In a surprise appearance, Sen. Herman E. Talmadge (D-Ga.) led off his own misconduct hearings before the Senate ethics committee yesterday, declaring that he had done no wrong and assailing Daniel Minchew, his chief accuser, as a "proven liar, cheat and embezzler."

The Georgia Democrat, looking fit and with an angry ring in his voice, dismissed as "trivial" three of the five charges brought against him by the Senate Select Committee on Ethics. He denied the remaining allegations: that he was responsible for the submission of false Senate expense vouchers and for converting campaign contributions to his own use through a secret bank account.

Ethics committee members had not expected to hear from Talmadge during the opening session of the hearings. Last week he told the panel he would not be the leadoff witness, as the committee had planned, and he did not indicate whether he would testify at all.

But yesterday, after being introduced by his lawyer James Hamilton as a "great attorney from Georgia," Talmadge stole the opening day show with his statement to the packed Senate hearing room.

The senator challenged Minchew's allegation that he knew of a secret account set up in 1971 at the Riggs Bank here. A total of $39,000 in improper Senate expense reimbursements and mostly unreported campaign contributions were funneled through the account up to 1974, when Minchew resigned as Talmadge's administrative aide and campaign manager.

Talmadge said Minchew forged his name on the account and forged his signature on two Senate expense vouchers worth a total of $13,000. The expense money-for nonexistent items-was eventually deposited in the secret account.

Minchew has testified to the committee that another $26,000 in mostly unreported campaign contributions to Talmadge also went through the account. Minchew has said that Talmadge knew of and benefited from the secret bank account.

But yesterday Talmadge said Minchew had lied about the account to the committee, the Justice Department and the Internal Revenue Service about secret funds allegedly known to Talmadge.

He noted that he had ordered an audit last June when, he said, Minchew first told him of the existence of the secret account. Talmadge also said he promptly reported the fact that the two Senate expense checks were processed through the Riggs Bank account when he first learned of them.

"These are steps that only a fool would take if he were aware. . . that there was a hidden phony bank account waiting to be found." Talmadge said. "Even my enemies don't claim I'm stupid."

According to Talmadge, it was Michew who badly needed money at the time the funds were diverted throught the secret bank account. He said Minchew in 1974 owed or spent $40,000 more than "his legitimate sources of income."

The senator said that, by contrast, his income in 1974 exceeded $80,000, that his wife made $80,000 in the same year and his son made $50,000. Talmadge said his net worth at the time was more than $1.5 million, and said he turned down speaking engagements with "generous honoraria" that he would have accepted if he needed money.

"A critical question is," he said, "who was motivated to steal money-Daniel Minchew or Herman Talmadge?"

On the matter of false office expense vouchers, Talmadge repeated previous assertions that the fault lay with sloppy bookkeeping by his staff. He said he knew nothing of the expense vouchers until they were reported in news accoutns last summer.

Once they were reported, he said, he ordered an audit, instructing the auditors "to be stringent and to resolve all doubts against me." The audit found that, between 1972 and 1978, the Senate overpaid Talmadge's office a total of $37,125. Talmadge has repaid the Senate.

Following the Talmadge's opening statement T. Rogers Wade, who replace Minchew as senior staff aide, told the committee that records of office expenses were poorly kept. He said it was standard practice under him and under Minchew to draw office expense money up to the maximum from the Senate, even if the expenses had not been occured.

In his statement, Talmadge also addressed the other three charges.

On the tax question, he said all 15 stock transactions with his former wife could bring a total gift tax levy of $1,070. But he denied any tax was due, because most of the transactions, he said, were not gifts. The others, said Talmadge, were lawful gift-tax exclusions and deductions. "At most," he said, "it is an argument between accountants."

On the unreported gifts, he said the total amount came to "around $1,000." He did not explain why they were not originally reported on his Senate disclosures, but he said they were declared on amended reports. He said he did not declare the disputed piece of property claimed by both himself and his former wife because his accountant advised him not to. Talmadge also said he didn't list 13 free plane rides from "friends" between 1970 and 1976 because the Senate rule on declaring the rides was unclear.

On the allegation of filing false campaign reports in 1973 and 1974, Talmadge said the major problem was confusion among his staff about the election law. In 1973, he said, he had not yet declared himself a candidate in Georgia and his staff concluded that since technically he was not a candidate, he did not have to file federal campaign reports. The following year he said his staff became confused with the amended federal campaign law and failed to file. Talmadge was later reimbursed a total of $26,00 for his personal 1973 and 1974 campaign expenses from the campaign fund, but he did not declare the reimbursements until they were reported in news accounts last year.

"The erros have been corrected," Talmadge said. "There is not the slightest basis for concluding that those errors were due to anything but inadvertence and confusion."

The senator left one major question about his finances still unanswered.

His former wife has told the ethics committee that a cache of $100 bills was kept in an overcoat in their Washington apartment until 1974. Yesterday Talmadge said his attorneys would prove the money did not come from the secret Riggs bank account and that it could not have been in his overcoat in 1974. He offered no elaboration but said his former wife's testimony conflicted with her earlier sworn accounts.

Following his half-hour opening statement Talmadge was not questioned by the committee; and he refused to answer questions from reporters. Instead he sat through the remainder of the first-day hearing puffing on a long cigar. Committee special counsel Carl Eardley called the Talmadge statement "a very good jury speech."

Talmadge's statements yesterday were not under oath. A spokesman fro the senator said after the hearing that no decision has been made on whether he will return to answer questions under oath. The committee has left that decision up to Talmadge. CAPTION: Picture 1, Talmadge, right, waits to defend himself as the Senate ethics committee opens its hearings on misconduct charges against the Georgia senator. Photos by James K. W. Atherton-The Washington Post; Picture 2, "Even my enemies don't claim I'm stupid."; Picture 3, In Dirksen Building, special counsel Carl Eardley of Senate ethics committee is silhouetted against files on Herman Talmadge; hearings continue today. By James K. W. Atherton-The Washington Post