Op-amp substitutes + transistor heat issues
The NE5534 is listed as the op-amp for U1 but my local supplier only has NE5532 (dual version). Can I use that, or do you strictly need the single NE5534? Also would a TL072 work as a substitute for U2?
The NE5532 is a dual NE5534 in a single package — you can absolutely use it for U1, just use one of the two op-amp sections. For U2 (the buffer), a TL072 will work fine since it's only copying the signal and bandwidth requirements are more relaxed there. The critical specs are on U1: high bandwidth and low noise.
Don't use LM358 as a substitute — learned this the hard way. The overdrive sounds genuinely terrible with it, lots of weird aliasing and the clean signal isn't actually clean. The NE5534 is cheap on eBay/AliExpress and makes a real audible difference. Worth waiting the extra week for shipping.
Ordered the proper parts. Got it all built now — sounds amazing! One concern: after about 10 minutes of playing, the TIP31 and TIP32 transistors are getting quite hot to the touch. I don't have heatsinks on them yet. Is that dangerous or can I just leave it?
Don't leave it without heatsinks — the TIP31/32 are rated for 2W dissipation and at 12V you're right at that limit. Small TO-220 aluminum heatsinks cost about $2 for a pack of 10 on Amazon and clip straight onto the transistors. Also a thin layer of thermal paste between the transistor and heatsink makes a big difference. With heatsinks mine runs barely warm after an hour of overdrive.
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