Futterman H3a OTL amp for sale seen on on eBay!

Just saw a Julius Futterman H3a power amp listed for sale on eBay today. Great piece of audio history to own !

rdiiorio04-10-2020 5:56pm

noromance Details Discussions Posts This discussion Message User 9,833 posts 04-18-2020 at 02:26pm Copy Comment URL Report Comment Edit Delete Early ones had stability issues. http://www.atma-sphere.com/Resources/OTLs_Defined.php Reading more, it seems JF improved the circuit and fixed the issue.
stevecutler Details Discussions Posts This discussion Message User 20 posts 04-20-2020 at 07:05pm Copy Comment URL Report Comment Edit Delete Futtermans like high impedance and were designed for 16 ohm and above speakers. These were very popular in early hi-fi. Will work OK but the low limit is 8 ohms. They are much more efficient the higher the impedance. Never heard of them taking out output tubes. A very novel tranformerless design.
rdiiorio OP Details Discussions Posts This discussion Message User 35 posts 11-18-2024 at 02:58pm Copy Comment URL Report Comment Edit Delete Also note many tried to drive low imp speakers with Futterman OTL which looked for high imp ( such as KLH Model Nine 16~30 ohm ) loads. When driving low imp designs when the impedance dipped to 4 ohms or less it would oscillate. ALL speakers have a varying impedance, it depends at what frequency is being delivered to the speaker when the measurement is being taken.