Matching transistor pairs?

SMR-4 BOM mentions that “Filter tuning is improved by using matched, thermocoupled transistor pairs”

What is the recommended method of matching transistor pairs?

Is simple gain matching (hfe) enough? Like the one described here:

Or one should go for a more complex method suggested by Music From Outer Space here:

Thank you,

Matching the gain should be enough ; but the most significant improvement is to make sure both transistors are at the same temperature.

Sounds good, thank you!

What do you recommend for having them at the same temperature? Glue both transistors to heat conducting material (like copper?) using J-B Weld ?

Thank you,

Yes, that would be a good idea to glue them.

I have hFE tested and matched some transistors for the smr-4 filter and have a few questions.

I have 2 matched pairs of 2n3904
The hFE reading of the first pair is approx 120 and the second pair is 152
Is a higher reading better? Which pair should i use?

secondly would this apply to the 2n3906?
I also have similarly matched 2n3906’s ranging from 151 to 223
Since there are multiple pairs of these on the board is there a preference as to where the higher or lower valued pairs should be placed in the signal path?

lastly has anyone used any other transistors other than the ones specified in the kit.
there is an electronic parts store near me that has quite a lot.
Any suggestions?

Is there really much of a substantial difference to the sound to be bothered worrying about all this


This doesn’t affect the sound at all, only the tuning (ability of the sine wave emitted when the filter goes into self oscillation to be playable “in tune” like an oscillator). I don’t bother with this in the unit I build, and get 4 octaves tracking.

Sorry that’s what I meant to say tuning not sound.
Late night post

So it doesn’t make a difference which i use of the( higher or lower) hfe valued pairs
as long as they are a matched pair? Sorry if this is stupid question
I’m just trying to keep myself occupied while I wait for some poly caps.
The reason I ask is I have a lot of 3904-3906 transistors with varying values and since i have them already i would like to use the best choice. Thanks

Also I would like to add that I haven’t been this excited about a new piece of gear in a long time
Amazing job Oliver. Im already thinking about buying another one maybe two :slight_smile:


To be honest you don’t even need a matched pair for nominal operation…

The circuit doesn’t rely on a particular value of hfe.

Thank for the quick response.

Looks like I’ve been worrying over this for no reason.
I guess its time to finish up then. Sooo excited can’t wait to be done