F2-B Output Impedance to Input of Poweramp

[slawie]

Hello Folks,  
 
The F2-B has an output impedence of 40k in stereo (how I wish to run it) and my  
power amp has an input impedance of 20k balanced (preferred) or 10k unbalanced.  
 
Ron,  
 
If I change F2-B plate resistors R21 and R23 both currently 100k to both 33k value this should alter the  
output impedance to 21.596858638743456 which would be  
slightly over the input impedance requirement of the power amp.  
 
Or maybe it would be best to change the plate resistors to 27k thereby altering the output impedance to 18.854748603351958 which is slightly under the power amp input requirement.  
 
The changes as is with triode preamp would the gain be thrown out with the  
cathode bypass capacitor value coming into play and would I be required to change that also?  
 
Are my calculations and logic correct or am I missing something or over thinking the whole thing?  
 
Please help.  
 
slawie
 
 
(Message edited by slawie on June 25, 2010)

[edwin]

Is there a problem you are noticing? In running my F2B into a Crest CA9, which has an unbalanced input impedance of >10k (how much greater is not specified), I have noticed no ill effects.
 
Even with my McIntosh 2105, with an input impedance of 200 ohms, I haven't really heard anything that sounded like a problem.
 
If you are thinking about adjusting the F2B impedance to go lower than the input impedance, you might consider using a transformer of decent quality. You could even use one that will give you a balanced signal. I think this would be a better solution than trying to adjust the output impedance of the tube section. It could throw things out of whack, but I am speaking outside of my expertise here.
 
What amp are you using?

[sonicus]

I have been speculating on the possibility of adding balanced line outs on an  F-2B as well.  As far has high quality transformers to do the job I think that you will find non finer then Lundahl  Transformers.  At a recent AES convention (Audio Engineering Society) I had the pleasure to meet
Per Lundahl in person and he is very helpful and knowledgeable.  I think that before this mod you might want to consult  with Alembic personnel on such a mod for any suggestions.   I have used  2 F-1X units in a situation where 2 balanced outs were useful.
Here is the link for Lundahl Transformers;       http://www.lundahl.se/
 
             Sonic Regards_________

[sonicus]

Slawie,
               I have used a  stock unmodified  F-2B with many different power amps and have had no problems.

[slawie]

I am getting, for my birthday, a Phonic XP2000 power amp as per the below link.
 
http://www.phonic.com/en/xp-2000-en.html
 
The unit comes in a roadcase with space for the F2-B (bonus).
 
My question, in as much, is theoretical as I have not had the opportunity to rig up.
The speakers driven by this will be bought/found at some stage in the future.
 
My concern was that my loved ones would go out and buy something that was not suited for purpose
or incompatible with the gear I am collecting and already have.
My limited audio engineering knowledge reminded me of gain (db) attenuation at variable frequencies through the diffference between preamp output impedance and amplifier input impedance. Particular in this instance the lower freqs (that would be a good band name lower freqs)
 
I really want to be sure that every bit of my bass was being sent out through the signal path.
The proof of the pudding of course would be of plugging it all together and hearing the end result.
 
It is tough being type A with OCD!
 
Edwin your suggestion seems to be reinforced by Sonicus. That must be that great minds think alike.
 
Sonicus I'll will get to work on designing an amp-amp to acheive the balanced line out mod it will be shared here unless you already have one.
 
Most likely I am, as I said in my initial email  overthinking the whole thing.
In a perfect world a jumpered capable resistor bank add-on board so that impedance matching to suit
various manufacturers input requirements could (theoretically) be a value added upgrade/retrofit.
 
Thanks guys I knew I could count on you for advice.
 
slawie

[sonicus]

Slawie,  
            All I have so far is the Lundahl Transformer contact. I am still torn between acting on this mod and leaving my very sweet little blue case F-2B stock .(metal knob)
I ended up using  2  discrete F-1X units  with my  Series II in a studio project  experiment for direct outs rather then modify my F-2B.  I share with you  my brother also  being Type A with OCD.  
       Sonic Regards ___

[edwin]

Well, guys, I would say that unless you are dealing with an amplifier with incredibly high fidelity (Bryston, Levinson, Pass, McIntosh), etc. and speakers with equally high fidelity, I think that this might be serious overkill. Keep in mind that any circuitry that you add will also have a downside. Even resistors add noise as do opamps and other active circuits. Transformers can alter frequency response and cause phase shift, audible as smearing of the tone (although Lundahl is an excellent choice, as is Jensen). I think you are coming up with complicated answers to a question that isn't really there. I don't want to rain on your parades and I'm definitely not one to discourage experimentation and the never ending search of better sound, but I think in this particular equation, there might be better places to focus your most excellent energies. You might look at getting the best possible caps in the signal path and the quietest plate resistors in the preamp.  While I'm sure the Phonic amplifier is most capable of providing what you need, it's made to a price point that I might suggest would cut a lot more corners than something made to Alembic specs, so there might be opportunities to clean up the signal path in there. Again, good caps in the signal path can make a real difference way beyond what the preamp to amp connection would.  
 
However, mods in there would certainly void the warranty and dealing with power amp voltages and currents is best left to the pros (said as a disclaimer because I don't really know your abilities. Might be a piece of cake for you!).
 
Carry on!
Edwin

[sonicus]

As far as balanced outs go in  the Alembic product category I would love to find an  Alembic  IN-2  for a rack accessory . The IN-2 had 2 balanced outs ;is that correct ?  It was a power supply and direct out interface , right ?    
 
       There are many choices out there for a quiet direct box but I would like to have an IN-2 in my rack of Alembic gear.
It would be so cool to get a re-issue.

[sonicus]

This is a similar thread regarding balanced direct outs ;       http://www.alembic.com/club/messages/402/24871.html?1145398481
Reference  is made to an IN-2 unit

[rjw]

Slawie,  connecting the output of the F-2B to your power amp without modification is very likely to work well.    while your calculations to modify the plate resistor value is correct, such a change would not work in practice.    
 
for some circuit configurations your concern about loading a higher impedance output to a lower impedance input is correct and performance would be compromised.   but for a common-cathode tube configuration with signal levels in the range expected to drive your power amplifier, it will work fine.
 
the final tube stage will be loaded down by the power amplifier, requiring the volume to be raised to compensate.
 
but there will not be loss of bass response with the original 1/10 micro-farad output capacitor.  (understand the concern if the output were from a cathode-follower or op-amp, both of which would have low source impedances, but consider that the output impedance of approx 40 kilo-ohms and the load impedance of approx 20 kilo-ohms gives the total impedance of 60 kilo-ohms, for which the capacitor low-frequency roll-over is ok).  
 
connect with the balanced input, which will give you all the benefits of a fully-balanced interconnect with an unbalanced drive side.   connect the + amplifier input to the tip, and both the shield and - amplifier inputs to the sleve of the 1/4 inch plug on the pre-amp end.    
 
this is just the short answer for both impedance and balanced circuit aspects of your post, and sometime later plan to elaborate on these issues generalizing them to cover other circumstances, since all the texts i'm aware of that cover them well are long out of print, and no single reference covers all aspects.

[slawie]

Thanks so much Ron,
 
I appreciate that you had offered me your valuable time as well as your learned response.
 
I believe that at this stage it will be a case of try it and see but I am more confident that
it will reproduce the sound of my bass accurately.
 
Thanks to everyone for their input.
 
I am in awe that the founder of the company would reply to a technical question from a pleb.
 
That just reinforces my belief that the quality of a product starts at the top.
 
slawie

[jseitang]

or then if youre talking about balanced outs, you could always get an in-2 like i do!