Ju-Jutsu: Ultimate Monoblock (Pt 8) Early Builds

To give a sense of what the physical build was like for this topology, I'm posting some photos of the original prototype for the studio-model we used at the 64 track studio as monitors.

This set of rack-mount monoblocks served also as the breadboard for the finalization of design specs
(and yes, we tried almost every topology including SRPP+ styles, and modifications on the taps, before settling on the Mu-Follower configuration, because of sound quality, the only object in this build)

The Redline Pro Rack-mount Series, serial #002L



We chose rack-mount to integrate these power amps into the studio environment.
This entailed a separately housed powersupply, and heavy-duty cabling to protect sound techs.

The case allowed a quick-hinge to access tubes and do testing.
The amps were fully functional with the cases closed,
but the power tubes (sockets) must be oriented such that grid-sag cannot occur
when running full-tilt. Later versions of the amp reverted to upright positions,
because questions were raised regarding future reliability and standardization of available tubes.



most of the wiring-harnesses below in the prototype were taken in and out multiple times,
so we weren't too concerned about final layout, but only easy point-testing etc.

The configuration shown (6550 / 6L6GC / 12AX7A) was an implementation
which delivered the right amount of power to compete with the MacKintosh's,
which at the time were the hands-down industry standard for mixdown.
Our amp blew the Mackintosh away in both power and clarity, quite a feat at the time,
although we weren't sure just how good our design was, until we A/B'ed it against the Bryston SS power amps, considered the cleanest high-power amps available commercially in the 90s.

here in the side-view, you can see the simple layout for the tube circuits,
consisting of perf-board units mounted close to the tubes.
It is important to realise that the two 12AX7s here are each running at different heater-voltage offsets, about 300 volts apart, and so each partial mini-board is a different set of components (i.e., amplifier and CCS top).



Another view of the simple layout for the power-stage is shown below:



The reason you see some resistors end to end (besides for connecting),
is that standard resistors are only rated for about 300 volts, but in this circuit,
there could be transients 5 times that high. In key positions, after voltage-testing,
we sometimes made up a value by series, doubling the voltage rating, and power handling.

The signal caps were gooped to prevent vibrations in the signal-path.

The inside back panel held both the OT and the input circuitry, as well as a DC cooling fan.
This was added because in the studio environment, the amps were in a separate sealed room,
mounted alongside other devices (amps, board PSs for the Neves etc.).
The noise of the fan was small, and did not intrude into the circuit which was nearly impervious to hum or noise pickup.



The use of the Jensen input transformer allowed a choice of balanced and unbalanced inputs,
allowing flexibility in application in studio situations.



The back of the panel shows the triple-insulated PS cabling, also protected by heat-shrink and cut-resistant plastic fibre tubing, which provides safety in the event of potential cable damage from door slamming, pulling, sharp edges in the rack etc.

All in all, these amps performed well above the Mackintoshes that they replaced,
creating a stunning staging clarity which was very useful for both recording and mixdown sessions.
Most of the engineers were shocked by the imaging,
which however was not always easy to reproduce on cheaper commercial units
which consumers were expected to be playing back the mixes on.

This forced us to use a variety of mixdown tests, so that we were not misled by the great sound,
as to what a mixdown was going to sound like in a real-world home or club environment.
For those purposes, mixdowns were also tested on Cerwin-Vega club gear, and typical home stereo units.
The idea of a 'near-field' standard was not yet solidified in the big studios at that time.

Ju-Jutsu: Ultimate Monoblock (Pt 7) The Amp

Now we can see the Ju-Jutsu Amp (SE Stereo) in its true light and simplicity.
It is simply a pure two-stage amplifier, in real linear operation,
with a pristine DC supply, and no current in the OT.
   

 


I've stuck in some LEDs to bias the first stage, as some people might want to try it.

When the Mu-Follower is understood as it really is,
a sophisticated power-supply regulator circuit combined with an ideal load,
we can see plainly how the amp achieves near-perfect distortion-free operation.

The only two caps in the signal path are in the PS chassis.

Ju-Jutsu: Ultimate Monoblock (Pt 6) PSRR

 Lets rearrange the Top Tubes in our amp, and put them on a separate Power-Supply Chassis, with the HV B+.




For SE operation, we have to do a little extra isolating of HV power supply to the top tubes,
using additional Cap/Resistor legs. (top of diagram).

Also, we install the signal caps on the PS chassis, so that at least two of the connecting lines
have no High Voltage DC (this is blocked by the caps.
Even the two lines with DC only have half the full B+ voltage,
since it is divided ('dropped') across the Top Tubes.

You can use ordinary 600 volt insulation wire (shielded) between PS and Amp Chassis.
Make sure you orient any connectors safely (males on receiving circuits, females on power sources).

Although there are now two tubes (a 12AX7 and a 5998 for instance) on the PS chassis,
it really isn't any more than you'd find on almost any regulated HV powersupply.
What has happened is that we have simply made more intelligent use of similar tubes,
with much more spectacular results.
Why have a mere 'regulated' supply, when you can have a 'feedback-controlled' CSS supply?
Don't forget to add a good heavy gauge ground line between the two chassis.

"Nazaroo, I'm not seeing what is the advantage of relocating the upper tubes of the Mu-Follower to the P.S. chasiss. Please clarify. Thanks."

 

(1) It helps isolate the real dangerous high voltage and keep it contained in the powersupply.

(2) Teaching purposes, so that people can understand the true meaning of the Mu-Follower.

(3) The separate heater supply for the top tubes (with HV offset) is also isolated.

The heater supply is perhaps the most dangerous, because even seasoned technicians
forget that it can have lethal HV riding on it.

