Hi,
I need some advice, how to change gain in this balanced preamp board? Which resistors I should to change?
There is small description about this board:
The distortion and noise of this line is much lower than that of a normal circuit, so the circuit looks less complicated and slightly more complicated, and the purpose is low distortion and low noise. In general, to convert the balanced signal into an unbalanced (single-ended) output signal, you can use 1/2 NE5532 plus 4 10K resistors, which can be achieved by connecting IC2A in the figure. At this time, R5~R8 are taken as 10K, a pair of balanced signals are input from the left end of R5 and R6 respectively, and a single-ended output signal can be obtained from the right end of R9. This differential amplifier circuit is quite simple. The output noise is also low, which can reach -104dBu (0dBu=775mV), but still feels a lot of noise. For this reason, the so-called double balance stage is adopted, that is, two differential amplifier stages are composed of IC2A and IC2B, and the balance is received at the same time. The input signals are added together to partially cancel out the uncorrelated noise, resulting in a 3dB noise reduction and improved circuit common-mode rejection ratio (CMRR).
Further improvements are also added to the first stage voltage followers IC1A, IC1B before IC2A and IC2B. This not only improves the input impedance, but also reduces the resistance of the eight resistors R5, R6, R7, R8, R13, R14, R15, and R16 of IC2A and IC2B from the original 10K to 820 ohms, greatly reducing the thermal noise of the resistor. Finally, the noise of this stage is reduced by -112dBu, which is 8dB better than the traditional single-difference circuit above.
My amp Class D need at least input 200mV-6V 102db. At the moment I have only +-120mV
Thank u!
I need some advice, how to change gain in this balanced preamp board? Which resistors I should to change?
There is small description about this board:
The distortion and noise of this line is much lower than that of a normal circuit, so the circuit looks less complicated and slightly more complicated, and the purpose is low distortion and low noise. In general, to convert the balanced signal into an unbalanced (single-ended) output signal, you can use 1/2 NE5532 plus 4 10K resistors, which can be achieved by connecting IC2A in the figure. At this time, R5~R8 are taken as 10K, a pair of balanced signals are input from the left end of R5 and R6 respectively, and a single-ended output signal can be obtained from the right end of R9. This differential amplifier circuit is quite simple. The output noise is also low, which can reach -104dBu (0dBu=775mV), but still feels a lot of noise. For this reason, the so-called double balance stage is adopted, that is, two differential amplifier stages are composed of IC2A and IC2B, and the balance is received at the same time. The input signals are added together to partially cancel out the uncorrelated noise, resulting in a 3dB noise reduction and improved circuit common-mode rejection ratio (CMRR).
Further improvements are also added to the first stage voltage followers IC1A, IC1B before IC2A and IC2B. This not only improves the input impedance, but also reduces the resistance of the eight resistors R5, R6, R7, R8, R13, R14, R15, and R16 of IC2A and IC2B from the original 10K to 820 ohms, greatly reducing the thermal noise of the resistor. Finally, the noise of this stage is reduced by -112dBu, which is 8dB better than the traditional single-difference circuit above.
My amp Class D need at least input 200mV-6V 102db. At the moment I have only +-120mV
Thank u!
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I don't know where you got this, but R4 and R12 are noise and offset generators. Make those 10k or so.
Double R7, R8, R15 and R16 for double gain.
Jan
Double R7, R8, R15 and R16 for double gain.
Jan
Thanks Jan,
I dont know but no background noise at all at the moment with this board, but if u saying R4 and R12 are noise generators... So I will change R4;R12 to 10Kohm and R7; R8; R15; R16 to 2kohm. I hope it will be enough mV to drive amp without background noise.
I dont know but no background noise at all at the moment with this board, but if u saying R4 and R12 are noise generators... So I will change R4;R12 to 10Kohm and R7; R8; R15; R16 to 2kohm. I hope it will be enough mV to drive amp without background noise.
It won't change the bass, but you will not have to turn up the volume as much.
If your sound is loud enough without having the volume control at max, this will not help with anything.
If it is not loud enough, turn up the volume.
If your volume is at max and it is still not loud enough, do the mod.
Jan
If your sound is loud enough without having the volume control at max, this will not help with anything.
If it is not loud enough, turn up the volume.
If your volume is at max and it is still not loud enough, do the mod.
Jan
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R4 and R12 are indeed offset generators but the offsets they generate are in common mode and cancelled by the following diff amps. The loss in headroom from the offset, around 200 mV with a 3µA bias current, is insignificant.
Whether R4 and R12 are significant noise generators depends on the source impedance feeding them which is in parallel (through R3/11) with the source.
A "double-balanced" stage is usually cross-coupled at the differential inputs and produces a balanced output.
The differential stages in what you show, as you point out, are in parallel.
My amp can handle easy 2x225w (2ohms) RMS. Now I did set full volume on amp, speakers are 200W RMS, but they play only half what they can. Problem is not enough input signal to drive aftermarket amp. Story is: my BMW has logic 7 audio system in car. From stock BMW L7 amp I found clean balanced audio signal and rewired to aftermarket amp. Aftermarket amp has only unbalanced signal, impossible connect balanced signal, so I upgraded aftermarket amp. Fitted these balanced unbalanced converters boards inside the amp, tested and works really great, no background noise good sound quality, but later figure out that not enough input signal.. So I did dicide tune this balanced preamp board, I need more gain.
You can if you want to. The argument for making them smaller wasn't particularly valid in the full context of the circuit.
who build this circuit (famous British power amplifier designer Douglas Self), it uses R7/R8/R15 and R16 10Kohm resistors but chinese tuned with 820ohms 🙂
Refer to Line Level standards here: https://en.wikipedia.org/wiki/Line_level then match sensitivity to be the same relative to power amp sensitivity, you then avoid adding unnecessary distortion, by amplifying any where else.
I did replace resistors to 2.2k, but still not enough gain. Yes, its feel difference but not enough. Maybe I will choose 10k resistors. Thank`s
jan.didden & mediatechnology 👍
Attachments
The issue only applies if the input is unplugged. So test it without an input and see if the noise warrants reducing.
Yes indeed. It still generates large offset due to imbalance in input bias DC resistance to gnd.
But maybe that is cancelled by tge diff amp downstream.
Jan
But maybe that is cancelled by tge diff amp downstream.
Jan