Uber Rare Triple Tube Hybrid Phaser In Action (JMJ)

[HideawayStudio]

There is gentle tube non-linearity to have but not wild overdrive. The tube B+ to all 6 triodes is around 200V (ie its not a low voltage "starved plate" design) and actually require a fair amount of signal to push them into distortion.

With the phaser enabled, if you push white noise into it and crank up the input gain control you get a very similar curious bubbling water effect as you get on the Small Stone which is quite cool. Most of these artefacts are from the OTA stages but the unit also runs with a tube in line even in bypass mode so it is possible to impart some more obvious tube overdrive if required. The inputs and outputs are balanced.

What was the goal of a high B+ for unity gain stages? More headroom, better linearity? I´ve seen on preamps as low as 12V. A single secondary powering up the plates and the heaters

The curves seem linear under 100V. Linear curves, what

Its not exactly quiet though - with so much regeneration rattling around three phaser blocks tapped in series/parallel its not really surprising. The tube stages are pretty quiet in comparison with regulated HT and heater rails.

Are the heaters supplies AC, or DC unregulated? I don´t notice any noise or hum. Sounds pretty clean to me!

The EQ sounds nice as well.

And the Chroma has poly aftertouch on top of it all! What a fine synth, all these years under the radar. Thanks for sharing.

It does indeed have regulated HT (around 180V) and regulated heater rails driving three JJ ECC83S (six triodes in total). The input and output are both balanced and on XLR and TRS. The last few built also had custom magnetically shielded torodial transformers.

The tube stages are actually pretty quiet - the main issue has always been the tortuous regeneration paths tend to exacerbate the hiss levels significantly when the regen is turned right up. As you can hear, the phaser has quite a sheen to it and the balance was not killing too much of the HF response for the sake of taming the hiss.

As ever with analog effects, its all about making sure the gain structure is sensible and the unit is receiving a healthily strong signal.

Regarding the Chroma - just bear in mind that Poly A/T was an option and not so many have been retrofitted with pressure sensors over the years. That said, I have to say I have been extremely impressed by the sheer level of technical support offered to and among the Chroma community via the Rhodes Chroma website and user group:

http://www.rhodeschroma.com/

I wish to heck more vintage instruments were supported with such a noble open community spirit!

....the Synclavier not being one of them!!!!

[Mooger5]

I (think) I see the point: 12V might be OK for consumer-level applications, but for pro audio signals it won´t be up to it. Plus at the input stage, the conversion from a balanced signal to single-end doubles the level up.
So 200V makes sure every detail of the music signal is processed effortlessly.

Interesting, the compromise regarding full spectrum vs. build-up noise. I guess it´s achieved by carefully selecting the cutoff point of an RC filter in the regeneration path? Is it of a fixed value or proportional i.e. the more feedback applied, the lower the cutoff?

Sorry for all the questions I´m dying to see a schematic which you aren´t obviously post, because clones...
It´s genuinely geeky curiosity.

So one tube for input stage and output (phase-splitter?) stage, plus four triodes for 360º rotation? I know very little and right now, as the phaser doesn´t seem to use interstage transformers I can´t think of another way to make an all-pass filter with a single triode, except using something like this:



Would the negative Vout tie to the ground of the tube buffer? Or the cathode??

Thanks for your patience!

[Mooger5]

Another thing: did you use "regular" OTAs for variable resistors? In the above picture it would go across R1? The voltage swing might be high but the current flowing through it would actually be very small? Would such a thing be feasible?

[HideawayStudio]

I (think) I see the point: 12V might be OK for consumer-level applications, but for pro audio signals it won´t be up to it. Plus at the input stage, the conversion from a balanced signal to single-end doubles the level up.
So 200V makes sure every detail of the music signal is processed effortlessly.

Interesting, the compromise regarding full spectrum vs. build-up noise. I guess it´s achieved by carefully selecting the cutoff point of an RC filter in the regeneration path? Is it of a fixed value or proportional i.e. the more feedback applied, the lower the cutoff?

Sorry for all the questions I´m dying to see a schematic which you aren´t obviously post, because clones...
It´s genuinely geeky curiosity.

So one tube for input stage and output (phase-splitter?) stage, plus four triodes for 360º rotation? I know very little and right now, as the phaser doesn´t seem to use interstage transformers I can´t think of another way to make an all-pass filter with a single triode, except using something like this:

Would the negative Vout tie to the ground of the tube buffer? Or the cathode??

Thanks for your patience!

Right up to the end of the tube era it was almost unheard of to see low voltage designs (ie. with B+ below 100V) with the exception of some mic preamps and miniature tube designs. The whole notion of running a tube at much lower voltages may have come about because of safety and cost implications in inexpensive designs rather than trying to achieve a sonic character. That said, it often amazes me how low a B+ many a tube design will run on. I see this all the time when I'm waking up vintage tube gear often for the first time in 30+ years whereby the heaters are run at 100% but the B+ is fed externally on a current limited HT bench supply which can run right down to less than 40 volts. Its not at all unusual for a mixer or a preamp or even a complete radio receiver to start running at 60 volts B+.

