Andrea- Depending on the requirements there are several vendors of differential amp-probes. For quick and dirty testing like you mentioned look at this: https://www.quantasylum.com/content/Products/QA190.aspx For less than $100 and including a current clamp (AC only) it’s a good deal. QA has a number of tools (scope, source, audio analyzer) that are very high value. There are other diff probes. A Google search will find a lot of options depending on whether you are looking for high gain or high voltage, but they start at around $500. They have replaced the older model of the preamp + probes it seems. Demian > Hello Mark. > > Yes, the AM502 is the thing I am searching, but they are not very > cheap for a 1977 vintage, in particular adding the mainframe. > > It's possible that in 38 years no maker did anything like? > Looking better, all other modules I saw aren't equipped with front > panel output, because are in effect vertical oscilloscope amplifiers > and thus the output is on the backplane. And they are not cheap also. > > Best regards, > Andrea Baldoni