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In Reply to: RE: 26 preamp -- Assault & Battery vs. Phaeton LC posted by Tube747 on March 31, 2017 at 08:38:51
The first one is active crossover. 2C51 amplifies high frequencies, with 22 nF capacitor acting as a high pass filter. The 26 section amplifies low frequencies, with output transformer acting as low pass filter. In this implementation, a cheap output transformer is needed with relatively high leakage inductance.
The choice of 2C51 is strange. Its amplification factor of 35 is grossly mismatched with that of 26 (mu=8). Otherwise, it is a nice linear triode with low Miller capacitance and low Rp, if sections are paralleled.
At what frequency point for the high pass filter. Also, is this a preamp set up for "universal" speaker system?
High pass frequency will depend on the input impedance of HF power amplifier. It can be determined using an online RC calculator.
Active crossovers are used to eliminate passive speaker crossovers. If a standard two-way speaker is used in an active crossover system, its own crossover should be gutted; woofer and tweeter should be then directly connected to outputs of their respective power amplifiers. This is called bi-amping.
Would it be a better idea to have a larger output cap value for 2C51 section and let the 26 section to feed the adjustable XO of the active subwoofer? Let's say 1uf.
A fellow I know who has a pair of stacked 3 x Quad ESL 57 and wired in series, and running by a pair of Transcendent Sound Beast OTL, and system has Paradigm Sub II for under 40Hz and the subwoofer has built in room correction, so it can be perfectly matched between the gain of 26 and 2C51 in his system.
I will suggest him to build such an amplifier but the drawback is the impedance matching between the preamplifier and amplifier (or active amp of Paradigm subwoofer). This is because the output impedance of 26 section should be high, not sure how much, the input impedance of the Paradigm Sub II is 10K!!
He has Aikido linestage in his system but his choice of tube is 6GU7 and E288CC.
So this is an electronc crossover, not a preamp circuit?
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