The ratification of the US Constitution in 1788 was by the state legislatures, and only free white-male property owners were allowed to vote in state elections. The total popular vote in the first election for the House of Representatives is based on the same electorate and it was only about 21,000 or about .5% of the total population in the 1790 census. http://althistory.wikia.com/wiki/United_States_House_of_Representatives_elections,1788-89(Election_Insurrection)
In addition, ratification by the Rhode Island only occurred after a threatened trade embargo from the rest of the states passed the US Senate. In spite of the tiny number in the electorate, most of the language of the original Constitution along with the entire Bill of Rights still has the force of law.
Any change to the this language with a constitutional amendment would require 2/3 majorities in both the House and Senate along with ratification by 3/4 of the state legislatures. The number of voters in the last presidential election was over 168 million, so effectively it would take the voice of roughly 10,000 modern voters to override the voice of one voter in 1788.
How does that square with equal protection and equal voting rights that are guaranteed under the constitution? Is the constitutional amendment process really unconstitutional?
In Britain, a simple majority in the House of Commons is sufficient to change the constitution. Is that a better system?
On the other hand, the European Union requires the approval of every member state to amend the treaty used to form the union. How is that likely to work over time?