QRV-05 - The high performance SMD Diamond buffer

 

The component side and also solder side. The component print

The performance is very good. See the technical data!

Interesting features:

  1. The pcb fits virtually any SOT23 small signal transistor and almost any SOT223 power transistor.
  2. The emitter loads have cascodes in order to increase linearity.
  3. SMD caps, including electrolytic caps.
  4. SMD parts which makes the buffer very small and thin.
  5. Input LP-filter if you have problems with RF interference, if not just omit it.
  6. Crossover distortion reducer for the opamp. This is a popular circuit in the DIY community.
  7. Useful for anything as long as power supply voltage is within limits and load not to heavy.
  8. Professional pcb with groundplane which establish good cooling and shielding.

 


 

The background

This type of buffer is called "diamond buffer" but I don't where the name comes from. I have also seen the name "Jung Super Buffer" (from his article). Many IC's have this diamond buffer, LT1223BUF634LME49600 and many more.

This buffer can be used in a preamp as line stage without gain, as a buffer for DAC's and CD-players. You can also use it for driving headphones. The buffer is rather small and very thin so it can be placed in narrow spaces.

There are lot's of good info out there on this subject. Walter Jung has written a couple of good articles and one of them is about this particular buffer. In fact his article is THE best for a normal DIY'er.

Reference articles

Realizing High Performance: Buffers (Part I)

Realizing High Performance: Buffers (Part II)

The ancient LH0002 is also a simple diamond buffer. The equally ancient LH0033 consists of a JFET buffer.

AN-47 from Linear is a big document, very well written. Look especially at page 45-47. Those pages describes a discrete diamond buffer.

AN-227 from National is also worth reading.

This Texas document has also a part about diamond buffers.

 


 

The schematics

The picture shows the schematics of the amp. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

The design fits in only one schematic page.

 


 

Ciruit description

I have chosen to use current generators with cascodes as emitter loads. How much good thecascodes do is unknown at the moment. The only obvious disadvantage is a little bit reduced output voltage swing, otherwise only positive things.

The current in the first stages is set by T1, T3 and T2, T4 along with R6 and R7. I have transistors as diodes just to make it simpler when it comes to ordering parts. The current is 0.65/220= 3 mA and the output stage will have approx. 6 mA. The class A circuit will have 1.5mA. My choice of currents is "average", because you both increase them och also decrease them. The max current is set by the max allowable power dissipation of the transistors. The smallSOT23 types can take 310mW or so and the SOT223, output transistors can dissipate 1.3 W but not in this application I imagine. More current mean slightly higher speed.

The input filter R5, C1 may be necessary if you are having trouble with RF interference. The values can be set to almost anything. The frequency is calculated by f= 1/(2*pi*R5*C1).

Please note that this buffer has no current limitation in order to protect the circuit so a short circuit may damage the output transistors. The buffer is intended to be used in controlled environments. If your load is unknown you may add a resistor at the output, 220 ohms or higher.

 


 

The pcb layout

Click on the picture to see a preview of the pcb.

The component print

The picture shows the component print of the PCB. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

The printed circuit board is made for one channel. It requires stabilized power supply.

The pcb have groundplane on the lower (solder) side (not shown in pictures) which is essential in order to get good results.

Almost everyone of the traces are on the solder side. The groundplane is very covering, only a few traces on the solder side.

The component side

The picture shows the component side of the PCB. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

Almost everyone of the traces are on the component side as you can see. The groundplane is on the solder side.

 


 

Build directions

This design is very easy to build. You should start with all low SMD parts. Solder in the electrolytic caps at last. For this solder job I recommend a good pair of tweezers. I use a type especially made for SMD. With this pair of tweezers I get a very good grip. You must also have a soldering iron with a small tip but.

Make sure if you want the class A circuit or not, T13, T14, R14, R15. Determine also if you want any of the R1, R2 or R3 resistors. If you don't want the input filter just short R5 with tin or a small wire.

Start with all low SMD parts

  1. Resistors
    Transistors
    Capacitors, not the electrolytic ones
  2. Then continue with the high electrolytic caps
  3. Wires

Check the finished amp

Apply voltage and check the output voltage. Measure the DC voltage at the output. It should be the specified offset voltage, a couple of millivolts.

Measure the quiescent current.

The whole buffer is ready.

Good luck and happy listening. Welcome to Per-Anders Sjöström's hifi pages.

 


 

Technical data

Click on the picture to see a preview of the pcb.

Frequency response: 0 Hz - 100 MHz(?), more than 10 MHz
Frequency response with input filter 0 Hz - 1.29 MHz, -3 dB
Power bandwidth at 7 Vrms, 50 ohms: 0 Hz - 100 MHz(?), more than 10 MHz
Equivalent input noise: 0,6 µV
Signal to noise ratio: 124 dB at 1 V out
Dynamic headroom: 141 dB
Distortion: 0,002%
Slew rate: 3000 V/us
Step response: Perfect without overshoot
Gain: 0.999 (0 dB)
Input impedance: >1 Mohms, pullup, pulldown resistors not included.
Output impedance: 2.5 ohms
Output current: 1 A peak
Supply voltage: +- 6 to 24 V DC with chosen transistors
Dimensions: 37,5 (1,475") x 35,6 (1,4") mm