All about Tubes, Tube Circuits, Tube Gear

Wednesday, November 26, 2014

Ashly PQX-572 Dual Parametric 7 band EQ

This is a simple, practical Parametric EQ design,
which is very flexible, and replaces/increases the options
on any input channel of your mixer via insert.

It can also be very effectively used for tone shaping of
PA systems via inserts on the Main Outputs of your mix.

One example use would be to dedicate one "channel"
for a mono stage-monitor/foldback mix, and the other
"channel" for the Mains FOH (Front of House) Mono mix.

It is good enough quality to also apply to a simple mixdown,
for testing or tone-shaping or corrective EQ.

On some units for sale used, you will see damaged front knobs.
The front knobs stick out a significant distance from the panel,
making them susceptable to damage, unless precautions are taken. A metal railing or recessed mounting is advisable.

The rear XLR / TRS in/out as well as screw-strips make
hookup very flexible.

I suspect that some units get damaged in live gigs due to abuse, rather than poor construction.

TOA E-1231 Series 1000, 31 band Graphic EQ

Two things commend these 31 band EQs:

(1)  They were manufactured in Japan, where quality control standards are very high.

This results in quality performance of electronic circuits,
and also reliability over time.


(2)  These particular units have full size, high quality slider-pots,
with a professional quality feel usually found only on high end stereos. 

They are very smooth and have a nice resistance to the touch,
and hold their positions very accurately.   The Knobs themselves
are also well designed for visibility and durability.
All three units I have purchased have all their knobs and
exhibit no damage from 'roading'.


The one drawback is a minor one.  These units come
by default without input or output transformers,
and are expected to be hard-wired into permanent systems,
such as church and public address PA systems, and so
TOA has left it as an option to order them with transformers
installed.   Thus all my units came without transformers.

In fact, Graphic 31 band EQs are best used next to your mixer board when doing live gigs, so transformers are not necessary, since the EQs can be inserted at the board,
and the unit shouldn't need to drive long balanced lines.

In case one does want to install them say between monitor sends and the stage, and still keep them next to the board,
one will probably want either output transformers (only),
or have input transformers on your power amps. 

The real issue is that they are by default only provided with
terminal strips, which means you must either install
XLR / TRS jacks, or make up short cable adapters.

This is not a real problem as there is plenty of space inside
the units and on the back panel for additional jacks.

I installed TRS jacks on two of my units to make them
easy to plug in and remove, as per the photos.

Friday, September 19, 2014

How to Wrap Cable without Damaging it

 There is an important trick that nobody is explaining but is paramount if you are hand winding cables on the elbow.

People do the elbow wind for speed.

With a rope, it doesn't matter, or at least it isn't likely to damage the rope for a long time,
and you can untangle it.

With a cable, you need to

Pro Method: (fast)

(1) automatically "roll" the cable in your fingers with your winding hand,

(2) do it by feel, in such a way that you can have zero 'twist' tension
in the direction of the roll of wire on your arm.

(3) keep shaking out the unwound end, so that the curl,
which usually BUILDS, as you ROLL the wire during the wind,
gets relieved by the unwound portion spinning to undo the twist you're adding.

This technique is critically important,
so practice it until you are sure you really know what you're doing.
And practice on one shitty cable of no value,
before you wind up your whole set of cables after a gig.

Here is my assistant demonstrating the method:

B. SAFE Method: (slower)

If you don't have a really strong mechanical sense of
what is going on with the cable, and you can't feel the
twist of the cable in your fingers,
DON'T use the elbow-winding technique.

Instead do it the slow way,
(1) Holding the loops at the top,

(2) pulling up a loop, and twisting the whole roll of loops
from above, in the direction that relieves the tension,

(3) seating the new loop on top of the others, always same side up.

Here is my assistant demonstrating the method:


 Fixing Cables:

You can also restore a cable sometimes, depending upon how the cable has been twisted.

