i guess i did not knwo this
got this from the API tech i talked to
P.S. All of these eq's are unbalanced, so if you put a balanced signal into them your level will be about 6 dB low.
and got this from him, great service!
Regarding your 550, there is little difference between the 550 and the 550A. Circuitry-wise, they are the virtually the same, sonically they are the same. The only big difference is in the hi and low frequency selectors. The 550 has two less frequencies on these two bands than the 550A. The 550A was just the rirst revision of that original 550 so there really is not much difference. The 550B, however, is a whole different animal. It has 4 bands instead of three, plus it includes peak and shelf toggle switches. The 550B does not include the hi-pass/lo-pass switches. You can visit our website to see the 550A and 550B to see the differences between them and yours. As for service, you can call us any time and we can give you a Return Authorization number so we can fix your unit. Shop time is $110 an hour, plus parts cost.