Improving upon the Alembic preamp - diyAudio
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Old 26th January 2013, 12:34 AM   #1
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Default Improving upon the Alembic preamp

Hello everyone. I have had an old alembic F2-B preamp for many years now. I have always enjoyed its sound. I have decided to build a clone, but want to implement a few changes.
The original circuit is pretty primitive. (as it has been simply cut out of a fender amp) But what if a slightly more engineered approach was taken with its design? What if this circuit was revved-up a little bit to resemble something like what you would find in a recording studio?

So here is my question for all you audiophiles:
What modifications and add-on would you recommend to further improve the sound?
What about the use of input and/or output buffers? (possibly JFET?)
What about plate voltages? Higher? Lower?
How about changing how the 12AX7 is biased?
Anything else? Open for any suggestions

Pleases provide some schematics if possible. Thanks for all you input!
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Old 26th January 2013, 06:58 AM   #2
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Hi, the schematic seems, is from guitar amplifier /not for Hi-Fi/.
You can change 12AX7 to 12AT7 for better sounding. Two plate resistors must be about 20-30 kohm, two cathode res. - about 400-600 ohm. You can cut potentiometers for treble, bass and med. and connect cap 0.33-0.47 uF between first plate and volume potentiometer, cut resistor 120 pF for bright.
Connect serial to grid of second triod resistor about 1 kohm. Every current plate better be about 4-5 ma, U plates about 160-180 v, adjusting with plates and cathodes resistors. +U PS - about 250-280 v. This is my aproximate advise, You can adjust schematic trying all changes, that can be less or more. /Sorry for bad English!/.
Regards!
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Old 26th January 2013, 01:44 PM   #3
DF96 is offline DF96  England
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Should be in Instruments & Amps section?
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Old 26th January 2013, 02:26 PM   #4
Enzo is offline Enzo  United States
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Indeed, it is not a hifi preamp, nor is it intended to be. Basic, not primitive. It does its job, you say you have enjoyed its sound for years. It already is the sort of thing found in a recording studio.

So the question: how would YOU define "improving" the sound of this unit? We need a goal before we come up with a plan to achieve it.

I think azazello is suggesting to remove the tone shaping circuits and lower the gain. But I also think he is trying to convert this guitar preamp into a hifi preamp.

I would suggest that most changes would turn this unit into something else than it is. If that is the goal, fine, but if you value the character of this sound, you don't want to stray far.

SO decide what it is the thing does not do to your satisfaction, and what you wish it could do but does not now. Then we will have a goal. I don't know that buffers add much, the input impedance is already high. Your cathode bypass caps are already large enough to cover the freq range of your bass guitar. You could fiddle with the resistor values in the plate leads for gain, and the cathode resistors for bias, but towards what? You could do something trendy like replace a 1.5k cathode resistor with an LED or othe diode, but I have no reason to think it an improvement
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Old 28th January 2013, 05:27 PM   #5
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Love the FB2, I've had a few.

OK, each of the two Alembic channels, like most middle-era Fender guitar preamps, doesn't have a lot of gain, and has two channels without any channel-swithcing, which is a pretty useless waste of hardware.

But its real beauty is when you replace the second channel inputs with a push-pull knob switch so that you can run one channel or both in series. An appropriate small relay is even better, with a small LED to indicate series status.

I don't find a parallel-channel mode (like y-cording into both channel inputs) very useful, but some people use both channels in parallel in order to get extreme EQ like an extreme mid-scoop with high & low boost by setting one channel to an extreme bright & treble with no mid or bass, and the other channel with extreme bass, and mixing both to the mono output. But I've never found such Marshall tricks necessary with more versatile tone controls.

Jerry Garica first turned me onto running one channel's output into the other channel's input in series, via a cable from the back channel 1 output to the front channel 2 input, then channel 2 output into a power amp (he used a McIntosh 2300 with a volume control, a nice choice at the time).