What appears in the OP photo is that the outer protective rubber has been pulled away from the inner wires and fiber,
and also that the inner wire package has been twisted in relation to the outer housing.

In this case, sometimes the cable can be easily fixed (if not too severe),
by a combination of stretching lengths of the cable by hand,
and shaking down the section to cause the inner cluster of wires
to return to their natural positioning (twist-wise).

Also, pulling on the outer housing can also relieve the built-up stress
in any cable housing, and stroking the cable through the hands
in the appropriate direction.
You pull on the outer housing, with a heavy slow friction,
and this will relieve the tension, twist, or bunching that
has built up inside the cable between the outer rubber cover,
and the inner wire cluster and reinforcement rope/housing.

With MICROPHONE cables you have to try these techniques very gently,
since you can "unwind" the shielding, and cause the cable to lose its
electromagnetic field and RF protection, becoming a 'noisey' cable.

Good cables use a braided shield, which can't be damaged by unwinding.

CHEAP mic cables use a simpler wound "wrap" which is easily mucked up.

You can check the type of cable by pulling back an end and looking at the shielding
to see if its braided. If it isn't, sell the cable and by a better one.

Wednesday, September 17, 2014

Yorkville Sound TL352 PA Cabinet

 Yorkville Sound TL352 PA Cabinet.

I have been working with PAs off and on for decades,
starting way back in the 80s with Cerwin Vega bins, and JBL horns.

I have always kept an eye on Yorkville Sound's products,
as they are a local manufacturer here in Toronto, and are
famous for their Trainor Guitar amps, used by rock players
for years, and now coveted tube gear.

They have now been in the PA supplying business for quite a while,
and have been a staple supplier of PA gear through
Long & McQuade Music Stores across Canada.

This meant their gear had to survive the inevitable torture
and abuse of both touring and rentals. As a result,
Yorkville Sound have taken that experience to heart,
and produced some of the most rugged and durable,
and even near-bulletproof PA gear around.

I finally purchased a couple of PA cabinets from their "Pulse Series" line, which is a medium cost build.

The first thing I did was pull the horn to see what kind of
thing was going on. I was astounded; inside as advertised
was a Celestion 100 watt compression driver, but most
importantly, was the way they had the driver bolted into
a 1" thick plywood framework to make it immovable and
immune to the cabinet dropping 10 feet.

(most cheaper cabinets have drivers screwed onto horns
which support the weight of the driver, up to 7 llbs.
When a cabinet like that is dropped, the driver can be sheared
off of the horn, or the ABS plastic horn can be simply snapped off.)

Next, I pulled the woofer, and was very impressed with this
massive custom Eminence Driver rated at 400 watts, with giant
black enameled steel plates and cooling vent.

The biggest surprise was the huge heavy-duty crossover,
which actually stunned me. I have never seen a bigger,
heavier, tougher-made crossover than the monster bolted
into the bottom of this cabinet.

Even with 40 years on an electronic tech bench,
I have never even seen the gigantic (50-100watt?) resistors
they used here. (Of course I've seen power resistors,
in Hydro gear, but not in audio equipment before.)

The coils were similarly massive, and plainly made
very credible their claim of a 400 watt power handling capability.

If you ever rent one of these cabinets, take a peek inside;
I think your jaw will drop when you compare this tank
to average PA cabinets.

I know they have had a longstanding policy of building tough,
but this cabinet surpised me even though I was prepared
to see a good build.

As to the sound:

They have chosen a design in this series with crossover points
between 2000 Hz and 2100 Hz
. This results in a very smooth
upper mid between say 2000 and 6000 Hz.
Other manufacturers crossover much higher, to protect
cheaper horn drivers from overload and burnouts, but
making the 15" woofer try to cover between 3000 and 5000 Hz
usually results in wonky uneven frequency response, as
the inductance of the speaker kicks in and the piston surface
begins to "break up" (flex and develop standing waves).

