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.

1.
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:


2.
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:


3.
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:


4.
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:


5.
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.

See also:

• Simulation of favorite unbalanced/balanced converter
• 4 input headphone amplifier
• 2 input headphone amplifier
• Modifier microphone preamplifier
  

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:


See also:

• Simulation of several unbalanced/balanced converters
  
• 4 input headphone amplifier
• 2 input headphone amplifier
• Modifier microphone preamplifier

Modified microphone preamplifier with INA217

I really like the microphone preamplifier with INA217. No noise, very good sound and dynamic, no feedback, very high (max 10.000x) gain, very simple application. Lot of websites testing and comparing this design with another industrial devices. The article, when this is the best mic preamp and the cost is USD5 only, maybe not true. Not only the INA217 required, you need PCB, transformer, 2 potentiometers, input and output connectors, power supply.

I modified the original design recommended on the datasheet. The original application circuit on datasheet is really simple and very good microphone preamplifier. I added a better unbalanced/balanced converter after unbalanced jack input. I have this feature before, but now I have better circuit. The another modification is the output. Because this preamp is mono, the simple connection to stereo system is not the best. The result may increase the crosstalk between left and right channels. This is the reason why I included two line driver circuits to separate left and right channels. Finally because this new design contains 3 integrated circuit, I inserted 3 "power filter" for less noise.

The modified preamp design with input unbalanced/balanced converter and output line drivers:


The PCB with simple connectors:


The same design without connectors, because this PCB can be inserted to the mainboards of modular mixers. The connectors changed to P4 header:


Now here is the list of features:

• Balanced input
  
• 6.3 jack as Unbalanced input, but with unbalanced/balanced converter
  
• Gain adjustment
• Volume adjustment
• Connector for 48V phantom power
  
• Switch for 48V phantom power
  
• Switch between unbalanced jack and balanced XLR input
  
• Divided line driver inputs connect to stereo systems
  
• Power filter circuits for all 3 integrated circuits for less noise
  

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des. Sim.1 Full2 Sim.1 Full2 Man3 Mic preamp pro with connectors 79x104 82 Yes Yes Yes - - No 13 21 Ask Mic preamp pro as mixer module 67x103 69 Yes Yes Yes - - No 11 19 Ask How to order? Please read the rules carefully!

Preamplifiers

I finished two of my new projects with preamplifiers. One of them contains independent preamplifier schematics, another one is an audio mixer with modular system where the preamplifier-modules have to be connected to the mainboard.

The simplest preamplifier with very high gain contains only 1 dual opamp. The sound quality depend on the amplifier circuit. Recommended to try the PCB with TL072, NE5532, or LT1124. Maybe the maximum gain will lower with better quality circuits, but the noise and the sound quality will better with NExxxx or LTxxxx dual operational amplifiers. The board of this schematic contains potentiometers for gain and volume adjustment, and 6,3 mm stereo jack input. Stereo circuit, like all others except microphone preamp. The board contains all required potentiometers, jack input, and two switches for stereo/mono and true-bypass. The switches have to be soldered to the small PCB with 12 pins "L" connector. This switch-board have to be connected to the preamplifier's PCB if required, if not, just wire the required pins of the connector on the preamp-board to ignore these switches. This very simple schematic have very big gain, but for microphone is not enough, for guitars too much. The sound of circuit with TL072 dual opamp is good for mixed music, but not for instruments like guitar or guitar effect. This is the reason why I recommended for home music listening. The "P1" connector on board is option for LED output to indicate if something connected to the jack-input.

Schematic:


PCB:


The second version of above schematic with two single operational amplifier instead of one dual:

This PCB is bigger, but very good solution for single operational amplifiers. The ultra low noise single OpAmps are not cheaper, but easier to find than dual. LF356, OPAxxx, LT1028, LT1115 (instead of dual LT1124) are the alternatives for better solutions than the cheapest TL071. The board contains one "power filter" circuit to reduce the noise of power supply.

