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Old 5th July 2005, 09:44 PM   #1
name773 is offline name773  United States
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Default amplifier attempt #1

here's an amplifier i just designed with great assistance from Rod Elliot's articles and a select few of his designs. It is intended to drive a pair of grado sr80s, any ideas if it will sound ok/be stable/etc? thank you for any shared insight
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Old 5th July 2005, 10:18 PM   #2
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Looks sweet. But im thinking some thermal resistors could come in handy on the emitter of bd139/140.. then youre very close to a standard amp. Just change the input stage before the VAS to a differential input. And maybe there would be good to have a dominant capacitor to erase oscillation. Also a capacitor series with resistor on the output ?
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Old 5th July 2005, 10:23 PM   #3
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Hi
this baby seems to have unity gain, create a V divider with a resistor from output to emitter of Q1.
You can also give some smallish (like 0R33) emitter resistors for output transistors and create a 'diamond' at input (add a npn emitter follower) to minimize offset voltage.
Just a first glance, might be wrong.
best regards
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Old 5th July 2005, 10:47 PM   #4
name773 is offline name773  United States
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oops, i forgot the other resistor that determines the gain...

i would like to stay away from the differential input to preserve a smallish component count (i hope to make this thing portable and stereo)
unless of course, that would help minimize voltage offset. i'd rather not add another transistor between the input transistor and the output driver, because the collector current on the input transistor should be around 120uA to drive the base of the driver transistor, and that's pretty low as it is.

would it be ok to make R4 1k8 and C2 10uf? that gives a roll off frequency of ~8.84 hz, but will it allow enough current through?
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Old 5th July 2005, 10:48 PM   #5
Eva is offline Eva  Spain
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I think that the input transistor is not properly biased. The DC output of the circuit will be more than one diode drop below ground, so an output coupling capacitor may be required.
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Old 5th July 2005, 11:02 PM   #6
PRR is offline PRR  United States
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The output DC voltage will be around -0.6VDC, which will not do the Grados any good. Either add a second input device to give near-zero offset, or re-cast the whole thing with +12V supply and capacitor coupling input and output.

There is no shorted-load protection. Headphone cables get pinched, Q2 and Q6 try to pass infinite current. Your second iteration has 0.33 ohm resistors, cutting shorted-load current to only 18 Amps, a little more than Q6 is rated for and far more heat than you can handle.

I'm glad to see R10 R11 added (how would you adjust bias without a resistor somewhere to stick your voltmeter?) but still think they are too low. 10% of load impedance is a better goal. A more subtle goal (for Class AB operation) is 30mV-100mV at idle current. Assuming AB idle bias is 1/10th of peak current, both "rules" lead to about 3 ohms, not 0.3 ohms.
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Old 5th July 2005, 11:04 PM   #7
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Now it looks way better. I think bigger R4 may play even better, but remember to make R9 bigger too. In project this simple the best thing to do is build and experiment a bit, maybe some simulation first. Try doing it, don't let any one to put you off.
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Old 6th July 2005, 12:40 AM   #8
name773 is offline name773  United States
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thank you all for the excellent replies!

here is an updated version
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Old 6th July 2005, 01:01 AM   #9
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shouldn't base of q5 be to the right 'leg'?
Now you have much more amplifying devices and more open loop gain. Give q1 q2 q3 q4 and q6 emitter resistors of value somewhere in range 47R-330R for better performance and stability.
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Old 6th July 2005, 01:10 AM   #10
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Yes Darkfenriz,

Other leg for Q5 base and all the resistors stated. Q6 emitter should be just below R7 in value so keep the others small.

Cheers,
Greg
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