One area of common confusion about my phaser... The "Triple" bit doesn't refer to three OTA shifter blocks but in fact three complete 4-stage phase shifters (ie. 12 shifters in total) each with their own regeneration path (ie. like having three Small Stones in a rack).

The signal is fed into the first phaser then tube buffered and fed into the next, tube buffered and fed into the final phaser but the signal is also tapped off at each phaser and tube mixed so end up with a very complex signal path of:

Ph1 + (Ph1 + Ph2) + (Ph1 + Ph2 + Ph3)

Each tapping is effectively weighted as there is a gain drop through the chain. This was intentional to try to add some depth to the swirly effect when the phasers are run from the triple LFO at phase spreads beyond 0 degrees (eg. 120 degrees).

Very similarly to the regen (resonance) signal path in a VCF it quickly transpired during the prototyping phase that the regen path dramatically effects the character of the phaser and I played with all manner of equalisation in the regen paths. The reason why the phaser sounds so shimmery is because I eventually settled on a relatively dramatic filter rolloff in order to accentuate the HF and tone down the lower mids as I was having some issues with the design sounding rather muddy at first.

[HideawayStudio]

Another thing: did you use "regular" OTAs for variable resistors? In the above picture it would go across R1? The voltage swing might be high but the current flowing through it would actually be very small? Would such a thing be feasible?

The OTAs are indeed used as voltage controlled resistors in a very similar manner to many VCFs. The history behind this is that in the early days filters were simply hand cranked and if more poles were required then multiple pots were either ganged or geared together (the Bruel & Kjaer 2107 being a beautiful example!.. the filters on the EML-200 synth work this way too). Later designs were automated often by substituting the pots with LDRs or Vactrols. The infamous Gerd Shulte Compact Phasing A Krautrock Phaser worked this way by literally surrounding MES light bulbs with clusters of 4 LDRs. The lamps were then modulated which is kind of crazy to see inside those old phasers!

Eventually switching the LDRs for OTAs was a natural design progression.

However..... There is an historical twist to this that I find fascinating...

Possibly as early as the late 50s Hammond already had a phaser design which in block form (but not implementation) is really rather similar to all of the later phaser designs!.. does this mean everything else was derivative?... I don't know.. it would be interesting to know.. Its certainly not the first time Hammond were there before the rest of the pack.

The AO41 and AO47 Vibrato Phase Shift Amplifier chassis is something I've been curious about for years now. Last year I finally got one to play with and two things very quickly became apparent. Firstly it is a fascinating design - the VPSA also uses R/C phase shifters like the Small Stone but the controlled Rs (or Cs) aren't mechanical or Varicap based or diode/transistor conduction curve based or LDR based or OTA based.. it in fact uses a technique that hasn't really been around since the days of large CRT based color TVs (and some arc welders) called Variable Saturation Transformers. This is where an additional DC control winding is wrapped on the core of a transformer whose magnetic properties are such that its a little easier to push the transformer into saturation. When a signal is passed through the transformer it is possible to attenuate it by changing the inductive properties of the transformer depending on how much current is fed through the control winding.

When I experimented with the Hammond chassis on the bench I quickly found that they'd setup the unit primarily as a sort of weird vibrato effect but to my amazement, if I added a regen loop, the thing immediately started to sound like what we normally think of as a classic phaser!!

There is also at least one very desirable guitar amp that features a very similar circuit although its interesting that neither had been setup to produce that wooshy phaser sound which begs the question - who discovered that effect first and was it just possibly whilst noodling around with a much older design??

[Mooger5]

Fascinating story. I´ve read that phase shifters were originally intended to simulate Leslie rotary speakers. Which makes total sense. And that Laurens Hammond hated Leslies! He tolerated Leslies because they helped selling organs and because of pressure from musicians and company employees. Laurens must have found them too gimmicky and unrelated to church pipe organs, which the B3 was meant to replace. Plus it would go against the notion of portability!

More later.

[Mooger5]

You know, before permanent magnets were widely available, loudspeakers used field-coil magnets. They often powered the coils with the B+ so they´d serve as smoothing chokes at the same time.

I wonder now how an L/C LPF would sound. With an OTA and a triangle oscillator you get a "triangular response". The lamps in the Schulte Compact Phasing A had a curve of their own. The only let-down was the lamps faded out much faster than they faded in. The variable inductors may also respond in a non-linear fashion.

Was the next phaser the MXR Phase 90? Then Keith Barr might have been the first to employ positive feedback for a wooshier sound. But prior to that there was already the Moog filter with emphasis control, so the idea wasn´t exactly new.

[Mooger5]

And speaking of Keith Barr, he posted elsewhere his idea of the perfect reverb algorithm. It´s simply a looped chain where each link consists of a 180º all pass filter followed by a delay line. Like the simulation of a Leslie rotary it makes absolute sense. Now I wonder how a "Phaserb" would sound like

Cheers!