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Old 01-29-2013, 03:31 PM
rs_ rs_ is offline
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DIY Thread

I'm posting DIY stuff here so I don't clutter up the Pictures thread with it. Feel free to post your Homebrew/DIY projects here, too. I'm mostly just posting my findings here in case they help somebody else.

I modified my Soul Sonic mixer pretty heavily (added transformers galore, simplified the cueing section, made a new faceplate with fewer channels and bigger knobs) and realized that, for me, mods and clone-building were a stepping stone, ultimately, to designing my own mixer from scratch.

Arlo is my 6-month old son


It will be 4U high with most of the "performance" controls as 2" knobs. I find that this unusually-large size really does offer a precision and "feel" advantage. Ergonomics studies have shown this as well: "Optimum knob crowding" Bradley, James V. 1969 Knob spacing and Knob size are significant (but more expensive of course).

Originally I was going to go with transformer-based summing since it's only 3 channels, but apparently there are a lot of crosstalk issues and other weirdness that can crop up. A smart old guy said that there was a reason they stopped doing it this way. The lower dB loss of TX-based mixing isn't worth the tradeoffs. So, plain old resistor-based mixing it is.

The core of the mixer is the Jensen 990 Discrete Op-Amp, which is definitely worth Googling for the history lesson, if nothing else. It was basically the precursor to a modern "open-source" project, as its plans were released in 1979 by Jensen and anybody was encouraged to make one. I'm building (9) of them from the recently-released Hairball kits. The 990 is basically a small power amplifier; in terms of real power mixed with fidelity, it blows virtually any line-level output stage away. It easily matches the original Bozak output stages for instance. I won't say that it surpasses it, because that would be blasphemy.

I looked into the newest op-amps, a variety of passive and discrete designs, and even tube designs and the 990 won out. I am adding a pair of UREI 1620 output transformers to the Master Out though, just because I like their style and the "American Thing" they do to the sound.

The EQ/Isolator section is unique. It's built around some old White inductors I ran across. White, along with his friend Dr. Boner (haha), were the pioneers in installing room-correction EQ. Before commercial EQ's existed, they would wire a bunch of these "inductor cans", along with resistors and capacitors in certain values, to correct room sound issues in a completely passive (power-free) fashion:

I had (6) of the Inductors but I needed (8). After scouring the internet and not finding any, I learned that Boner and Associates had an office down the street from me in Austin, TX, in an old house just north of the UT Campus. (It was about a mile from my old house in Austin, actually).

Anyway, I ended up talking to the deceased Dr. Boner's partner, I told him what I was doing, and he said he had some inductors laying around that I could have!

The EQ/Isolator is entirely passive and cut-only (unpowered), relying on the 990's enormous makeup gain to restore any signal losses. It consists of a few 6dB/octave, LR, lowpass and highpass filters in parallel that are re-summed. Phase distortion and frequency errors are minimal, considering. The crossover points are adjustable without using any servos or other circuitry, as the old-school White Inductor values were carefully chosen to make such things easy to do with just potentiometers.

All the knobs are ALPS RK27's, of course. There are some Quad knobs used, for the Crossover selection and for the FX Wet-Dry. Power supply is an internal switching unit by Mean Well. I'm running the 990's at 24V, for greater gain and headroom. There aren't any Capacitors in the signal path; Telco-grade matching TXes are used for noise and DC rejection instead.

More as it comes. I'm awaiting the 4U case from Par-Metal and then I'm going to stuff the circuits inside it. I've found that the casework/mounting is actually much trickier than designing the circuits, at least for me. I still want to get really good at the metalwork part.


