When I tried to find something about the Avalon U5, I realise that there was almost nothing about it.
The Avalon U5 is really different of other classical DI :
- it is a preamp : gain of maxi +30 dB is set by 3dB steps
- it's an equalizer : some useful Eq curves are already programmed in it.
- it has a very high clipping level : +24dBu.
A Classical DI box takes is power from Phantom power of the console, or from a 9V battery, giving a poor +4.5V / -4.5V power supply for the internal circuitry.
Here is the big difference of the U5 : you need a mains plug for it, and the power rails are much higher : + 32V / -32V.
When the box is opened, we could see that the DI contains 3 PCB :
- the main PCB, which includes pre amplifier, amplifiers , power supply
- the GAIN PCB which contains only resistors and gain switc. it is connected to main PCB by 2 wires
- the EQ PCB, which contains the passive EQ. it is connected to main PCB by 3 wires
Let's have a look inside :
On the main PCB, we could see that very few active components are used :
- BC546 : NPN Transistor 65V 100mA
- BC556 : PNP Transistor 65V 100mA
- BD139 : NPN 80V , 1.5A
- BD140 : PNP 80V , 1.5A
- LF412 : dual opamp
That's all for audio ! we also have two transistors for the power supply :
- TIP31
- TIP32
This is a discrete preamp / DI : that means audio amplification is made by transistors rather than integrated circuits.
And that's why we could get a High headroom.
With classical Opamps design, the limitation for power supply voltage is around +/-17V.
Here, the Avalon is going to +/- 32V .
The DI must have a very high input impedance : the datasheet claims that it is 3Meg.
how do we get such high impedance with bipolar transistor ?
to answer to this question, I have to do a bit of "reverse", as no schematic is available on the net for the U5.
By the way, we could verify by ourself if the mention :"DC coupling" "Super Bass" is true (and of course...it is not !)
Here is the schematic of the input circuit (simplified and partial) :
as it could be seen, the input stage has no differential pair : a single PNP transistoris used to create an asymetrical input with a very high input impedance.
Here is the real mystery of this circuit : the input impedance is approx 3Meg : that's rougthly the 3 resistors of 10Meg in parallel.
It seems that the input impedance of the bipolar transistor (which is quite low) does not count.