Can I spell a voiced "th" without using IPA?

Solution 1:

This list of pronunciation respelling systems on Wikipedia gives a good idea as to what's used in various systems.

Here is a screenshot (irrelevant rows removed):

IPA1  IPA2    APA NOAD    AHD RHD WBO MECD    DPL DPN TBD NBC MWCD    COD4    POD Cham    SD  AB  Dictcom BBC Wikipedia¹  Wikipedia²  Examples//θ θ   θ   ᴛʜ  th  t͡h th  th  th  th  th  th  th  th  th  th  th  th  th  th  th  th//ð   ð   ð   ᴛʜ  th  th̸ th  th  th  t̷h TH  th: th  dh  th  dh  TH  dh th   dh  dh  dh

So, /θ/ (unvoiced th) is usually written as "th", while /ð/ (voiced th) has more variation:

  • "dh"
  • underlined t̲h̲
  • capitalized TH
  • italic th
  • various strikethroughs (th̸, t̷h, TH)

Solution 2:

The American Heritage Dictionary uses:

  • 'th' for unvoiced th /θ/
  • 'th' (italicized) for voiced th /ð/.

Reference (PDF).