Last edited by rs_ : 01-29-2013 at 05:05 PM.
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Old 02-04-2013, 09:48 AM
charles0322 charles0322 is offline
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Keep us posted _rs! Looking good
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Old 02-04-2013, 01:53 PM
misterharrison misterharrison is offline
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Originally Posted by rs_
The EQ/Isolator section is unique. It's built around some old White inductors I ran across. White, along with his friend Dr. Boner (haha)...
I'm sorry but I couldn't concentrate after this bit. Brilliant. You couldn't make it up! I think you should call your mixer the "boner" mixer instead of Arlo.
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Old 02-04-2013, 03:23 PM
vinyl_junkie vinyl_junkie is offline
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This looks amazing, can't wait to see how it pans out :-)

Personally I wouldn't of put the master next to the input knobs and probs based the aesthetics a lot on the 1620 but it's your mixer, so as I said to Dolores Montenergo in "Calling All Quakers" Have it your way, Baby!

You thought about putting Alps RK-40's instead of 27's? Considering the quality of the rest of the mixer I'd go all out
--The future scares me, any chance of a lift back to the past any one?--

Last edited by vinyl_junkie : 02-04-2013 at 03:27 PM.
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Old 02-04-2013, 08:41 PM
rs_ rs_ is offline
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Thanks for the kind words.

misterharrison: I told my wife is was named "Arlo" and she is rather attached to the name now...can't change it...

vinyl_junkie: I actually prefer the feel of the RK27, having used and tested both extensively. The smaller size is also a slight advantage to me, as a beginner designer.

I added a VU Meter to the design, it sums L and R and is Master Out only. It's a Hairball 8037, which is a very nice meter with an integrated LED.

Unfortunate bit of news: apparently a fire at the Par-Metal factory has them closed until further notice. So, I'm fabricating the entire case (not just the panels) through Front Panel Express.

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Old 02-04-2013, 11:52 PM
rs_ rs_ is offline
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Some revisions to the layout. I carefully studied Urei 1620, Pioneer DJM-800, and Bozak mixer layouts, paying special attention to the way they make ergonomic cues. The DJ should be able to walk up to the mixer, and figure it out almost instantly.

Added 30-segment LED Meter from eBay seller "ledbargraph". No room for an analog VU.

I'm trying to pay special attention to "hand feel". Basically the area around the "business part" of the mixer (volumes, EQ knobs) is as sacrosanct as the area around a crossfader on a scratch mixer.


Last edited by rs_ : 02-05-2013 at 01:57 PM.
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Old 02-05-2013, 02:10 PM
rs_ rs_ is offline
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Details about the circuit: the Jensen 990 Discrete Opamp will put out 13.8 Volts at full bandwidth, into a 75 ohm load. (That's pretty incredible for a line driver circuit.) It is actually capable of a lot more output than that (it can drive a small speaker) but you start to lose bandwidth.

Between the summing circuitry, the Passive EQ/Isolator section, and the Wet/Dry Effects Summing, I'm losing about 33dB. So, there's a 990 summing amp stage to make that up, then that's fed into another 990 stage for the output, which I'll probably set at about 33dB again and it will effectively do 36dB gain including the Urei 1620 output transformer.

The Booth Output is another 990 pair, and the mic preamp is a 990 at 45dB gain, plus a Cinemag transformer I had laying around. The headphone output is another 990 pair.

Looking for advice on "Quasi-PTP" vs actually making a PCB for the circuit. I've had good luck with PTP in the past, but I'm concerned about long-term reliability. Open to opinions on this.

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Old 02-07-2013, 10:46 PM
rs_ rs_ is offline
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Getting close to final version of layout. Experimenting with color as an orientation mechanism.

It's really important to be sure that all the parts internally clear the edges, and each other. Scouring datasheets for exact dimensions is probably my least favorite part of the process.

Finalizing PCB design as well. I'd always avoided it but it's actually easier than designing a front panel, I think. I'm using BatchPCB in the end -- they specialize in short-run orders. I'm actually able to make most of the mixer circuitry out of repeats of the same board with different resistor values and a few chassis-mount transformers. That's a testament to the great versatility of the Jensen 990 Opamp.

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