getting hold of the 2SC2291 is usually very hard. I didn't want to rely on builders getting this
part so I again have chosen the THAT300P which has been used successfully in the earlier
TM3030 issues and Oakley Diode Superladder modules. The THAT300 is four very well
matched NPN devices in one package. In my tests I found that this part sounds identical to the
Japanese parts and is therefore an excellent substitute.
THAT300P can be obtained from Profusion and Farnell in the UK, Small Bear in the US, and
also at Mouser (part #887-300P14-U). It is a good part but is somewhat expensive.
The differential amplifier of the filter uses a single 2SC1583F in the original design. The
TM3030 allows for this if you can get hold of the 2SC1583. If not, then do not worry, as the
board can also be fitted with a hand matched pair of BC549B, BC550B, BC549C or BC550C
transistors in positions Q36 and Q37. Again, like the FET in the VCO buffer either the two
transistors or the 2SC1583 need to be fitted. Do not fit both U12, and Q36 and Q37.
As stated, if you are fitting them, Q36 and Q37 should be a matched BC549B, BC550B,
BC549C or BC550C pair. This means that both Q36 and Q37 should have identical
characteristics. It is unlikely that two randomly picked NPN transistors will be exactly the
same since the manufacturing process is not that precise. However, it is of my opinion that if
both transistors are of the same type, eg. both BC550C, and are bought new from the same
vendor and at the same time there is a very good chance that they will be matched well enough
for this application. Modern fabrication methods are much better than they used to be.
The VCA circuit also allows the builder some choice in the construction. The board is laid out
to accept either the original R-Ohm BA662 OTA chip, the excellent BA662 clone part from
'Open Music Labs', or a my own VCA design based around the CA3080 OTA and a discrete
buffer circuit. I have tested this part of the circuit very thoroughly, and I am convinced there is
little or no perceivable audible difference in the two designs.
It should be said that both OTA chips are now obsolete, but the CA3080 is still available from
many sources. The original BA662 is nigh on impossible to find without paying very high
prices so Open Music Lab's clone is the best bet if you want to go the BA662 route.
In either case, do ensure that you build your board to suit your choice. The 3080 design uses
an additional FET (Q35) and resistor (R118) and neither of these should be fitted if you are
using a BA662 or the new BA662 clone.
For the dual op-amps in the TM3030 I have specified the same parts as used in the original
unit. These are AN6562 devices. They have a very large input voltage range, that includes
zero volts, so they cannot be substituted with more common op-amps like the TL072.
Alternatively you can use the LM358 instead. This is a good part and although it does behave
slightly differently to the AN6562 in some conditions it does not appear to affect the
TM3030's sound in any way.
The AN6562 devices are available from various online places for a modest amount. You need
two of them, one for the VCO circuit, the other for the power supply. The LM358 is more
readily available.
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