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V=IR, Db=20 log V1/V2 , Bat 51SE

I am considering mating a Bat 51Se with Pass X350.5. Input impedance of the pass is 20K ohms balanced. Bat output impedance was specified at 200 ohms. If data is true, than the match should be great at least on paper.

According to stereophile measurements, the output impedance of the Bat rose to 4.7K ohm at 20Hz. Assuming this is true, I am interested in calculating the roll off I'd experience.

R1 output imped of preamp
R2 input resistance of amp

assuming preamp does not increase current to maintain constant output voltage, the drop in voltage should be proportion to the drop in resistance ( V=IR).

Ideally: R2>>R1 so R=R1 (1/R=1/R1+ 1/R2) or R=R1R2/(R1+R2)

My situation: R2=4.26 R1
so: R = 0.81 R1

therefore voltage drop is 19% at 20Hz

Using the conversion to decibel

drop in Db= 20 log V2/V1 with V1 (ideal voltage), V2 (actual output)

= 20 log 0.81
= -1.83 Db

If sterophile measurements and the calculations are correct, I would experience a 1.8 Db drop at 20Hz.

1) I have not done this stuff for a long time, please verify whether I am right or wrong.

2) If I amp calculating system gain in Db, it should be

db= 10 log (X1/X2) , should the loss be 0.9db instead of 1.8db?

3) Does preamp increase current to maintain constant voltage?

Thanks in advance.
=

glai

Showing 1 response by bombaywalla

bombaywalla's avatar
bombaywalla
1,958 posts
 
>> 2) If I amp calculating system gain in Db, it should be
>> db= 10 log (X1/X2) , should the loss be 0.9db instead
>> of 1.8db?
No, I think that 20log() is correct as we are dealing with power ratios: V2^2/R2 div-by V1^2/R1 where the 2 voltages cancel out & the impedances are ratioed 1:0.81.

>> 3) Does preamp increase current to maintain constant
>> voltage?
yes. that's what Eldartford was stating as well. The preamp should have some negative feedback around it. Thus, as its output impedance increases, it'll draw more current to output the same voltage per the volume knob setting. The power supply should have enough capacity for the job. If it didn't then only would the output voltage drop. So, under the elevated output impedance condition, there'll be more power dissipated in the preamp i.e. lossier load power transfer.

>> 11-03-06: Bob_reynolds
>> But, since its internal impedance increases there will
>> be a larger voltage drop internally. So less voltage
>> will reach the amp.
no, more power dissipated in the pre internally.
less voltage ONLY if the preamp power supply runs out of ability to supply current. Hopefully an expensive pre like the 51SE will have that headroom!
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