Alembic F2B power amp matching

[lprainard]

I just got my hands on an used F2B preamp. I have read that they are picky about the input impedance of the power amp. Alembic says that anything <10k ohms is not good. I am currently running mine with a Pevey IPR 1600 (only power amp I have at the moment) that has an input impedance of 7.5k ohms unbalanced and 15k balanced. So far it sounds good and I am able to drive the hell out of my power amp. So my question is: does this mismatch effect fidelity or do I just have to crank the volume on the preamp to make up for it? I like how the F2B sounds. The Fender tone stack comes with a bit of a learning curve. I realize the simple solution it to make a cable to go from unbalanced to balanced (- and ground tied together) thus getting to 15k ohms. I am just wondering if the mismatch is huge deal or not?

 

[SGD Lutherie]

No, it won't affect the tone, just the output.

Leave it unbalanced.

The way the Fender tone stacks work is flat is actually about bass=3, mid=10, treb=2.

 

[line6man]

What is the output impedance of the preamp that is driving the 7.5k input stage?

Are you sure you have your inequality sign the right way? You generally want the input impedance to be above some value rather than below. In this application, the only real disadvantage to an overly high input impedance is the increase in noise attributed to the extra resistance. If the input impedance is too low, however, it has the effect of loading down the signal.

 

[SGD Lutherie]

What is the output impedance of the preamp that is driving the 7.5k input stage?

Are you sure you have your inequality sign the right way? You generally want the input impedance to be above some value rather than below. In this application, the only real disadvantage to an overly high input impedance is the increase in noise attributed to the extra resistance. If the input impedance is too low, however, it has the effect of loading down the signal.

I was thinking the same thing.

That preamp should be able to drive any power amp made.

I went over to the Alembic website and it states:

[Invalid or Expired Link Removed]

The signal loss of the passive tone controls and the volume control is made up by the second stage, also a common-cathode vacuum tube stage. The second stage plate circuit feeds the output jack. Since the output impedance of the tube circuit is fairly high, the power amplifier(s) should be located adjacent to the F-2B so that short connecting cords can be used.

While the output circuitry of the F-2B can satisfactorily interface with the vast majority of power amplifiers, the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity.

The fact that it's working well shows it's working well.

 

[Passinwind]

What is the output impedance of the preamp that is driving the 7.5k input stage?

~80K ohms if only one channel is being used, since it comes more or less right off a 12AX7 plate with the standard Fender Rp value. The OP has good reason to ask his question, but in this case the IPR 1600 amp only needs .775 volts RMS to drive it fully, so he should be good to go.

 

[line6man]

~80K ohms if only one channel is being used, since it comes more or less right off a 12AX7 plate with the standard Fender Rp value. The OP has good reason to ask his question, but in this case the IPR 1600 amp only needs .775 volts RMS to drive it fully, so he should be good to go.

It would be fine if the numbers were the other way around, but 80k into 7.5k doesn't sound good. How about throwing in a buffer to get the impedances closer to what they should be?

 

[SGD Lutherie]

~80K ohms if only one channel is being used, since it comes more or less right off a 12AX7 plate with the standard Fender Rp value. The OP has good reason to ask his question, but in this case the IPR 1600 amp only needs .775 volts RMS to drive it fully, so he should be good to go.

The F2-B has an output impedence of 40k when run in stereo. Mono out is 90k. What will happen is the tube stage will be loaded down, and that would result in a loss of volume. The fact that he's able to drive the power amp shows it's sensitive enough to work with the high output impedance of the F2-B. But they should have used an output buffer.

My TubeWorks BlueTube suffers from this issue, because the output is off the tube. In some situations I have had to use another pedal to buffer the output from the BlueTube.

Glad I bought a Harke LH-500 instead of the F2B and a power amp!

 

[Passinwind]

It would be fine if the numbers were the other way around, but 80k into 7.5k doesn't sound good. How about throwing in a buffer to get the impedances closer to what they should be?

The F1X does just that, but with a so-so at best opamp. This guy did it right though, IMHO.

 

[SGD Lutherie]

The F1X does just that, but with a so-so at best opamp. This guy did it right though, IMHO.

That's a nice looking preamp. But you don't need balanced outs unless you plan on running a 50 foot cable between the preamp and power amp.

Balanced outs are to eliminate noise from feeble microphone outs after a long snake run, and then into the mic preamps.

But if you power amp has balanced ins it can't hurt.

 

[Passinwind]

The F2-B has an output impedence of 40k when run in stereo. Mono out is 90k.

Right you are. In this case stereo=using one individual channel, which most people tend to do most of the time IME, or going 2-in 2-out (true stereo) rather than summing the two channels to mono through the Alembic summing network. So not quite as bad as what I said above, but still far from ideal.

That's a nice looking preamp. But you don't need balanced outs unless you plan on running a 50 foot cable between the preamp and power amp.

Balanced outs are to eliminate noise from feeble microphone outs after a long snake run, and then into the mic preamps.

But if your power amp has balanced ins it can't hurt.

The balancing part is nice (6dB free gain makes me happy in those cases where it applies) and I always use it when available with line level gear, but the 15K:600 ohm transformers being driven by the White Cathode Follower circuits is what I was getting at.

I owned an F2B briefly and modified Channel 2 to allow more midrange adjustment -- boost even. Nice sounding piece, but I have way too many DIY rack preamps kicking around already that I like a little more for one reason or another. Selling the Alembic was a snap, lots of people still covet them.

