A simple stereo Amp aroud a 2N3773

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I have been using CE1704 for decades.

Until very recently when MY MISTAKE took out a module, I have never damaged any of the 6 modules I bought.
They are rated as 170W into 4ohms for domestic type use.

I use them exclusively as 100W into 8ohms amplifiers. They actually measure as 110W into 8r0 on my UK voltage.
Two 150W devices for a total Pdis of only 300W would indicate a maximum output power of ~60W, but with a cool to cold heatsink and doestic type use they can easily manage as 100W into 8ohms and can be tested into a 4r0 dummy load for a few seconds (heatsink and Tc still cold).
 
Hi Andrew

I'm sure that it would be possible to get 150W into 4 ohm loads with a pair of 2N3773's
, but my concerns are that the data sheets still show 200kHz fT. To design a power amp with these which operate to 20kHz would mean having a gain of only 10 (lower at higher frequencies) and although the gain is only 15 anyway at 8A, it does mean that the drivers need to be quite beefy - typically having to drive at least 0.6A - and if these have a gain of only 50 (say) then the base current they need would be at least 12mA which means running the VAS at least 25mA if not more. I've no doubt it CAN be done, and with higher fT devices the task is easier.

I use a pair of MJ21193/4 for 100W into 8 ohms myself (0.003% thd at 20kHz) and would recommend these. Higher gain, more linear gain, better fT etcetc.

What fT do your 3773's measure?
 
typically 3773 outputs are used in emitter follower stages so the voltage gains are less than 1. the amplifier loop stability can be a concern for low Ft devices in amps not designed for it. not plug n play in modern designs.
I think the overall open loop gain would have to be reduced for dominate pole compensation or other types of compensation ( circuit changes) employed to have closed loop gains f-3db beyond 20KHz. with resultant lower overall negative feedback, giving less ideal performance as a result.
 
Hi Andrew

I'm sure that it would be possible to get 150W into 4 ohm loads with a pair of 2N3773's
, but my concerns are that the data sheets still show 200kHz fT. To design a power amp with these which operate to 20kHz would mean having a gain of only 10 (lower at higher frequencies) and although the gain is only 15 anyway at 8A, it does mean that the drivers need to be quite beefy - typically having to drive at least 0.6A - and if these have a gain of only 50 (say) then the base current they need would be at least 12mA which means running the VAS at least 25mA if not more. I've no doubt it CAN be done, and with higher fT devices the task is easier.

I use a pair of MJ21193/4 for 100W into 8 ohms myself (0.003% thd at 20kHz) and would recommend these. Higher gain, more linear gain, better fT etcetc.

What fT do your 3773's measure?
4ohms is a speaker load, i.e a reactive load.
4r0 is resistor, i.e. a dummy/test load.

They are very different in what effect they have on the stresses on the amplifier.

A 8ohms capable amplifier is stressed more by a 8ohms speaker than by a test to full power into a 4r0 test load.

I said "can be tested into a 4r0 dummy load for a few seconds (heatsink and Tc still cold)".
I said "I use them exclusively as 100W into 8ohms amplifiers".

B.Powell has kindly posted the sch of some of the Crimson amplifiers. They are 3stage outputs with a pre-driver and include 1.5A high speed drivers. They also are of the "British style" for compensation, i.e. they do not use an added Cdom around the VAS stage. There will only be the inherent C of the To92 VAS device.

I have read about testing fT on the Forum, but I have not tried it on any of my transistors.
Have you found that easy to achieve with normal "amateur" equipment?
 
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To make an fT measurement I usually make a simple test with a spot measurement which is intended to be in the gain roll-off region. I use a simple filament lamp stabilised wien bridge oscillator (not an LDO) with a unity gain buffer (to drive high currents) which is just a wide bandwidth amp using BD131/BD132 outputs. This is generally fixed at 1 MHz. The transistor under test is set up in a classic CLass A bias circuit using four resistors, but the collector load is low to keep the collector voltage swing low. The emitter resistor is bypassed with a capacitor or three to ensure it is held at AC ground. The oscillator is fed through a fifth resistor, connected to the base which I generally use an amplitude and resistor combination to give approx. 10% modulation of the DC value, and measure the collector voltage on a scope.

Generally for BD139's etc the gain is 100 or so and the roll-off slope is not apparent. For lower frequency transistors the fT can be estimated from the measured gain.

For more accurate results, you can plot several frequency response points to check if you are on the slope, but I find a spot measurement is usually adequate.
 
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