Some random notes, especially clues for running both channels in series:
- Both channels in series is not only great for more gain and overdrive. Assuming you have volume controls on each channel and added another volume between channels, and a master volume on the power amp or between preamp and power amp, you can run each stage very conservatively and get the most wonderful BRIGHT clean tone, and truly remarkable treble response. Much brighter than just one channel, and still low noise.
- The network between channel 1 input jack and the first grid is really critical, and has to match your guitar. Various grid stopper resistors, grid leak resistors, caps from grid to cathode or anode, impedance resistor from grid to ground...these all have a truly profound effect on the way it loads the guitar pickups and the frequency resonse of the preamp. Anything you lose in the first stage is gone for good. Some Alembic preamps had a note with the pink-sheet schematic explaining the jack switching and how plugging into channel 1 jack 1 loaded your guitar pickups differently. Later it seemed Alembic forgot about their own best feature. But the resistor between jack and first grid can be much lower if your guitar pickups have less output. It really should be adjustable via a pot within some small range limited by resistors. Hitting that first grid harder works wonders for low-output strats without Lace sensors, Travis Bean jazzy lower-output humbuckers, etc. or for a biting mesa-boogie or Texas tone with hotter pickups. The resistor to ground and any caps you add can be good things to limit bandwidth and eliminate any squeals, but the Alembic circuit is not very prone to such problems until you mess with it.
- There's a pretty creepy mixer resistor network for mixing channel 1 and channel 2 for a mono output yet allowing seperate outputs for each channel. This just doesn't work very well unless you have a stereo guitar and want a mono output with two identical channels and no channel switching; but that's aomost useless waste of hardware unless you have a stereo guitar and mono amp. Eliminate the mixing network and just have two seperate outputs, or at least provide a switch or switching jack to disconnect that mixing network between channels when operating the channels in series or in stereo. Otherwise when you put the channels in series that 'mono-out' network feeds a little of the channel 2 output into its own input at extreme gain.
- At extreme gain levels with the channels in series you can get quite a bit of extreme compression and then some grid-blocking. There are actually some musical applications for this "problem" but you can do all kinds of things to minimize the recovery time after grid blocking, but take care not to limit the high-frequency response or you will lose some of the magical capability of the Alembic.
- At extreme gain levels with channels in series you will get some power supply motor-boating. You don't need much power, so there's no excuse not to use a choke in that power supply ladder filter network, except they don't fit into the 1-rack-unit case. Use smaller caps earlier in the filter ladder and bigger caps later. You can consider adding sag/stiff resistors, or a switchable regulator, for the second channel.
- Make sure you use low-noise resistors, especially around the first stage.
- Key to the Alembic feel is those heavy allen-bradley pots, and orange-drop caps.
- AC heaters are the old-school Fender type. Either raise the heater AC 12 V. with a DC bias floating around 1/10 of B+ or convert to DC heaters. Stiffening up a DC heater supply with another choke isn't ridiculous, as it prevents capacitive coupling from one stage's cathode to the heater supply and then to another cathode. Some people favor one small choke and cap for each tube. Personally I'm building a 2-channel Alembic as two monoblocks with seperate power supplies for each channel.
- Some people like grid chokes instead of the grid leak resistor. I haven't tried it except on power sections. It gives a stiffer bias yet high-impedance to the audio signal, and low noise.
- Some people like anod chokes to the B+ instead of load resistors. Gives a good B+ supply yet high-impedance to the audio signal, and low noise.
- A real low-noise 7025 as the first tube gives you lower noise the way Fender intended.
- Two channels in series with two sets of tone controls is really required IMHO for being able to "tune" your preamp for many different sounds. Done right, it can allow you real flexibility. For extreme clean, you can just turn off one channel. For extreme bright, you use both in series with just a little gain in each, and the bright switch on the first channel. For a little grit, just turn it up a bit and use the second channel treble control to determine how much bite. For extreme gain it's critical to have seperate tone control over just the distortion: the way this is accomplished is via pre-emphasis, distortion generation, and final de-emphasis. For instance, for a creamy distortion you use the bright switch and lots of treble on the first channel, turn up the volume on the first channel quite a bit, then turn down the treble in the second channel; the guitar sound got its treble boosted & cut but the distortion generated between channels only got its treble cut, so with a bit of conscious adjustment you have the ability to control the tone of the distortion independently from the tone of the basic guitar.
- The bright switch toggle on the first channel should be two mini toggles, with different value caps on each. This is just a cap across the volume control. With two, you can turn on one, the other, or both, and alter the knee frequency where the treble-boost comes in.
- The second channel doesn't really need a bright switch.
- A switch to bypass the tone control network on the first channel is very useful. It gives you a lot more gain and some reduction in noise. Putting it on a footswitch is another Mesa trick. You still have the second channel in series for tone controls if you need them.
- Multiple effects loops between various stages are really useful. For instance, dynamic range is greatest after the first stage with raw tone (before any tone control network). That's a great place for the envelope-following of a volume-to-voltage converter for a subsequent voltage-controlled filter. But much later, like around the final master volume, is a better place for applying any filter. So if you want a responsive trigger-wah or auto-fitler you should hack into the unit and feed the envelope-following circuit from a jack after the first triode stage before even tone controls, but have the auto-wah apply its fitlering to the sound much later in the chain after all compression, distortion, and tone. Reverb, tremolo, echo belong very late in the chain, on either side of the master volume. Many distortion pedals work better right after the first stage instead of at guitar level, though some FET boosters work well before the preamp on the guitar line. The point is that between the first stage and channel 1 tone controls is sometimes a really good place for an effects loop, as well as between the channels, and around the master volume.
- A Peavey Valverb goes very nicely with an Alembic preamp.
- A resistor dropping network and pot to a Jensen BM series or direct-box series output transformer is an expensive but incredibly flexible way to drive a low-impedance line to a board or to the low-impdance input of a power amp. If you don't go all-out on the transformer or don't need any long line, then don't bother with a crappy direct-out. WELCOME TO JENSEN TRANSFORMERS, INC. Call them for advice. Their output transformers have a nice wide operating range so you're not tweeking the line-out on-stage to optimize VU every time you change the volume knob on your guitar or amp.

I'm building my Alembic clone as dual monoblock preamps into two large footswitch boxes with a true bypass switch on each. That keeps the cable runs from the guitar short, and gives plenty of drive to the lines to the backline amps. Very low noise and I can use cheaper cords to the power amp. It also allows me to use stomp boxes on the preamp output and effects loops with short cables. The 'spacial environment' effects like reverb, echo etc. can be either place, at my feet or at the amp.

Last edited by cyclecamper; 28th January 2013 at 05:30 PM.
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Old 29th January 2013, 04:59 PM   #6
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Any FET input buffer or for cleaner high gain can easily be implemented (or purchased) either in a stomp-box or in the same chassis. The first stage is really all about 'clean' and not losing any treble, and an FET can do that perfectly well, still keeping the sound of the tube stages for distortion generation. This is something I'm just learning, to tolerate FETs in a tube-sound amp. The emotive aspect of playing across the clean/dirty line can be enhanced by improving the clean as well as adding more stages of dirty. We need some new 'scientific' measurable names for describing that clean/dirty line, how thick that transition-line area is, and the amount of difference between clean and dirty.

Last edited by cyclecamper; 29th January 2013 at 05:14 PM.
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