Ju-Jutsu: Ultimate Monoblock (Pt 5) SE versions

Here's a circuit diagram of the Stereo SE version of my Ju-Jutsu Topology:



The cost difference involves swapping out the input tranny for another output tranny,
and buying one more RCA jack.

The power supply can remain the same.

Now you can all build SE versions at half the cost!
If you like the sound, but want more power, build a second unit.

A 5-tube stereo SE 10 watt amp of unsurpassed purity,
for under $100 plus trannys.

Let the rich cry the blues.
You can listen to their songs with better performance
than they are getting with their $10,000 rigs.

Ju-Jutsu: Ultimate Monoblock (Pt 3) First Stage

Okay lets look at the input / stage 1.



Overview:

What is going on here?

First of all, I chose balanced stages all the way through the amp.
We have to get to balanced anyway, for a Push-Pull at the output stage,
so at some point I'd need an interstage transformer or a splitter, with all those problems:
added stages, another layer of distortion etc.
Well I learned a lesson working with professional studio gear, which is balanced all the way from the microphone.
It works best, and gives the highest noise immunity and lowest distortion.
All the advantages of the Push-Pull (harmonic cancellation, Powersupply noise cancellation)
are now transferred to the whole amp.
If the Mu-Follower wasn't clean enough, its now bullet-proof.

What you are looking at is two Mu-followers, one for each side of a balanced input.
One is drawn upside down for symmetry and beauty.
The input transformer is a Jensen.
Simply put they make the best input transformers you can buy.
We use them in everything in a studio environment, for isolation, balance, and frequency-response, noise immunity, and low distortion.
They are a cost-effective solution and replace an interstage transformer, which,
to get the same specs would cost a fortune, and they don't make them like this anyway.
Jensen can make near-perfect transformers because of the signal amplitude and size.
Nuff said. No one in their right mind would prefer an interstage to an input transformer.

That leaves the resistor values.
The input stage here could be any tube, but lets pick a 12AX7A
just to keep it easy to get good quality cheaply available parts.

Well, we'd like about 250 volts across each tube.
We'd like to bias them according to my little method.
But people have designed plenty of 12AX7 Mu-Followers,
and you can use tested circuits that are known to deliver high fidelity.

Some things never change.
The input grid resistor for each triode should be 500k to 1Meg R.
This provides an appropriate input impedance for the transformer,
and a path to ground to (self-)bias the grid.
The cathode resistor should be about 1k R to give a volt or two of self-bias.
It's positive voltage at the top (due to the idle current) makes the grid relatively negative.
Input tubes should pretty much always be self-biased unless you have a good reason not to.
If you are using a 6922 or some other more sensitive tube, don't forget a grid-stopper (shown).
Otherwise you risk hidden parasitic oscillation and poor performance.

You'll notice I have a three-resistor network to ground for the two bottom tubes (one upside-down).
That's Hafler's minimum IM distortion trick, discussed in the literature.
You can just as easily use a couple of LED diodes to ground on the bottom tubes (NOT the top ones!!!),
as some people are doing these days (they weren't available in Hafler's day).

The top tubes are set up similarly, but the extra resistor on the cathode is another parasitics-stopper (horizontal, optional).
Again a 500K resistor (or 1 Meg R) is a grid-leak or input resistor (and a horizontal grid stopper if needed: try 1k).

The top resistor is not really a load resistor, its again a 'parasitics stopper' resistor. (500 R would do, or even skip it).
The top tube is operating as a cathode follower.

The input cap for the top tubes are .22 uF, but you could use a .1 uF, its not critical.
What's left? The input is grounded to the chassis- via a star-ground at the bottom of the input tubes.
This is for shielding, but it can be skipped, and you could just use an XLR input for a balanced line input.
You could even eliminate the input transformer entirely if you have balanced lines.

Any questions? No, then we can move on to the next (last) stage.

Ju-Jutsu: The Ultimate Monoblock (Pt 2)

Here is the circuit for the WWII version of my amp, one of the first I built.
You can see why it is the cleanest of sounds:

Only two stages. Less than most preamps!

Two transformers.

3 caps.

It is an elegant solution to sonic purity.

Here are some notes:

I used Jensen transformers for the input;
any good output transformer will do.

It requires two heater supplies, one for the upper tubes, one for the lower.
Either float the upper or DC offset it from the powersupply at around 300 volts.

It requires two power supplies, 400-500 for the 1st stage, 700-800 for the 2nd.
The PSRR is so good, you only need simple, modest PS circuits.
You can run the 1st stage off a resistor/cap leg from the main B+.

The top tube was a 5998 in the power section (V3 A, V3 B).
You can also try a couple of 6L6s or 6V6s, in triode-mode.
The bottoms were a 1625s. 807s are equivalents (triode mode with the usual precautions)
6SN7s or 12AX7s will do in the first stage.

 At one point, the man who reviewed the amps was still in shock,
so he set up a series of A/B tests, and drove me and the amps all over Ontario.

At one particular location, a fellow had spent several thousand in parts,
for an elaborate SE amp,he had designed using very expensive transformers, silver wire etc., and some $10,000 pair of speakers.

He played me his rig for about an hour.
I had to admit it was beautiful, rich and warm, quite an accomplishment.
I said nothing, and neither did the reviewer.
We set up my monoblocks quietly, then spent the next 3 hours listening to everything he could throw at them.
It really sounded like Diana Krall et al, were in the room, saxes, accoustic basses and all.

He said the monoblocks were incredible.
I felt kind of sad that my amps had defeated his greatest efforts and expense.
We parted friends.

In a similar A/B shootout with Bryston's best and purest solid-state offerings,
we blew their amps out of the water.

It was a thrilling experience, never to be repeated.
I felt dishonest in a way, because I knew a key point of my innovation was based on the work of others.

Here's a pic of the pair that went on the A/B challenge: