Earfonia PBA
(Pseudo Balanced Amplifier)
2 channels, 3 channels, or 4 channels amplifier (balanced), how much improvement will we get by having more channels? I started a proof of concept project last year to test it. This project is not about making an amp to be compared with other amp, but a proof of concept project to compare the sound characteristic of 2, 3, and 4 channels topology within the same amplifier.
To simplified the circuit, I use Op-Amp with simple non-inverting circuit (a.k.a CMoy) for every channel. I initially tried to make a balanced amp with 4 non-inverting channels (not bridge configuration), using resistor voltage divider to get the inverted signal, but failed on the grounding. Then I make use the board and the power supply to make 4 channels amplifier, to make what I called a Pseudo Balanced Amplifier. It is a 4 channels amp, looks like a balanced amp, but not a balanced amplifier because there is no inverted signal, only 2 independent ground channels for each driver, as shown in the diagram below. The 4 channels output is what I called the Pseudo Balanced Amplifier (since I haven't seen this topology on the web yet, so let me coin the term for it, Pseudo Balanced Amplifier, or PBA).
Some information about the amplifier circuit:
DC Coupled
Op-Amp: OPA552 (200 mA output current, 60 volt max supply)
Resistors: Vishay Dale
Gain: 13
Volume Pot: ALPS RK27 20k log.
Power Supply: Dual supply +/- 28 volt regulated (LM317 & LM337), 375 mA per channel.
Electrolytic Capacitors: Panasonic FC series, except the biggest cap is panasonic EC series.
The schematic for the power supply:
So it is a kind of high voltage, high power, 4 channels CMoy
OPA552 is a cranky op-amp, rather high DC offset at the output. I've tried many resistor combinations, and I found that to lower the DC offset, I have to use low value resistors. I also found that in the same circuit, different OP552 chip will have different DC offset value by a lot of margin. I bought 8x OPA552 chips to select 4 chips with DC offset less than 4 mV in the PBA circuit.
Since OPA552 is stable for gain 5 or higher, I set the gain at 13, rather high, my purpose is to drive the Orthos The ground channels gain is also set at the same gain, 13. So far It sounds alright with gain 13 on the ground channels It seems it is not necessary to set unity gain on the ground channel
The 2 empty op-amp sockets in the middle of the op-amp row, are from the previous project (SBA Project), and not used in the PBA circuit.
The setup I use to test my PBA:
Balanced headphone, Hifiman HE5-LE with DIY Silver Dragon v2 cable.
Computer with Dr. DAC2 DX (function as DAC), mostly playing 24bit / 96kHz or 24bit / 88kHz FLAC files.
FYI, my Dr. DAC2 DX Op-Amps have been upgraded with OPA2107 for the FDO section, and OPA1642 for the Line Out and Headphone Amp section.
I use DIY 4 pin XLR to TRS jack adapter to test the 2 channels and 3 channels output.
Sound Characteristic
The overall OPA552 sound characteristic in this high voltage circuit, Burr Brown sound, a tad midcentric, with nice liquid sounding, full bodied midrange. Midrange is thicker than OPA1642, more neutral and not as warm as OPA2604. Bass is proportionally good, full bodied with good control and texture. Treble is smooth, not as bright and transparent as most of the AD op-amps, also less transparent than the OPA1642, but overall sound is not dark, still quite neutral. Detail is good, better than OPA2604, but not as detailed as OPA1642. Presentation also quite neutral, not as laid back as OPA2107 or OPA627/637.
The overall sound differences between the 2, 3, and 4 channels are minimal, not much. Untrained ears might not be able to distinguish the difference. But when listened carefully, with some songs, the differences is quite noticeable.
2 Channels
The 2 channels output ground connected directly to the power supply ground.
The tonal balance is slightly more forward, less smooth, more grain compared to the 3 & 4 channels. Vocal sounds thinner and less warm. The Xylophone sound in song 'Nows The Time' from the album Jazz At The Pawnshop, sounds less weight and dry. The overall sound is also less airy and spacious.
3 Channels
The 3 channels use one active ground channel. One channel of the non-inverting OPA552 circuit is used for the ground channel. It sounds more natural overall. More body to the midrange, vocal sounds fuller and slightly (just slightly) warmer. The Xylophone sound in song 'Nows The Time' from the album Jazz At The Pawnshop, sounds weightier with more body. It sounds like the Xylophone metal is thicker. Imaging and instruments separation is about the same as the 2 channels, maybe slightly better in 3 channels, but not much.
4 Channels
The 4 channels output (PBA output) use independent active ground for each driver. The PBA output sounds best IMO. Vocal and instruments sounds fuller. Tonal balance is similar as the 3 channels, but bass is tighter with better control, imaging is noticeably better, more 3D sounding, instruments separation is also clearer, better focus. For comparison, The Xylophone sound in song 'Nows The Time' from the album Jazz At The Pawnshop, sounds more realistic than the 2 and 3 channels output, with better body to the sound.
That's my impression so far. On my PBA, I found the 3 channels output sounds better than the 2 channels, and the PBA output (4 channels) sounds best. Will test with more balanced headphone later, need to find time to re-cable some of my Headphones. My PBA is currently being tested by my friend Mike from Headfonia, he tested it with Hifiman HE6 which is famous for being difficult to drive. He will post his impression as a short article in Headfonia later.
Hopefully PBA topology can be another simple alternative for balanced headphones
18 May 2011 update:
Short article about PBA in Headfonia:
http://www.headfonia.com/earfonias-pseudo-balanced-amplifier-pba/
I agree with Mike:
...So affordable that the PBA can also stand for the “Poor-man’s Balanced Amplifier”.
