Read Custom Bass Purity preamp- the inside scoop

Hi All!

I was told that there was some talk about the Bass Purity preamp so I
thought I'd pop my head in and give you all some details. I'll try to reply
to responses, but I don't get to visit TalkBass all that often (much to my
dismay). I try to keep myself shackled to my workbench whenever possible.

Here's a little history of the Bass Purity for those who really have too
much time on their hands. The rest of you can skip right over this... The
development of the Bass Purity has taken place over the last year or so
(with many years of electronic design and amp work behind it). The very
first prototype is still in existence, although rather mauled by
modification on top of modification. It was pretty cool, though. It was as
close to a straight-wire preamp as I could get. The bass signal went through
two stages of tube gain and then out to the power amp. Nothing too exciting
there, but for kicks I had two sets of tubes: one 12AX7 and one 6SL7, with a
toggle switch to select one, the other, or cascade both for an interesting
effect. Lots of folks tried it out and really liked the sound, but said "Where's the eq?", so
I added an Alembic/Kern/Fender tweed style eq section. I was not happy with
that at all. I changed it to more of an audio style eq, but wasn't happy
with that either. I eventually developed my own eq design, which is a 4 band
eq (bass, low mid, high mid, treble) along with a unique mid emphasis
control. There are also deep and bright switches which do a very good job of
helping to shape the overall tone without ever mucking it up. My primary
design goal was to keep the bass signal Pure, yet offer tonal flexibility.
The eq system took a long time to develop and is certainly the most complex
part of the design. Speaking of complexity, I spent far more time
eliminating components from the signal path than I did designing them in. The first "real" unit was a 2 space box finished in Shoreline Gold with
white chicken head knobs. Why? Partly because I had the paint and the knobs laying
here. Don't ask me for pictures of that one! After that was built I had some
custom toroidal power transformers made so I could get this all down to one
rack space. I also incorporated an effects loop, a DI level control,
transformer coupling for the output signals, and an EQ Bypass switch. The
latter was something I had in mind from the very beginning and is an
indispensable feature. I strived to make the preamp as open and natural
sounding as possible; capture some of that tube magic (warmth, depth, and a
certain three dimensional quality that I have never found in solid state
designs); and maintain quick response and clarity without any harshness. I
think I can probably walk a tight rope now, though I've never tried. I
use very high grade components in every part of the signal path wherever it
makes a difference. I build high end tube audio gear now and then and much of the Bass Purity design was influenced by that. I don't
use any printed circuit boards (not that there's anything wrong with them if
they are designed properly). Components are mounted on a 1/8"
epoxy/fiberglass board and are soldered to turrets (not eyelets). Everything
is point-to-point wired. Tubes are 12AX7 and 12AU7.

The final result will show here if I can figure out how to post a picture.
Hey, I'm a TalkBass newbie, what can I say!

Some of the features of the final design I've already mentioned. I've
included an impedance selector switch for the input. I find that most basses
sound great using the high impedance setting (equivalent to the "passive"
input on some amps), although there is a slight difference in tone with some
active basses if you use the low impedance (aka "active input"). The setting
to use is the one that sounds best, really.

I'm also offering a version of the Bass Purity with a 6SL7 preamp tube as a
custom order. It's pretty cool- a slightly softer, more rounded sound. If
someone wants a full blown custom preamp I can build a 2 rack space unit
that has *both* sets of preamp tubes and can be switched back and forth
between the two, along with lots of other neat tricks. I'm afraid it would
not be cheap, though. I was interested to read that the Alessandro preamp
uses 6SL7s. I've seen that he's fond of them for his guitar amps. I tried 6SL7s,
6SN7s, and a host of others, but I liked the 6SL7 the best of all the
non-12AX7 variants that I tested.

The Bass Purity will be up on the Read Custom website as soon as I can get
around to it. Full production starts in a couple of months, and right now
I'm building each one by hand.

Now, to clear up any possible misconceptions:

-I did not reverse engineer from alien space craft, and I am not related to
Corso. As far as you know.
-I didn't invent the EQ bypass, by any means! Lots of amps have it, and many
high end audio preamps have it.
-Plugging a bass directly into a power amp with no preamp works, but
generally not all that well. The voltage swing at the output of your bass is
not enough to drive the power amp to its full potential, and the low input
impedance of your power amp will probably suck some of the high end out of
your bass (especially if it's a passive bass). You might get away with it using an active bass depending upon the impedance match/mismatch.
-A few folks tried out some of the later prototypes; the real thing is a big
step up.

I hope this didn't bore anyone to tears, and I hope this isn't the longest
post to ever appear in this forum! Sorry for yammering on like this.

Cheers,

Jack Read- Read Custom Instruments, Inc.
Custom Guitars & Basses
Custom Tube Amps & Tube Preamps
Pay us a virtual visit: http://www.readcustom.com
(978) 779-0075
Also offering Stewart Audio power amps and more.

Read Custom is featured in the new book, American Basses, by Jim Roberts
(Backbeat Books, http://www.backbeatbooks.com).

 

Hi,

No wall warts!

A 230V/50Hz version is imminent. The toroidal transformers are on order and are due within weeks- there are a couple of folks in Europe eagerly awaiting them.

Thanks,

Jack

 

Anything you like, really. "Standard" is JJ/Tesla. I have found them to offer consistent quality and excellent tone. I have NOS tubes as well (5751, RCA black plates, Telefunkens... the usual suspects). Some of the NOS tubes are horrendously expensive (like the Telefunkens). I test each set of tubes befor the preamp leaves here and make sure they are low noise and no microphonics.

Some of the best current production preamp tubes include JJ/Tesla, Ei, and "winged C" (former Svetlana factory... *not* the tubes currently being sold under the Svetlana name).

Cheers,

Jack

 

Sorry for talking over the PA system, but here goes:

Fran, I got your email about the 230V/50Hz preamp but I can't answer it- my replies get bounced. Do you have another email account, or can you turn off whatever filter is causing the bounced messages?

Thanks,

Jack

 

A: The emphasis knob works by boosting a broad range of middle frequneces along a unique response curve. This is an excellent tool for cutting through a mix, or for making tweaks to your eq without losing "your sound". As you turn the emphasis knob clockwise you can hear your bass cut through more. Flat is full counterclockwise. It can still be used when the eq is bypassed.

A: Incomiiiiinnnngggg!

A: Absolutely. I'd be happy to offer it as an option. I could even employ a two-button footswtich so you could control the mute as well.

Cheers,

Jack

 

I hope a picture of the back of the preamp appears here... If it doesn't, can someone tell me how to do it?

Thanks,

Jack