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Introducing Ventus

Transistor matching

Elsewhere it is indicated that specification can be improved by matching the transistors. Since these are very inexpensive, one can buy a bunch, and match them by measuring hfe with a capable Multimeter.

Although I find no claims of audible differences, matching the transistors is part of the fun of diy. Which of the transistors would benefit from hfe matching?

Thanks and Merry Christmas to all...
 
Hi glt,

If you feel like matching the best way to do it would be to create matched pairs for each pair in the schematic.

For example QN2/3, QP1/2, QP3/4, QN5/6, QN7/8, QN1 and QN4 do not need to be matched at all. All pairs need only be matched to their mate. You do not need to match "all" the transistors.

One other change that people might want to play with is adjusting R6/7. The value of those resistors sets the current through the front end. They should be 1% or better.

Cheers!
Russ
 
Russ/Brian,

A few questions that I figured I'd ask here so that they could be used by others.

1. If I want to run both SE and Bal inputs is that easily possible? From my reading here it sounds like there are some adjustments that need to be made to the amp boards to run SE inputs.

2. Is there any risk (oscillation) running the amp at unity gain? I'd like to set this up to use with sensitive headphones like IEMs to start. Any issues with running a switch that would jumper R3 when enabled?

3. Could you list the complete set of recommendations if one is planning to power the Ventus with the Placid BP? As a start do I need to remove the 1000uF caps that I already installed on the ventus boards? Second, what's the recommended initial setting for the shunt current? I saw mentioned somewhere that the expected draw is ~30mA so I'm thinking 35mA through the shunt would fit your normal +10% recommendation that I've seen elsewhere.

Thanks in advance, this looks like a really cool project. Well, for a headphone geek anyway. ;)

Happy Holidays!

Nate
 
Thanks glt, I'd seen that post and was looking for confirmation that the caps could stay without penalty. I may remove them, since it would allow for much lower profile stacking of the boards.

Also, what's the output impedance of this amp? I'm curious given the inductor and resistor that are in series with the output.
 
Thanks glt, I'd seen that post and was looking for confirmation that the caps could stay without penalty. I may remove them, since it would allow for much lower profile stacking of the boards.

Also, what's the output impedance of this amp? I'm curious given the inductor and resistor that are in series with the output.

The output impedance of the Ventus at Audio frequencies is virtually the same as the DCR of the inductor. Which is something less than .05R (I don't have the datasheet open). At frequencies where the inductor impedance begins to rise the parallel resistor will dictate the output impedance. This would be in the Mhz frequency range.

The output inductor is actually optional. I added as a safety measure against capacitive cables. It will keep the amplifier very stable. If you find you don't need it you can simply use a jumper there instead.

Cheers!
Russ
 
Russ,

Thanks for the output impedance explanation, I'd measured the resistance across the inductor/resistor assembly and found it to essentially be 0 but just wanted to make sure I wasn't missing something.

If you get a chance could you bounce back to page 3 and have a look at a couple of the questions I asked there? Specifically I'm curious about implementing a pseudo gain switch and also how best to approach offering both SE and balanced inputs.

Thanks in advance.

Nate
 
A couple of questions before I start building the kit

1- L1 and R1 are both optional? I can connect the output directly to a headphone such as HD580?

2- With balanced input, I can DC couple the output of OPUS?. In fact, no dc blocking caps all the way to the headphone?

3- To nulll the offset: What to do with the inputs when nulling the output

Thanks!
 
1- L1 and R1 are both optional? I can connect the output directly to a headphone such as HD580?

2- With balanced input, I can DC couple the output of OPUS?. In fact, no dc blocking caps all the way to the headphone?

3- To nulll the offset: What to do with the inputs when nulling the output

Thanks!

1) Yes, but you will need to short one of them :)

2) Yes.

3) The best is to connect them to your source. Or short the inputs to GND.

Cheers!
Russ
 
2) For non-inverting use: Reduce R2 to 221R and make R4 47K (this will set the input impedance). Connect -IN to GND. Then adjust R5 and R3 set the gain. Gain = 1+(R5/R3). For unity gain simply omit R3. For example. Using R5 of 1K and R3 of 1K would get you a voltage gain of 2, which is 6db.

Russ,

Could you please clarify how to configure the amp for unity gain? Do you completely omit (do not populate) R3 for unity gain or should you jumper this position? I'm assuming it's the former and not the latter.

Any progress on the manual?