Harrison Labs - TWG - DSP Ambience Processor

[dw154515]

For those of you who haven't chimed in to my previous threads...

I recently acquired a 1975 Allen 632 MOS-1
http://www.organforum.com/forums/sho...is-Allen-organ

It needs some help in terms of reverb - badly! I have a ton of experience with outboard "rack" gear and DSP processors, etc. with my work in electronic music, recording/production, etc. so I know there are some very high-end units out there. However, I do love the convenience of the Harrison Labs units - they can be mounted inside the console easily, are relatively inexpensive and have a small footprint.

I know I've read on other threads that these units yield some very satisfactory results but I just want to make sure that these Harrison Labs products are as easy to install as I'm thinking they are (or "should be").

After reading these forums and reviewing the Harrison website (which took hours...) I ordered 2x TWG's, 2x DSP Ambience Processors, and 2x Pedal Isolators.

Harrison Labs seem to be an honest, straight-shooting, polite American business but their website is AWFUL and I do not fully understand their "manuals." I see the flow diagram, and I assume that's the easiest thing to follow when doing the install?

My question is this:
Does anyone have any tips, tricks, warnings, advice for this total newbie to Allen organs? Or is it just a simple "hook up and go?"

[don60]

The only non-standard aspect of the hookup is Allen's odd implementation of the expression pedals. As you probably know, each pedal contains a light source and one or more light-dependent resistors. The resistors, one per channel, are connected through capacitors to the organ's amplifier inputs. As the LDRs vary their resistance, they shunt varying amounts of signal through the capacitors to ground; the capacitor values are chosen to give a frequency-dependent expression that mimicks the action of real swell shutters.

This Rube Goldberg arrangement depends on the signal being supplied to the amplifier inputs from high-impedance sources so that the LDRs form the lower legs of voltage dividers. Normally, the signal source for the amplifiers is the digital-to-analog converter outputs from the DAC board. But when you install an HLabs device such as a reverb unit, it goes between the DAC and the amplifiers, rendering the expression ineffective because the HLabs component has a low-impedance output. The "isolators" that they sell for way too much money are nothing more than series resistors in little cans with RCA connectors. By plugging these isolators in between the HLabs outputs and the amplifier inputs, you restore the high-impedance drive and make the expression work properly.

Don

[circa1949]

Don...I think "Rube Goldberg" is a subtle and unintentional slight. That term implies an extremely over-complicated mechanism that accomplishes a trivial, trifling task, that could have otherwise been completed more simply. Allen's MOS/MADC expression system seems 1) highly reliable 2) immune from static/interference 3) easily adjustable and 4) not dependent on many and/or expensive components. (As you know, your more advanced ADC full-cage organ has an entire board of OPAMPs to handle expression.)

BTW - I do thank you for helping though the BS from this company. How much are these small boxes with a 79 cent resistor? The fact they are apparently poorly supported (according to the OP) only adds to the skepticism many here have expressed about them for several years now. The whole way the "traveling wave generators" were marketed was a joke...if something can magically improve the sound of MOS organs, post a couple before/after youtube videos. They are free to post. There are SO many used, high quality reverbs units for sale on ebay these days as small project studios go 100% virtual...why bother with something that is probably a rebadged, lesser reverb unit that's been marked up 10X?

[dw154515]

Well, aside from the very detailed and "over my head" information, do you have any first-hand experience with these devices?

[don60]

Well, aside from the very detailed and "over my head" information . . .

Well, you did ask! As you may gather, a number of electrical engineers frequent these pages and always feel free to expound on the details of the various makers' circuitry.

Circa, your point is well taken about my Rube Goldberg comment. On reflection, I would say that Allen's scheme has at least a hint of elegance in it. The fault I find with it is the non-conventional impedance level coming out of the DACs that requires work-arounds such as the "isolators" whenver additional equipment is connected. You are correct that this system looks very appealing compared with the analog-to-digital-to-analog scheme in the ADCs.

Never having dealt with HLabs, I will reserve judgment on their products. Since I do teach technical writing, however, I will state that their Web site stinks--it is long on puffery and short on substantitve technical data.

Don

[dw154515]

Yes, I agree with you about their website - it's atrocious. I know nothing "technical" about what you and circa1949 are discussing I'm certainly willing to learn, though. My initial concern was this; If their products are designed and implemented anything like their website or their "manuals," I'm in for a severe disappointment. On the contrary, however, I've read good things about their products here on the forums.