Finally I have version no.3 for the Opamp solutions. The stereo circuit contains 2 single Opamps instead of one dual like above, but two power filter circuits included, one-one for both operation amplifiers. This solution is better for ultra low noise circuits like LT1028 or LT11115.


The small circuit contains switches:

This circuit contains 3 switches, but SW1 and SW2 are same. Only the place of these parts are different. If required only one switch, use SW3 only.

The PCB of switches:

This really small board required for all preamplifiers, except Dual-jFET preamplifier. If the switches are not required, just wire the correct pins of the P1 connector on the amplifier's board.

The dual-jFET preamplifier.

This is my favorite schematic for guitars or guitar-effects. The circuit contains one dual jFET for stereo input. No noise, good dynamics, warm sound. No symmetrical power required, therefore the power filter is smaller and simpler. The gain and volume potentiometers, and the stereo jack input connected to the PCB. The maximum gain is lower than the Opamp preamplifier's, but enough for guitars, guitar effects, and keyboards, or another instruments, except microphones.

See the schematic:


PCB:

This PCB have no bypass and stereo/mono switches. Maybe later will be included. The originally planned part 2N3958 Dual jFET is not available now, but any low frequency dual N-channel jFET compatible with this solution. Here is the table to find another dual jFET.

The next schematic is a microphone preamplifier with very cheap and good quality operational amplifier INA217. This is the recommended application schematic of this integrated instrument preamplifier. The PCB contains two mono potentiometers for gain and volume, and one 6,3mm jack input for unbalanced inputs like guitars. The switch-module contains only one switch to select between unbalanced/balanced inputs. The unbalanced input is the 6,3 jack, the balanced have 3 pins connector, and external XLR input required. For 48V phantom power required an independent external switch to the case if you want to use condenser microphones.

Very cheap and good quality circuit. No noise, no feedback, very high gain, good dynamics. Compared to cheap Phonix mixer, this circuit was much better with Sennheiser and Behringer dynamic vocal microphones. The 48V input for phantom power included to the PCB but power supply required for this feature. Unbalanced to balanced converter circuit included after jack input to the board. This is cheap and simple converter, but with the recommendation of Jensen. Later will be changed this converter to better one. The PCB contains 3 integrated circuits, but one power filter circuit included.

The PCB:


The PCBs of these preamplifier circuits drilled with same positions on the bottom. With these holes possible to fix to the case of mixer or preamplifiers. Later, all of these preamplifiers modified to module of audio mixer. The inputs, outputs, and powers wired to the pins of connectors.

Finally here is the recommended +-15V power supply:


PCB:


The PDF about preamplifier projects.

As I posted, all preamplifiers converted to module of audio mixer with 12 pins connectors. With these connectors the preaplifiers can be connected to the mainboards of mixer. All of the power filter circuits connected as module to the new PCB of preamp-modules. The mixer-mainboard contains only mixing circuits and 2 stereo line outputs:


Gallery about preamplifiers:



Picasa gallery about modular mixer:

PCB sales of this project Module name Size (mm) Area (cm2) PDF SCH PCB image Tested Price (US$) Des. Sim.1 Full2 Sim.1 Full2 Man3 Microphone preamp V:2 130x105 137 Yes Yes Yes Yes - No 20 30 Ask 1 dual opamp preamp 95x104 99 Yes Yes Yes Yes - No 15 24 Ask 2 single opamp preamp 89x110 98 Yes Yes Yes Yes - No 15 24 Ask HQ 2 single opamp 105x112 118 Yes Yes Yes - - No 18 27 Ask jFET guitar preamp 70x108 76 Yes Yes Yes Yes - No 12 20 Ask Switch module 18x34 6 Yes Yes Yes Yes - No 3 11 Ask Simple Power supply 121x57 69 Yes Yes Yes Yes - No 11 20 Ask Power supply with 48V phantom 168x89 150 Yes Yes Yes - - No 22 32 Ask How to order? Please read the rules carefully!