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AudioGeek 25th June 2006 11:17 PM

inductance required for a BUZ900 / 23K135?
If i wanted to load a BUZ900 or 2SK135 with a transformer or inductor as a load... what would be a good minimum inductance required say for:

-3db at 10Hz or so?


Christer 25th June 2006 11:54 PM

Unless I misunderstand your question it will depend on the output impedance of the circuit, and thus depend both on the topology and of the other components in the circuit. Thus there is no answer to your question, without showing the whole circuit diagram.

AudioGeek 26th June 2006 02:55 AM

it is a buz900 connected to a 3:1 step down loaded with 4-8 ohms.

power supply 40vdc and 1A of bias current on mosfet.

need to figure out how much inductance i need on the primary.


AndrewT 26th June 2006 07:09 AM

that sounds more like a tube (valve) type question, where you must match impedances to allow the tubes to work correctly.

For solid state, which work down to very low impedances since they are effectively constant voltage sources, you simply have to use the turns ratio squared to get the effective impedance at the FET. ie. about 36r to 72r.
By design you seem to have chosen your bias to just meet the minimum requirement for current into your load.

The big thing here will be ensuring that the iron in the transformer does not become saturated at the higher current values. That is down to area of the core and limbs and loosely (maybe?) connected to the primary turns.

richie00boy 26th June 2006 08:11 AM

The inductance is not the issue. The bass -3dB point is determined by the core and it's saturation limits. For 10Hz you will need a BIG transformer.

Ouroboros 26th June 2006 09:52 AM

If the output transformer is in the drain circuit of the MOSFET, then the primary inductance will have a substantial effect on the open-loop LF response, as the drain circuit of the MOSFET approximates to a constant-current sink.

If the transformer ratio is 3:1 then it will provide an impedance transformation of 9:1, so that a 4R speaker provides a 36R load at the drain.

You want the XL of the transformer to be much higher than the reflected load. If the transformer has an inductance of 1H (say), then at 30Hz the XL will be 2*PI*F*L = 188 Ohms. So a 1H primary is OK as 188R is adequately higher than the 36R presented by the load.

smms73 27th June 2006 09:56 PM

i think you are making a single end, so its very dificult to make the transformer with a very hight inductance because the transformer need a air gap to prevent him from saturate. Ouroboros is right in the value of 1h for 3:1 but the transformer have to be big .

some year ago i have done a single end with a inductance of 65mH as charge for a mosfet, (4homs speakers), and the bass was very good.

Ouroboros 28th June 2006 06:55 AM

I have built a single-ended source-follower power amplifier using a 125mH 5A choke as the load (together with a 10,000uF capacitor to connect to the speaker) and this works very well indeed. Luckily I had obtained two of the 125mH power chokes from an old and very large power supply. To buy them new would have cost a lot of money.

To buy a suitable 3:1 transformer to use in the drain circuit (or collector circuit) of a single-ended amplifier will seriously deplete your finances!

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