(Pseudo Balanced Amplifier)
2 channels, 3 channels, or 4 channels amplifier (balanced), how much improvement will we get by having more channels? I started a proof of concept project last year to test it. This project is not about making an amp to be compared with other amp, but a proof of concept project to compare the sound characteristic of 2, 3, and 4 channels topology within the same amplifier.
To simplified the circuit, I use Op-Amp with simple non-inverting circuit (a.k.a CMoy) for every channel. I initially tried to make a balanced amp with 4 non-inverting channels (not bridge configuration), using resistor voltage divider to get the inverted signal, but failed on the grounding. Then I make use the board and the power supply to make 4 channels amplifier, to make what I called a Pseudo Balanced Amplifier. It is a 4 channels amp, looks like a balanced amp, but not a balanced amplifier because there is no inverted signal, only 2 independent ground channels for each driver, as shown in the diagram below. The 4 channels output is what I called the Pseudo Balanced Amplifier (since I haven't seen this topology on the web yet, so let me coin the term for it, Pseudo Balanced Amplifier, or PBA).
Some information about the amplifier circuit:
DC Coupled
Op-Amp: OPA552 (200 mA output current, 60 volt max supply)
Resistors: Vishay Dale
Gain: 13
Volume Pot: ALPS RK27 20k log.
Power Supply: Dual supply +/- 28 volt regulated (LM317 & LM337), 375 mA per channel.
Electrolytic Capacitors: Panasonic FC series, except the biggest cap is panasonic EC series.
The schematic for the power supply:
So it is a kind of high voltage, high power, 4 channels CMoy
OPA552 is a cranky op-amp, rather high DC offset at the output. I've tried many resistor combinations, and I found that to lower the DC offset, I have to use low value resistors. I also found that in the same circuit, different OP552 chip will have different DC offset value by a lot of margin. I bought 8x OPA552 chips to select 4 chips with DC offset less than 4 mV in the PBA circuit.
Since OPA552 is stable for gain 5 or higher, I set the gain at 13, rather high, my purpose is to drive the Orthos
The ground channels gain is also set at the same gain, 13. So far It sounds alright with gain 13 on the ground channels It seems it is not necessary to set unity gain on the ground channel The 2 empty op-amp sockets in the middle of the op-amp row, are from the previous project (SBA Project), and not used in the PBA circuit.
The setup I use to test my PBA:
Balanced headphone, Hifiman HE5-LE with DIY Silver Dragon v2 cable.
Computer with Dr. DAC2 DX (function as DAC), mostly playing 24bit / 96kHz or 24bit / 88kHz FLAC files.
FYI, my Dr. DAC2 DX Op-Amps have been upgraded with OPA2107 for the FDO section, and OPA1642 for the Line Out and Headphone Amp section.
I use DIY 4 pin XLR to TRS jack adapter to test the 2 channels and 3 channels output.
Sound Characteristic
The overall OPA552 sound characteristic in this high voltage circuit, Burr Brown sound, a tad midcentric, with nice liquid sounding, full bodied midrange. Midrange is thicker than OPA1642, more neutral and not as warm as OPA2604. Bass is proportionally good, full bodied with good control and texture. Treble is smooth, not as bright and transparent as most of the AD op-amps, also less transparent than the OPA1642, but overall sound is not dark, still quite neutral. Detail is good, better than OPA2604, but not as detailed as OPA1642. Presentation also quite neutral, not as laid back as OPA2107 or OPA627/637.
The overall sound differences between the 2, 3, and 4 channels are minimal, not much. Untrained ears might not be able to distinguish the difference. But when listened carefully, with some songs, the differences is quite noticeable.
2 Channels
The 2 channels output ground connected directly to the power supply ground.
The tonal balance is slightly more forward, less smooth, more grain compared to the 3 & 4 channels. Vocal sounds thinner and less warm. The Xylophone sound in song 'Nows The Time' from the album Jazz At The Pawnshop, sounds less weight and dry. The overall sound is also less airy and spacious.
3 Channels
The 3 channels use one active ground channel. One channel of the non-inverting OPA552 circuit is used for the ground channel. It sounds more natural overall. More body to the midrange, vocal sounds fuller and slightly (just slightly) warmer. The Xylophone sound in song 'Nows The Time' from the album Jazz At The Pawnshop, sounds weightier with more body. It sounds like the Xylophone metal is thicker. Imaging and instruments separation is about the same as the 2 channels, maybe slightly better in 3 channels, but not much.
4 Channels
The 4 channels output (PBA output) use independent active ground for each driver. The PBA output sounds best IMO. Vocal and instruments sounds fuller. Tonal balance is similar as the 3 channels, but bass is tighter with better control, imaging is noticeably better, more 3D sounding, instruments separation is also clearer, better focus. For comparison, The Xylophone sound in song 'Nows The Time' from the album Jazz At The Pawnshop, sounds more realistic than the 2 and 3 channels output, with better body to the sound.
That's my impression so far. On my PBA, I found the 3 channels output sounds better than the 2 channels, and the PBA output (4 channels) sounds best. Will test with more balanced headphone later, need to find time to re-cable some of my Headphones. My PBA is currently being tested by my friend Mike from Headfonia, he tested it with Hifiman HE6 which is famous for being difficult to drive. He will post his impression as a short article in Headfonia later.
Hopefully PBA topology can be another simple alternative for balanced headphones
18 May 2011 update:
Short article about PBA in Headfonia:
http://www.headfonia.com/earfonias-pseudo-balanced-amplifier-pba/
I agree with Mike:
...So affordable that the PBA can also stand for the “Poor-man’s Balanced Amplifier”.