By crossing so low, they let the horn handle this critical range.
The horn is then protected from overload by various circuits
such as the 'lamp bypass' and varistors etc.
This PA bin is similarly protected with an automatically resetting
circuit incase of feedback or overload.

In a previous set of different cabinets, I replaced the crossover
with a 4th order LR at 3.5 kHz, and got a very acceptable sound. I had chosen the higher frequency to protect the horn compression driver also.

I don't think the Yorkville Sound PA box has such a high order crossover, but they have approached the problem differently,
with a lower frequency and protection circuits.
The result is a very smooth sound in which the woofer is
relieved of having to provide higher frequency coverage.

The birch plywood box is built like a tank, and I expect that
this unit can take a lot of abuse, and is the type that you'd
rather maintain or repair than replace.

Hats off to whoever is in charge of their quality control and
design specs, as they have produced something like the highest quality product one could hope for, with top notch components and drivers, and a supremely heavy-duty build.

What are the drawbacks?

Only one I can see: These cabinets are heavy,
and although not that large, two will fill the backseat of a car.
You will need a pickup to move an entire PA built around these cabinets. And they are alot easier to lift with two people.

I would have liked to have seen metal handles,
but the recessed ABS-style handles seem well-made.

The cabinet is covered with a tough plastic/fibreglass type coating that seems very scuff resistant (unlike rugs and paint), and looks like it will hold up visually over time.

I thought I was going to swap out the crossover,
but once I saw the monster that was in there, I decided
to leave it as is.

Very pleased with this purchase, so far, and
I have to say picking up a used pair at Long & McQuade
was the bargain of the century.
Not only a great product, but L&M gave me a warrantee as well!!!

I'm going to road these locally and I consider this
one of the most robust components of my rigs.


Sound - Great.

Build - Awesome.

Drivers - Top Quality.

Crossover - powerhouse.

Box - a tank.

Handles - (minor worry: wish they were metal)

Finish - durable.

Grill - metal curved grill, non-existant rattle issues.

Connectors - Speakon / Phone.

My score: 95% of my wishlist met.

Friday, July 11, 2014

RCA Golden Age Microphone Preamps BA-71B

 What equipment did Elvis and Harry Belafonte use to get those sweet sounds?
Well, RCA equipment back in the day (1950-1970).

There were the now famous 44 and 77 Ribbon microphones,
and of course the RCA consoles, with their modular microphone preamps,
first off the OP7 (tube gear) and finally, in the 60s and 70s, the first transistor preamps, like the BA-71B.

The care in design and pioneering skills of the early RCA engineers became legendary, alongside the artists that helped make recording equipment coveted.

Being a bit of a packrat, I had picked these up surplus and squirrelled them away to restore and use them. 

RCA BA-71-B Microphone Preamps! 

I replaced the wiring and removed the connector sockets, in order to hook them up to XLR plugs in the early 1990s to test them as outboard units.   My boss at the time didn't think they amounted to much, as people back then didn't think of these as 'vintage' but just 'recently old'.   Now however, interest in vintage gear has both revived and expanded to include a wide array of audio gear.

 I've decided to finally follow through and restore at least one for myself, and possibly sell the others.  These units are now fetching interest at a much higher price-point than what I first got them at.

Here are some examples of prices (at least asked) on Ebay for both unrestored ('As Is') and fully restored and enhanced versions:

So it doesn't seem unreasonable for me to ask about $275 each for mine,
'As Is'.

Friday, July 4, 2014

Fixing Up a Behringer 31 Band EQ

On Amazon, under the listing for the FBQ3102 31-Band Stereo Graphic Equalizer, you may find the following notes in the review section:

"This equalizer works fine and is easy to use. But I am uncomfortable with the high heat it generates. It gets VERY hot on the back panel around the sub output. It gets so hot in that area, it hurts to keep a finger there. This is very strange as the heat is generated even when the EQ it not wired to anything but the A/C power. It is not on top of warm equipment either. I have observed this high-heat behavior on two units. Two different calls to Behringer yielded two results. The first time it was suggested that I exchange the device, which I did. The second time I was told the heat was O.K. Weee, multiple choice from customer support."  (By Dr. James Williamson on July 15, 2010)"

In fact, one will find (on Musician's Friend, Amazon, and other sites) many mixed reviews with this and other similar products (low end home-studio, inexpensive semi-pro equipment).