Thank you for the "over my head" information, by the way. I truly wish I understood it all better.

[circa1949]

"with the analog-to-digital-to-analog scheme in the ADCs"

When I once got a close look at an AP expression control board from an MDS organ, I noted it had many, many opamps, but only 7 DACs, no ADCs. My thinking was a binary word of expression was somehow created at a small PCB near the expression shoe, and then the digital signal incoming from that or from the MDS integrated MIDI would get converted by one of the 7 DACS into a sort of leveling voltage for the Opamp configuration, raising or lowering the volume by selective shunting more or less of the signal to ground. (Nowadays there are actually ICs called "digital pots" that accomplish this function with a minimum of components) To re-quantize using the 8 bit PMI chips would have resulted in hideously grainy sound quality...it's amazing the ADC organs sound as clean as they do even with one DAC08 output stage. It is possible it was different on the full cage ADC, but I really think they would have avoided an unnecessary set of conversions.

[don60]

I'm sure that the expression is implemented in analog form on the large ADC organs.

The signals are converted from digital to analog on their respective tone generator boards and passed through the backplane wiring to the audio processor (AP) board. The expression pedal positions (as sensed by the light-dependent resistors) are converted to digital form on the console multiplex board and passed in the bit streams to the card cages. The expression bits are decoded in a DAC (not the infamous DAC08 chip, however) for each channel and applied to some analog op-amp circuit (technically known as a "black magic" modulator) that adjusts the amplitude of the corresponding signal.

I am kidding about the black magic modulator of course--I simply have not had the interest or time to dissect the circuit; I can imagine several simple approaches that would work adequately. Circa, you are correct in suggesting that the expression was not done digitally because this approach would have required an additional digital-to-analog conversion with its attendant noise.

One might wonder why expression was not implemented at the tone generator boards when the signals were already in digital form. I am guessing that applying expression there would have required fast and expensive multiplier circuits as opposed to the simple analog modulators that were actually used.

Allen was all about keeping as much functionality as possible in the memory chips and as little as possible in the surrounding hardware since processing power was very limited. Truly, there is no "central processor unit" at all in these so-called computer organs, and by most experts' definition they are not really digital computers.

Don

[circa1949]

One might wonder why expression was not implemented at the tone generator boards when the signals were already in digital form. I am guessing that applying expression there would have required fast and expensive multiplier circuits as opposed to the simple analog modulators that were actually used.

Well, one obvious thing is that they didn't have many bits to work with, period. If you digitally "turn down" a sample already quantized at 8 bits, you get one quantized at 4 bits, which is going to sound atrocious. As the great Walter Greenwood (ADC system designer) pointed out on the yahoo board, some quiet ADC voices like the Flauto Dolce sounded terrible as it was. The push to 20 and 24 bit in pro audio production was all about being able to record quiet nuances. In fact in the late 1990s when there was a digital mixing console "arms race", manufacturers kept trying to outdo each with how many virtual bits their busses were quantizing to mix and level many audio signals. IIRC the 1 million dollar Sony Oxford consoles boasted of 56 bit floating! (these behemoths are now desupported and literally worth a couple pennies on the dollar...a cautionary tale about digital audio technology)

The expression bits are decoded in a DAC (not the infamous DAC08 chip, however)
Yep, they probably found a cheaper non-audio rated one! The 16 bit DAC used in the MDS organs had a unit price of something like $35 in 1989 when those models started coming out, meaning for a large 3 manual organ just those chips were close to $1000 of the manufacturing price.

[don60]

One might wonder why expression was not implemented at the tone generator boards when the signals were already in digital form. I am guessing that applying expression there would have required fast and expensive multiplier circuits as opposed to the simple analog modulators that were actually used.

Well, one obvious thing is that they didn't have many bits to work with, period. If you digitally "turn down" a sample already quantized at 8 bits, you get one quantized at 4 bits, which is going to sound atrocious. As the great Walter Greenwood (ADC system designer) pointed out on the yahoo board, some quiet ADC voices like the Flauto Dolce sounded terrible as it was.

Excellent point--that is undoubtedly the primary motivation for the design. I imagine that the attack/release envelopes in the ADC were implemented in analog for the same reason. Interestingly, though, the original Allen/Rockwell patents describe a purely digital envelope generation--apparently after actually hearing the terrible results of such a scheme, the engineers rethought the process.