.

    It's ridiculous but the 10lb preamps I use are major contributors to the weight of my rack rigs :)
  4. vin*tone

    vin*tone

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    Wow. Amazing.

    Can you explain what you meant by "...to be somewhat along the lines of an Ampeg and/or G-K vibe"

    ?
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  6. UncleFluffy

    UncleFluffy Gold Supporting Member

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    Very interesting ... what sw/hw combination are you using for the measurement and analysis?
  7. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Sure. My last build was pretty much a straight-up solid state version of a Dual Showman preamp section. This time I started in my mind at B-15-meets-RB 800, hopefully wher it can cop either vibe. So, a simple passive bass/treble tone stack, but with one or two active mids added to the mix, and a seriously kicking brightness boost section. And the whole things drives a tube line driver with 6SL7s or 6SN7s if I want a little more "tubiness." The main idea was that I could boost mids if desired, not just cut only like the Fender stack does unless you add a few tweaks elsewhere in the circuit.
  8. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    Wow Charlie!
    that little baby is gorgeous! your work is absolutely pristine! nice job!
    .........and i'd be willing to bet that it sounds as good as it looks.
  9. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Rightmark for the S/N and distortion specs, True RTA 1/24th octave for the frequency graphs. Basic EMU 1820M sound card, which is becoming a limiting factor as I improve my builds. At this point I don't think I can really measure improvements too meaningfully, and of course I take all these homespun specs with a big grain of salt. Time to go visit a friend with good outboard test gear I guess...
  10. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    tests and specs can be very useful, but i wouldn't worry too much about them, since, IMO, your ears are the ultimate test anyway.
  11. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Yeah, absolutely. My basic MO is very much to go by the seat of my pants. I still haven't even drawn up the midrange circuit, but I'm guessing it may look goofy to an EE. I use those scary graphs and specs more for troubleshooting and developing proof of concept than anything else. They were a really big help in that regard this time too, and I learned some things that ultimately improved both of my other preamps.

    I'll definitely post some sound clips later this week. But again, I like it, and that's all that really matters when you're doing a DIY build, eh?:cool:
  12. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    yeah, that's what i use them for too, as sometimes its nice to actually SEE what's going on.

    it would be great to hear the clips when you have them.
    and yes, that is ALL that matters.
  13. greenboy

    greenboy

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    You think Charlie isn't looking at specs practically every step of the way? He'd better be if he's even bothering with this. That's how you improve things. You can't just go by subjectives and take it forward in every area. Right down to selecting components I'm sure he's not playing blind.

    Better S/N and dynamic range and control over distortion characteristics, EQ curves that don't make him later wonder what he was thinking... That doesn't come from just messing with luck-chosen components and then stopping there.

    And later if someone who also can interpret specs is given the chance, they have a very good idea whether it's an item they want to show further interest in. It's certainly not that persons' failing if they can make judgment on specs as well as listening.

    You think your beloved JBL engineers didn't measure every which way they could as part of the development process and to further understand what was going on in relation to what they were hearing?
    maestrovert likes this.
  14. fdeck

    fdeck Supporting Member

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    Subscribed. Very nice job!
  15. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    i remember when Curt Knoppel was trying to design and build the ultimate 'colorless' studio/hifi preamp back in 1975. he based it on the finest components and circuits that he could obtain, and when he finished building/tweaking it, he listened to it with some of his esoteric audio buddies. they all loved it. then, he subsequently went and measured its specs on some state of the art testing equipment for distortion, frequency response, noise etc. to his surprise, in the lab, it didn't measure up very well at all, especially in the distortion and phasing department, but he and his small group of friends, still loved it anyway. all of them agreed that it made recordings seem to sound more 'real' and dynamic, so instead of scrapping it and starting all over again, he figured out why it sounded so good, started replicating it, and started renting them to studio's as an 'aural exciter'.

    a good friend of mine, Fane Opperman, had a beautiful 24 track studio in Santa Cruz, CA back in the late 70's, had one of the very first prototypes made, and i must admit, it did sound really good, albeit 'colored'. we used it on a few of my bands tracks on our record at the time.

    so this another one of those stories about discovering/inventing something by mistake, rather than making a product that he set out to do in the first place.

    just a little anecdote that i thought i'd throw in.
    maestrovert likes this.
  16. R Baer

    R Baer Supporting Member

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    Nice work Charlie. Between your's and Fdeck's preamps, I am starting to think we have put entirely too many parts in ours! ;)
  17. fdeck

    fdeck Supporting Member

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    You'll have to use the Muntz Method, named after legendary California huckster Madman Muntz:

    Muntz developed a television chassis that produced an acceptable monochrome picture with 17 tubes. He often carried a pair of wire clippers, and when he thought that one of his employees was "over-engineering" a circuit, he would begin snipping components out until the picture or sound stopped working. At that point, he would tell the engineer "Well, I guess you have to put that last part back in" and walk away.

    http://en.wikipedia.org/wiki/Madman_Muntz

    :D
  18. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Something like that happened the other day with the tube line driver in my other current DIY thread. On a lark I tried just ignoring the distortion specs and optimizing for signal/noise instead, figuring .5% isn't all that much anyway in the grand scheme of bass reinforcement. Sounds much better that way IMHO, at least in this particular case. And yeah, there are a few "mistakes" in the preamp in this thread too, and I'm most likely going to leave it that way! :p

    Thanks for the kind words from my TB techie friends, it means a lot.
  19. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Nah, everyone knows that more knobs = more tone. ;)
  20. superbassman2000

    superbassman2000

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    Wow! that was exactly what i was looking for 3 years ago! I was looking for a simple preamp because i liked the sound of my bass and I used a bit of effects, so I never needed a graphic-eq or compressors or anything beyond the basics--I couldn't find anything so I gave up haha

    nice work!
  21. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    love 'yer attitude and design sense, not to mention the immaculate physical execution of your work. sometimes, a preamp or amplifier may not look 'perfect' on paper, but has a 'charm' to it that would be unachievable if it did, and to me, that's a big part of what custom builds are all about.

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