High quality preamp. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Preamplifiers
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"After a long search and a series of failures" - that's exactly what happened to me. I looked through a hundred of various magazines and brochures in search of a decent preamplifier circuit, and now it seems I found ... I will not hide, in the process of assembly and testing, I had to modify the circuit - remove and add something. In particular, an additional signal level control was introduced at the input of the PU (R1) with its help the input signal is attenuated to the nominal level and a loudly compensated volume control (R21) as well as the possibility of stepwise attenuation of the signal by 20 decibels (SA2).
The results exceeded all expectations - a correctly assembled PU according to the scheme I proposed in terms of sound quality far exceeds active PUs and many passive ones installed in high-end equipment. Therefore, I recommend to everyone who uses domestic amplifiers (regardless of class) to replace the preliminary amplifiers with the one brought to your attention. The sound will change beyond recognition, naturally for the better, transparency and purity will appear with a low level of noise and non-linear distortion.
The amplifier uses dual variable resistors R11 and R17 of any type of group B, R1 and R21 of any type of group C or A. As a heavily compensated volume control (R21), you can use a 100 kΩ variable resistor (with a tap from the middle). Transistors can be replaced with KT3107I, KT313B, KT361V, K (VT1, VT4) and KT312V, KT315V (the rest). Replacing the K574UD1 op amp with other types of op amp is not recommended. With a significant level of the constant component (in rare cases) at point A, it is necessary to install a capacitor with a capacity of 2.2 - 5 microfarads.
The described PU is connected to an AF power amplifier with an input impedance of at least 10 kOhm. With a significant increase in Kg, this PU can also be loaded on the UMZCH with Rin up to 2 kOhm (which is highly undesirable), in such cases (if the Rin of your UMZCH is less than 10 kOhm), you just need to power up the output stage again (a copy of the circuit section VT1-VT2- VT3-VT4-R4-R5-R6-R7, connect to the DA2 output), connect resistors R23 and R24 in the same way as resistors R2 and R3, although in this case the noise level may increase. And if Rin of your UMZCH is greater than or equal to 100 kOhm, then it is recommended to use K2UD574A (B) as an operational amplifier DA1, this will reduce the level of distortion and noise.
Possible changes in the scheme (improving):
The amplifier circuit is shown below:
(click to enlarge)
To exclude the P2K switches (very unreliable in operation) from the audio signal path, it is recommended to exclude the SA1 switch from the circuit (together with resistors R8, R9), and transfer the SA2 switch to the last stage by short-circuiting resistor R23 (resistors R13, R14 are excluded from scheme).
If you have any questions about this amplifier, please send me to bolero@chat.ru
Created on the basis of VPU Yu. Solntsev.
Publication: cxem.net
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