Monday, February 20, 2012

4 input headphone amplifier with TPA6120 chip

I already have schematic and PCB design with TPA6120 chip with 2 inputs. Now I made same design but with 4 inputs and one line output. The new design contains PCB transformer, dual +/- 15V power supply, 4 channel mixer without preamplifiers, unbalanced/balanced converters, and line output with level adjustment.

The schematic:

The device contains 4 audio inputs with volume adjustment. The input connector must be soldered to the bottom of PCB (blue) and the volume must be potentiometers soldered to the top (yellow) of PCB. The input and line output connectors can be 6.3mm jack OR stereo RCA connector. The headphone output can be 6.3mm jack only. The PCB contains header for 4 power filter circuits (whats are optional if not required) for 4 dual operational amplifiers. The operational amplifier chips are compatible with TL072 for cheapest solution, NE5532 for better quality, and with LT1124 for the best quality. These chips used for the input mixer, for unbalanced/balanced converters, and for the line output amplifiers.

The PCB design:

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PCB sales of this project
Module name Size
PDF SCH PCB image Tested Price (US$)
Des. Sim.1 Full2 Sim.1 Full2 Man3
4 input headphone amplifier
189x112 212 No
Yes Yes - - No 30
39 Ask
How to order? Please read the rules carefully!

Sunday, February 19, 2012

More unbalanced to balanced converters

At my previous post I simulated the unbalanced/balanced converter I using for unbalanced microphone inputs and headphone amplifier. The used chips (INA217 and TPA6120) have symmetrical inputs. These chips with balanced source have much better dynamics, noise, and sound quality. I think the selected converter I used is very good, but I wanted to simulate another solutions just for fun.

This very simple schematic without required capacitors. This is the reason why the AC analysis have very good result, but with required parts the result of stability and the phases would be changed:

The AC analysis of the pure schematic:

This sample is more complex, but the result is not the best. The reason is that the operation amplifiers not same with inverted and non-inverted mode, but the schematic is symmetrical for - and + outputs:

The result of AC analysis:

Simple "pure" solution again without capacitors and another required parts. This "base" circuit working well but with whole system have to be modified:

The AC analysis is perfect, but we got another result with required parts:

On the previous 3 examples have different method negative feedback on operational amplifier. Sometime the input connected to inverted and non-inverted inputs, and the required amplifiers have several feedback. By this example the inverter circuit getting the input signal from the output of first stage. Very stable example with really good AC analysis result:

The AC analysis:

The final example have more than one versions. The benefit of these schematics are the symmetrical solution of + and - outputs, which have same (or very closed) frequency responses and phases. The previous circuits have no capacitors on the negative feedback, or if have the capacitors modified the inverted stage only, the original signal more linear than the inverted. With the current solution, the frequency response and the phase of inverted and non-inverted stages are relative parallel, not like in the 2nd example what is serious problem.

The simplest version:

The AC analysis where the frequency responses are same, the phases are not linear, but running parallel, and the difference is very closed to required 180 degree:

The first modification is the active feedback between the outputs and inverted input for the adjust of inverted stage output level by the R15 resistor:

The final version of this really interesting converter is two operational amplifiers added for the output. This modification have same AC result than the previous version:

The AC analysis:

This example have gain from 707mV to 11V what is not required. If this gain is too much, modify R4/R5 and R9/R11 resistors to adjust the output gain. When the gain of stages modified, set again the output amplitude to exactly same by R15 resistor what can be trimmer potentiometer:

The result of AC analysis is very closed but modified when the gain changed:

The question is, what is the best solution if unbalanced/balanced converter required. The most simple versions are looks like perfect solutions, but with additional (and required) capacitors and with non exactly same resistors the phase and the amplitudes has been shifted. The another reason of differences is the difference between inverted and non-inverted mode of same operational amplifier stage. The simplest (and the best) schematics are block diagrams only, very good base but have to be modified. The useful solutions have two useful version: the input signal connected to same inverted and non-inverted stage. The another one is the input signal connected to non-inverted stage, and the output of this stage connected to inverted-mode operational amplifier. The another differences between the negative feedback. The second example, where the original input signal uses same stage but one with inverted one with non-inverted mode, the result is not really useful. This is the difference between modes of operational amplifiers.

The another question is the same frequency response of - and + outputs. If the original signal uses the simplest solution with direct negative feedback (like in the 1st example), and the inverted stage have negative feedback with capacitor, the frequency response (and the phases) will be different. If the difference shows after 40kHz by AC analysis, I think the result is very useful.

The last 5th example is interesting only. Not the simplest solution, and the problem is, the phase modified by the input frequency. The difference between + and - outputs is constant 180 degree, but always modified the phase values. This is the reason why I think the one of the best result for balanced conversion is on my previous post.

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Saturday, February 18, 2012

Simulation of unbalanced - balanced converter

I using unbalanced/balanced converters with my circuits like microphone preamplifiers and headphone amp because the used chips (INA217 and TPA6120) have symmetrical balanced inputs. This is very important, the sound and the dynamics are much better with this solution. I simulated the schematic of converter, and here are the results:

With scope and AC voltmeter on real time simulation:

AC analysis:

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Tuesday, February 7, 2012

Simulation of UREI LP/HP filter module

Maybe this is the last post about UREI parametric filter projects. On the previous post I finalized the design of modular parametric EQ. I have only one idea for the future, the stereo version of this project, but the biggest problem the potentiometer with 4 adjustable resistors, what is not impossible. The simulation of parametric EQ successfully done, but the original values of high pass/low pass filter have to be modified. This filter circuit with original values started too high, and the possible highest frequency is around 15 kHz what is too low.

This is the schematic of HP/LP filter module:

In this module the most important parts are C2 C3 and C4 C5 capacitors. Noted, this equalizer made for instruments amplification like guitar and vocal, not for home hi-fi, for these applications the middle range of audible frequency is good (better) solution than the full frequency range. But I wanted more, because this module is adjustable like all others in this project. I think, if the middle positions of potentiometers have good result, I have reserved range and more possibilities.

On the first version, the capacitor values like in the schematic, and the simulation result of LP filter is here:

This solution with the end position of potentiometer cutting the highest frequency around 40kHz what is too much. Not a big failure, because the value is adjustable, but not required - compared to the original EQ where the end value is around 15kHz. This is the reason why the capacitors changed to 5.6 nF.

The AC analysis of HP filter:

The start value is around 14Hz what is too low. The end value of this filter is around 326Hz what is too low again. This is the reason why decreased the capacitor value to 330nF.

The simulation of both LP/HP filter at one graph:

With the new simulation the capacitors of LP filter changed to 5.6nF, the capacitors of HP filter changed to 330nF. Here is the AC analysis of HP filter:

The LP filter with 5.6 nF:

Both analysis at one graph:

Not impossible to use this filter module alone, but I analyzed how modified the frequency response of parametric EQ modules. The potentiometers of LP/HP filters set to 50% middle positions:

  • The red line if all modules off (bypassed)
  • The blue is the result if only LP/HP filter on with middle positioned potentiometers
  • The green line if all EQ modules on but the LP/HP filter module off
  • The maroon line if all modules on.

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