 

[SGD Lutherie]

Right you are. In this case stereo=using one individual channel rather than summing to mono, as most people tend to do most of the time IME. So not quite as bad as what I said above, but still far from ideal.

I agree. Makes you wonder why they left it like that (and for all that money).

The balancing part is nice (6dB free gain makes me happy in those cases where it applies) and I always use it when available with line level gear, but the 15K:600 ohm transformers being driven by the White Cathode Follower circuits is what I was getting at.

I agree, that's a nice way to fix the problem, and transformer tend to warm up the tone.

 

[Passinwind]

I agree. Makes you wonder why they left it like that (and for all that money).



I agree, that's a nice way to fix the problem, and transformer tend to warm up the tone.

Yeah, I use output transformers on my solid state builds for that reason.

Have you seen the thread on my DIY tube preamp? I'm more of a solid state guy for bass, but I do like to change things up once in a while.

 

[SGD Lutherie]

Yeah, I use output transformers on my solid state builds for that reason.

You and Jonas Hellborg!

Have you seen the thread on my DIY tube preamp? I'm more of a solid state guy for bass, but I do like to change things up once in a while.

No, I haven't seen that. Looks cool. I'll have to take the time to read it after posting this.

I'm also a solid state guy for bass, even though I own a Mesa 400+, which I think is a great sounding amp. Just too heavy.

But since the early 90s I have been using a BlueTube pedal, with a 12AU7 tube replacing the 12AX7. It warms up the tone as you would expect. I used to use it with a GK-800RB, and for the past nine years or so I was using that in front of a Trace Elliot head. But the BlueTube hums, and the Trace was only 150 watts, albeit a loud 150 watts.

So I thought about doing the F2B and a power amp setup, but instead got the LH-500, and I couldn't be happier. It gets the tone I was getting from the last setup, and then some. It's got the class A 12AX7 preamp with the passive tone stack like with the F2B, and a hefty MOSFET power amp section.

 

[Passinwind]

You and Jonas Hellborg!

Nah, not the power amp part...

So I thought about doing the F2B and a power amp setup, but instead got the LH-500, and I couldn't be happier. It gets the tone I was getting from the last setup, and then some. It's got the class A 12AX7 preamp with the passive tone stack like with the F2B, and a hefty MOSFET power amp section.

I've played through one at a GTG, cool amp.

 

[lprainard]

Thanks for the advice. I am going to try running it unbalanced into balanced to get that 6db back. It also gets me within (just barely) Alembic's specs. I will also try running a buffered fx pedal in line. According to some folks online a >200k ohm input impedance would be ideal. I am nowhere close to that and most modern power amps are 10k unbalanced 20k balanced. My goal with this thing was to combine a classic preamp with some class d power. As it turns out it is not so simple. I got my F2B in trade, so i did't pay the stupid price that they are going for. I will let you know how this works out.

 

[Passinwind]

Thanks for the advice. I am going to try running it unbalanced into balanced to get that 6db back.

Sorry for the diversions and derails!

You can't readily get back that 6dB with a cabling change since the output was never active-balanced to begin with. A buffer will work fine, that's how I ran mine. If the buffer is active-balanced you may get a 6dB bump, but you may as well just use a clean boost of some sort at that point, if in fact you even need it.

 

[MPU]

I have a copy of single Alembic tube preamp. I made a cathode follower to match it with almost anything I plug it in. Now it works great. Before adding that cathode follower output was tinny and weak.
Marko

 

[bongomania]

That preamp should be able to drive any power amp made.

No, only ones with very low input sensitivity ratings. Fortunately, that is the case with his Peavey.

But you don't need balanced outs unless you plan on running a 50 foot cable between the preamp and power amp. Balanced outs are to eliminate noise from feeble microphone outs after a long snake run, and then into the mic preamps. But if you power amp has balanced ins it can't hurt.

The large majority of PA-type power amps have balanced inputs, and in most of those cases there is a 6 dB penalty if you connect an unbalanced preamp to them. So the "free 6 dB" that Passinwind mentioned is very important in the many cases where a preamp needs a little gain help for properly driving a power amp. This is an instance where balancing has a purpose and meaning unrelated to noise cancellation.

You can't readily get back that 6dB with a cabling change since the output was never active-balanced to begin with. A buffer will work fine, that's how I ran mine. If the buffer is active-balanced you may get a 6dB bump, but you may as well just use a clean boost of some sort at that point, if in fact you even need it.

All true! I've used both methods, either balancing the line or adding more gain on the unbalanced line, or both, depending on the specific needs of the power amp. I recommend this to anyone who has found their new preamp-poweramp setup doesn't sound as loud, ballsy, or deep as they hoped it would.

Again, the OP's Peavey probably has no problem being driven by the Alembic; I'm posting these clarifications for later readers who may want to use a different PA-style power amp.

Another angle: in today's market one of the best size-weight-power ratios in a power amp is to use a "micro head" amp and just bypass its preamp stage. For example I use a GK MB500 this way, as a power amp. A huge yet "hidden" benefit of this approach is that the FX loop return jack that is used as an input for this purpose, to connect your preamp, has a very low input sensitivity rating, and is unbalanced. So the head's power section is driven very well by low-output preamps! This is true of most integrated heads; maybe not all, but I haven't found an exception yet, apart from the Markbass heads where using the loop return doesn't bypass their preamp stage.

 

[lprainard]

Any suggestions for buffers? I have a few old boss pedals I am trying now. Some have said the MXR micro amp will work well. I would like to use a active DI but most of them rely on batteries and phantom power so that would be adding more components than I would like.