These units (many brands and models) all seem to fail frequently in partial functions or even completely, after heavy use or leaving them on for extended periods.

This can almost always be traced to HEAT PROBLEMS as a result mainly of cutting corners and assuming that consumers will not run the units 24/7 or for more than an hour or so at a time.

The manufacturers typically use smaller heat-sinks (or no heat-sinks at all),
or they skip on fans, or provide poor ventilation to allow heat to escape at
an adequate heat-transfer rate for continuous use (i.e., extended on-times).

On the one hand, you will read great reviews praising the performance of the products, and on the other hand you will also read seemingly contradictory horror stories of how these products fail after only moderate extended use, often just outside the warranty time limits!.

This is precisely what is to be expected.

Luckily there are solutions.

If you're trying to save a few dollars with these inexpensive units,
and since the warranty will rarely work in your favour anyway,
you can take a bit of risk voiding the warranty and instead modify
the unit so that it at least has a chance of functioning normally
and without problems under extended or continuous use.

This usually means providing better ventilation and heat-transfer,
and even adding fan-cooling or larger heat-sinks or metal components.

Here's what we did to deal with the heat problem on our 31 Band EQ:

First, we opened up the unit to discover a lot of empty space in there,
and one seemingly adequate, but unfortunately sealed, heat-sink
in the input area at the top, which accounts for the high heat you can
feel on the top of the left side of the case when its powered on for any
length of time.

I was initially thinking I would add a heat-sink to the outside, so as
not to deface the unit, by gluing it on.    But inspecting the inside showed
me that this would not really work.  The chip doing the heating was not
screwed to the case as I anticipated, but had its own heat-sink already,
which was not touching the case at all!

This is doubly disturbing, since the case was incredibly hot in spite of
not being connected directly to the heat-sink.

Obviously the real problem was no ventilation, not no heat-sink.

Heat-sinks can't operate properly unless there is some air-flow that can
transfer heat continuously to the outside without build-up.

So I took a 'nibbler-tool' and cut a square hole right above the heat-sink,
and also one on the bottom of the unit about the same size,
so that air can flow naturally by convection up through the bottom,
over the heat-sink fins, and exit the unit at the top.


Also of concern if you are 'roading' the unit is to make sure things you do can't introduce more problems: I used some Locktite on the nuts holding the screen in place above, just to make sure that they didn't work loose and start bouncing around inside the case, shorting out electrical components. You can also just use a bit of paint or glue, to make sure screws don't come loose with jiggling over time.

To prevent injury, I also filed the sharp edges of the hole I cut with a file,
and then 'painted' the edges with a permanent magic-marker to remove
any unsightliness.


This seems to have solved the heat problem, but keep in mind that
these window-vents might get blocked still, if the unit is mounted tight against
other equipment in a rack.


There is also plenty of space on the back of the unit to cut a hole,
instead of the bottom, if that is better in your situation. (e.g., if the bottom hole will be blocked otherwise.)  I cut the bottom because it would be less visible,
and I had planned to tilt the unit in a special rack.

Another way to protect EQ units is to use them properly.

Here you can see I have left EQ centered and flat except for
subtractive notches to take out room-boom and feedback points.
No frequencies are boosted, because that is better done with wider Q
tone controls on the channels and mains, i.e., a parametric EQ.

The very ends of bass and treble can be rolled off too,
to protect bass speakers and remove harshness from horns.

EQ should be used in an insert on the mixer board so that what it
is doing to the sound can be monitored at the board with headphones,
for both Front of House (main FOH) or monitors (e.g., stage monitors with
a separate EQ channel).