see this preamp circuit diagram

[ataul]

I have build this Leach inspired Pre amp/line amp few days ago. sound comes wonderful.
48 volt power supply is simple just diod+2200mfd+220mfd+100Ohm+2200mfd+220mfd.
no any hum even at supper quite mid night in full volume.bass,presence and highs are well balanced.
I have a question, if anybody clarify this i will pleased to him.
see the circuit diagram, why input dc blocking capacitor so high value (47mfd)
and output dc blocking capacitor is only 2.2mfd.

 

[Calvin]

Hi,

the source will see R10 and Q1´s input impedance in series as R-part of the Input RC-Highpass-filter. Depending on the collector current of Q1 the input impedance may be only a few kOhms low.
The Output Cap C2 sees R7 in parallel to RL1.
R7 seems too low after my taste and its not clear which pupose it serves.
I´d rather choose 50k-100k for R7.
Then C2 would see an impedance of 25k-48k.
As the R-part would be alot greater in the Output than in the Input, the Cap value for the same cut-off frequency of the RC-filter could be smaller.

jauu
Calvin

ps. still not idea why R7 is chosen so low in value. Doesn´t make sense to me.

 

[ataul]

calvine i appreciate for ur discussions.

 

[juma]

...the source will see R10 and Q1´s input impedance in series as R-part of the Input RC-Highpass-filter. Depending on the collector current of Q1 the input impedance may be only a few kOhms low...

It seems that you overlooked R16 (100k) which will predominantly determine the input resistance of the preamp in the very wide freq. range. The Ib of Q1 (BC550C) is less than 1uA so it doesn't play a significant role - same as very mild HP input filter.
So it's perfectly OK to use 1uF for C1.

As for the output cap's (C4) value, it's OK (2u2) if we give to R7 (whose only job is to discharge the C4) a realistic value of about 220k-330k and if the input impedance of the next stage (power amp) is not lower than 45k-50k.

 

[ataul]

It seems that you overlooked R16 (100k) which will predominantly determine the input resistance of the preamp in the very wide freq. range. The Ib of Q1 (BC550C) is less than 1uA so it doesn't play a significant role - same as very mild HP input filter.
So it's perfectly OK to use 1uF for C1.

As for the output cap's (C4) value, it's OK (2u2) if we give to R7 (whose only job is to discharge the C4) a realistic value of about 220k-330k and if the input impedance of the next stage (power amp) is not lower than 45k-50k.

thanks Juma

here u see the R10, only 390 Ohm; so Q1 collect more current through C1.i think -3db low frequency roll-off going high around 8.6Hz to around 400Hz if i place 1mfd in input.it need R10 around 18kOhm if i place 1mfd in input if i maintain roll-off. as Mr.calvin discussions. what u think?

 

[juma]

R9 is in series with the source so its value is added to Q1's base resistance.
C1=1uF (not 1mF, it's 1000uF) will give you flat frequency/phase response from below 20Hz and upwards to at least 100kHz where the R10,R11,C5 filter kicks in.