[sdiy] Moog ladder filter transistor pairs matched or not..??

[Andrew Simper]

On 25 February 2014 02:03, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> You won't be able to match the resonance without simple calibration.
>
> I own an early RA Moog Minimoog with all five transistor pairs matched
> in the filter.  I have played two other RA Moog Minimoogs earlier than
> mine and the resonance quality of the filter of both were very close
> to mine.
>
> When I play later minimoogs with only the top and bottom transistor
> pairs matched, the resonance quality is different from mine and they
> were all different from one another.
>
> The big reason is the unmatched emitter resistance slope between the
> unmatched pairs, thus the poles of the 6dB/oct RC product of those
> pairs will not be in the same position between filters (read: the
> resonance changes).  The only calibration that will fix this is one
> that varies with cutoff current and feedback (resonance) signal in the
> ladder and that is far more trouble than it is worth.
>
> MC

With older transistors that is probably true, but I have a feeling
that for modern designs the capacitors not being matched will make
more difference to the 6 dB point. I had a chat with Cyril Lance about
the matching of the central transistors a while back at Musikmesse and
he said that they deliberately don't match them in their current
designs.

--Andy



>
> On 2/24/14, Mattias Rickardsson <mr at analogue.org> wrote:
>> ...the resonance will not match between voices *without proper calibration*
>> ,
>>
>> or
>>
>> ...the resonance will not *even with calibration be able to* match between
>> voices?
>>
>> /mr
>>  Den 23 feb 2014 20:56 skrev "Michael E Caloroso"
>> <mec.forumreader at gmail.com
>>>:
>>
>>> If you're only using it in a monophonic system, you can get away with
>>> the top and bottom matched pairs.
>>>
>>> The minute you use it in a polyphonic system, you need all 5 pairs
>>> matched or the resonance will not match between voices.
>>>
>>> MC
>>>
>>> On 2/18/14, Gordon JC Pearce <gordonjcp at gjcp.net> wrote:
>>> > On Tue, Feb 18, 2014 at 09:12:22AM -0500, Jean-Pierre Desrochers wrote:
>>> >> You are probably right Tom,
>>> >> I remember now reading something like what you just stated..
>>> >> In fact what I need to know for my future projects
>>> >> is what are the 'picky' transistors that absolutely need matching..
>>> >> in that design. I don't care spending some time
>>> >> on my tranny matching rig to make sure the VCF
>>> >> works as it's supposed to.
>>> >>
>>> >> JP
>>> >
>>> > Having tried matching diodes and transistors, I have come to the
>>> conclusion
>>> > that - particularly with cheap Chinese parts bought for pennies per
>>> hundred
>>> > - any random two you pull out of the bag will be Pretty Damn Close.
>>> >
>>> > --
>>> > Gordonjcp MM0YEQ
>>> >
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>>
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